1 .. SPDX-License-Identifier: GPL-2.0
7 /proc/sys/net/ipv4/* Variables
8 ==============================
11 - 0 - disabled (default)
14 Forward Packets between interfaces.
16 This variable is special, its change resets all configuration
17 parameters to their default state (RFC1122 for hosts, RFC1812
20 ip_default_ttl - INTEGER
21 Default value of TTL field (Time To Live) for outgoing (but not
22 forwarded) IP packets. Should be between 1 and 255 inclusive.
23 Default: 64 (as recommended by RFC1700)
25 ip_no_pmtu_disc - INTEGER
26 Disable Path MTU Discovery. If enabled in mode 1 and a
27 fragmentation-required ICMP is received, the PMTU to this
28 destination will be set to the smallest of the old MTU to
29 this destination and min_pmtu (see below). You will need
30 to raise min_pmtu to the smallest interface MTU on your system
31 manually if you want to avoid locally generated fragments.
33 In mode 2 incoming Path MTU Discovery messages will be
34 discarded. Outgoing frames are handled the same as in mode 1,
35 implicitly setting IP_PMTUDISC_DONT on every created socket.
37 Mode 3 is a hardened pmtu discover mode. The kernel will only
38 accept fragmentation-needed errors if the underlying protocol
39 can verify them besides a plain socket lookup. Current
40 protocols for which pmtu events will be honored are TCP, SCTP
41 and DCCP as they verify e.g. the sequence number or the
42 association. This mode should not be enabled globally but is
43 only intended to secure e.g. name servers in namespaces where
44 TCP path mtu must still work but path MTU information of other
45 protocols should be discarded. If enabled globally this mode
46 could break other protocols.
53 default 552 - minimum Path MTU. Unless this is changed mannually,
54 each cached pmtu will never be lower than this setting.
56 ip_forward_use_pmtu - BOOLEAN
57 By default we don't trust protocol path MTUs while forwarding
58 because they could be easily forged and can lead to unwanted
59 fragmentation by the router.
60 You only need to enable this if you have user-space software
61 which tries to discover path mtus by itself and depends on the
62 kernel honoring this information. This is normally not the
72 fwmark_reflect - BOOLEAN
73 Controls the fwmark of kernel-generated IPv4 reply packets that are not
74 associated with a socket for example, TCP RSTs or ICMP echo replies).
75 If unset, these packets have a fwmark of zero. If set, they have the
76 fwmark of the packet they are replying to.
80 fib_multipath_use_neigh - BOOLEAN
81 Use status of existing neighbor entry when determining nexthop for
82 multipath routes. If disabled, neighbor information is not used and
83 packets could be directed to a failed nexthop. Only valid for kernels
84 built with CONFIG_IP_ROUTE_MULTIPATH enabled.
93 fib_multipath_hash_policy - INTEGER
94 Controls which hash policy to use for multipath routes. Only valid
95 for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.
103 - 2 - Layer 3 or inner Layer 3 if present
104 - 3 - Custom multipath hash. Fields used for multipath hash calculation
105 are determined by fib_multipath_hash_fields sysctl
107 fib_multipath_hash_fields - UNSIGNED INTEGER
108 When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
109 fields used for multipath hash calculation are determined by this
112 This value is a bitmask which enables various fields for multipath hash
117 ====== ============================
118 0x0001 Source IP address
119 0x0002 Destination IP address
121 0x0008 Unused (Flow Label)
123 0x0020 Destination port
124 0x0040 Inner source IP address
125 0x0080 Inner destination IP address
126 0x0100 Inner IP protocol
127 0x0200 Inner Flow Label
128 0x0400 Inner source port
129 0x0800 Inner destination port
130 ====== ============================
132 Default: 0x0007 (source IP, destination IP and IP protocol)
134 fib_sync_mem - UNSIGNED INTEGER
135 Amount of dirty memory from fib entries that can be backlogged before
136 synchronize_rcu is forced.
138 Default: 512kB Minimum: 64kB Maximum: 64MB
140 ip_forward_update_priority - INTEGER
141 Whether to update SKB priority from "TOS" field in IPv4 header after it
142 is forwarded. The new SKB priority is mapped from TOS field value
143 according to an rt_tos2priority table (see e.g. man tc-prio).
145 Default: 1 (Update priority.)
149 - 0 - Do not update priority.
150 - 1 - Update priority.
152 route/max_size - INTEGER
153 Maximum number of routes allowed in the kernel. Increase
154 this when using large numbers of interfaces and/or routes.
156 From linux kernel 3.6 onwards, this is deprecated for ipv4
157 as route cache is no longer used.
159 neigh/default/gc_thresh1 - INTEGER
160 Minimum number of entries to keep. Garbage collector will not
161 purge entries if there are fewer than this number.
165 neigh/default/gc_thresh2 - INTEGER
166 Threshold when garbage collector becomes more aggressive about
167 purging entries. Entries older than 5 seconds will be cleared
168 when over this number.
172 neigh/default/gc_thresh3 - INTEGER
173 Maximum number of non-PERMANENT neighbor entries allowed. Increase
174 this when using large numbers of interfaces and when communicating
175 with large numbers of directly-connected peers.
179 neigh/default/unres_qlen_bytes - INTEGER
180 The maximum number of bytes which may be used by packets
181 queued for each unresolved address by other network layers.
184 Setting negative value is meaningless and will return error.
186 Default: SK_WMEM_MAX, (same as net.core.wmem_default).
188 Exact value depends on architecture and kernel options,
189 but should be enough to allow queuing 256 packets
192 neigh/default/unres_qlen - INTEGER
193 The maximum number of packets which may be queued for each
194 unresolved address by other network layers.
196 (deprecated in linux 3.3) : use unres_qlen_bytes instead.
198 Prior to linux 3.3, the default value is 3 which may cause
199 unexpected packet loss. The current default value is calculated
200 according to default value of unres_qlen_bytes and true size of
205 neigh/default/interval_probe_time_ms - INTEGER
206 The probe interval for neighbor entries with NTF_MANAGED flag,
211 mtu_expires - INTEGER
212 Time, in seconds, that cached PMTU information is kept.
214 min_adv_mss - INTEGER
215 The advertised MSS depends on the first hop route MTU, but will
216 never be lower than this setting.
218 fib_notify_on_flag_change - INTEGER
219 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
220 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
222 After installing a route to the kernel, user space receives an
223 acknowledgment, which means the route was installed in the kernel,
224 but not necessarily in hardware.
225 It is also possible for a route already installed in hardware to change
226 its action and therefore its flags. For example, a host route that is
227 trapping packets can be "promoted" to perform decapsulation following
228 the installation of an IPinIP/VXLAN tunnel.
229 The notifications will indicate to user-space the state of the route.
231 Default: 0 (Do not emit notifications.)
235 - 0 - Do not emit notifications.
236 - 1 - Emit notifications.
237 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
241 ipfrag_high_thresh - LONG INTEGER
242 Maximum memory used to reassemble IP fragments.
244 ipfrag_low_thresh - LONG INTEGER
245 (Obsolete since linux-4.17)
246 Maximum memory used to reassemble IP fragments before the kernel
247 begins to remove incomplete fragment queues to free up resources.
248 The kernel still accepts new fragments for defragmentation.
250 ipfrag_time - INTEGER
251 Time in seconds to keep an IP fragment in memory.
253 ipfrag_max_dist - INTEGER
254 ipfrag_max_dist is a non-negative integer value which defines the
255 maximum "disorder" which is allowed among fragments which share a
256 common IP source address. Note that reordering of packets is
257 not unusual, but if a large number of fragments arrive from a source
258 IP address while a particular fragment queue remains incomplete, it
259 probably indicates that one or more fragments belonging to that queue
260 have been lost. When ipfrag_max_dist is positive, an additional check
261 is done on fragments before they are added to a reassembly queue - if
262 ipfrag_max_dist (or more) fragments have arrived from a particular IP
263 address between additions to any IP fragment queue using that source
264 address, it's presumed that one or more fragments in the queue are
265 lost. The existing fragment queue will be dropped, and a new one
266 started. An ipfrag_max_dist value of zero disables this check.
268 Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
269 result in unnecessarily dropping fragment queues when normal
270 reordering of packets occurs, which could lead to poor application
271 performance. Using a very large value, e.g. 50000, increases the
272 likelihood of incorrectly reassembling IP fragments that originate
273 from different IP datagrams, which could result in data corruption.
276 bc_forwarding - INTEGER
277 bc_forwarding enables the feature described in rfc1812#section-5.3.5.2
278 and rfc2644. It allows the router to forward directed broadcast.
279 To enable this feature, the 'all' entry and the input interface entry
286 inet_peer_threshold - INTEGER
287 The approximate size of the storage. Starting from this threshold
288 entries will be thrown aggressively. This threshold also determines
289 entries' time-to-live and time intervals between garbage collection
290 passes. More entries, less time-to-live, less GC interval.
292 inet_peer_minttl - INTEGER
293 Minimum time-to-live of entries. Should be enough to cover fragment
294 time-to-live on the reassembling side. This minimum time-to-live is
295 guaranteed if the pool size is less than inet_peer_threshold.
298 inet_peer_maxttl - INTEGER
299 Maximum time-to-live of entries. Unused entries will expire after
300 this period of time if there is no memory pressure on the pool (i.e.
301 when the number of entries in the pool is very small).
308 Limit of socket listen() backlog, known in userspace as SOMAXCONN.
309 Defaults to 4096. (Was 128 before linux-5.4)
310 See also tcp_max_syn_backlog for additional tuning for TCP sockets.
312 tcp_abort_on_overflow - BOOLEAN
313 If listening service is too slow to accept new connections,
314 reset them. Default state is FALSE. It means that if overflow
315 occurred due to a burst, connection will recover. Enable this
316 option _only_ if you are really sure that listening daemon
317 cannot be tuned to accept connections faster. Enabling this
318 option can harm clients of your server.
320 tcp_adv_win_scale - INTEGER
321 Count buffering overhead as bytes/2^tcp_adv_win_scale
322 (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
325 Possible values are [-31, 31], inclusive.
329 tcp_allowed_congestion_control - STRING
330 Show/set the congestion control choices available to non-privileged
331 processes. The list is a subset of those listed in
332 tcp_available_congestion_control.
334 Default is "reno" and the default setting (tcp_congestion_control).
336 tcp_app_win - INTEGER
337 Reserve max(window/2^tcp_app_win, mss) of window for application
338 buffer. Value 0 is special, it means that nothing is reserved.
342 tcp_autocorking - BOOLEAN
343 Enable TCP auto corking :
344 When applications do consecutive small write()/sendmsg() system calls,
345 we try to coalesce these small writes as much as possible, to lower
346 total amount of sent packets. This is done if at least one prior
347 packet for the flow is waiting in Qdisc queues or device transmit
348 queue. Applications can still use TCP_CORK for optimal behavior
349 when they know how/when to uncork their sockets.
353 tcp_available_congestion_control - STRING
354 Shows the available congestion control choices that are registered.
355 More congestion control algorithms may be available as modules,
358 tcp_base_mss - INTEGER
359 The initial value of search_low to be used by the packetization layer
360 Path MTU discovery (MTU probing). If MTU probing is enabled,
361 this is the initial MSS used by the connection.
363 tcp_mtu_probe_floor - INTEGER
364 If MTU probing is enabled this caps the minimum MSS used for search_low
369 tcp_min_snd_mss - INTEGER
370 TCP SYN and SYNACK messages usually advertise an ADVMSS option,
371 as described in RFC 1122 and RFC 6691.
373 If this ADVMSS option is smaller than tcp_min_snd_mss,
374 it is silently capped to tcp_min_snd_mss.
376 Default : 48 (at least 8 bytes of payload per segment)
378 tcp_congestion_control - STRING
379 Set the congestion control algorithm to be used for new
380 connections. The algorithm "reno" is always available, but
381 additional choices may be available based on kernel configuration.
382 Default is set as part of kernel configuration.
383 For passive connections, the listener congestion control choice
386 [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
389 Allows TCP to send "duplicate" SACKs.
391 tcp_early_retrans - INTEGER
392 Tail loss probe (TLP) converts RTOs occurring due to tail
393 losses into fast recovery (draft-ietf-tcpm-rack). Note that
394 TLP requires RACK to function properly (see tcp_recovery below)
404 Control use of Explicit Congestion Notification (ECN) by TCP.
405 ECN is used only when both ends of the TCP connection indicate
406 support for it. This feature is useful in avoiding losses due
407 to congestion by allowing supporting routers to signal
408 congestion before having to drop packets.
412 = =====================================================
413 0 Disable ECN. Neither initiate nor accept ECN.
414 1 Enable ECN when requested by incoming connections and
415 also request ECN on outgoing connection attempts.
416 2 Enable ECN when requested by incoming connections
417 but do not request ECN on outgoing connections.
418 = =====================================================
422 tcp_ecn_fallback - BOOLEAN
423 If the kernel detects that ECN connection misbehaves, enable fall
424 back to non-ECN. Currently, this knob implements the fallback
425 from RFC3168, section 6.1.1.1., but we reserve that in future,
426 additional detection mechanisms could be implemented under this
427 knob. The value is not used, if tcp_ecn or per route (or congestion
428 control) ECN settings are disabled.
430 Default: 1 (fallback enabled)
433 This is a legacy option, it has no effect anymore.
435 tcp_fin_timeout - INTEGER
436 The length of time an orphaned (no longer referenced by any
437 application) connection will remain in the FIN_WAIT_2 state
438 before it is aborted at the local end. While a perfectly
439 valid "receive only" state for an un-orphaned connection, an
440 orphaned connection in FIN_WAIT_2 state could otherwise wait
441 forever for the remote to close its end of the connection.
448 Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
449 F-RTO is an enhanced recovery algorithm for TCP retransmission
450 timeouts. It is particularly beneficial in networks where the
451 RTT fluctuates (e.g., wireless). F-RTO is sender-side only
452 modification. It does not require any support from the peer.
454 By default it's enabled with a non-zero value. 0 disables F-RTO.
456 tcp_fwmark_accept - BOOLEAN
457 If set, incoming connections to listening sockets that do not have a
458 socket mark will set the mark of the accepting socket to the fwmark of
459 the incoming SYN packet. This will cause all packets on that connection
460 (starting from the first SYNACK) to be sent with that fwmark. The
461 listening socket's mark is unchanged. Listening sockets that already
462 have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
467 tcp_invalid_ratelimit - INTEGER
468 Limit the maximal rate for sending duplicate acknowledgments
469 in response to incoming TCP packets that are for an existing
470 connection but that are invalid due to any of these reasons:
472 (a) out-of-window sequence number,
473 (b) out-of-window acknowledgment number, or
474 (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
476 This can help mitigate simple "ack loop" DoS attacks, wherein
477 a buggy or malicious middlebox or man-in-the-middle can
478 rewrite TCP header fields in manner that causes each endpoint
479 to think that the other is sending invalid TCP segments, thus
480 causing each side to send an unterminating stream of duplicate
481 acknowledgments for invalid segments.
483 Using 0 disables rate-limiting of dupacks in response to
484 invalid segments; otherwise this value specifies the minimal
485 space between sending such dupacks, in milliseconds.
487 Default: 500 (milliseconds).
489 tcp_keepalive_time - INTEGER
490 How often TCP sends out keepalive messages when keepalive is enabled.
493 tcp_keepalive_probes - INTEGER
494 How many keepalive probes TCP sends out, until it decides that the
495 connection is broken. Default value: 9.
497 tcp_keepalive_intvl - INTEGER
498 How frequently the probes are send out. Multiplied by
499 tcp_keepalive_probes it is time to kill not responding connection,
500 after probes started. Default value: 75sec i.e. connection
501 will be aborted after ~11 minutes of retries.
503 tcp_l3mdev_accept - BOOLEAN
504 Enables child sockets to inherit the L3 master device index.
505 Enabling this option allows a "global" listen socket to work
506 across L3 master domains (e.g., VRFs) with connected sockets
507 derived from the listen socket to be bound to the L3 domain in
508 which the packets originated. Only valid when the kernel was
509 compiled with CONFIG_NET_L3_MASTER_DEV.
511 Default: 0 (disabled)
513 tcp_low_latency - BOOLEAN
514 This is a legacy option, it has no effect anymore.
516 tcp_max_orphans - INTEGER
517 Maximal number of TCP sockets not attached to any user file handle,
518 held by system. If this number is exceeded orphaned connections are
519 reset immediately and warning is printed. This limit exists
520 only to prevent simple DoS attacks, you _must_ not rely on this
521 or lower the limit artificially, but rather increase it
522 (probably, after increasing installed memory),
523 if network conditions require more than default value,
524 and tune network services to linger and kill such states
525 more aggressively. Let me to remind again: each orphan eats
526 up to ~64K of unswappable memory.
528 tcp_max_syn_backlog - INTEGER
529 Maximal number of remembered connection requests (SYN_RECV),
530 which have not received an acknowledgment from connecting client.
532 This is a per-listener limit.
534 The minimal value is 128 for low memory machines, and it will
535 increase in proportion to the memory of machine.
537 If server suffers from overload, try increasing this number.
539 Remember to also check /proc/sys/net/core/somaxconn
540 A SYN_RECV request socket consumes about 304 bytes of memory.
542 tcp_max_tw_buckets - INTEGER
543 Maximal number of timewait sockets held by system simultaneously.
544 If this number is exceeded time-wait socket is immediately destroyed
545 and warning is printed. This limit exists only to prevent
546 simple DoS attacks, you _must_ not lower the limit artificially,
547 but rather increase it (probably, after increasing installed memory),
548 if network conditions require more than default value.
550 tcp_mem - vector of 3 INTEGERs: min, pressure, max
551 min: below this number of pages TCP is not bothered about its
554 pressure: when amount of memory allocated by TCP exceeds this number
555 of pages, TCP moderates its memory consumption and enters memory
556 pressure mode, which is exited when memory consumption falls
559 max: number of pages allowed for queueing by all TCP sockets.
561 Defaults are calculated at boot time from amount of available
564 tcp_min_rtt_wlen - INTEGER
565 The window length of the windowed min filter to track the minimum RTT.
566 A shorter window lets a flow more quickly pick up new (higher)
567 minimum RTT when it is moved to a longer path (e.g., due to traffic
568 engineering). A longer window makes the filter more resistant to RTT
569 inflations such as transient congestion. The unit is seconds.
571 Possible values: 0 - 86400 (1 day)
575 tcp_moderate_rcvbuf - BOOLEAN
576 If set, TCP performs receive buffer auto-tuning, attempting to
577 automatically size the buffer (no greater than tcp_rmem[2]) to
578 match the size required by the path for full throughput. Enabled by
581 tcp_mtu_probing - INTEGER
582 Controls TCP Packetization-Layer Path MTU Discovery. Takes three
586 - 1 - Disabled by default, enabled when an ICMP black hole detected
587 - 2 - Always enabled, use initial MSS of tcp_base_mss.
589 tcp_probe_interval - UNSIGNED INTEGER
590 Controls how often to start TCP Packetization-Layer Path MTU
591 Discovery reprobe. The default is reprobing every 10 minutes as
594 tcp_probe_threshold - INTEGER
595 Controls when TCP Packetization-Layer Path MTU Discovery probing
596 will stop in respect to the width of search range in bytes. Default
599 tcp_no_metrics_save - BOOLEAN
600 By default, TCP saves various connection metrics in the route cache
601 when the connection closes, so that connections established in the
602 near future can use these to set initial conditions. Usually, this
603 increases overall performance, but may sometimes cause performance
604 degradation. If set, TCP will not cache metrics on closing
607 tcp_no_ssthresh_metrics_save - BOOLEAN
608 Controls whether TCP saves ssthresh metrics in the route cache.
610 Default is 1, which disables ssthresh metrics.
612 tcp_orphan_retries - INTEGER
613 This value influences the timeout of a locally closed TCP connection,
614 when RTO retransmissions remain unacknowledged.
615 See tcp_retries2 for more details.
617 The default value is 8.
619 If your machine is a loaded WEB server,
620 you should think about lowering this value, such sockets
621 may consume significant resources. Cf. tcp_max_orphans.
623 tcp_recovery - INTEGER
624 This value is a bitmap to enable various experimental loss recovery
627 ========= =============================================================
628 RACK: 0x1 enables the RACK loss detection for fast detection of lost
629 retransmissions and tail drops. It also subsumes and disables
630 RFC6675 recovery for SACK connections.
632 RACK: 0x2 makes RACK's reordering window static (min_rtt/4).
634 RACK: 0x4 disables RACK's DUPACK threshold heuristic
635 ========= =============================================================
639 tcp_reflect_tos - BOOLEAN
640 For listening sockets, reuse the DSCP value of the initial SYN message
641 for outgoing packets. This allows to have both directions of a TCP
642 stream to use the same DSCP value, assuming DSCP remains unchanged for
643 the lifetime of the connection.
645 This options affects both IPv4 and IPv6.
647 Default: 0 (disabled)
649 tcp_reordering - INTEGER
650 Initial reordering level of packets in a TCP stream.
651 TCP stack can then dynamically adjust flow reordering level
652 between this initial value and tcp_max_reordering
656 tcp_max_reordering - INTEGER
657 Maximal reordering level of packets in a TCP stream.
658 300 is a fairly conservative value, but you might increase it
659 if paths are using per packet load balancing (like bonding rr mode)
663 tcp_retrans_collapse - BOOLEAN
664 Bug-to-bug compatibility with some broken printers.
665 On retransmit try to send bigger packets to work around bugs in
668 tcp_retries1 - INTEGER
669 This value influences the time, after which TCP decides, that
670 something is wrong due to unacknowledged RTO retransmissions,
671 and reports this suspicion to the network layer.
672 See tcp_retries2 for more details.
674 RFC 1122 recommends at least 3 retransmissions, which is the
677 tcp_retries2 - INTEGER
678 This value influences the timeout of an alive TCP connection,
679 when RTO retransmissions remain unacknowledged.
680 Given a value of N, a hypothetical TCP connection following
681 exponential backoff with an initial RTO of TCP_RTO_MIN would
682 retransmit N times before killing the connection at the (N+1)th RTO.
684 The default value of 15 yields a hypothetical timeout of 924.6
685 seconds and is a lower bound for the effective timeout.
686 TCP will effectively time out at the first RTO which exceeds the
687 hypothetical timeout.
689 RFC 1122 recommends at least 100 seconds for the timeout,
690 which corresponds to a value of at least 8.
692 tcp_rfc1337 - BOOLEAN
693 If set, the TCP stack behaves conforming to RFC1337. If unset,
694 we are not conforming to RFC, but prevent TCP TIME_WAIT
699 tcp_rmem - vector of 3 INTEGERs: min, default, max
700 min: Minimal size of receive buffer used by TCP sockets.
701 It is guaranteed to each TCP socket, even under moderate memory
706 default: initial size of receive buffer used by TCP sockets.
707 This value overrides net.core.rmem_default used by other protocols.
708 Default: 131072 bytes.
709 This value results in initial window of 65535.
711 max: maximal size of receive buffer allowed for automatically
712 selected receiver buffers for TCP socket. This value does not override
713 net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables
714 automatic tuning of that socket's receive buffer size, in which
715 case this value is ignored.
716 Default: between 131072 and 6MB, depending on RAM size.
719 Enable select acknowledgments (SACKS).
721 tcp_comp_sack_delay_ns - LONG INTEGER
722 TCP tries to reduce number of SACK sent, using a timer
723 based on 5% of SRTT, capped by this sysctl, in nano seconds.
724 The default is 1ms, based on TSO autosizing period.
726 Default : 1,000,000 ns (1 ms)
728 tcp_comp_sack_slack_ns - LONG INTEGER
729 This sysctl control the slack used when arming the
730 timer used by SACK compression. This gives extra time
731 for small RTT flows, and reduces system overhead by allowing
732 opportunistic reduction of timer interrupts.
734 Default : 100,000 ns (100 us)
736 tcp_comp_sack_nr - INTEGER
737 Max number of SACK that can be compressed.
738 Using 0 disables SACK compression.
742 tcp_slow_start_after_idle - BOOLEAN
743 If set, provide RFC2861 behavior and time out the congestion
744 window after an idle period. An idle period is defined at
745 the current RTO. If unset, the congestion window will not
746 be timed out after an idle period.
751 Use the Host requirements interpretation of the TCP urgent pointer field.
752 Most hosts use the older BSD interpretation, so if you turn this on
753 Linux might not communicate correctly with them.
757 tcp_synack_retries - INTEGER
758 Number of times SYNACKs for a passive TCP connection attempt will
759 be retransmitted. Should not be higher than 255. Default value
760 is 5, which corresponds to 31seconds till the last retransmission
761 with the current initial RTO of 1second. With this the final timeout
762 for a passive TCP connection will happen after 63seconds.
764 tcp_syncookies - INTEGER
765 Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
766 Send out syncookies when the syn backlog queue of a socket
767 overflows. This is to prevent against the common 'SYN flood attack'
770 Note, that syncookies is fallback facility.
771 It MUST NOT be used to help highly loaded servers to stand
772 against legal connection rate. If you see SYN flood warnings
773 in your logs, but investigation shows that they occur
774 because of overload with legal connections, you should tune
775 another parameters until this warning disappear.
776 See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
778 syncookies seriously violate TCP protocol, do not allow
779 to use TCP extensions, can result in serious degradation
780 of some services (f.e. SMTP relaying), visible not by you,
781 but your clients and relays, contacting you. While you see
782 SYN flood warnings in logs not being really flooded, your server
783 is seriously misconfigured.
785 If you want to test which effects syncookies have to your
786 network connections you can set this knob to 2 to enable
787 unconditionally generation of syncookies.
789 tcp_migrate_req - BOOLEAN
790 The incoming connection is tied to a specific listening socket when
791 the initial SYN packet is received during the three-way handshake.
792 When a listener is closed, in-flight request sockets during the
793 handshake and established sockets in the accept queue are aborted.
795 If the listener has SO_REUSEPORT enabled, other listeners on the
796 same port should have been able to accept such connections. This
797 option makes it possible to migrate such child sockets to another
798 listener after close() or shutdown().
800 The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
801 usually be used to define the policy to pick an alive listener.
802 Otherwise, the kernel will randomly pick an alive listener only if
803 this option is enabled.
805 Note that migration between listeners with different settings may
806 crash applications. Let's say migration happens from listener A to
807 B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
808 the requests migrated from A. To avoid such a situation, cancel
809 migration by returning SK_DROP in the type of eBPF program, or
814 tcp_fastopen - INTEGER
815 Enable TCP Fast Open (RFC7413) to send and accept data in the opening
818 The client support is enabled by flag 0x1 (on by default). The client
819 then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
820 rather than connect() to send data in SYN.
822 The server support is enabled by flag 0x2 (off by default). Then
823 either enable for all listeners with another flag (0x400) or
824 enable individual listeners via TCP_FASTOPEN socket option with
825 the option value being the length of the syn-data backlog.
827 The values (bitmap) are
829 ===== ======== ======================================================
830 0x1 (client) enables sending data in the opening SYN on the client.
831 0x2 (server) enables the server support, i.e., allowing data in
832 a SYN packet to be accepted and passed to the
833 application before 3-way handshake finishes.
834 0x4 (client) send data in the opening SYN regardless of cookie
835 availability and without a cookie option.
836 0x200 (server) accept data-in-SYN w/o any cookie option present.
837 0x400 (server) enable all listeners to support Fast Open by
838 default without explicit TCP_FASTOPEN socket option.
839 ===== ======== ======================================================
843 Note that additional client or server features are only
844 effective if the basic support (0x1 and 0x2) are enabled respectively.
846 tcp_fastopen_blackhole_timeout_sec - INTEGER
847 Initial time period in second to disable Fastopen on active TCP sockets
848 when a TFO firewall blackhole issue happens.
849 This time period will grow exponentially when more blackhole issues
850 get detected right after Fastopen is re-enabled and will reset to
851 initial value when the blackhole issue goes away.
852 0 to disable the blackhole detection.
854 By default, it is set to 0 (feature is disabled).
856 tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
857 The list consists of a primary key and an optional backup key. The
858 primary key is used for both creating and validating cookies, while the
859 optional backup key is only used for validating cookies. The purpose of
860 the backup key is to maximize TFO validation when keys are rotated.
862 A randomly chosen primary key may be configured by the kernel if
863 the tcp_fastopen sysctl is set to 0x400 (see above), or if the
864 TCP_FASTOPEN setsockopt() optname is set and a key has not been
865 previously configured via sysctl. If keys are configured via
866 setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
867 per-socket keys will be used instead of any keys that are specified via
870 A key is specified as 4 8-digit hexadecimal integers which are separated
871 by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
872 omitted. A primary and a backup key may be specified by separating them
873 by a comma. If only one key is specified, it becomes the primary key and
874 any previously configured backup keys are removed.
876 tcp_syn_retries - INTEGER
877 Number of times initial SYNs for an active TCP connection attempt
878 will be retransmitted. Should not be higher than 127. Default value
879 is 6, which corresponds to 63seconds till the last retransmission
880 with the current initial RTO of 1second. With this the final timeout
881 for an active TCP connection attempt will happen after 127seconds.
883 tcp_timestamps - INTEGER
884 Enable timestamps as defined in RFC1323.
887 - 1: Enable timestamps as defined in RFC1323 and use random offset for
888 each connection rather than only using the current time.
889 - 2: Like 1, but without random offsets.
893 tcp_min_tso_segs - INTEGER
894 Minimal number of segments per TSO frame.
896 Since linux-3.12, TCP does an automatic sizing of TSO frames,
897 depending on flow rate, instead of filling 64Kbytes packets.
898 For specific usages, it's possible to force TCP to build big
899 TSO frames. Note that TCP stack might split too big TSO packets
900 if available window is too small.
904 tcp_tso_rtt_log - INTEGER
905 Adjustment of TSO packet sizes based on min_rtt
907 Starting from linux-5.18, TCP autosizing can be tweaked
908 for flows having small RTT.
910 Old autosizing was splitting the pacing budget to send 1024 TSO
913 tso_packet_size = sk->sk_pacing_rate / 1024;
915 With the new mechanism, we increase this TSO sizing using:
917 distance = min_rtt_usec / (2^tcp_tso_rtt_log)
918 tso_packet_size += gso_max_size >> distance;
920 This means that flows between very close hosts can use bigger
921 TSO packets, reducing their cpu costs.
923 If you want to use the old autosizing, set this sysctl to 0.
925 Default: 9 (2^9 = 512 usec)
927 tcp_pacing_ss_ratio - INTEGER
928 sk->sk_pacing_rate is set by TCP stack using a ratio applied
929 to current rate. (current_rate = cwnd * mss / srtt)
930 If TCP is in slow start, tcp_pacing_ss_ratio is applied
931 to let TCP probe for bigger speeds, assuming cwnd can be
932 doubled every other RTT.
936 tcp_pacing_ca_ratio - INTEGER
937 sk->sk_pacing_rate is set by TCP stack using a ratio applied
938 to current rate. (current_rate = cwnd * mss / srtt)
939 If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
940 is applied to conservatively probe for bigger throughput.
944 tcp_tso_win_divisor - INTEGER
945 This allows control over what percentage of the congestion window
946 can be consumed by a single TSO frame.
947 The setting of this parameter is a choice between burstiness and
948 building larger TSO frames.
952 tcp_tw_reuse - INTEGER
953 Enable reuse of TIME-WAIT sockets for new connections when it is
954 safe from protocol viewpoint.
958 - 2 - enable for loopback traffic only
960 It should not be changed without advice/request of technical
965 tcp_window_scaling - BOOLEAN
966 Enable window scaling as defined in RFC1323.
968 tcp_wmem - vector of 3 INTEGERs: min, default, max
969 min: Amount of memory reserved for send buffers for TCP sockets.
970 Each TCP socket has rights to use it due to fact of its birth.
974 default: initial size of send buffer used by TCP sockets. This
975 value overrides net.core.wmem_default used by other protocols.
977 It is usually lower than net.core.wmem_default.
981 max: Maximal amount of memory allowed for automatically tuned
982 send buffers for TCP sockets. This value does not override
983 net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables
984 automatic tuning of that socket's send buffer size, in which case
985 this value is ignored.
987 Default: between 64K and 4MB, depending on RAM size.
989 tcp_notsent_lowat - UNSIGNED INTEGER
990 A TCP socket can control the amount of unsent bytes in its write queue,
991 thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
992 reports POLLOUT events if the amount of unsent bytes is below a per
993 socket value, and if the write queue is not full. sendmsg() will
994 also not add new buffers if the limit is hit.
996 This global variable controls the amount of unsent data for
997 sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
998 to the global variable has immediate effect.
1000 Default: UINT_MAX (0xFFFFFFFF)
1002 tcp_workaround_signed_windows - BOOLEAN
1003 If set, assume no receipt of a window scaling option means the
1004 remote TCP is broken and treats the window as a signed quantity.
1005 If unset, assume the remote TCP is not broken even if we do
1006 not receive a window scaling option from them.
1010 tcp_thin_linear_timeouts - BOOLEAN
1011 Enable dynamic triggering of linear timeouts for thin streams.
1012 If set, a check is performed upon retransmission by timeout to
1013 determine if the stream is thin (less than 4 packets in flight).
1014 As long as the stream is found to be thin, up to 6 linear
1015 timeouts may be performed before exponential backoff mode is
1016 initiated. This improves retransmission latency for
1017 non-aggressive thin streams, often found to be time-dependent.
1018 For more information on thin streams, see
1019 Documentation/networking/tcp-thin.rst
1023 tcp_limit_output_bytes - INTEGER
1024 Controls TCP Small Queue limit per tcp socket.
1025 TCP bulk sender tends to increase packets in flight until it
1026 gets losses notifications. With SNDBUF autotuning, this can
1027 result in a large amount of packets queued on the local machine
1028 (e.g.: qdiscs, CPU backlog, or device) hurting latency of other
1029 flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes
1030 limits the number of bytes on qdisc or device to reduce artificial
1031 RTT/cwnd and reduce bufferbloat.
1033 Default: 1048576 (16 * 65536)
1035 tcp_challenge_ack_limit - INTEGER
1036 Limits number of Challenge ACK sent per second, as recommended
1037 in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
1043 udp_l3mdev_accept - BOOLEAN
1044 Enabling this option allows a "global" bound socket to work
1045 across L3 master domains (e.g., VRFs) with packets capable of
1046 being received regardless of the L3 domain in which they
1047 originated. Only valid when the kernel was compiled with
1048 CONFIG_NET_L3_MASTER_DEV.
1050 Default: 0 (disabled)
1052 udp_mem - vector of 3 INTEGERs: min, pressure, max
1053 Number of pages allowed for queueing by all UDP sockets.
1055 min: Number of pages allowed for queueing by all UDP sockets.
1057 pressure: This value was introduced to follow format of tcp_mem.
1059 max: This value was introduced to follow format of tcp_mem.
1061 Default is calculated at boot time from amount of available memory.
1063 udp_rmem_min - INTEGER
1064 Minimal size of receive buffer used by UDP sockets in moderation.
1065 Each UDP socket is able to use the size for receiving data, even if
1066 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
1070 udp_wmem_min - INTEGER
1071 UDP does not have tx memory accounting and this tunable has no effect.
1076 raw_l3mdev_accept - BOOLEAN
1077 Enabling this option allows a "global" bound socket to work
1078 across L3 master domains (e.g., VRFs) with packets capable of
1079 being received regardless of the L3 domain in which they
1080 originated. Only valid when the kernel was compiled with
1081 CONFIG_NET_L3_MASTER_DEV.
1083 Default: 1 (enabled)
1088 cipso_cache_enable - BOOLEAN
1089 If set, enable additions to and lookups from the CIPSO label mapping
1090 cache. If unset, additions are ignored and lookups always result in a
1091 miss. However, regardless of the setting the cache is still
1092 invalidated when required when means you can safely toggle this on and
1093 off and the cache will always be "safe".
1097 cipso_cache_bucket_size - INTEGER
1098 The CIPSO label cache consists of a fixed size hash table with each
1099 hash bucket containing a number of cache entries. This variable limits
1100 the number of entries in each hash bucket; the larger the value is, the
1101 more CIPSO label mappings that can be cached. When the number of
1102 entries in a given hash bucket reaches this limit adding new entries
1103 causes the oldest entry in the bucket to be removed to make room.
1107 cipso_rbm_optfmt - BOOLEAN
1108 Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
1109 the CIPSO draft specification (see Documentation/netlabel for details).
1110 This means that when set the CIPSO tag will be padded with empty
1111 categories in order to make the packet data 32-bit aligned.
1115 cipso_rbm_structvalid - BOOLEAN
1116 If set, do a very strict check of the CIPSO option when
1117 ip_options_compile() is called. If unset, relax the checks done during
1118 ip_options_compile(). Either way is "safe" as errors are caught else
1119 where in the CIPSO processing code but setting this to 0 (False) should
1120 result in less work (i.e. it should be faster) but could cause problems
1121 with other implementations that require strict checking.
1128 ip_local_port_range - 2 INTEGERS
1129 Defines the local port range that is used by TCP and UDP to
1130 choose the local port. The first number is the first, the
1131 second the last local port number.
1132 If possible, it is better these numbers have different parity
1133 (one even and one odd value).
1134 Must be greater than or equal to ip_unprivileged_port_start.
1135 The default values are 32768 and 60999 respectively.
1137 ip_local_reserved_ports - list of comma separated ranges
1138 Specify the ports which are reserved for known third-party
1139 applications. These ports will not be used by automatic port
1140 assignments (e.g. when calling connect() or bind() with port
1141 number 0). Explicit port allocation behavior is unchanged.
1143 The format used for both input and output is a comma separated
1144 list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
1145 10). Writing to the file will clear all previously reserved
1146 ports and update the current list with the one given in the
1149 Note that ip_local_port_range and ip_local_reserved_ports
1150 settings are independent and both are considered by the kernel
1151 when determining which ports are available for automatic port
1154 You can reserve ports which are not in the current
1155 ip_local_port_range, e.g.::
1157 $ cat /proc/sys/net/ipv4/ip_local_port_range
1159 $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
1162 although this is redundant. However such a setting is useful
1163 if later the port range is changed to a value that will
1164 include the reserved ports. Also keep in mind, that overlapping
1165 of these ranges may affect probability of selecting ephemeral
1166 ports which are right after block of reserved ports.
1170 ip_unprivileged_port_start - INTEGER
1171 This is a per-namespace sysctl. It defines the first
1172 unprivileged port in the network namespace. Privileged ports
1173 require root or CAP_NET_BIND_SERVICE in order to bind to them.
1174 To disable all privileged ports, set this to 0. They must not
1175 overlap with the ip_local_port_range.
1179 ip_nonlocal_bind - BOOLEAN
1180 If set, allows processes to bind() to non-local IP addresses,
1181 which can be quite useful - but may break some applications.
1185 ip_autobind_reuse - BOOLEAN
1186 By default, bind() does not select the ports automatically even if
1187 the new socket and all sockets bound to the port have SO_REUSEADDR.
1188 ip_autobind_reuse allows bind() to reuse the port and this is useful
1189 when you use bind()+connect(), but may break some applications.
1190 The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
1191 option should only be set by experts.
1194 ip_dynaddr - INTEGER
1195 If set non-zero, enables support for dynamic addresses.
1196 If set to a non-zero value larger than 1, a kernel log
1197 message will be printed when dynamic address rewriting
1202 ip_early_demux - BOOLEAN
1203 Optimize input packet processing down to one demux for
1204 certain kinds of local sockets. Currently we only do this
1205 for established TCP and connected UDP sockets.
1207 It may add an additional cost for pure routing workloads that
1208 reduces overall throughput, in such case you should disable it.
1212 ping_group_range - 2 INTEGERS
1213 Restrict ICMP_PROTO datagram sockets to users in the group range.
1214 The default is "1 0", meaning, that nobody (not even root) may
1215 create ping sockets. Setting it to "100 100" would grant permissions
1216 to the single group. "0 4294967295" would enable it for the world, "100
1217 4294967295" would enable it for the users, but not daemons.
1219 tcp_early_demux - BOOLEAN
1220 Enable early demux for established TCP sockets.
1224 udp_early_demux - BOOLEAN
1225 Enable early demux for connected UDP sockets. Disable this if
1226 your system could experience more unconnected load.
1230 icmp_echo_ignore_all - BOOLEAN
1231 If set non-zero, then the kernel will ignore all ICMP ECHO
1232 requests sent to it.
1236 icmp_echo_enable_probe - BOOLEAN
1237 If set to one, then the kernel will respond to RFC 8335 PROBE
1238 requests sent to it.
1242 icmp_echo_ignore_broadcasts - BOOLEAN
1243 If set non-zero, then the kernel will ignore all ICMP ECHO and
1244 TIMESTAMP requests sent to it via broadcast/multicast.
1248 icmp_ratelimit - INTEGER
1249 Limit the maximal rates for sending ICMP packets whose type matches
1250 icmp_ratemask (see below) to specific targets.
1251 0 to disable any limiting,
1252 otherwise the minimal space between responses in milliseconds.
1253 Note that another sysctl, icmp_msgs_per_sec limits the number
1254 of ICMP packets sent on all targets.
1258 icmp_msgs_per_sec - INTEGER
1259 Limit maximal number of ICMP packets sent per second from this host.
1260 Only messages whose type matches icmp_ratemask (see below) are
1261 controlled by this limit. For security reasons, the precise count
1262 of messages per second is randomized.
1266 icmp_msgs_burst - INTEGER
1267 icmp_msgs_per_sec controls number of ICMP packets sent per second,
1268 while icmp_msgs_burst controls the burst size of these packets.
1269 For security reasons, the precise burst size is randomized.
1273 icmp_ratemask - INTEGER
1274 Mask made of ICMP types for which rates are being limited.
1276 Significant bits: IHGFEDCBA9876543210
1278 Default mask: 0000001100000011000 (6168)
1280 Bit definitions (see include/linux/icmp.h):
1282 = =========================
1284 3 Destination Unreachable [1]_
1285 4 Source Quench [1]_
1288 B Time Exceeded [1]_
1289 C Parameter Problem [1]_
1294 H Address Mask Request
1295 I Address Mask Reply
1296 = =========================
1298 .. [1] These are rate limited by default (see default mask above)
1300 icmp_ignore_bogus_error_responses - BOOLEAN
1301 Some routers violate RFC1122 by sending bogus responses to broadcast
1302 frames. Such violations are normally logged via a kernel warning.
1303 If this is set to TRUE, the kernel will not give such warnings, which
1304 will avoid log file clutter.
1308 icmp_errors_use_inbound_ifaddr - BOOLEAN
1310 If zero, icmp error messages are sent with the primary address of
1311 the exiting interface.
1313 If non-zero, the message will be sent with the primary address of
1314 the interface that received the packet that caused the icmp error.
1315 This is the behaviour many network administrators will expect from
1316 a router. And it can make debugging complicated network layouts
1319 Note that if no primary address exists for the interface selected,
1320 then the primary address of the first non-loopback interface that
1321 has one will be used regardless of this setting.
1325 igmp_max_memberships - INTEGER
1326 Change the maximum number of multicast groups we can subscribe to.
1329 Theoretical maximum value is bounded by having to send a membership
1330 report in a single datagram (i.e. the report can't span multiple
1331 datagrams, or risk confusing the switch and leaving groups you don't
1334 The number of supported groups 'M' is bounded by the number of group
1335 report entries you can fit into a single datagram of 65535 bytes.
1337 M = 65536-sizeof (ip header)/(sizeof(Group record))
1339 Group records are variable length, with a minimum of 12 bytes.
1340 So net.ipv4.igmp_max_memberships should not be set higher than:
1342 (65536-24) / 12 = 5459
1344 The value 5459 assumes no IP header options, so in practice
1345 this number may be lower.
1347 igmp_max_msf - INTEGER
1348 Maximum number of addresses allowed in the source filter list for a
1354 Controls the IGMP query robustness variable (see RFC2236 8.1).
1356 Default: 2 (as specified by RFC2236 8.1)
1358 Minimum: 1 (as specified by RFC6636 4.5)
1360 force_igmp_version - INTEGER
1361 - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
1362 allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
1363 Present timer expires.
1364 - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
1365 receive IGMPv2/v3 query.
1366 - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
1367 IGMPv1 query message. Will reply report if receive IGMPv3 query.
1368 - 3 - Enforce to use IGMP version 3. The same react with default 0.
1372 this is not the same with force_mld_version because IGMPv3 RFC3376
1373 Security Considerations does not have clear description that we could
1374 ignore other version messages completely as MLDv2 RFC3810. So make
1375 this value as default 0 is recommended.
1377 ``conf/interface/*``
1378 changes special settings per interface (where
1379 interface" is the name of your network interface)
1382 is special, changes the settings for all interfaces
1384 log_martians - BOOLEAN
1385 Log packets with impossible addresses to kernel log.
1386 log_martians for the interface will be enabled if at least one of
1387 conf/{all,interface}/log_martians is set to TRUE,
1388 it will be disabled otherwise
1390 accept_redirects - BOOLEAN
1391 Accept ICMP redirect messages.
1392 accept_redirects for the interface will be enabled if:
1394 - both conf/{all,interface}/accept_redirects are TRUE in the case
1395 forwarding for the interface is enabled
1399 - at least one of conf/{all,interface}/accept_redirects is TRUE in the
1400 case forwarding for the interface is disabled
1402 accept_redirects for the interface will be disabled otherwise
1409 forwarding - BOOLEAN
1410 Enable IP forwarding on this interface. This controls whether packets
1411 received _on_ this interface can be forwarded.
1413 mc_forwarding - BOOLEAN
1414 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
1415 and a multicast routing daemon is required.
1416 conf/all/mc_forwarding must also be set to TRUE to enable multicast
1417 routing for the interface
1420 Integer value used to differentiate the devices by the medium they
1421 are attached to. Two devices can have different id values when
1422 the broadcast packets are received only on one of them.
1423 The default value 0 means that the device is the only interface
1424 to its medium, value of -1 means that medium is not known.
1426 Currently, it is used to change the proxy_arp behavior:
1427 the proxy_arp feature is enabled for packets forwarded between
1428 two devices attached to different media.
1433 proxy_arp for the interface will be enabled if at least one of
1434 conf/{all,interface}/proxy_arp is set to TRUE,
1435 it will be disabled otherwise
1437 proxy_arp_pvlan - BOOLEAN
1438 Private VLAN proxy arp.
1440 Basically allow proxy arp replies back to the same interface
1441 (from which the ARP request/solicitation was received).
1443 This is done to support (ethernet) switch features, like RFC
1444 3069, where the individual ports are NOT allowed to
1445 communicate with each other, but they are allowed to talk to
1446 the upstream router. As described in RFC 3069, it is possible
1447 to allow these hosts to communicate through the upstream
1448 router by proxy_arp'ing. Don't need to be used together with
1451 This technology is known by different names:
1453 In RFC 3069 it is called VLAN Aggregation.
1454 Cisco and Allied Telesyn call it Private VLAN.
1455 Hewlett-Packard call it Source-Port filtering or port-isolation.
1456 Ericsson call it MAC-Forced Forwarding (RFC Draft).
1458 shared_media - BOOLEAN
1459 Send(router) or accept(host) RFC1620 shared media redirects.
1460 Overrides secure_redirects.
1462 shared_media for the interface will be enabled if at least one of
1463 conf/{all,interface}/shared_media is set to TRUE,
1464 it will be disabled otherwise
1468 secure_redirects - BOOLEAN
1469 Accept ICMP redirect messages only to gateways listed in the
1470 interface's current gateway list. Even if disabled, RFC1122 redirect
1473 Overridden by shared_media.
1475 secure_redirects for the interface will be enabled if at least one of
1476 conf/{all,interface}/secure_redirects is set to TRUE,
1477 it will be disabled otherwise
1481 send_redirects - BOOLEAN
1482 Send redirects, if router.
1484 send_redirects for the interface will be enabled if at least one of
1485 conf/{all,interface}/send_redirects is set to TRUE,
1486 it will be disabled otherwise
1490 bootp_relay - BOOLEAN
1491 Accept packets with source address 0.b.c.d destined
1492 not to this host as local ones. It is supposed, that
1493 BOOTP relay daemon will catch and forward such packets.
1494 conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
1499 Not Implemented Yet.
1501 accept_source_route - BOOLEAN
1502 Accept packets with SRR option.
1503 conf/all/accept_source_route must also be set to TRUE to accept packets
1504 with SRR option on the interface
1511 accept_local - BOOLEAN
1512 Accept packets with local source addresses. In combination with
1513 suitable routing, this can be used to direct packets between two
1514 local interfaces over the wire and have them accepted properly.
1517 route_localnet - BOOLEAN
1518 Do not consider loopback addresses as martian source or destination
1519 while routing. This enables the use of 127/8 for local routing purposes.
1524 - 0 - No source validation.
1525 - 1 - Strict mode as defined in RFC3704 Strict Reverse Path
1526 Each incoming packet is tested against the FIB and if the interface
1527 is not the best reverse path the packet check will fail.
1528 By default failed packets are discarded.
1529 - 2 - Loose mode as defined in RFC3704 Loose Reverse Path
1530 Each incoming packet's source address is also tested against the FIB
1531 and if the source address is not reachable via any interface
1532 the packet check will fail.
1534 Current recommended practice in RFC3704 is to enable strict mode
1535 to prevent IP spoofing from DDos attacks. If using asymmetric routing
1536 or other complicated routing, then loose mode is recommended.
1538 The max value from conf/{all,interface}/rp_filter is used
1539 when doing source validation on the {interface}.
1541 Default value is 0. Note that some distributions enable it
1544 src_valid_mark - BOOLEAN
1545 - 0 - The fwmark of the packet is not included in reverse path
1546 route lookup. This allows for asymmetric routing configurations
1547 utilizing the fwmark in only one direction, e.g., transparent
1550 - 1 - The fwmark of the packet is included in reverse path route
1551 lookup. This permits rp_filter to function when the fwmark is
1552 used for routing traffic in both directions.
1554 This setting also affects the utilization of fmwark when
1555 performing source address selection for ICMP replies, or
1556 determining addresses stored for the IPOPT_TS_TSANDADDR and
1557 IPOPT_RR IP options.
1559 The max value from conf/{all,interface}/src_valid_mark is used.
1563 arp_filter - BOOLEAN
1564 - 1 - Allows you to have multiple network interfaces on the same
1565 subnet, and have the ARPs for each interface be answered
1566 based on whether or not the kernel would route a packet from
1567 the ARP'd IP out that interface (therefore you must use source
1568 based routing for this to work). In other words it allows control
1569 of which cards (usually 1) will respond to an arp request.
1571 - 0 - (default) The kernel can respond to arp requests with addresses
1572 from other interfaces. This may seem wrong but it usually makes
1573 sense, because it increases the chance of successful communication.
1574 IP addresses are owned by the complete host on Linux, not by
1575 particular interfaces. Only for more complex setups like load-
1576 balancing, does this behaviour cause problems.
1578 arp_filter for the interface will be enabled if at least one of
1579 conf/{all,interface}/arp_filter is set to TRUE,
1580 it will be disabled otherwise
1582 arp_announce - INTEGER
1583 Define different restriction levels for announcing the local
1584 source IP address from IP packets in ARP requests sent on
1587 - 0 - (default) Use any local address, configured on any interface
1588 - 1 - Try to avoid local addresses that are not in the target's
1589 subnet for this interface. This mode is useful when target
1590 hosts reachable via this interface require the source IP
1591 address in ARP requests to be part of their logical network
1592 configured on the receiving interface. When we generate the
1593 request we will check all our subnets that include the
1594 target IP and will preserve the source address if it is from
1595 such subnet. If there is no such subnet we select source
1596 address according to the rules for level 2.
1597 - 2 - Always use the best local address for this target.
1598 In this mode we ignore the source address in the IP packet
1599 and try to select local address that we prefer for talks with
1600 the target host. Such local address is selected by looking
1601 for primary IP addresses on all our subnets on the outgoing
1602 interface that include the target IP address. If no suitable
1603 local address is found we select the first local address
1604 we have on the outgoing interface or on all other interfaces,
1605 with the hope we will receive reply for our request and
1606 even sometimes no matter the source IP address we announce.
1608 The max value from conf/{all,interface}/arp_announce is used.
1610 Increasing the restriction level gives more chance for
1611 receiving answer from the resolved target while decreasing
1612 the level announces more valid sender's information.
1614 arp_ignore - INTEGER
1615 Define different modes for sending replies in response to
1616 received ARP requests that resolve local target IP addresses:
1618 - 0 - (default): reply for any local target IP address, configured
1620 - 1 - reply only if the target IP address is local address
1621 configured on the incoming interface
1622 - 2 - reply only if the target IP address is local address
1623 configured on the incoming interface and both with the
1624 sender's IP address are part from same subnet on this interface
1625 - 3 - do not reply for local addresses configured with scope host,
1626 only resolutions for global and link addresses are replied
1628 - 8 - do not reply for all local addresses
1630 The max value from conf/{all,interface}/arp_ignore is used
1631 when ARP request is received on the {interface}
1633 arp_notify - BOOLEAN
1634 Define mode for notification of address and device changes.
1636 == ==========================================================
1637 0 (default): do nothing
1638 1 Generate gratuitous arp requests when device is brought up
1639 or hardware address changes.
1640 == ==========================================================
1642 arp_accept - INTEGER
1643 Define behavior for accepting gratuitous ARP (garp) frames from devices
1644 that are not already present in the ARP table:
1646 - 0 - don't create new entries in the ARP table
1647 - 1 - create new entries in the ARP table
1648 - 2 - create new entries only if the source IP address is in the same
1649 subnet as an address configured on the interface that received the
1652 Both replies and requests type gratuitous arp will trigger the
1653 ARP table to be updated, if this setting is on.
1655 If the ARP table already contains the IP address of the
1656 gratuitous arp frame, the arp table will be updated regardless
1657 if this setting is on or off.
1659 arp_evict_nocarrier - BOOLEAN
1660 Clears the ARP cache on NOCARRIER events. This option is important for
1661 wireless devices where the ARP cache should not be cleared when roaming
1662 between access points on the same network. In most cases this should
1663 remain as the default (1).
1665 - 1 - (default): Clear the ARP cache on NOCARRIER events
1666 - 0 - Do not clear ARP cache on NOCARRIER events
1668 mcast_solicit - INTEGER
1669 The maximum number of multicast probes in INCOMPLETE state,
1670 when the associated hardware address is unknown. Defaults
1673 ucast_solicit - INTEGER
1674 The maximum number of unicast probes in PROBE state, when
1675 the hardware address is being reconfirmed. Defaults to 3.
1677 app_solicit - INTEGER
1678 The maximum number of probes to send to the user space ARP daemon
1679 via netlink before dropping back to multicast probes (see
1680 mcast_resolicit). Defaults to 0.
1682 mcast_resolicit - INTEGER
1683 The maximum number of multicast probes after unicast and
1684 app probes in PROBE state. Defaults to 0.
1686 disable_policy - BOOLEAN
1687 Disable IPSEC policy (SPD) for this interface
1689 disable_xfrm - BOOLEAN
1690 Disable IPSEC encryption on this interface, whatever the policy
1692 igmpv2_unsolicited_report_interval - INTEGER
1693 The interval in milliseconds in which the next unsolicited
1694 IGMPv1 or IGMPv2 report retransmit will take place.
1696 Default: 10000 (10 seconds)
1698 igmpv3_unsolicited_report_interval - INTEGER
1699 The interval in milliseconds in which the next unsolicited
1700 IGMPv3 report retransmit will take place.
1702 Default: 1000 (1 seconds)
1704 ignore_routes_with_linkdown - BOOLEAN
1705 Ignore routes whose link is down when performing a FIB lookup.
1707 promote_secondaries - BOOLEAN
1708 When a primary IP address is removed from this interface
1709 promote a corresponding secondary IP address instead of
1710 removing all the corresponding secondary IP addresses.
1712 drop_unicast_in_l2_multicast - BOOLEAN
1713 Drop any unicast IP packets that are received in link-layer
1714 multicast (or broadcast) frames.
1716 This behavior (for multicast) is actually a SHOULD in RFC
1717 1122, but is disabled by default for compatibility reasons.
1721 drop_gratuitous_arp - BOOLEAN
1722 Drop all gratuitous ARP frames, for example if there's a known
1723 good ARP proxy on the network and such frames need not be used
1724 (or in the case of 802.11, must not be used to prevent attacks.)
1730 Allows you to write a number, which can be used as required.
1734 xfrm4_gc_thresh - INTEGER
1735 (Obsolete since linux-4.14)
1736 The threshold at which we will start garbage collecting for IPv4
1737 destination cache entries. At twice this value the system will
1738 refuse new allocations.
1740 igmp_link_local_mcast_reports - BOOLEAN
1741 Enable IGMP reports for link local multicast groups in the
1747 kuznet@ms2.inr.ac.ru
1754 delon.nicolas@wanadoo.fr
1759 /proc/sys/net/ipv6/* Variables
1760 ==============================
1762 IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
1763 apply to IPv6 [XXX?].
1765 bindv6only - BOOLEAN
1766 Default value for IPV6_V6ONLY socket option,
1767 which restricts use of the IPv6 socket to IPv6 communication
1770 - TRUE: disable IPv4-mapped address feature
1771 - FALSE: enable IPv4-mapped address feature
1773 Default: FALSE (as specified in RFC3493)
1775 flowlabel_consistency - BOOLEAN
1776 Protect the consistency (and unicity) of flow label.
1777 You have to disable it to use IPV6_FL_F_REFLECT flag on the
1785 auto_flowlabels - INTEGER
1786 Automatically generate flow labels based on a flow hash of the
1787 packet. This allows intermediate devices, such as routers, to
1788 identify packet flows for mechanisms like Equal Cost Multipath
1789 Routing (see RFC 6438).
1791 = ===========================================================
1792 0 automatic flow labels are completely disabled
1793 1 automatic flow labels are enabled by default, they can be
1794 disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
1796 2 automatic flow labels are allowed, they may be enabled on a
1797 per socket basis using the IPV6_AUTOFLOWLABEL socket option
1798 3 automatic flow labels are enabled and enforced, they cannot
1799 be disabled by the socket option
1800 = ===========================================================
1804 flowlabel_state_ranges - BOOLEAN
1805 Split the flow label number space into two ranges. 0-0x7FFFF is
1806 reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
1807 is reserved for stateless flow labels as described in RFC6437.
1814 flowlabel_reflect - INTEGER
1815 Control flow label reflection. Needed for Path MTU
1816 Discovery to work with Equal Cost Multipath Routing in anycast
1817 environments. See RFC 7690 and:
1818 https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
1822 - 1: enabled for established flows
1824 Note that this prevents automatic flowlabel changes, as done
1825 in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
1826 and "tcp: Change txhash on every SYN and RTO retransmit"
1828 - 2: enabled for TCP RESET packets (no active listener)
1829 If set, a RST packet sent in response to a SYN packet on a closed
1830 port will reflect the incoming flow label.
1832 - 4: enabled for ICMPv6 echo reply messages.
1836 fib_multipath_hash_policy - INTEGER
1837 Controls which hash policy to use for multipath routes.
1839 Default: 0 (Layer 3)
1843 - 0 - Layer 3 (source and destination addresses plus flow label)
1844 - 1 - Layer 4 (standard 5-tuple)
1845 - 2 - Layer 3 or inner Layer 3 if present
1846 - 3 - Custom multipath hash. Fields used for multipath hash calculation
1847 are determined by fib_multipath_hash_fields sysctl
1849 fib_multipath_hash_fields - UNSIGNED INTEGER
1850 When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
1851 fields used for multipath hash calculation are determined by this
1854 This value is a bitmask which enables various fields for multipath hash
1857 Possible fields are:
1859 ====== ============================
1860 0x0001 Source IP address
1861 0x0002 Destination IP address
1865 0x0020 Destination port
1866 0x0040 Inner source IP address
1867 0x0080 Inner destination IP address
1868 0x0100 Inner IP protocol
1869 0x0200 Inner Flow Label
1870 0x0400 Inner source port
1871 0x0800 Inner destination port
1872 ====== ============================
1874 Default: 0x0007 (source IP, destination IP and IP protocol)
1876 anycast_src_echo_reply - BOOLEAN
1877 Controls the use of anycast addresses as source addresses for ICMPv6
1885 idgen_delay - INTEGER
1886 Controls the delay in seconds after which time to retry
1887 privacy stable address generation if a DAD conflict is
1890 Default: 1 (as specified in RFC7217)
1892 idgen_retries - INTEGER
1893 Controls the number of retries to generate a stable privacy
1894 address if a DAD conflict is detected.
1896 Default: 3 (as specified in RFC7217)
1899 Controls the MLD query robustness variable (see RFC3810 9.1).
1901 Default: 2 (as specified by RFC3810 9.1)
1903 Minimum: 1 (as specified by RFC6636 4.5)
1905 max_dst_opts_number - INTEGER
1906 Maximum number of non-padding TLVs allowed in a Destination
1907 options extension header. If this value is less than zero
1908 then unknown options are disallowed and the number of known
1909 TLVs allowed is the absolute value of this number.
1913 max_hbh_opts_number - INTEGER
1914 Maximum number of non-padding TLVs allowed in a Hop-by-Hop
1915 options extension header. If this value is less than zero
1916 then unknown options are disallowed and the number of known
1917 TLVs allowed is the absolute value of this number.
1921 max_dst_opts_length - INTEGER
1922 Maximum length allowed for a Destination options extension
1925 Default: INT_MAX (unlimited)
1927 max_hbh_length - INTEGER
1928 Maximum length allowed for a Hop-by-Hop options extension
1931 Default: INT_MAX (unlimited)
1933 skip_notify_on_dev_down - BOOLEAN
1934 Controls whether an RTM_DELROUTE message is generated for routes
1935 removed when a device is taken down or deleted. IPv4 does not
1936 generate this message; IPv6 does by default. Setting this sysctl
1937 to true skips the message, making IPv4 and IPv6 on par in relying
1938 on userspace caches to track link events and evict routes.
1940 Default: false (generate message)
1942 nexthop_compat_mode - BOOLEAN
1943 New nexthop API provides a means for managing nexthops independent of
1944 prefixes. Backwards compatibilty with old route format is enabled by
1945 default which means route dumps and notifications contain the new
1946 nexthop attribute but also the full, expanded nexthop definition.
1947 Further, updates or deletes of a nexthop configuration generate route
1948 notifications for each fib entry using the nexthop. Once a system
1949 understands the new API, this sysctl can be disabled to achieve full
1950 performance benefits of the new API by disabling the nexthop expansion
1951 and extraneous notifications.
1952 Default: true (backward compat mode)
1954 fib_notify_on_flag_change - INTEGER
1955 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
1956 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
1958 After installing a route to the kernel, user space receives an
1959 acknowledgment, which means the route was installed in the kernel,
1960 but not necessarily in hardware.
1961 It is also possible for a route already installed in hardware to change
1962 its action and therefore its flags. For example, a host route that is
1963 trapping packets can be "promoted" to perform decapsulation following
1964 the installation of an IPinIP/VXLAN tunnel.
1965 The notifications will indicate to user-space the state of the route.
1967 Default: 0 (Do not emit notifications.)
1971 - 0 - Do not emit notifications.
1972 - 1 - Emit notifications.
1973 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
1976 Define the IOAM id of this node. Uses only 24 bits out of 32 in total.
1983 ioam6_id_wide - LONG INTEGER
1984 Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
1985 total. Can be different from ioam6_id.
1988 Max: 0xFFFFFFFFFFFFFF
1990 Default: 0xFFFFFFFFFFFFFF
1994 ip6frag_high_thresh - INTEGER
1995 Maximum memory used to reassemble IPv6 fragments. When
1996 ip6frag_high_thresh bytes of memory is allocated for this purpose,
1997 the fragment handler will toss packets until ip6frag_low_thresh
2000 ip6frag_low_thresh - INTEGER
2001 See ip6frag_high_thresh
2003 ip6frag_time - INTEGER
2004 Time in seconds to keep an IPv6 fragment in memory.
2007 Change the interface-specific default settings.
2009 These settings would be used during creating new interfaces.
2013 Change all the interface-specific settings.
2015 [XXX: Other special features than forwarding?]
2017 conf/all/disable_ipv6 - BOOLEAN
2018 Changing this value is same as changing ``conf/default/disable_ipv6``
2019 setting and also all per-interface ``disable_ipv6`` settings to the same
2022 Reading this value does not have any particular meaning. It does not say
2023 whether IPv6 support is enabled or disabled. Returned value can be 1
2024 also in the case when some interface has ``disable_ipv6`` set to 0 and
2025 has configured IPv6 addresses.
2027 conf/all/forwarding - BOOLEAN
2028 Enable global IPv6 forwarding between all interfaces.
2030 IPv4 and IPv6 work differently here; e.g. netfilter must be used
2031 to control which interfaces may forward packets and which not.
2033 This also sets all interfaces' Host/Router setting
2034 'forwarding' to the specified value. See below for details.
2036 This referred to as global forwarding.
2041 fwmark_reflect - BOOLEAN
2042 Controls the fwmark of kernel-generated IPv6 reply packets that are not
2043 associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
2044 If unset, these packets have a fwmark of zero. If set, they have the
2045 fwmark of the packet they are replying to.
2049 ``conf/interface/*``:
2050 Change special settings per interface.
2052 The functional behaviour for certain settings is different
2053 depending on whether local forwarding is enabled or not.
2056 Accept Router Advertisements; autoconfigure using them.
2058 It also determines whether or not to transmit Router
2059 Solicitations. If and only if the functional setting is to
2060 accept Router Advertisements, Router Solicitations will be
2063 Possible values are:
2065 == ===========================================================
2066 0 Do not accept Router Advertisements.
2067 1 Accept Router Advertisements if forwarding is disabled.
2068 2 Overrule forwarding behaviour. Accept Router Advertisements
2069 even if forwarding is enabled.
2070 == ===========================================================
2074 - enabled if local forwarding is disabled.
2075 - disabled if local forwarding is enabled.
2077 accept_ra_defrtr - BOOLEAN
2078 Learn default router in Router Advertisement.
2082 - enabled if accept_ra is enabled.
2083 - disabled if accept_ra is disabled.
2085 ra_defrtr_metric - UNSIGNED INTEGER
2086 Route metric for default route learned in Router Advertisement. This value
2087 will be assigned as metric for the default route learned via IPv6 Router
2088 Advertisement. Takes affect only if accept_ra_defrtr is enabled.
2093 Default: IP6_RT_PRIO_USER i.e. 1024.
2095 accept_ra_from_local - BOOLEAN
2096 Accept RA with source-address that is found on local machine
2097 if the RA is otherwise proper and able to be accepted.
2099 Default is to NOT accept these as it may be an un-intended
2104 - enabled if accept_ra_from_local is enabled
2105 on a specific interface.
2106 - disabled if accept_ra_from_local is disabled
2107 on a specific interface.
2109 accept_ra_min_hop_limit - INTEGER
2110 Minimum hop limit Information in Router Advertisement.
2112 Hop limit Information in Router Advertisement less than this
2113 variable shall be ignored.
2117 accept_ra_pinfo - BOOLEAN
2118 Learn Prefix Information in Router Advertisement.
2122 - enabled if accept_ra is enabled.
2123 - disabled if accept_ra is disabled.
2125 accept_ra_rt_info_min_plen - INTEGER
2126 Minimum prefix length of Route Information in RA.
2128 Route Information w/ prefix smaller than this variable shall
2133 * 0 if accept_ra_rtr_pref is enabled.
2134 * -1 if accept_ra_rtr_pref is disabled.
2136 accept_ra_rt_info_max_plen - INTEGER
2137 Maximum prefix length of Route Information in RA.
2139 Route Information w/ prefix larger than this variable shall
2144 * 0 if accept_ra_rtr_pref is enabled.
2145 * -1 if accept_ra_rtr_pref is disabled.
2147 accept_ra_rtr_pref - BOOLEAN
2148 Accept Router Preference in RA.
2152 - enabled if accept_ra is enabled.
2153 - disabled if accept_ra is disabled.
2155 accept_ra_mtu - BOOLEAN
2156 Apply the MTU value specified in RA option 5 (RFC4861). If
2157 disabled, the MTU specified in the RA will be ignored.
2161 - enabled if accept_ra is enabled.
2162 - disabled if accept_ra is disabled.
2164 accept_redirects - BOOLEAN
2169 - enabled if local forwarding is disabled.
2170 - disabled if local forwarding is enabled.
2172 accept_source_route - INTEGER
2173 Accept source routing (routing extension header).
2175 - >= 0: Accept only routing header type 2.
2176 - < 0: Do not accept routing header.
2181 Autoconfigure addresses using Prefix Information in Router
2186 - enabled if accept_ra_pinfo is enabled.
2187 - disabled if accept_ra_pinfo is disabled.
2189 dad_transmits - INTEGER
2190 The amount of Duplicate Address Detection probes to send.
2194 forwarding - INTEGER
2195 Configure interface-specific Host/Router behaviour.
2199 It is recommended to have the same setting on all
2200 interfaces; mixed router/host scenarios are rather uncommon.
2202 Possible values are:
2204 - 0 Forwarding disabled
2205 - 1 Forwarding enabled
2209 By default, Host behaviour is assumed. This means:
2211 1. IsRouter flag is not set in Neighbour Advertisements.
2212 2. If accept_ra is TRUE (default), transmit Router
2214 3. If accept_ra is TRUE (default), accept Router
2215 Advertisements (and do autoconfiguration).
2216 4. If accept_redirects is TRUE (default), accept Redirects.
2220 If local forwarding is enabled, Router behaviour is assumed.
2221 This means exactly the reverse from the above:
2223 1. IsRouter flag is set in Neighbour Advertisements.
2224 2. Router Solicitations are not sent unless accept_ra is 2.
2225 3. Router Advertisements are ignored unless accept_ra is 2.
2226 4. Redirects are ignored.
2228 Default: 0 (disabled) if global forwarding is disabled (default),
2229 otherwise 1 (enabled).
2232 Default Hop Limit to set.
2237 Default Maximum Transfer Unit
2239 Default: 1280 (IPv6 required minimum)
2241 ip_nonlocal_bind - BOOLEAN
2242 If set, allows processes to bind() to non-local IPv6 addresses,
2243 which can be quite useful - but may break some applications.
2247 router_probe_interval - INTEGER
2248 Minimum interval (in seconds) between Router Probing described
2253 router_solicitation_delay - INTEGER
2254 Number of seconds to wait after interface is brought up
2255 before sending Router Solicitations.
2259 router_solicitation_interval - INTEGER
2260 Number of seconds to wait between Router Solicitations.
2264 router_solicitations - INTEGER
2265 Number of Router Solicitations to send until assuming no
2266 routers are present.
2270 use_oif_addrs_only - BOOLEAN
2271 When enabled, the candidate source addresses for destinations
2272 routed via this interface are restricted to the set of addresses
2273 configured on this interface (vis. RFC 6724, section 4).
2277 use_tempaddr - INTEGER
2278 Preference for Privacy Extensions (RFC3041).
2280 * <= 0 : disable Privacy Extensions
2281 * == 1 : enable Privacy Extensions, but prefer public
2282 addresses over temporary addresses.
2283 * > 1 : enable Privacy Extensions and prefer temporary
2284 addresses over public addresses.
2288 * 0 (for most devices)
2289 * -1 (for point-to-point devices and loopback devices)
2291 temp_valid_lft - INTEGER
2292 valid lifetime (in seconds) for temporary addresses.
2294 Default: 172800 (2 days)
2296 temp_prefered_lft - INTEGER
2297 Preferred lifetime (in seconds) for temporary addresses.
2299 Default: 86400 (1 day)
2301 keep_addr_on_down - INTEGER
2302 Keep all IPv6 addresses on an interface down event. If set static
2303 global addresses with no expiration time are not flushed.
2306 * 0 : system default
2309 Default: 0 (addresses are removed)
2311 max_desync_factor - INTEGER
2312 Maximum value for DESYNC_FACTOR, which is a random value
2313 that ensures that clients don't synchronize with each
2314 other and generate new addresses at exactly the same time.
2315 value is in seconds.
2319 regen_max_retry - INTEGER
2320 Number of attempts before give up attempting to generate
2321 valid temporary addresses.
2325 max_addresses - INTEGER
2326 Maximum number of autoconfigured addresses per interface. Setting
2327 to zero disables the limitation. It is not recommended to set this
2328 value too large (or to zero) because it would be an easy way to
2329 crash the kernel by allowing too many addresses to be created.
2333 disable_ipv6 - BOOLEAN
2334 Disable IPv6 operation. If accept_dad is set to 2, this value
2335 will be dynamically set to TRUE if DAD fails for the link-local
2338 Default: FALSE (enable IPv6 operation)
2340 When this value is changed from 1 to 0 (IPv6 is being enabled),
2341 it will dynamically create a link-local address on the given
2342 interface and start Duplicate Address Detection, if necessary.
2344 When this value is changed from 0 to 1 (IPv6 is being disabled),
2345 it will dynamically delete all addresses and routes on the given
2346 interface. From now on it will not possible to add addresses/routes
2347 to the selected interface.
2349 accept_dad - INTEGER
2350 Whether to accept DAD (Duplicate Address Detection).
2352 == ==============================================================
2354 1 Enable DAD (default)
2355 2 Enable DAD, and disable IPv6 operation if MAC-based duplicate
2356 link-local address has been found.
2357 == ==============================================================
2359 DAD operation and mode on a given interface will be selected according
2360 to the maximum value of conf/{all,interface}/accept_dad.
2362 force_tllao - BOOLEAN
2363 Enable sending the target link-layer address option even when
2364 responding to a unicast neighbor solicitation.
2368 Quoting from RFC 2461, section 4.4, Target link-layer address:
2370 "The option MUST be included for multicast solicitations in order to
2371 avoid infinite Neighbor Solicitation "recursion" when the peer node
2372 does not have a cache entry to return a Neighbor Advertisements
2373 message. When responding to unicast solicitations, the option can be
2374 omitted since the sender of the solicitation has the correct link-
2375 layer address; otherwise it would not have be able to send the unicast
2376 solicitation in the first place. However, including the link-layer
2377 address in this case adds little overhead and eliminates a potential
2378 race condition where the sender deletes the cached link-layer address
2379 prior to receiving a response to a previous solicitation."
2381 ndisc_notify - BOOLEAN
2382 Define mode for notification of address and device changes.
2384 * 0 - (default): do nothing
2385 * 1 - Generate unsolicited neighbour advertisements when device is brought
2386 up or hardware address changes.
2388 ndisc_tclass - INTEGER
2389 The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
2390 Discovery (Router Solicitation, Router Advertisement, Neighbor
2391 Solicitation, Neighbor Advertisement, Redirect) messages.
2392 These 8 bits can be interpreted as 6 high order bits holding the DSCP
2393 value and 2 low order bits representing ECN (which you probably want
2398 ndisc_evict_nocarrier - BOOLEAN
2399 Clears the neighbor discovery table on NOCARRIER events. This option is
2400 important for wireless devices where the neighbor discovery cache should
2401 not be cleared when roaming between access points on the same network.
2402 In most cases this should remain as the default (1).
2404 - 1 - (default): Clear neighbor discover cache on NOCARRIER events.
2405 - 0 - Do not clear neighbor discovery cache on NOCARRIER events.
2407 mldv1_unsolicited_report_interval - INTEGER
2408 The interval in milliseconds in which the next unsolicited
2409 MLDv1 report retransmit will take place.
2411 Default: 10000 (10 seconds)
2413 mldv2_unsolicited_report_interval - INTEGER
2414 The interval in milliseconds in which the next unsolicited
2415 MLDv2 report retransmit will take place.
2417 Default: 1000 (1 second)
2419 force_mld_version - INTEGER
2420 * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
2421 * 1 - Enforce to use MLD version 1
2422 * 2 - Enforce to use MLD version 2
2424 suppress_frag_ndisc - INTEGER
2425 Control RFC 6980 (Security Implications of IPv6 Fragmentation
2426 with IPv6 Neighbor Discovery) behavior:
2428 * 1 - (default) discard fragmented neighbor discovery packets
2429 * 0 - allow fragmented neighbor discovery packets
2431 optimistic_dad - BOOLEAN
2432 Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
2434 * 0: disabled (default)
2437 Optimistic Duplicate Address Detection for the interface will be enabled
2438 if at least one of conf/{all,interface}/optimistic_dad is set to 1,
2439 it will be disabled otherwise.
2441 use_optimistic - BOOLEAN
2442 If enabled, do not classify optimistic addresses as deprecated during
2443 source address selection. Preferred addresses will still be chosen
2444 before optimistic addresses, subject to other ranking in the source
2445 address selection algorithm.
2447 * 0: disabled (default)
2450 This will be enabled if at least one of
2451 conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
2453 stable_secret - IPv6 address
2454 This IPv6 address will be used as a secret to generate IPv6
2455 addresses for link-local addresses and autoconfigured
2456 ones. All addresses generated after setting this secret will
2457 be stable privacy ones by default. This can be changed via the
2458 addrgenmode ip-link. conf/default/stable_secret is used as the
2459 secret for the namespace, the interface specific ones can
2460 overwrite that. Writes to conf/all/stable_secret are refused.
2462 It is recommended to generate this secret during installation
2463 of a system and keep it stable after that.
2465 By default the stable secret is unset.
2467 addr_gen_mode - INTEGER
2468 Defines how link-local and autoconf addresses are generated.
2470 = =================================================================
2471 0 generate address based on EUI64 (default)
2472 1 do no generate a link-local address, use EUI64 for addresses
2473 generated from autoconf
2474 2 generate stable privacy addresses, using the secret from
2475 stable_secret (RFC7217)
2476 3 generate stable privacy addresses, using a random secret if unset
2477 = =================================================================
2479 drop_unicast_in_l2_multicast - BOOLEAN
2480 Drop any unicast IPv6 packets that are received in link-layer
2481 multicast (or broadcast) frames.
2483 By default this is turned off.
2485 drop_unsolicited_na - BOOLEAN
2486 Drop all unsolicited neighbor advertisements, for example if there's
2487 a known good NA proxy on the network and such frames need not be used
2488 (or in the case of 802.11, must not be used to prevent attacks.)
2490 By default this is turned off.
2492 accept_untracked_na - INTEGER
2493 Define behavior for accepting neighbor advertisements from devices that
2494 are absent in the neighbor cache:
2496 - 0 - (default) Do not accept unsolicited and untracked neighbor
2499 - 1 - Add a new neighbor cache entry in STALE state for routers on
2500 receiving a neighbor advertisement (either solicited or unsolicited)
2501 with target link-layer address option specified if no neighbor entry
2502 is already present for the advertised IPv6 address. Without this knob,
2503 NAs received for untracked addresses (absent in neighbor cache) are
2506 This is as per router-side behavior documented in RFC9131.
2508 This has lower precedence than drop_unsolicited_na.
2510 This will optimize the return path for the initial off-link
2511 communication that is initiated by a directly connected host, by
2512 ensuring that the first-hop router which turns on this setting doesn't
2513 have to buffer the initial return packets to do neighbor-solicitation.
2514 The prerequisite is that the host is configured to send unsolicited
2515 neighbor advertisements on interface bringup. This setting should be
2516 used in conjunction with the ndisc_notify setting on the host to
2517 satisfy this prerequisite.
2519 - 2 - Extend option (1) to add a new neighbor cache entry only if the
2520 source IP address is in the same subnet as an address configured on
2521 the interface that received the neighbor advertisement.
2523 enhanced_dad - BOOLEAN
2524 Include a nonce option in the IPv6 neighbor solicitation messages used for
2525 duplicate address detection per RFC7527. A received DAD NS will only signal
2526 a duplicate address if the nonce is different. This avoids any false
2527 detection of duplicates due to loopback of the NS messages that we send.
2528 The nonce option will be sent on an interface unless both of
2529 conf/{all,interface}/enhanced_dad are set to FALSE.
2537 Limit the maximal rates for sending ICMPv6 messages.
2539 0 to disable any limiting,
2540 otherwise the minimal space between responses in milliseconds.
2544 ratemask - list of comma separated ranges
2545 For ICMPv6 message types matching the ranges in the ratemask, limit
2546 the sending of the message according to ratelimit parameter.
2548 The format used for both input and output is a comma separated
2549 list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
2550 129). Writing to the file will clear all previous ranges of ICMPv6
2551 message types and update the current list with the input.
2553 Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
2554 for numerical values of ICMPv6 message types, e.g. echo request is 128
2555 and echo reply is 129.
2557 Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)
2559 echo_ignore_all - BOOLEAN
2560 If set non-zero, then the kernel will ignore all ICMP ECHO
2561 requests sent to it over the IPv6 protocol.
2565 echo_ignore_multicast - BOOLEAN
2566 If set non-zero, then the kernel will ignore all ICMP ECHO
2567 requests sent to it over the IPv6 protocol via multicast.
2571 echo_ignore_anycast - BOOLEAN
2572 If set non-zero, then the kernel will ignore all ICMP ECHO
2573 requests sent to it over the IPv6 protocol destined to anycast address.
2577 xfrm6_gc_thresh - INTEGER
2578 (Obsolete since linux-4.14)
2579 The threshold at which we will start garbage collecting for IPv6
2580 destination cache entries. At twice this value the system will
2581 refuse new allocations.
2585 Pekka Savola <pekkas@netcore.fi>
2586 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
2589 /proc/sys/net/bridge/* Variables:
2590 =================================
2592 bridge-nf-call-arptables - BOOLEAN
2593 - 1 : pass bridged ARP traffic to arptables' FORWARD chain.
2598 bridge-nf-call-iptables - BOOLEAN
2599 - 1 : pass bridged IPv4 traffic to iptables' chains.
2604 bridge-nf-call-ip6tables - BOOLEAN
2605 - 1 : pass bridged IPv6 traffic to ip6tables' chains.
2610 bridge-nf-filter-vlan-tagged - BOOLEAN
2611 - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
2616 bridge-nf-filter-pppoe-tagged - BOOLEAN
2617 - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
2622 bridge-nf-pass-vlan-input-dev - BOOLEAN
2623 - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
2624 interface on the bridge and set the netfilter input device to the
2625 vlan. This allows use of e.g. "iptables -i br0.1" and makes the
2626 REDIRECT target work with vlan-on-top-of-bridge interfaces. When no
2627 matching vlan interface is found, or this switch is off, the input
2628 device is set to the bridge interface.
2630 - 0: disable bridge netfilter vlan interface lookup.
2634 ``proc/sys/net/sctp/*`` Variables:
2635 ==================================
2637 addip_enable - BOOLEAN
2638 Enable or disable extension of Dynamic Address Reconfiguration
2639 (ADD-IP) functionality specified in RFC5061. This extension provides
2640 the ability to dynamically add and remove new addresses for the SCTP
2643 1: Enable extension.
2645 0: Disable extension.
2650 Enable or disable pf (pf is short for potentially failed) state. A value
2651 of pf_retrans > path_max_retrans also disables pf state. That is, one of
2652 both pf_enable and pf_retrans > path_max_retrans can disable pf state.
2653 Since pf_retrans and path_max_retrans can be changed by userspace
2654 application, sometimes user expects to disable pf state by the value of
2655 pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
2656 or path_max_retrans is changed by the user application, this pf state is
2657 enabled. As such, it is necessary to add this to dynamically enable
2658 and disable pf state. See:
2659 https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
2669 Unset or enable/disable pf (pf is short for potentially failed) state
2670 exposure. Applications can control the exposure of the PF path state
2671 in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
2672 sockopt. When it's unset, no SCTP_PEER_ADDR_CHANGE event with
2673 SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
2674 can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it's enabled,
2675 a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
2676 SCTP_PF state and a SCTP_PF-state transport info can be got via
2677 SCTP_GET_PEER_ADDR_INFO sockopt; When it's diabled, no
2678 SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
2679 trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
2682 0: Unset pf state exposure, Compatible with old applications.
2684 1: Disable pf state exposure.
2686 2: Enable pf state exposure.
2690 addip_noauth_enable - BOOLEAN
2691 Dynamic Address Reconfiguration (ADD-IP) requires the use of
2692 authentication to protect the operations of adding or removing new
2693 addresses. This requirement is mandated so that unauthorized hosts
2694 would not be able to hijack associations. However, older
2695 implementations may not have implemented this requirement while
2696 allowing the ADD-IP extension. For reasons of interoperability,
2697 we provide this variable to control the enforcement of the
2698 authentication requirement.
2700 == ===============================================================
2701 1 Allow ADD-IP extension to be used without authentication. This
2702 should only be set in a closed environment for interoperability
2703 with older implementations.
2705 0 Enforce the authentication requirement
2706 == ===============================================================
2710 auth_enable - BOOLEAN
2711 Enable or disable Authenticated Chunks extension. This extension
2712 provides the ability to send and receive authenticated chunks and is
2713 required for secure operation of Dynamic Address Reconfiguration
2716 - 1: Enable this extension.
2717 - 0: Disable this extension.
2721 prsctp_enable - BOOLEAN
2722 Enable or disable the Partial Reliability extension (RFC3758) which
2723 is used to notify peers that a given DATA should no longer be expected.
2725 - 1: Enable extension
2731 The limit of the number of new packets that can be initially sent. It
2732 controls how bursty the generated traffic can be.
2736 association_max_retrans - INTEGER
2737 Set the maximum number for retransmissions that an association can
2738 attempt deciding that the remote end is unreachable. If this value
2739 is exceeded, the association is terminated.
2743 max_init_retransmits - INTEGER
2744 The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
2745 that an association will attempt before declaring the destination
2746 unreachable and terminating.
2750 path_max_retrans - INTEGER
2751 The maximum number of retransmissions that will be attempted on a given
2752 path. Once this threshold is exceeded, the path is considered
2753 unreachable, and new traffic will use a different path when the
2754 association is multihomed.
2758 pf_retrans - INTEGER
2759 The number of retransmissions that will be attempted on a given path
2760 before traffic is redirected to an alternate transport (should one
2761 exist). Note this is distinct from path_max_retrans, as a path that
2762 passes the pf_retrans threshold can still be used. Its only
2763 deprioritized when a transmission path is selected by the stack. This
2764 setting is primarily used to enable fast failover mechanisms without
2765 having to reduce path_max_retrans to a very low value. See:
2766 http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
2767 for details. Note also that a value of pf_retrans > path_max_retrans
2768 disables this feature. Since both pf_retrans and path_max_retrans can
2769 be changed by userspace application, a variable pf_enable is used to
2774 ps_retrans - INTEGER
2775 Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
2776 from section-5 "Primary Path Switchover" in rfc7829. The primary path
2777 will be changed to another active path when the path error counter on
2778 the old primary path exceeds PSMR, so that "the SCTP sender is allowed
2779 to continue data transmission on a new working path even when the old
2780 primary destination address becomes active again". Note this feature
2781 is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
2782 and its value can't be less than 'pf_retrans' when changing by sysctl.
2786 rto_initial - INTEGER
2787 The initial round trip timeout value in milliseconds that will be used
2788 in calculating round trip times. This is the initial time interval
2789 for retransmissions.
2794 The maximum value (in milliseconds) of the round trip timeout. This
2795 is the largest time interval that can elapse between retransmissions.
2800 The minimum value (in milliseconds) of the round trip timeout. This
2801 is the smallest time interval the can elapse between retransmissions.
2805 hb_interval - INTEGER
2806 The interval (in milliseconds) between HEARTBEAT chunks. These chunks
2807 are sent at the specified interval on idle paths to probe the state of
2808 a given path between 2 associations.
2812 sack_timeout - INTEGER
2813 The amount of time (in milliseconds) that the implementation will wait
2818 valid_cookie_life - INTEGER
2819 The default lifetime of the SCTP cookie (in milliseconds). The cookie
2820 is used during association establishment.
2824 cookie_preserve_enable - BOOLEAN
2825 Enable or disable the ability to extend the lifetime of the SCTP cookie
2826 that is used during the establishment phase of SCTP association
2828 - 1: Enable cookie lifetime extension.
2833 cookie_hmac_alg - STRING
2834 Select the hmac algorithm used when generating the cookie value sent by
2835 a listening sctp socket to a connecting client in the INIT-ACK chunk.
2842 Ability to assign md5 or sha1 as the selected alg is predicated on the
2843 configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
2844 CONFIG_CRYPTO_SHA1).
2846 Default: Dependent on configuration. MD5 if available, else SHA1 if
2847 available, else none.
2849 rcvbuf_policy - INTEGER
2850 Determines if the receive buffer is attributed to the socket or to
2851 association. SCTP supports the capability to create multiple
2852 associations on a single socket. When using this capability, it is
2853 possible that a single stalled association that's buffering a lot
2854 of data may block other associations from delivering their data by
2855 consuming all of the receive buffer space. To work around this,
2856 the rcvbuf_policy could be set to attribute the receiver buffer space
2857 to each association instead of the socket. This prevents the described
2860 - 1: rcvbuf space is per association
2861 - 0: rcvbuf space is per socket
2865 sndbuf_policy - INTEGER
2866 Similar to rcvbuf_policy above, this applies to send buffer space.
2868 - 1: Send buffer is tracked per association
2869 - 0: Send buffer is tracked per socket.
2873 sctp_mem - vector of 3 INTEGERs: min, pressure, max
2874 Number of pages allowed for queueing by all SCTP sockets.
2876 min: Below this number of pages SCTP is not bothered about its
2877 memory appetite. When amount of memory allocated by SCTP exceeds
2878 this number, SCTP starts to moderate memory usage.
2880 pressure: This value was introduced to follow format of tcp_mem.
2882 max: Number of pages allowed for queueing by all SCTP sockets.
2884 Default is calculated at boot time from amount of available memory.
2886 sctp_rmem - vector of 3 INTEGERs: min, default, max
2887 Only the first value ("min") is used, "default" and "max" are
2890 min: Minimal size of receive buffer used by SCTP socket.
2891 It is guaranteed to each SCTP socket (but not association) even
2892 under moderate memory pressure.
2896 sctp_wmem - vector of 3 INTEGERs: min, default, max
2897 Only the first value ("min") is used, "default" and "max" are
2900 min: Minimum size of send buffer that can be used by SCTP sockets.
2901 It is guaranteed to each SCTP socket (but not association) even
2902 under moderate memory pressure.
2906 addr_scope_policy - INTEGER
2907 Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
2909 - 0 - Disable IPv4 address scoping
2910 - 1 - Enable IPv4 address scoping
2911 - 2 - Follow draft but allow IPv4 private addresses
2912 - 3 - Follow draft but allow IPv4 link local addresses
2917 The listening port for the local UDP tunneling sock. Normally it's
2918 using the IANA-assigned UDP port number 9899 (sctp-tunneling).
2920 This UDP sock is used for processing the incoming UDP-encapsulated
2921 SCTP packets (from RFC6951), and shared by all applications in the
2922 same net namespace. This UDP sock will be closed when the value is
2925 The value will also be used to set the src port of the UDP header
2926 for the outgoing UDP-encapsulated SCTP packets. For the dest port,
2927 please refer to 'encap_port' below.
2931 encap_port - INTEGER
2932 The default remote UDP encapsulation port.
2934 This value is used to set the dest port of the UDP header for the
2935 outgoing UDP-encapsulated SCTP packets by default. Users can also
2936 change the value for each sock/asoc/transport by using setsockopt.
2937 For further information, please refer to RFC6951.
2939 Note that when connecting to a remote server, the client should set
2940 this to the port that the UDP tunneling sock on the peer server is
2941 listening to and the local UDP tunneling sock on the client also
2942 must be started. On the server, it would get the encap_port from
2943 the incoming packet's source port.
2947 plpmtud_probe_interval - INTEGER
2948 The time interval (in milliseconds) for the PLPMTUD probe timer,
2949 which is configured to expire after this period to receive an
2950 acknowledgment to a probe packet. This is also the time interval
2951 between the probes for the current pmtu when the probe search
2954 PLPMTUD will be disabled when 0 is set, and other values for it
2959 reconf_enable - BOOLEAN
2960 Enable or disable extension of Stream Reconfiguration functionality
2961 specified in RFC6525. This extension provides the ability to "reset"
2962 a stream, and it includes the Parameters of "Outgoing/Incoming SSN
2963 Reset", "SSN/TSN Reset" and "Add Outgoing/Incoming Streams".
2965 - 1: Enable extension.
2966 - 0: Disable extension.
2970 intl_enable - BOOLEAN
2971 Enable or disable extension of User Message Interleaving functionality
2972 specified in RFC8260. This extension allows the interleaving of user
2973 messages sent on different streams. With this feature enabled, I-DATA
2974 chunk will replace DATA chunk to carry user messages if also supported
2975 by the peer. Note that to use this feature, one needs to set this option
2976 to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2
2977 and SCTP_INTERLEAVING_SUPPORTED to 1.
2979 - 1: Enable extension.
2980 - 0: Disable extension.
2984 ecn_enable - BOOLEAN
2985 Control use of Explicit Congestion Notification (ECN) by SCTP.
2986 Like in TCP, ECN is used only when both ends of the SCTP connection
2987 indicate support for it. This feature is useful in avoiding losses
2988 due to congestion by allowing supporting routers to signal congestion
2989 before having to drop packets.
2997 ``/proc/sys/net/core/*``
2998 ========================
3000 Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
3003 ``/proc/sys/net/unix/*``
3004 ========================
3006 max_dgram_qlen - INTEGER
3007 The maximum length of dgram socket receive queue