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 mtu_expires - INTEGER
206 Time, in seconds, that cached PMTU information is kept.
208 min_adv_mss - INTEGER
209 The advertised MSS depends on the first hop route MTU, but will
210 never be lower than this setting.
212 fib_notify_on_flag_change - INTEGER
213 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
214 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
216 After installing a route to the kernel, user space receives an
217 acknowledgment, which means the route was installed in the kernel,
218 but not necessarily in hardware.
219 It is also possible for a route already installed in hardware to change
220 its action and therefore its flags. For example, a host route that is
221 trapping packets can be "promoted" to perform decapsulation following
222 the installation of an IPinIP/VXLAN tunnel.
223 The notifications will indicate to user-space the state of the route.
225 Default: 0 (Do not emit notifications.)
229 - 0 - Do not emit notifications.
230 - 1 - Emit notifications.
231 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
235 ipfrag_high_thresh - LONG INTEGER
236 Maximum memory used to reassemble IP fragments.
238 ipfrag_low_thresh - LONG INTEGER
239 (Obsolete since linux-4.17)
240 Maximum memory used to reassemble IP fragments before the kernel
241 begins to remove incomplete fragment queues to free up resources.
242 The kernel still accepts new fragments for defragmentation.
244 ipfrag_time - INTEGER
245 Time in seconds to keep an IP fragment in memory.
247 ipfrag_max_dist - INTEGER
248 ipfrag_max_dist is a non-negative integer value which defines the
249 maximum "disorder" which is allowed among fragments which share a
250 common IP source address. Note that reordering of packets is
251 not unusual, but if a large number of fragments arrive from a source
252 IP address while a particular fragment queue remains incomplete, it
253 probably indicates that one or more fragments belonging to that queue
254 have been lost. When ipfrag_max_dist is positive, an additional check
255 is done on fragments before they are added to a reassembly queue - if
256 ipfrag_max_dist (or more) fragments have arrived from a particular IP
257 address between additions to any IP fragment queue using that source
258 address, it's presumed that one or more fragments in the queue are
259 lost. The existing fragment queue will be dropped, and a new one
260 started. An ipfrag_max_dist value of zero disables this check.
262 Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
263 result in unnecessarily dropping fragment queues when normal
264 reordering of packets occurs, which could lead to poor application
265 performance. Using a very large value, e.g. 50000, increases the
266 likelihood of incorrectly reassembling IP fragments that originate
267 from different IP datagrams, which could result in data corruption.
270 bc_forwarding - INTEGER
271 bc_forwarding enables the feature described in rfc1812#section-5.3.5.2
272 and rfc2644. It allows the router to forward directed broadcast.
273 To enable this feature, the 'all' entry and the input interface entry
280 inet_peer_threshold - INTEGER
281 The approximate size of the storage. Starting from this threshold
282 entries will be thrown aggressively. This threshold also determines
283 entries' time-to-live and time intervals between garbage collection
284 passes. More entries, less time-to-live, less GC interval.
286 inet_peer_minttl - INTEGER
287 Minimum time-to-live of entries. Should be enough to cover fragment
288 time-to-live on the reassembling side. This minimum time-to-live is
289 guaranteed if the pool size is less than inet_peer_threshold.
292 inet_peer_maxttl - INTEGER
293 Maximum time-to-live of entries. Unused entries will expire after
294 this period of time if there is no memory pressure on the pool (i.e.
295 when the number of entries in the pool is very small).
302 Limit of socket listen() backlog, known in userspace as SOMAXCONN.
303 Defaults to 4096. (Was 128 before linux-5.4)
304 See also tcp_max_syn_backlog for additional tuning for TCP sockets.
306 tcp_abort_on_overflow - BOOLEAN
307 If listening service is too slow to accept new connections,
308 reset them. Default state is FALSE. It means that if overflow
309 occurred due to a burst, connection will recover. Enable this
310 option _only_ if you are really sure that listening daemon
311 cannot be tuned to accept connections faster. Enabling this
312 option can harm clients of your server.
314 tcp_adv_win_scale - INTEGER
315 Count buffering overhead as bytes/2^tcp_adv_win_scale
316 (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
319 Possible values are [-31, 31], inclusive.
323 tcp_allowed_congestion_control - STRING
324 Show/set the congestion control choices available to non-privileged
325 processes. The list is a subset of those listed in
326 tcp_available_congestion_control.
328 Default is "reno" and the default setting (tcp_congestion_control).
330 tcp_app_win - INTEGER
331 Reserve max(window/2^tcp_app_win, mss) of window for application
332 buffer. Value 0 is special, it means that nothing is reserved.
336 tcp_autocorking - BOOLEAN
337 Enable TCP auto corking :
338 When applications do consecutive small write()/sendmsg() system calls,
339 we try to coalesce these small writes as much as possible, to lower
340 total amount of sent packets. This is done if at least one prior
341 packet for the flow is waiting in Qdisc queues or device transmit
342 queue. Applications can still use TCP_CORK for optimal behavior
343 when they know how/when to uncork their sockets.
347 tcp_available_congestion_control - STRING
348 Shows the available congestion control choices that are registered.
349 More congestion control algorithms may be available as modules,
352 tcp_base_mss - INTEGER
353 The initial value of search_low to be used by the packetization layer
354 Path MTU discovery (MTU probing). If MTU probing is enabled,
355 this is the initial MSS used by the connection.
357 tcp_mtu_probe_floor - INTEGER
358 If MTU probing is enabled this caps the minimum MSS used for search_low
363 tcp_min_snd_mss - INTEGER
364 TCP SYN and SYNACK messages usually advertise an ADVMSS option,
365 as described in RFC 1122 and RFC 6691.
367 If this ADVMSS option is smaller than tcp_min_snd_mss,
368 it is silently capped to tcp_min_snd_mss.
370 Default : 48 (at least 8 bytes of payload per segment)
372 tcp_congestion_control - STRING
373 Set the congestion control algorithm to be used for new
374 connections. The algorithm "reno" is always available, but
375 additional choices may be available based on kernel configuration.
376 Default is set as part of kernel configuration.
377 For passive connections, the listener congestion control choice
380 [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
383 Allows TCP to send "duplicate" SACKs.
385 tcp_early_retrans - INTEGER
386 Tail loss probe (TLP) converts RTOs occurring due to tail
387 losses into fast recovery (draft-ietf-tcpm-rack). Note that
388 TLP requires RACK to function properly (see tcp_recovery below)
398 Control use of Explicit Congestion Notification (ECN) by TCP.
399 ECN is used only when both ends of the TCP connection indicate
400 support for it. This feature is useful in avoiding losses due
401 to congestion by allowing supporting routers to signal
402 congestion before having to drop packets.
406 = =====================================================
407 0 Disable ECN. Neither initiate nor accept ECN.
408 1 Enable ECN when requested by incoming connections and
409 also request ECN on outgoing connection attempts.
410 2 Enable ECN when requested by incoming connections
411 but do not request ECN on outgoing connections.
412 = =====================================================
416 tcp_ecn_fallback - BOOLEAN
417 If the kernel detects that ECN connection misbehaves, enable fall
418 back to non-ECN. Currently, this knob implements the fallback
419 from RFC3168, section 6.1.1.1., but we reserve that in future,
420 additional detection mechanisms could be implemented under this
421 knob. The value is not used, if tcp_ecn or per route (or congestion
422 control) ECN settings are disabled.
424 Default: 1 (fallback enabled)
427 This is a legacy option, it has no effect anymore.
429 tcp_fin_timeout - INTEGER
430 The length of time an orphaned (no longer referenced by any
431 application) connection will remain in the FIN_WAIT_2 state
432 before it is aborted at the local end. While a perfectly
433 valid "receive only" state for an un-orphaned connection, an
434 orphaned connection in FIN_WAIT_2 state could otherwise wait
435 forever for the remote to close its end of the connection.
442 Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
443 F-RTO is an enhanced recovery algorithm for TCP retransmission
444 timeouts. It is particularly beneficial in networks where the
445 RTT fluctuates (e.g., wireless). F-RTO is sender-side only
446 modification. It does not require any support from the peer.
448 By default it's enabled with a non-zero value. 0 disables F-RTO.
450 tcp_fwmark_accept - BOOLEAN
451 If set, incoming connections to listening sockets that do not have a
452 socket mark will set the mark of the accepting socket to the fwmark of
453 the incoming SYN packet. This will cause all packets on that connection
454 (starting from the first SYNACK) to be sent with that fwmark. The
455 listening socket's mark is unchanged. Listening sockets that already
456 have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
461 tcp_invalid_ratelimit - INTEGER
462 Limit the maximal rate for sending duplicate acknowledgments
463 in response to incoming TCP packets that are for an existing
464 connection but that are invalid due to any of these reasons:
466 (a) out-of-window sequence number,
467 (b) out-of-window acknowledgment number, or
468 (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
470 This can help mitigate simple "ack loop" DoS attacks, wherein
471 a buggy or malicious middlebox or man-in-the-middle can
472 rewrite TCP header fields in manner that causes each endpoint
473 to think that the other is sending invalid TCP segments, thus
474 causing each side to send an unterminating stream of duplicate
475 acknowledgments for invalid segments.
477 Using 0 disables rate-limiting of dupacks in response to
478 invalid segments; otherwise this value specifies the minimal
479 space between sending such dupacks, in milliseconds.
481 Default: 500 (milliseconds).
483 tcp_keepalive_time - INTEGER
484 How often TCP sends out keepalive messages when keepalive is enabled.
487 tcp_keepalive_probes - INTEGER
488 How many keepalive probes TCP sends out, until it decides that the
489 connection is broken. Default value: 9.
491 tcp_keepalive_intvl - INTEGER
492 How frequently the probes are send out. Multiplied by
493 tcp_keepalive_probes it is time to kill not responding connection,
494 after probes started. Default value: 75sec i.e. connection
495 will be aborted after ~11 minutes of retries.
497 tcp_l3mdev_accept - BOOLEAN
498 Enables child sockets to inherit the L3 master device index.
499 Enabling this option allows a "global" listen socket to work
500 across L3 master domains (e.g., VRFs) with connected sockets
501 derived from the listen socket to be bound to the L3 domain in
502 which the packets originated. Only valid when the kernel was
503 compiled with CONFIG_NET_L3_MASTER_DEV.
505 Default: 0 (disabled)
507 tcp_low_latency - BOOLEAN
508 This is a legacy option, it has no effect anymore.
510 tcp_max_orphans - INTEGER
511 Maximal number of TCP sockets not attached to any user file handle,
512 held by system. If this number is exceeded orphaned connections are
513 reset immediately and warning is printed. This limit exists
514 only to prevent simple DoS attacks, you _must_ not rely on this
515 or lower the limit artificially, but rather increase it
516 (probably, after increasing installed memory),
517 if network conditions require more than default value,
518 and tune network services to linger and kill such states
519 more aggressively. Let me to remind again: each orphan eats
520 up to ~64K of unswappable memory.
522 tcp_max_syn_backlog - INTEGER
523 Maximal number of remembered connection requests (SYN_RECV),
524 which have not received an acknowledgment from connecting client.
526 This is a per-listener limit.
528 The minimal value is 128 for low memory machines, and it will
529 increase in proportion to the memory of machine.
531 If server suffers from overload, try increasing this number.
533 Remember to also check /proc/sys/net/core/somaxconn
534 A SYN_RECV request socket consumes about 304 bytes of memory.
536 tcp_max_tw_buckets - INTEGER
537 Maximal number of timewait sockets held by system simultaneously.
538 If this number is exceeded time-wait socket is immediately destroyed
539 and warning is printed. This limit exists only to prevent
540 simple DoS attacks, you _must_ not lower the limit artificially,
541 but rather increase it (probably, after increasing installed memory),
542 if network conditions require more than default value.
544 tcp_mem - vector of 3 INTEGERs: min, pressure, max
545 min: below this number of pages TCP is not bothered about its
548 pressure: when amount of memory allocated by TCP exceeds this number
549 of pages, TCP moderates its memory consumption and enters memory
550 pressure mode, which is exited when memory consumption falls
553 max: number of pages allowed for queueing by all TCP sockets.
555 Defaults are calculated at boot time from amount of available
558 tcp_min_rtt_wlen - INTEGER
559 The window length of the windowed min filter to track the minimum RTT.
560 A shorter window lets a flow more quickly pick up new (higher)
561 minimum RTT when it is moved to a longer path (e.g., due to traffic
562 engineering). A longer window makes the filter more resistant to RTT
563 inflations such as transient congestion. The unit is seconds.
565 Possible values: 0 - 86400 (1 day)
569 tcp_moderate_rcvbuf - BOOLEAN
570 If set, TCP performs receive buffer auto-tuning, attempting to
571 automatically size the buffer (no greater than tcp_rmem[2]) to
572 match the size required by the path for full throughput. Enabled by
575 tcp_mtu_probing - INTEGER
576 Controls TCP Packetization-Layer Path MTU Discovery. Takes three
580 - 1 - Disabled by default, enabled when an ICMP black hole detected
581 - 2 - Always enabled, use initial MSS of tcp_base_mss.
583 tcp_probe_interval - UNSIGNED INTEGER
584 Controls how often to start TCP Packetization-Layer Path MTU
585 Discovery reprobe. The default is reprobing every 10 minutes as
588 tcp_probe_threshold - INTEGER
589 Controls when TCP Packetization-Layer Path MTU Discovery probing
590 will stop in respect to the width of search range in bytes. Default
593 tcp_no_metrics_save - BOOLEAN
594 By default, TCP saves various connection metrics in the route cache
595 when the connection closes, so that connections established in the
596 near future can use these to set initial conditions. Usually, this
597 increases overall performance, but may sometimes cause performance
598 degradation. If set, TCP will not cache metrics on closing
601 tcp_no_ssthresh_metrics_save - BOOLEAN
602 Controls whether TCP saves ssthresh metrics in the route cache.
604 Default is 1, which disables ssthresh metrics.
606 tcp_orphan_retries - INTEGER
607 This value influences the timeout of a locally closed TCP connection,
608 when RTO retransmissions remain unacknowledged.
609 See tcp_retries2 for more details.
611 The default value is 8.
613 If your machine is a loaded WEB server,
614 you should think about lowering this value, such sockets
615 may consume significant resources. Cf. tcp_max_orphans.
617 tcp_recovery - INTEGER
618 This value is a bitmap to enable various experimental loss recovery
621 ========= =============================================================
622 RACK: 0x1 enables the RACK loss detection for fast detection of lost
623 retransmissions and tail drops. It also subsumes and disables
624 RFC6675 recovery for SACK connections.
626 RACK: 0x2 makes RACK's reordering window static (min_rtt/4).
628 RACK: 0x4 disables RACK's DUPACK threshold heuristic
629 ========= =============================================================
633 tcp_reordering - INTEGER
634 Initial reordering level of packets in a TCP stream.
635 TCP stack can then dynamically adjust flow reordering level
636 between this initial value and tcp_max_reordering
640 tcp_max_reordering - INTEGER
641 Maximal reordering level of packets in a TCP stream.
642 300 is a fairly conservative value, but you might increase it
643 if paths are using per packet load balancing (like bonding rr mode)
647 tcp_retrans_collapse - BOOLEAN
648 Bug-to-bug compatibility with some broken printers.
649 On retransmit try to send bigger packets to work around bugs in
652 tcp_retries1 - INTEGER
653 This value influences the time, after which TCP decides, that
654 something is wrong due to unacknowledged RTO retransmissions,
655 and reports this suspicion to the network layer.
656 See tcp_retries2 for more details.
658 RFC 1122 recommends at least 3 retransmissions, which is the
661 tcp_retries2 - INTEGER
662 This value influences the timeout of an alive TCP connection,
663 when RTO retransmissions remain unacknowledged.
664 Given a value of N, a hypothetical TCP connection following
665 exponential backoff with an initial RTO of TCP_RTO_MIN would
666 retransmit N times before killing the connection at the (N+1)th RTO.
668 The default value of 15 yields a hypothetical timeout of 924.6
669 seconds and is a lower bound for the effective timeout.
670 TCP will effectively time out at the first RTO which exceeds the
671 hypothetical timeout.
673 RFC 1122 recommends at least 100 seconds for the timeout,
674 which corresponds to a value of at least 8.
676 tcp_rfc1337 - BOOLEAN
677 If set, the TCP stack behaves conforming to RFC1337. If unset,
678 we are not conforming to RFC, but prevent TCP TIME_WAIT
683 tcp_rmem - vector of 3 INTEGERs: min, default, max
684 min: Minimal size of receive buffer used by TCP sockets.
685 It is guaranteed to each TCP socket, even under moderate memory
690 default: initial size of receive buffer used by TCP sockets.
691 This value overrides net.core.rmem_default used by other protocols.
692 Default: 131072 bytes.
693 This value results in initial window of 65535.
695 max: maximal size of receive buffer allowed for automatically
696 selected receiver buffers for TCP socket. This value does not override
697 net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables
698 automatic tuning of that socket's receive buffer size, in which
699 case this value is ignored.
700 Default: between 131072 and 6MB, depending on RAM size.
703 Enable select acknowledgments (SACKS).
705 tcp_comp_sack_delay_ns - LONG INTEGER
706 TCP tries to reduce number of SACK sent, using a timer
707 based on 5% of SRTT, capped by this sysctl, in nano seconds.
708 The default is 1ms, based on TSO autosizing period.
710 Default : 1,000,000 ns (1 ms)
712 tcp_comp_sack_slack_ns - LONG INTEGER
713 This sysctl control the slack used when arming the
714 timer used by SACK compression. This gives extra time
715 for small RTT flows, and reduces system overhead by allowing
716 opportunistic reduction of timer interrupts.
718 Default : 100,000 ns (100 us)
720 tcp_comp_sack_nr - INTEGER
721 Max number of SACK that can be compressed.
722 Using 0 disables SACK compression.
726 tcp_slow_start_after_idle - BOOLEAN
727 If set, provide RFC2861 behavior and time out the congestion
728 window after an idle period. An idle period is defined at
729 the current RTO. If unset, the congestion window will not
730 be timed out after an idle period.
735 Use the Host requirements interpretation of the TCP urgent pointer field.
736 Most hosts use the older BSD interpretation, so if you turn this on
737 Linux might not communicate correctly with them.
741 tcp_synack_retries - INTEGER
742 Number of times SYNACKs for a passive TCP connection attempt will
743 be retransmitted. Should not be higher than 255. Default value
744 is 5, which corresponds to 31seconds till the last retransmission
745 with the current initial RTO of 1second. With this the final timeout
746 for a passive TCP connection will happen after 63seconds.
748 tcp_syncookies - INTEGER
749 Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
750 Send out syncookies when the syn backlog queue of a socket
751 overflows. This is to prevent against the common 'SYN flood attack'
754 Note, that syncookies is fallback facility.
755 It MUST NOT be used to help highly loaded servers to stand
756 against legal connection rate. If you see SYN flood warnings
757 in your logs, but investigation shows that they occur
758 because of overload with legal connections, you should tune
759 another parameters until this warning disappear.
760 See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
762 syncookies seriously violate TCP protocol, do not allow
763 to use TCP extensions, can result in serious degradation
764 of some services (f.e. SMTP relaying), visible not by you,
765 but your clients and relays, contacting you. While you see
766 SYN flood warnings in logs not being really flooded, your server
767 is seriously misconfigured.
769 If you want to test which effects syncookies have to your
770 network connections you can set this knob to 2 to enable
771 unconditionally generation of syncookies.
773 tcp_migrate_req - BOOLEAN
774 The incoming connection is tied to a specific listening socket when
775 the initial SYN packet is received during the three-way handshake.
776 When a listener is closed, in-flight request sockets during the
777 handshake and established sockets in the accept queue are aborted.
779 If the listener has SO_REUSEPORT enabled, other listeners on the
780 same port should have been able to accept such connections. This
781 option makes it possible to migrate such child sockets to another
782 listener after close() or shutdown().
784 The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
785 usually be used to define the policy to pick an alive listener.
786 Otherwise, the kernel will randomly pick an alive listener only if
787 this option is enabled.
789 Note that migration between listeners with different settings may
790 crash applications. Let's say migration happens from listener A to
791 B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
792 the requests migrated from A. To avoid such a situation, cancel
793 migration by returning SK_DROP in the type of eBPF program, or
798 tcp_fastopen - INTEGER
799 Enable TCP Fast Open (RFC7413) to send and accept data in the opening
802 The client support is enabled by flag 0x1 (on by default). The client
803 then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
804 rather than connect() to send data in SYN.
806 The server support is enabled by flag 0x2 (off by default). Then
807 either enable for all listeners with another flag (0x400) or
808 enable individual listeners via TCP_FASTOPEN socket option with
809 the option value being the length of the syn-data backlog.
811 The values (bitmap) are
813 ===== ======== ======================================================
814 0x1 (client) enables sending data in the opening SYN on the client.
815 0x2 (server) enables the server support, i.e., allowing data in
816 a SYN packet to be accepted and passed to the
817 application before 3-way handshake finishes.
818 0x4 (client) send data in the opening SYN regardless of cookie
819 availability and without a cookie option.
820 0x200 (server) accept data-in-SYN w/o any cookie option present.
821 0x400 (server) enable all listeners to support Fast Open by
822 default without explicit TCP_FASTOPEN socket option.
823 ===== ======== ======================================================
827 Note that additional client or server features are only
828 effective if the basic support (0x1 and 0x2) are enabled respectively.
830 tcp_fastopen_blackhole_timeout_sec - INTEGER
831 Initial time period in second to disable Fastopen on active TCP sockets
832 when a TFO firewall blackhole issue happens.
833 This time period will grow exponentially when more blackhole issues
834 get detected right after Fastopen is re-enabled and will reset to
835 initial value when the blackhole issue goes away.
836 0 to disable the blackhole detection.
838 By default, it is set to 0 (feature is disabled).
840 tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
841 The list consists of a primary key and an optional backup key. The
842 primary key is used for both creating and validating cookies, while the
843 optional backup key is only used for validating cookies. The purpose of
844 the backup key is to maximize TFO validation when keys are rotated.
846 A randomly chosen primary key may be configured by the kernel if
847 the tcp_fastopen sysctl is set to 0x400 (see above), or if the
848 TCP_FASTOPEN setsockopt() optname is set and a key has not been
849 previously configured via sysctl. If keys are configured via
850 setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
851 per-socket keys will be used instead of any keys that are specified via
854 A key is specified as 4 8-digit hexadecimal integers which are separated
855 by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
856 omitted. A primary and a backup key may be specified by separating them
857 by a comma. If only one key is specified, it becomes the primary key and
858 any previously configured backup keys are removed.
860 tcp_syn_retries - INTEGER
861 Number of times initial SYNs for an active TCP connection attempt
862 will be retransmitted. Should not be higher than 127. Default value
863 is 6, which corresponds to 63seconds till the last retransmission
864 with the current initial RTO of 1second. With this the final timeout
865 for an active TCP connection attempt will happen after 127seconds.
867 tcp_timestamps - INTEGER
868 Enable timestamps as defined in RFC1323.
871 - 1: Enable timestamps as defined in RFC1323 and use random offset for
872 each connection rather than only using the current time.
873 - 2: Like 1, but without random offsets.
877 tcp_min_tso_segs - INTEGER
878 Minimal number of segments per TSO frame.
880 Since linux-3.12, TCP does an automatic sizing of TSO frames,
881 depending on flow rate, instead of filling 64Kbytes packets.
882 For specific usages, it's possible to force TCP to build big
883 TSO frames. Note that TCP stack might split too big TSO packets
884 if available window is too small.
888 tcp_tso_rtt_log - INTEGER
889 Adjustment of TSO packet sizes based on min_rtt
891 Starting from linux-5.18, TCP autosizing can be tweaked
892 for flows having small RTT.
894 Old autosizing was splitting the pacing budget to send 1024 TSO
897 tso_packet_size = sk->sk_pacing_rate / 1024;
899 With the new mechanism, we increase this TSO sizing using:
901 distance = min_rtt_usec / (2^tcp_tso_rtt_log)
902 tso_packet_size += gso_max_size >> distance;
904 This means that flows between very close hosts can use bigger
905 TSO packets, reducing their cpu costs.
907 If you want to use the old autosizing, set this sysctl to 0.
909 Default: 9 (2^9 = 512 usec)
911 tcp_pacing_ss_ratio - INTEGER
912 sk->sk_pacing_rate is set by TCP stack using a ratio applied
913 to current rate. (current_rate = cwnd * mss / srtt)
914 If TCP is in slow start, tcp_pacing_ss_ratio is applied
915 to let TCP probe for bigger speeds, assuming cwnd can be
916 doubled every other RTT.
920 tcp_pacing_ca_ratio - INTEGER
921 sk->sk_pacing_rate is set by TCP stack using a ratio applied
922 to current rate. (current_rate = cwnd * mss / srtt)
923 If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
924 is applied to conservatively probe for bigger throughput.
928 tcp_tso_win_divisor - INTEGER
929 This allows control over what percentage of the congestion window
930 can be consumed by a single TSO frame.
931 The setting of this parameter is a choice between burstiness and
932 building larger TSO frames.
936 tcp_tw_reuse - INTEGER
937 Enable reuse of TIME-WAIT sockets for new connections when it is
938 safe from protocol viewpoint.
942 - 2 - enable for loopback traffic only
944 It should not be changed without advice/request of technical
949 tcp_window_scaling - BOOLEAN
950 Enable window scaling as defined in RFC1323.
952 tcp_wmem - vector of 3 INTEGERs: min, default, max
953 min: Amount of memory reserved for send buffers for TCP sockets.
954 Each TCP socket has rights to use it due to fact of its birth.
958 default: initial size of send buffer used by TCP sockets. This
959 value overrides net.core.wmem_default used by other protocols.
961 It is usually lower than net.core.wmem_default.
965 max: Maximal amount of memory allowed for automatically tuned
966 send buffers for TCP sockets. This value does not override
967 net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables
968 automatic tuning of that socket's send buffer size, in which case
969 this value is ignored.
971 Default: between 64K and 4MB, depending on RAM size.
973 tcp_notsent_lowat - UNSIGNED INTEGER
974 A TCP socket can control the amount of unsent bytes in its write queue,
975 thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
976 reports POLLOUT events if the amount of unsent bytes is below a per
977 socket value, and if the write queue is not full. sendmsg() will
978 also not add new buffers if the limit is hit.
980 This global variable controls the amount of unsent data for
981 sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
982 to the global variable has immediate effect.
984 Default: UINT_MAX (0xFFFFFFFF)
986 tcp_workaround_signed_windows - BOOLEAN
987 If set, assume no receipt of a window scaling option means the
988 remote TCP is broken and treats the window as a signed quantity.
989 If unset, assume the remote TCP is not broken even if we do
990 not receive a window scaling option from them.
994 tcp_thin_linear_timeouts - BOOLEAN
995 Enable dynamic triggering of linear timeouts for thin streams.
996 If set, a check is performed upon retransmission by timeout to
997 determine if the stream is thin (less than 4 packets in flight).
998 As long as the stream is found to be thin, up to 6 linear
999 timeouts may be performed before exponential backoff mode is
1000 initiated. This improves retransmission latency for
1001 non-aggressive thin streams, often found to be time-dependent.
1002 For more information on thin streams, see
1003 Documentation/networking/tcp-thin.rst
1007 tcp_limit_output_bytes - INTEGER
1008 Controls TCP Small Queue limit per tcp socket.
1009 TCP bulk sender tends to increase packets in flight until it
1010 gets losses notifications. With SNDBUF autotuning, this can
1011 result in a large amount of packets queued on the local machine
1012 (e.g.: qdiscs, CPU backlog, or device) hurting latency of other
1013 flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes
1014 limits the number of bytes on qdisc or device to reduce artificial
1015 RTT/cwnd and reduce bufferbloat.
1017 Default: 1048576 (16 * 65536)
1019 tcp_challenge_ack_limit - INTEGER
1020 Limits number of Challenge ACK sent per second, as recommended
1021 in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
1027 udp_l3mdev_accept - BOOLEAN
1028 Enabling this option allows a "global" bound socket to work
1029 across L3 master domains (e.g., VRFs) with packets capable of
1030 being received regardless of the L3 domain in which they
1031 originated. Only valid when the kernel was compiled with
1032 CONFIG_NET_L3_MASTER_DEV.
1034 Default: 0 (disabled)
1036 udp_mem - vector of 3 INTEGERs: min, pressure, max
1037 Number of pages allowed for queueing by all UDP sockets.
1039 min: Number of pages allowed for queueing by all UDP sockets.
1041 pressure: This value was introduced to follow format of tcp_mem.
1043 max: This value was introduced to follow format of tcp_mem.
1045 Default is calculated at boot time from amount of available memory.
1047 udp_rmem_min - INTEGER
1048 Minimal size of receive buffer used by UDP sockets in moderation.
1049 Each UDP socket is able to use the size for receiving data, even if
1050 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
1054 udp_wmem_min - INTEGER
1055 Minimal size of send buffer used by UDP sockets in moderation.
1056 Each UDP socket is able to use the size for sending data, even if
1057 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
1064 raw_l3mdev_accept - BOOLEAN
1065 Enabling this option allows a "global" bound socket to work
1066 across L3 master domains (e.g., VRFs) with packets capable of
1067 being received regardless of the L3 domain in which they
1068 originated. Only valid when the kernel was compiled with
1069 CONFIG_NET_L3_MASTER_DEV.
1071 Default: 1 (enabled)
1076 cipso_cache_enable - BOOLEAN
1077 If set, enable additions to and lookups from the CIPSO label mapping
1078 cache. If unset, additions are ignored and lookups always result in a
1079 miss. However, regardless of the setting the cache is still
1080 invalidated when required when means you can safely toggle this on and
1081 off and the cache will always be "safe".
1085 cipso_cache_bucket_size - INTEGER
1086 The CIPSO label cache consists of a fixed size hash table with each
1087 hash bucket containing a number of cache entries. This variable limits
1088 the number of entries in each hash bucket; the larger the value the
1089 more CIPSO label mappings that can be cached. When the number of
1090 entries in a given hash bucket reaches this limit adding new entries
1091 causes the oldest entry in the bucket to be removed to make room.
1095 cipso_rbm_optfmt - BOOLEAN
1096 Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
1097 the CIPSO draft specification (see Documentation/netlabel for details).
1098 This means that when set the CIPSO tag will be padded with empty
1099 categories in order to make the packet data 32-bit aligned.
1103 cipso_rbm_structvalid - BOOLEAN
1104 If set, do a very strict check of the CIPSO option when
1105 ip_options_compile() is called. If unset, relax the checks done during
1106 ip_options_compile(). Either way is "safe" as errors are caught else
1107 where in the CIPSO processing code but setting this to 0 (False) should
1108 result in less work (i.e. it should be faster) but could cause problems
1109 with other implementations that require strict checking.
1116 ip_local_port_range - 2 INTEGERS
1117 Defines the local port range that is used by TCP and UDP to
1118 choose the local port. The first number is the first, the
1119 second the last local port number.
1120 If possible, it is better these numbers have different parity
1121 (one even and one odd value).
1122 Must be greater than or equal to ip_unprivileged_port_start.
1123 The default values are 32768 and 60999 respectively.
1125 ip_local_reserved_ports - list of comma separated ranges
1126 Specify the ports which are reserved for known third-party
1127 applications. These ports will not be used by automatic port
1128 assignments (e.g. when calling connect() or bind() with port
1129 number 0). Explicit port allocation behavior is unchanged.
1131 The format used for both input and output is a comma separated
1132 list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
1133 10). Writing to the file will clear all previously reserved
1134 ports and update the current list with the one given in the
1137 Note that ip_local_port_range and ip_local_reserved_ports
1138 settings are independent and both are considered by the kernel
1139 when determining which ports are available for automatic port
1142 You can reserve ports which are not in the current
1143 ip_local_port_range, e.g.::
1145 $ cat /proc/sys/net/ipv4/ip_local_port_range
1147 $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
1150 although this is redundant. However such a setting is useful
1151 if later the port range is changed to a value that will
1152 include the reserved ports. Also keep in mind, that overlapping
1153 of these ranges may affect probability of selecting ephemeral
1154 ports which are right after block of reserved ports.
1158 ip_unprivileged_port_start - INTEGER
1159 This is a per-namespace sysctl. It defines the first
1160 unprivileged port in the network namespace. Privileged ports
1161 require root or CAP_NET_BIND_SERVICE in order to bind to them.
1162 To disable all privileged ports, set this to 0. They must not
1163 overlap with the ip_local_port_range.
1167 ip_nonlocal_bind - BOOLEAN
1168 If set, allows processes to bind() to non-local IP addresses,
1169 which can be quite useful - but may break some applications.
1173 ip_autobind_reuse - BOOLEAN
1174 By default, bind() does not select the ports automatically even if
1175 the new socket and all sockets bound to the port have SO_REUSEADDR.
1176 ip_autobind_reuse allows bind() to reuse the port and this is useful
1177 when you use bind()+connect(), but may break some applications.
1178 The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
1179 option should only be set by experts.
1182 ip_dynaddr - BOOLEAN
1183 If set non-zero, enables support for dynamic addresses.
1184 If set to a non-zero value larger than 1, a kernel log
1185 message will be printed when dynamic address rewriting
1190 ip_early_demux - BOOLEAN
1191 Optimize input packet processing down to one demux for
1192 certain kinds of local sockets. Currently we only do this
1193 for established TCP and connected UDP sockets.
1195 It may add an additional cost for pure routing workloads that
1196 reduces overall throughput, in such case you should disable it.
1200 ping_group_range - 2 INTEGERS
1201 Restrict ICMP_PROTO datagram sockets to users in the group range.
1202 The default is "1 0", meaning, that nobody (not even root) may
1203 create ping sockets. Setting it to "100 100" would grant permissions
1204 to the single group. "0 4294967295" would enable it for the world, "100
1205 4294967295" would enable it for the users, but not daemons.
1207 tcp_early_demux - BOOLEAN
1208 Enable early demux for established TCP sockets.
1212 udp_early_demux - BOOLEAN
1213 Enable early demux for connected UDP sockets. Disable this if
1214 your system could experience more unconnected load.
1218 icmp_echo_ignore_all - BOOLEAN
1219 If set non-zero, then the kernel will ignore all ICMP ECHO
1220 requests sent to it.
1224 icmp_echo_enable_probe - BOOLEAN
1225 If set to one, then the kernel will respond to RFC 8335 PROBE
1226 requests sent to it.
1230 icmp_echo_ignore_broadcasts - BOOLEAN
1231 If set non-zero, then the kernel will ignore all ICMP ECHO and
1232 TIMESTAMP requests sent to it via broadcast/multicast.
1236 icmp_ratelimit - INTEGER
1237 Limit the maximal rates for sending ICMP packets whose type matches
1238 icmp_ratemask (see below) to specific targets.
1239 0 to disable any limiting,
1240 otherwise the minimal space between responses in milliseconds.
1241 Note that another sysctl, icmp_msgs_per_sec limits the number
1242 of ICMP packets sent on all targets.
1246 icmp_msgs_per_sec - INTEGER
1247 Limit maximal number of ICMP packets sent per second from this host.
1248 Only messages whose type matches icmp_ratemask (see below) are
1249 controlled by this limit. For security reasons, the precise count
1250 of messages per second is randomized.
1254 icmp_msgs_burst - INTEGER
1255 icmp_msgs_per_sec controls number of ICMP packets sent per second,
1256 while icmp_msgs_burst controls the burst size of these packets.
1257 For security reasons, the precise burst size is randomized.
1261 icmp_ratemask - INTEGER
1262 Mask made of ICMP types for which rates are being limited.
1264 Significant bits: IHGFEDCBA9876543210
1266 Default mask: 0000001100000011000 (6168)
1268 Bit definitions (see include/linux/icmp.h):
1270 = =========================
1272 3 Destination Unreachable [1]_
1273 4 Source Quench [1]_
1276 B Time Exceeded [1]_
1277 C Parameter Problem [1]_
1282 H Address Mask Request
1283 I Address Mask Reply
1284 = =========================
1286 .. [1] These are rate limited by default (see default mask above)
1288 icmp_ignore_bogus_error_responses - BOOLEAN
1289 Some routers violate RFC1122 by sending bogus responses to broadcast
1290 frames. Such violations are normally logged via a kernel warning.
1291 If this is set to TRUE, the kernel will not give such warnings, which
1292 will avoid log file clutter.
1296 icmp_errors_use_inbound_ifaddr - BOOLEAN
1298 If zero, icmp error messages are sent with the primary address of
1299 the exiting interface.
1301 If non-zero, the message will be sent with the primary address of
1302 the interface that received the packet that caused the icmp error.
1303 This is the behaviour many network administrators will expect from
1304 a router. And it can make debugging complicated network layouts
1307 Note that if no primary address exists for the interface selected,
1308 then the primary address of the first non-loopback interface that
1309 has one will be used regardless of this setting.
1313 igmp_max_memberships - INTEGER
1314 Change the maximum number of multicast groups we can subscribe to.
1317 Theoretical maximum value is bounded by having to send a membership
1318 report in a single datagram (i.e. the report can't span multiple
1319 datagrams, or risk confusing the switch and leaving groups you don't
1322 The number of supported groups 'M' is bounded by the number of group
1323 report entries you can fit into a single datagram of 65535 bytes.
1325 M = 65536-sizeof (ip header)/(sizeof(Group record))
1327 Group records are variable length, with a minimum of 12 bytes.
1328 So net.ipv4.igmp_max_memberships should not be set higher than:
1330 (65536-24) / 12 = 5459
1332 The value 5459 assumes no IP header options, so in practice
1333 this number may be lower.
1335 igmp_max_msf - INTEGER
1336 Maximum number of addresses allowed in the source filter list for a
1342 Controls the IGMP query robustness variable (see RFC2236 8.1).
1344 Default: 2 (as specified by RFC2236 8.1)
1346 Minimum: 1 (as specified by RFC6636 4.5)
1348 force_igmp_version - INTEGER
1349 - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
1350 allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
1351 Present timer expires.
1352 - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
1353 receive IGMPv2/v3 query.
1354 - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
1355 IGMPv1 query message. Will reply report if receive IGMPv3 query.
1356 - 3 - Enforce to use IGMP version 3. The same react with default 0.
1360 this is not the same with force_mld_version because IGMPv3 RFC3376
1361 Security Considerations does not have clear description that we could
1362 ignore other version messages completely as MLDv2 RFC3810. So make
1363 this value as default 0 is recommended.
1365 ``conf/interface/*``
1366 changes special settings per interface (where
1367 interface" is the name of your network interface)
1370 is special, changes the settings for all interfaces
1372 log_martians - BOOLEAN
1373 Log packets with impossible addresses to kernel log.
1374 log_martians for the interface will be enabled if at least one of
1375 conf/{all,interface}/log_martians is set to TRUE,
1376 it will be disabled otherwise
1378 accept_redirects - BOOLEAN
1379 Accept ICMP redirect messages.
1380 accept_redirects for the interface will be enabled if:
1382 - both conf/{all,interface}/accept_redirects are TRUE in the case
1383 forwarding for the interface is enabled
1387 - at least one of conf/{all,interface}/accept_redirects is TRUE in the
1388 case forwarding for the interface is disabled
1390 accept_redirects for the interface will be disabled otherwise
1397 forwarding - BOOLEAN
1398 Enable IP forwarding on this interface. This controls whether packets
1399 received _on_ this interface can be forwarded.
1401 mc_forwarding - BOOLEAN
1402 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
1403 and a multicast routing daemon is required.
1404 conf/all/mc_forwarding must also be set to TRUE to enable multicast
1405 routing for the interface
1408 Integer value used to differentiate the devices by the medium they
1409 are attached to. Two devices can have different id values when
1410 the broadcast packets are received only on one of them.
1411 The default value 0 means that the device is the only interface
1412 to its medium, value of -1 means that medium is not known.
1414 Currently, it is used to change the proxy_arp behavior:
1415 the proxy_arp feature is enabled for packets forwarded between
1416 two devices attached to different media.
1421 proxy_arp for the interface will be enabled if at least one of
1422 conf/{all,interface}/proxy_arp is set to TRUE,
1423 it will be disabled otherwise
1425 proxy_arp_pvlan - BOOLEAN
1426 Private VLAN proxy arp.
1428 Basically allow proxy arp replies back to the same interface
1429 (from which the ARP request/solicitation was received).
1431 This is done to support (ethernet) switch features, like RFC
1432 3069, where the individual ports are NOT allowed to
1433 communicate with each other, but they are allowed to talk to
1434 the upstream router. As described in RFC 3069, it is possible
1435 to allow these hosts to communicate through the upstream
1436 router by proxy_arp'ing. Don't need to be used together with
1439 This technology is known by different names:
1441 In RFC 3069 it is called VLAN Aggregation.
1442 Cisco and Allied Telesyn call it Private VLAN.
1443 Hewlett-Packard call it Source-Port filtering or port-isolation.
1444 Ericsson call it MAC-Forced Forwarding (RFC Draft).
1446 shared_media - BOOLEAN
1447 Send(router) or accept(host) RFC1620 shared media redirects.
1448 Overrides secure_redirects.
1450 shared_media for the interface will be enabled if at least one of
1451 conf/{all,interface}/shared_media is set to TRUE,
1452 it will be disabled otherwise
1456 secure_redirects - BOOLEAN
1457 Accept ICMP redirect messages only to gateways listed in the
1458 interface's current gateway list. Even if disabled, RFC1122 redirect
1461 Overridden by shared_media.
1463 secure_redirects for the interface will be enabled if at least one of
1464 conf/{all,interface}/secure_redirects is set to TRUE,
1465 it will be disabled otherwise
1469 send_redirects - BOOLEAN
1470 Send redirects, if router.
1472 send_redirects for the interface will be enabled if at least one of
1473 conf/{all,interface}/send_redirects is set to TRUE,
1474 it will be disabled otherwise
1478 bootp_relay - BOOLEAN
1479 Accept packets with source address 0.b.c.d destined
1480 not to this host as local ones. It is supposed, that
1481 BOOTP relay daemon will catch and forward such packets.
1482 conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
1487 Not Implemented Yet.
1489 accept_source_route - BOOLEAN
1490 Accept packets with SRR option.
1491 conf/all/accept_source_route must also be set to TRUE to accept packets
1492 with SRR option on the interface
1499 accept_local - BOOLEAN
1500 Accept packets with local source addresses. In combination with
1501 suitable routing, this can be used to direct packets between two
1502 local interfaces over the wire and have them accepted properly.
1505 route_localnet - BOOLEAN
1506 Do not consider loopback addresses as martian source or destination
1507 while routing. This enables the use of 127/8 for local routing purposes.
1512 - 0 - No source validation.
1513 - 1 - Strict mode as defined in RFC3704 Strict Reverse Path
1514 Each incoming packet is tested against the FIB and if the interface
1515 is not the best reverse path the packet check will fail.
1516 By default failed packets are discarded.
1517 - 2 - Loose mode as defined in RFC3704 Loose Reverse Path
1518 Each incoming packet's source address is also tested against the FIB
1519 and if the source address is not reachable via any interface
1520 the packet check will fail.
1522 Current recommended practice in RFC3704 is to enable strict mode
1523 to prevent IP spoofing from DDos attacks. If using asymmetric routing
1524 or other complicated routing, then loose mode is recommended.
1526 The max value from conf/{all,interface}/rp_filter is used
1527 when doing source validation on the {interface}.
1529 Default value is 0. Note that some distributions enable it
1532 src_valid_mark - BOOLEAN
1533 - 0 - The fwmark of the packet is not included in reverse path
1534 route lookup. This allows for asymmetric routing configurations
1535 utilizing the fwmark in only one direction, e.g., transparent
1538 - 1 - The fwmark of the packet is included in reverse path route
1539 lookup. This permits rp_filter to function when the fwmark is
1540 used for routing traffic in both directions.
1542 This setting also affects the utilization of fmwark when
1543 performing source address selection for ICMP replies, or
1544 determining addresses stored for the IPOPT_TS_TSANDADDR and
1545 IPOPT_RR IP options.
1547 The max value from conf/{all,interface}/src_valid_mark is used.
1551 arp_filter - BOOLEAN
1552 - 1 - Allows you to have multiple network interfaces on the same
1553 subnet, and have the ARPs for each interface be answered
1554 based on whether or not the kernel would route a packet from
1555 the ARP'd IP out that interface (therefore you must use source
1556 based routing for this to work). In other words it allows control
1557 of which cards (usually 1) will respond to an arp request.
1559 - 0 - (default) The kernel can respond to arp requests with addresses
1560 from other interfaces. This may seem wrong but it usually makes
1561 sense, because it increases the chance of successful communication.
1562 IP addresses are owned by the complete host on Linux, not by
1563 particular interfaces. Only for more complex setups like load-
1564 balancing, does this behaviour cause problems.
1566 arp_filter for the interface will be enabled if at least one of
1567 conf/{all,interface}/arp_filter is set to TRUE,
1568 it will be disabled otherwise
1570 arp_announce - INTEGER
1571 Define different restriction levels for announcing the local
1572 source IP address from IP packets in ARP requests sent on
1575 - 0 - (default) Use any local address, configured on any interface
1576 - 1 - Try to avoid local addresses that are not in the target's
1577 subnet for this interface. This mode is useful when target
1578 hosts reachable via this interface require the source IP
1579 address in ARP requests to be part of their logical network
1580 configured on the receiving interface. When we generate the
1581 request we will check all our subnets that include the
1582 target IP and will preserve the source address if it is from
1583 such subnet. If there is no such subnet we select source
1584 address according to the rules for level 2.
1585 - 2 - Always use the best local address for this target.
1586 In this mode we ignore the source address in the IP packet
1587 and try to select local address that we prefer for talks with
1588 the target host. Such local address is selected by looking
1589 for primary IP addresses on all our subnets on the outgoing
1590 interface that include the target IP address. If no suitable
1591 local address is found we select the first local address
1592 we have on the outgoing interface or on all other interfaces,
1593 with the hope we will receive reply for our request and
1594 even sometimes no matter the source IP address we announce.
1596 The max value from conf/{all,interface}/arp_announce is used.
1598 Increasing the restriction level gives more chance for
1599 receiving answer from the resolved target while decreasing
1600 the level announces more valid sender's information.
1602 arp_ignore - INTEGER
1603 Define different modes for sending replies in response to
1604 received ARP requests that resolve local target IP addresses:
1606 - 0 - (default): reply for any local target IP address, configured
1608 - 1 - reply only if the target IP address is local address
1609 configured on the incoming interface
1610 - 2 - reply only if the target IP address is local address
1611 configured on the incoming interface and both with the
1612 sender's IP address are part from same subnet on this interface
1613 - 3 - do not reply for local addresses configured with scope host,
1614 only resolutions for global and link addresses are replied
1616 - 8 - do not reply for all local addresses
1618 The max value from conf/{all,interface}/arp_ignore is used
1619 when ARP request is received on the {interface}
1621 arp_notify - BOOLEAN
1622 Define mode for notification of address and device changes.
1624 == ==========================================================
1625 0 (default): do nothing
1626 1 Generate gratuitous arp requests when device is brought up
1627 or hardware address changes.
1628 == ==========================================================
1630 arp_accept - BOOLEAN
1631 Define behavior for gratuitous ARP frames who's IP is not
1632 already present in the ARP table:
1634 - 0 - don't create new entries in the ARP table
1635 - 1 - create new entries in the ARP table
1637 Both replies and requests type gratuitous arp will trigger the
1638 ARP table to be updated, if this setting is on.
1640 If the ARP table already contains the IP address of the
1641 gratuitous arp frame, the arp table will be updated regardless
1642 if this setting is on or off.
1644 arp_evict_nocarrier - BOOLEAN
1645 Clears the ARP cache on NOCARRIER events. This option is important for
1646 wireless devices where the ARP cache should not be cleared when roaming
1647 between access points on the same network. In most cases this should
1648 remain as the default (1).
1650 - 1 - (default): Clear the ARP cache on NOCARRIER events
1651 - 0 - Do not clear ARP cache on NOCARRIER events
1653 mcast_solicit - INTEGER
1654 The maximum number of multicast probes in INCOMPLETE state,
1655 when the associated hardware address is unknown. Defaults
1658 ucast_solicit - INTEGER
1659 The maximum number of unicast probes in PROBE state, when
1660 the hardware address is being reconfirmed. Defaults to 3.
1662 app_solicit - INTEGER
1663 The maximum number of probes to send to the user space ARP daemon
1664 via netlink before dropping back to multicast probes (see
1665 mcast_resolicit). Defaults to 0.
1667 mcast_resolicit - INTEGER
1668 The maximum number of multicast probes after unicast and
1669 app probes in PROBE state. Defaults to 0.
1671 disable_policy - BOOLEAN
1672 Disable IPSEC policy (SPD) for this interface
1674 disable_xfrm - BOOLEAN
1675 Disable IPSEC encryption on this interface, whatever the policy
1677 igmpv2_unsolicited_report_interval - INTEGER
1678 The interval in milliseconds in which the next unsolicited
1679 IGMPv1 or IGMPv2 report retransmit will take place.
1681 Default: 10000 (10 seconds)
1683 igmpv3_unsolicited_report_interval - INTEGER
1684 The interval in milliseconds in which the next unsolicited
1685 IGMPv3 report retransmit will take place.
1687 Default: 1000 (1 seconds)
1689 ignore_routes_with_linkdown - BOOLEAN
1690 Ignore routes whose link is down when performing a FIB lookup.
1692 promote_secondaries - BOOLEAN
1693 When a primary IP address is removed from this interface
1694 promote a corresponding secondary IP address instead of
1695 removing all the corresponding secondary IP addresses.
1697 drop_unicast_in_l2_multicast - BOOLEAN
1698 Drop any unicast IP packets that are received in link-layer
1699 multicast (or broadcast) frames.
1701 This behavior (for multicast) is actually a SHOULD in RFC
1702 1122, but is disabled by default for compatibility reasons.
1706 drop_gratuitous_arp - BOOLEAN
1707 Drop all gratuitous ARP frames, for example if there's a known
1708 good ARP proxy on the network and such frames need not be used
1709 (or in the case of 802.11, must not be used to prevent attacks.)
1715 Allows you to write a number, which can be used as required.
1719 xfrm4_gc_thresh - INTEGER
1720 (Obsolete since linux-4.14)
1721 The threshold at which we will start garbage collecting for IPv4
1722 destination cache entries. At twice this value the system will
1723 refuse new allocations.
1725 igmp_link_local_mcast_reports - BOOLEAN
1726 Enable IGMP reports for link local multicast groups in the
1732 kuznet@ms2.inr.ac.ru
1739 delon.nicolas@wanadoo.fr
1744 /proc/sys/net/ipv6/* Variables
1745 ==============================
1747 IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
1748 apply to IPv6 [XXX?].
1750 bindv6only - BOOLEAN
1751 Default value for IPV6_V6ONLY socket option,
1752 which restricts use of the IPv6 socket to IPv6 communication
1755 - TRUE: disable IPv4-mapped address feature
1756 - FALSE: enable IPv4-mapped address feature
1758 Default: FALSE (as specified in RFC3493)
1760 flowlabel_consistency - BOOLEAN
1761 Protect the consistency (and unicity) of flow label.
1762 You have to disable it to use IPV6_FL_F_REFLECT flag on the
1770 auto_flowlabels - INTEGER
1771 Automatically generate flow labels based on a flow hash of the
1772 packet. This allows intermediate devices, such as routers, to
1773 identify packet flows for mechanisms like Equal Cost Multipath
1774 Routing (see RFC 6438).
1776 = ===========================================================
1777 0 automatic flow labels are completely disabled
1778 1 automatic flow labels are enabled by default, they can be
1779 disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
1781 2 automatic flow labels are allowed, they may be enabled on a
1782 per socket basis using the IPV6_AUTOFLOWLABEL socket option
1783 3 automatic flow labels are enabled and enforced, they cannot
1784 be disabled by the socket option
1785 = ===========================================================
1789 flowlabel_state_ranges - BOOLEAN
1790 Split the flow label number space into two ranges. 0-0x7FFFF is
1791 reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
1792 is reserved for stateless flow labels as described in RFC6437.
1799 flowlabel_reflect - INTEGER
1800 Control flow label reflection. Needed for Path MTU
1801 Discovery to work with Equal Cost Multipath Routing in anycast
1802 environments. See RFC 7690 and:
1803 https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
1807 - 1: enabled for established flows
1809 Note that this prevents automatic flowlabel changes, as done
1810 in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
1811 and "tcp: Change txhash on every SYN and RTO retransmit"
1813 - 2: enabled for TCP RESET packets (no active listener)
1814 If set, a RST packet sent in response to a SYN packet on a closed
1815 port will reflect the incoming flow label.
1817 - 4: enabled for ICMPv6 echo reply messages.
1821 fib_multipath_hash_policy - INTEGER
1822 Controls which hash policy to use for multipath routes.
1824 Default: 0 (Layer 3)
1828 - 0 - Layer 3 (source and destination addresses plus flow label)
1829 - 1 - Layer 4 (standard 5-tuple)
1830 - 2 - Layer 3 or inner Layer 3 if present
1831 - 3 - Custom multipath hash. Fields used for multipath hash calculation
1832 are determined by fib_multipath_hash_fields sysctl
1834 fib_multipath_hash_fields - UNSIGNED INTEGER
1835 When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
1836 fields used for multipath hash calculation are determined by this
1839 This value is a bitmask which enables various fields for multipath hash
1842 Possible fields are:
1844 ====== ============================
1845 0x0001 Source IP address
1846 0x0002 Destination IP address
1850 0x0020 Destination port
1851 0x0040 Inner source IP address
1852 0x0080 Inner destination IP address
1853 0x0100 Inner IP protocol
1854 0x0200 Inner Flow Label
1855 0x0400 Inner source port
1856 0x0800 Inner destination port
1857 ====== ============================
1859 Default: 0x0007 (source IP, destination IP and IP protocol)
1861 anycast_src_echo_reply - BOOLEAN
1862 Controls the use of anycast addresses as source addresses for ICMPv6
1870 idgen_delay - INTEGER
1871 Controls the delay in seconds after which time to retry
1872 privacy stable address generation if a DAD conflict is
1875 Default: 1 (as specified in RFC7217)
1877 idgen_retries - INTEGER
1878 Controls the number of retries to generate a stable privacy
1879 address if a DAD conflict is detected.
1881 Default: 3 (as specified in RFC7217)
1884 Controls the MLD query robustness variable (see RFC3810 9.1).
1886 Default: 2 (as specified by RFC3810 9.1)
1888 Minimum: 1 (as specified by RFC6636 4.5)
1890 max_dst_opts_number - INTEGER
1891 Maximum number of non-padding TLVs allowed in a Destination
1892 options extension header. If this value is less than zero
1893 then unknown options are disallowed and the number of known
1894 TLVs allowed is the absolute value of this number.
1898 max_hbh_opts_number - INTEGER
1899 Maximum number of non-padding TLVs allowed in a Hop-by-Hop
1900 options extension header. If this value is less than zero
1901 then unknown options are disallowed and the number of known
1902 TLVs allowed is the absolute value of this number.
1906 max_dst_opts_length - INTEGER
1907 Maximum length allowed for a Destination options extension
1910 Default: INT_MAX (unlimited)
1912 max_hbh_length - INTEGER
1913 Maximum length allowed for a Hop-by-Hop options extension
1916 Default: INT_MAX (unlimited)
1918 skip_notify_on_dev_down - BOOLEAN
1919 Controls whether an RTM_DELROUTE message is generated for routes
1920 removed when a device is taken down or deleted. IPv4 does not
1921 generate this message; IPv6 does by default. Setting this sysctl
1922 to true skips the message, making IPv4 and IPv6 on par in relying
1923 on userspace caches to track link events and evict routes.
1925 Default: false (generate message)
1927 nexthop_compat_mode - BOOLEAN
1928 New nexthop API provides a means for managing nexthops independent of
1929 prefixes. Backwards compatibilty with old route format is enabled by
1930 default which means route dumps and notifications contain the new
1931 nexthop attribute but also the full, expanded nexthop definition.
1932 Further, updates or deletes of a nexthop configuration generate route
1933 notifications for each fib entry using the nexthop. Once a system
1934 understands the new API, this sysctl can be disabled to achieve full
1935 performance benefits of the new API by disabling the nexthop expansion
1936 and extraneous notifications.
1937 Default: true (backward compat mode)
1939 fib_notify_on_flag_change - INTEGER
1940 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
1941 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
1943 After installing a route to the kernel, user space receives an
1944 acknowledgment, which means the route was installed in the kernel,
1945 but not necessarily in hardware.
1946 It is also possible for a route already installed in hardware to change
1947 its action and therefore its flags. For example, a host route that is
1948 trapping packets can be "promoted" to perform decapsulation following
1949 the installation of an IPinIP/VXLAN tunnel.
1950 The notifications will indicate to user-space the state of the route.
1952 Default: 0 (Do not emit notifications.)
1956 - 0 - Do not emit notifications.
1957 - 1 - Emit notifications.
1958 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
1961 Define the IOAM id of this node. Uses only 24 bits out of 32 in total.
1968 ioam6_id_wide - LONG INTEGER
1969 Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
1970 total. Can be different from ioam6_id.
1973 Max: 0xFFFFFFFFFFFFFF
1975 Default: 0xFFFFFFFFFFFFFF
1979 ip6frag_high_thresh - INTEGER
1980 Maximum memory used to reassemble IPv6 fragments. When
1981 ip6frag_high_thresh bytes of memory is allocated for this purpose,
1982 the fragment handler will toss packets until ip6frag_low_thresh
1985 ip6frag_low_thresh - INTEGER
1986 See ip6frag_high_thresh
1988 ip6frag_time - INTEGER
1989 Time in seconds to keep an IPv6 fragment in memory.
1992 Change the interface-specific default settings.
1994 These settings would be used during creating new interfaces.
1998 Change all the interface-specific settings.
2000 [XXX: Other special features than forwarding?]
2002 conf/all/disable_ipv6 - BOOLEAN
2003 Changing this value is same as changing ``conf/default/disable_ipv6``
2004 setting and also all per-interface ``disable_ipv6`` settings to the same
2007 Reading this value does not have any particular meaning. It does not say
2008 whether IPv6 support is enabled or disabled. Returned value can be 1
2009 also in the case when some interface has ``disable_ipv6`` set to 0 and
2010 has configured IPv6 addresses.
2012 conf/all/forwarding - BOOLEAN
2013 Enable global IPv6 forwarding between all interfaces.
2015 IPv4 and IPv6 work differently here; e.g. netfilter must be used
2016 to control which interfaces may forward packets and which not.
2018 This also sets all interfaces' Host/Router setting
2019 'forwarding' to the specified value. See below for details.
2021 This referred to as global forwarding.
2026 fwmark_reflect - BOOLEAN
2027 Controls the fwmark of kernel-generated IPv6 reply packets that are not
2028 associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
2029 If unset, these packets have a fwmark of zero. If set, they have the
2030 fwmark of the packet they are replying to.
2034 ``conf/interface/*``:
2035 Change special settings per interface.
2037 The functional behaviour for certain settings is different
2038 depending on whether local forwarding is enabled or not.
2041 Accept Router Advertisements; autoconfigure using them.
2043 It also determines whether or not to transmit Router
2044 Solicitations. If and only if the functional setting is to
2045 accept Router Advertisements, Router Solicitations will be
2048 Possible values are:
2050 == ===========================================================
2051 0 Do not accept Router Advertisements.
2052 1 Accept Router Advertisements if forwarding is disabled.
2053 2 Overrule forwarding behaviour. Accept Router Advertisements
2054 even if forwarding is enabled.
2055 == ===========================================================
2059 - enabled if local forwarding is disabled.
2060 - disabled if local forwarding is enabled.
2062 accept_ra_defrtr - BOOLEAN
2063 Learn default router in Router Advertisement.
2067 - enabled if accept_ra is enabled.
2068 - disabled if accept_ra is disabled.
2070 ra_defrtr_metric - UNSIGNED INTEGER
2071 Route metric for default route learned in Router Advertisement. This value
2072 will be assigned as metric for the default route learned via IPv6 Router
2073 Advertisement. Takes affect only if accept_ra_defrtr is enabled.
2078 Default: IP6_RT_PRIO_USER i.e. 1024.
2080 accept_ra_from_local - BOOLEAN
2081 Accept RA with source-address that is found on local machine
2082 if the RA is otherwise proper and able to be accepted.
2084 Default is to NOT accept these as it may be an un-intended
2089 - enabled if accept_ra_from_local is enabled
2090 on a specific interface.
2091 - disabled if accept_ra_from_local is disabled
2092 on a specific interface.
2094 accept_ra_min_hop_limit - INTEGER
2095 Minimum hop limit Information in Router Advertisement.
2097 Hop limit Information in Router Advertisement less than this
2098 variable shall be ignored.
2102 accept_ra_pinfo - BOOLEAN
2103 Learn Prefix Information in Router Advertisement.
2107 - enabled if accept_ra is enabled.
2108 - disabled if accept_ra is disabled.
2110 accept_ra_rt_info_min_plen - INTEGER
2111 Minimum prefix length of Route Information in RA.
2113 Route Information w/ prefix smaller than this variable shall
2118 * 0 if accept_ra_rtr_pref is enabled.
2119 * -1 if accept_ra_rtr_pref is disabled.
2121 accept_ra_rt_info_max_plen - INTEGER
2122 Maximum prefix length of Route Information in RA.
2124 Route Information w/ prefix larger than this variable shall
2129 * 0 if accept_ra_rtr_pref is enabled.
2130 * -1 if accept_ra_rtr_pref is disabled.
2132 accept_ra_rtr_pref - BOOLEAN
2133 Accept Router Preference in RA.
2137 - enabled if accept_ra is enabled.
2138 - disabled if accept_ra is disabled.
2140 accept_ra_mtu - BOOLEAN
2141 Apply the MTU value specified in RA option 5 (RFC4861). If
2142 disabled, the MTU specified in the RA will be ignored.
2146 - enabled if accept_ra is enabled.
2147 - disabled if accept_ra is disabled.
2149 accept_redirects - BOOLEAN
2154 - enabled if local forwarding is disabled.
2155 - disabled if local forwarding is enabled.
2157 accept_source_route - INTEGER
2158 Accept source routing (routing extension header).
2160 - >= 0: Accept only routing header type 2.
2161 - < 0: Do not accept routing header.
2166 Autoconfigure addresses using Prefix Information in Router
2171 - enabled if accept_ra_pinfo is enabled.
2172 - disabled if accept_ra_pinfo is disabled.
2174 dad_transmits - INTEGER
2175 The amount of Duplicate Address Detection probes to send.
2179 forwarding - INTEGER
2180 Configure interface-specific Host/Router behaviour.
2184 It is recommended to have the same setting on all
2185 interfaces; mixed router/host scenarios are rather uncommon.
2187 Possible values are:
2189 - 0 Forwarding disabled
2190 - 1 Forwarding enabled
2194 By default, Host behaviour is assumed. This means:
2196 1. IsRouter flag is not set in Neighbour Advertisements.
2197 2. If accept_ra is TRUE (default), transmit Router
2199 3. If accept_ra is TRUE (default), accept Router
2200 Advertisements (and do autoconfiguration).
2201 4. If accept_redirects is TRUE (default), accept Redirects.
2205 If local forwarding is enabled, Router behaviour is assumed.
2206 This means exactly the reverse from the above:
2208 1. IsRouter flag is set in Neighbour Advertisements.
2209 2. Router Solicitations are not sent unless accept_ra is 2.
2210 3. Router Advertisements are ignored unless accept_ra is 2.
2211 4. Redirects are ignored.
2213 Default: 0 (disabled) if global forwarding is disabled (default),
2214 otherwise 1 (enabled).
2217 Default Hop Limit to set.
2222 Default Maximum Transfer Unit
2224 Default: 1280 (IPv6 required minimum)
2226 ip_nonlocal_bind - BOOLEAN
2227 If set, allows processes to bind() to non-local IPv6 addresses,
2228 which can be quite useful - but may break some applications.
2232 router_probe_interval - INTEGER
2233 Minimum interval (in seconds) between Router Probing described
2238 router_solicitation_delay - INTEGER
2239 Number of seconds to wait after interface is brought up
2240 before sending Router Solicitations.
2244 router_solicitation_interval - INTEGER
2245 Number of seconds to wait between Router Solicitations.
2249 router_solicitations - INTEGER
2250 Number of Router Solicitations to send until assuming no
2251 routers are present.
2255 use_oif_addrs_only - BOOLEAN
2256 When enabled, the candidate source addresses for destinations
2257 routed via this interface are restricted to the set of addresses
2258 configured on this interface (vis. RFC 6724, section 4).
2262 use_tempaddr - INTEGER
2263 Preference for Privacy Extensions (RFC3041).
2265 * <= 0 : disable Privacy Extensions
2266 * == 1 : enable Privacy Extensions, but prefer public
2267 addresses over temporary addresses.
2268 * > 1 : enable Privacy Extensions and prefer temporary
2269 addresses over public addresses.
2273 * 0 (for most devices)
2274 * -1 (for point-to-point devices and loopback devices)
2276 temp_valid_lft - INTEGER
2277 valid lifetime (in seconds) for temporary addresses.
2279 Default: 172800 (2 days)
2281 temp_prefered_lft - INTEGER
2282 Preferred lifetime (in seconds) for temporary addresses.
2284 Default: 86400 (1 day)
2286 keep_addr_on_down - INTEGER
2287 Keep all IPv6 addresses on an interface down event. If set static
2288 global addresses with no expiration time are not flushed.
2291 * 0 : system default
2294 Default: 0 (addresses are removed)
2296 max_desync_factor - INTEGER
2297 Maximum value for DESYNC_FACTOR, which is a random value
2298 that ensures that clients don't synchronize with each
2299 other and generate new addresses at exactly the same time.
2300 value is in seconds.
2304 regen_max_retry - INTEGER
2305 Number of attempts before give up attempting to generate
2306 valid temporary addresses.
2310 max_addresses - INTEGER
2311 Maximum number of autoconfigured addresses per interface. Setting
2312 to zero disables the limitation. It is not recommended to set this
2313 value too large (or to zero) because it would be an easy way to
2314 crash the kernel by allowing too many addresses to be created.
2318 disable_ipv6 - BOOLEAN
2319 Disable IPv6 operation. If accept_dad is set to 2, this value
2320 will be dynamically set to TRUE if DAD fails for the link-local
2323 Default: FALSE (enable IPv6 operation)
2325 When this value is changed from 1 to 0 (IPv6 is being enabled),
2326 it will dynamically create a link-local address on the given
2327 interface and start Duplicate Address Detection, if necessary.
2329 When this value is changed from 0 to 1 (IPv6 is being disabled),
2330 it will dynamically delete all addresses and routes on the given
2331 interface. From now on it will not possible to add addresses/routes
2332 to the selected interface.
2334 accept_dad - INTEGER
2335 Whether to accept DAD (Duplicate Address Detection).
2337 == ==============================================================
2339 1 Enable DAD (default)
2340 2 Enable DAD, and disable IPv6 operation if MAC-based duplicate
2341 link-local address has been found.
2342 == ==============================================================
2344 DAD operation and mode on a given interface will be selected according
2345 to the maximum value of conf/{all,interface}/accept_dad.
2347 force_tllao - BOOLEAN
2348 Enable sending the target link-layer address option even when
2349 responding to a unicast neighbor solicitation.
2353 Quoting from RFC 2461, section 4.4, Target link-layer address:
2355 "The option MUST be included for multicast solicitations in order to
2356 avoid infinite Neighbor Solicitation "recursion" when the peer node
2357 does not have a cache entry to return a Neighbor Advertisements
2358 message. When responding to unicast solicitations, the option can be
2359 omitted since the sender of the solicitation has the correct link-
2360 layer address; otherwise it would not have be able to send the unicast
2361 solicitation in the first place. However, including the link-layer
2362 address in this case adds little overhead and eliminates a potential
2363 race condition where the sender deletes the cached link-layer address
2364 prior to receiving a response to a previous solicitation."
2366 ndisc_notify - BOOLEAN
2367 Define mode for notification of address and device changes.
2369 * 0 - (default): do nothing
2370 * 1 - Generate unsolicited neighbour advertisements when device is brought
2371 up or hardware address changes.
2373 ndisc_tclass - INTEGER
2374 The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
2375 Discovery (Router Solicitation, Router Advertisement, Neighbor
2376 Solicitation, Neighbor Advertisement, Redirect) messages.
2377 These 8 bits can be interpreted as 6 high order bits holding the DSCP
2378 value and 2 low order bits representing ECN (which you probably want
2383 ndisc_evict_nocarrier - BOOLEAN
2384 Clears the neighbor discovery table on NOCARRIER events. This option is
2385 important for wireless devices where the neighbor discovery cache should
2386 not be cleared when roaming between access points on the same network.
2387 In most cases this should remain as the default (1).
2389 - 1 - (default): Clear neighbor discover cache on NOCARRIER events.
2390 - 0 - Do not clear neighbor discovery cache on NOCARRIER events.
2392 mldv1_unsolicited_report_interval - INTEGER
2393 The interval in milliseconds in which the next unsolicited
2394 MLDv1 report retransmit will take place.
2396 Default: 10000 (10 seconds)
2398 mldv2_unsolicited_report_interval - INTEGER
2399 The interval in milliseconds in which the next unsolicited
2400 MLDv2 report retransmit will take place.
2402 Default: 1000 (1 second)
2404 force_mld_version - INTEGER
2405 * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
2406 * 1 - Enforce to use MLD version 1
2407 * 2 - Enforce to use MLD version 2
2409 suppress_frag_ndisc - INTEGER
2410 Control RFC 6980 (Security Implications of IPv6 Fragmentation
2411 with IPv6 Neighbor Discovery) behavior:
2413 * 1 - (default) discard fragmented neighbor discovery packets
2414 * 0 - allow fragmented neighbor discovery packets
2416 optimistic_dad - BOOLEAN
2417 Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
2419 * 0: disabled (default)
2422 Optimistic Duplicate Address Detection for the interface will be enabled
2423 if at least one of conf/{all,interface}/optimistic_dad is set to 1,
2424 it will be disabled otherwise.
2426 use_optimistic - BOOLEAN
2427 If enabled, do not classify optimistic addresses as deprecated during
2428 source address selection. Preferred addresses will still be chosen
2429 before optimistic addresses, subject to other ranking in the source
2430 address selection algorithm.
2432 * 0: disabled (default)
2435 This will be enabled if at least one of
2436 conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
2438 stable_secret - IPv6 address
2439 This IPv6 address will be used as a secret to generate IPv6
2440 addresses for link-local addresses and autoconfigured
2441 ones. All addresses generated after setting this secret will
2442 be stable privacy ones by default. This can be changed via the
2443 addrgenmode ip-link. conf/default/stable_secret is used as the
2444 secret for the namespace, the interface specific ones can
2445 overwrite that. Writes to conf/all/stable_secret are refused.
2447 It is recommended to generate this secret during installation
2448 of a system and keep it stable after that.
2450 By default the stable secret is unset.
2452 addr_gen_mode - INTEGER
2453 Defines how link-local and autoconf addresses are generated.
2455 = =================================================================
2456 0 generate address based on EUI64 (default)
2457 1 do no generate a link-local address, use EUI64 for addresses
2458 generated from autoconf
2459 2 generate stable privacy addresses, using the secret from
2460 stable_secret (RFC7217)
2461 3 generate stable privacy addresses, using a random secret if unset
2462 = =================================================================
2464 drop_unicast_in_l2_multicast - BOOLEAN
2465 Drop any unicast IPv6 packets that are received in link-layer
2466 multicast (or broadcast) frames.
2468 By default this is turned off.
2470 drop_unsolicited_na - BOOLEAN
2471 Drop all unsolicited neighbor advertisements, for example if there's
2472 a known good NA proxy on the network and such frames need not be used
2473 (or in the case of 802.11, must not be used to prevent attacks.)
2475 By default this is turned off.
2477 accept_untracked_na - BOOLEAN
2478 Add a new neighbour cache entry in STALE state for routers on receiving a
2479 neighbour advertisement (either solicited or unsolicited) with target
2480 link-layer address option specified if no neighbour entry is already
2481 present for the advertised IPv6 address. Without this knob, NAs received
2482 for untracked addresses (absent in neighbour cache) are silently ignored.
2484 This is as per router-side behaviour documented in RFC9131.
2486 This has lower precedence than drop_unsolicited_na.
2488 This will optimize the return path for the initial off-link communication
2489 that is initiated by a directly connected host, by ensuring that
2490 the first-hop router which turns on this setting doesn't have to
2491 buffer the initial return packets to do neighbour-solicitation.
2492 The prerequisite is that the host is configured to send
2493 unsolicited neighbour advertisements on interface bringup.
2494 This setting should be used in conjunction with the ndisc_notify setting
2495 on the host to satisfy this prerequisite.
2497 By default this is turned off.
2499 enhanced_dad - BOOLEAN
2500 Include a nonce option in the IPv6 neighbor solicitation messages used for
2501 duplicate address detection per RFC7527. A received DAD NS will only signal
2502 a duplicate address if the nonce is different. This avoids any false
2503 detection of duplicates due to loopback of the NS messages that we send.
2504 The nonce option will be sent on an interface unless both of
2505 conf/{all,interface}/enhanced_dad are set to FALSE.
2513 Limit the maximal rates for sending ICMPv6 messages.
2515 0 to disable any limiting,
2516 otherwise the minimal space between responses in milliseconds.
2520 ratemask - list of comma separated ranges
2521 For ICMPv6 message types matching the ranges in the ratemask, limit
2522 the sending of the message according to ratelimit parameter.
2524 The format used for both input and output is a comma separated
2525 list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
2526 129). Writing to the file will clear all previous ranges of ICMPv6
2527 message types and update the current list with the input.
2529 Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
2530 for numerical values of ICMPv6 message types, e.g. echo request is 128
2531 and echo reply is 129.
2533 Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)
2535 echo_ignore_all - BOOLEAN
2536 If set non-zero, then the kernel will ignore all ICMP ECHO
2537 requests sent to it over the IPv6 protocol.
2541 echo_ignore_multicast - BOOLEAN
2542 If set non-zero, then the kernel will ignore all ICMP ECHO
2543 requests sent to it over the IPv6 protocol via multicast.
2547 echo_ignore_anycast - BOOLEAN
2548 If set non-zero, then the kernel will ignore all ICMP ECHO
2549 requests sent to it over the IPv6 protocol destined to anycast address.
2553 xfrm6_gc_thresh - INTEGER
2554 (Obsolete since linux-4.14)
2555 The threshold at which we will start garbage collecting for IPv6
2556 destination cache entries. At twice this value the system will
2557 refuse new allocations.
2561 Pekka Savola <pekkas@netcore.fi>
2562 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
2565 /proc/sys/net/bridge/* Variables:
2566 =================================
2568 bridge-nf-call-arptables - BOOLEAN
2569 - 1 : pass bridged ARP traffic to arptables' FORWARD chain.
2574 bridge-nf-call-iptables - BOOLEAN
2575 - 1 : pass bridged IPv4 traffic to iptables' chains.
2580 bridge-nf-call-ip6tables - BOOLEAN
2581 - 1 : pass bridged IPv6 traffic to ip6tables' chains.
2586 bridge-nf-filter-vlan-tagged - BOOLEAN
2587 - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
2592 bridge-nf-filter-pppoe-tagged - BOOLEAN
2593 - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
2598 bridge-nf-pass-vlan-input-dev - BOOLEAN
2599 - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
2600 interface on the bridge and set the netfilter input device to the
2601 vlan. This allows use of e.g. "iptables -i br0.1" and makes the
2602 REDIRECT target work with vlan-on-top-of-bridge interfaces. When no
2603 matching vlan interface is found, or this switch is off, the input
2604 device is set to the bridge interface.
2606 - 0: disable bridge netfilter vlan interface lookup.
2610 ``proc/sys/net/sctp/*`` Variables:
2611 ==================================
2613 addip_enable - BOOLEAN
2614 Enable or disable extension of Dynamic Address Reconfiguration
2615 (ADD-IP) functionality specified in RFC5061. This extension provides
2616 the ability to dynamically add and remove new addresses for the SCTP
2619 1: Enable extension.
2621 0: Disable extension.
2626 Enable or disable pf (pf is short for potentially failed) state. A value
2627 of pf_retrans > path_max_retrans also disables pf state. That is, one of
2628 both pf_enable and pf_retrans > path_max_retrans can disable pf state.
2629 Since pf_retrans and path_max_retrans can be changed by userspace
2630 application, sometimes user expects to disable pf state by the value of
2631 pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
2632 or path_max_retrans is changed by the user application, this pf state is
2633 enabled. As such, it is necessary to add this to dynamically enable
2634 and disable pf state. See:
2635 https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
2645 Unset or enable/disable pf (pf is short for potentially failed) state
2646 exposure. Applications can control the exposure of the PF path state
2647 in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
2648 sockopt. When it's unset, no SCTP_PEER_ADDR_CHANGE event with
2649 SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
2650 can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it's enabled,
2651 a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
2652 SCTP_PF state and a SCTP_PF-state transport info can be got via
2653 SCTP_GET_PEER_ADDR_INFO sockopt; When it's diabled, no
2654 SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
2655 trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
2658 0: Unset pf state exposure, Compatible with old applications.
2660 1: Disable pf state exposure.
2662 2: Enable pf state exposure.
2666 addip_noauth_enable - BOOLEAN
2667 Dynamic Address Reconfiguration (ADD-IP) requires the use of
2668 authentication to protect the operations of adding or removing new
2669 addresses. This requirement is mandated so that unauthorized hosts
2670 would not be able to hijack associations. However, older
2671 implementations may not have implemented this requirement while
2672 allowing the ADD-IP extension. For reasons of interoperability,
2673 we provide this variable to control the enforcement of the
2674 authentication requirement.
2676 == ===============================================================
2677 1 Allow ADD-IP extension to be used without authentication. This
2678 should only be set in a closed environment for interoperability
2679 with older implementations.
2681 0 Enforce the authentication requirement
2682 == ===============================================================
2686 auth_enable - BOOLEAN
2687 Enable or disable Authenticated Chunks extension. This extension
2688 provides the ability to send and receive authenticated chunks and is
2689 required for secure operation of Dynamic Address Reconfiguration
2692 - 1: Enable this extension.
2693 - 0: Disable this extension.
2697 prsctp_enable - BOOLEAN
2698 Enable or disable the Partial Reliability extension (RFC3758) which
2699 is used to notify peers that a given DATA should no longer be expected.
2701 - 1: Enable extension
2707 The limit of the number of new packets that can be initially sent. It
2708 controls how bursty the generated traffic can be.
2712 association_max_retrans - INTEGER
2713 Set the maximum number for retransmissions that an association can
2714 attempt deciding that the remote end is unreachable. If this value
2715 is exceeded, the association is terminated.
2719 max_init_retransmits - INTEGER
2720 The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
2721 that an association will attempt before declaring the destination
2722 unreachable and terminating.
2726 path_max_retrans - INTEGER
2727 The maximum number of retransmissions that will be attempted on a given
2728 path. Once this threshold is exceeded, the path is considered
2729 unreachable, and new traffic will use a different path when the
2730 association is multihomed.
2734 pf_retrans - INTEGER
2735 The number of retransmissions that will be attempted on a given path
2736 before traffic is redirected to an alternate transport (should one
2737 exist). Note this is distinct from path_max_retrans, as a path that
2738 passes the pf_retrans threshold can still be used. Its only
2739 deprioritized when a transmission path is selected by the stack. This
2740 setting is primarily used to enable fast failover mechanisms without
2741 having to reduce path_max_retrans to a very low value. See:
2742 http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
2743 for details. Note also that a value of pf_retrans > path_max_retrans
2744 disables this feature. Since both pf_retrans and path_max_retrans can
2745 be changed by userspace application, a variable pf_enable is used to
2750 ps_retrans - INTEGER
2751 Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
2752 from section-5 "Primary Path Switchover" in rfc7829. The primary path
2753 will be changed to another active path when the path error counter on
2754 the old primary path exceeds PSMR, so that "the SCTP sender is allowed
2755 to continue data transmission on a new working path even when the old
2756 primary destination address becomes active again". Note this feature
2757 is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
2758 and its value can't be less than 'pf_retrans' when changing by sysctl.
2762 rto_initial - INTEGER
2763 The initial round trip timeout value in milliseconds that will be used
2764 in calculating round trip times. This is the initial time interval
2765 for retransmissions.
2770 The maximum value (in milliseconds) of the round trip timeout. This
2771 is the largest time interval that can elapse between retransmissions.
2776 The minimum value (in milliseconds) of the round trip timeout. This
2777 is the smallest time interval the can elapse between retransmissions.
2781 hb_interval - INTEGER
2782 The interval (in milliseconds) between HEARTBEAT chunks. These chunks
2783 are sent at the specified interval on idle paths to probe the state of
2784 a given path between 2 associations.
2788 sack_timeout - INTEGER
2789 The amount of time (in milliseconds) that the implementation will wait
2794 valid_cookie_life - INTEGER
2795 The default lifetime of the SCTP cookie (in milliseconds). The cookie
2796 is used during association establishment.
2800 cookie_preserve_enable - BOOLEAN
2801 Enable or disable the ability to extend the lifetime of the SCTP cookie
2802 that is used during the establishment phase of SCTP association
2804 - 1: Enable cookie lifetime extension.
2809 cookie_hmac_alg - STRING
2810 Select the hmac algorithm used when generating the cookie value sent by
2811 a listening sctp socket to a connecting client in the INIT-ACK chunk.
2818 Ability to assign md5 or sha1 as the selected alg is predicated on the
2819 configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
2820 CONFIG_CRYPTO_SHA1).
2822 Default: Dependent on configuration. MD5 if available, else SHA1 if
2823 available, else none.
2825 rcvbuf_policy - INTEGER
2826 Determines if the receive buffer is attributed to the socket or to
2827 association. SCTP supports the capability to create multiple
2828 associations on a single socket. When using this capability, it is
2829 possible that a single stalled association that's buffering a lot
2830 of data may block other associations from delivering their data by
2831 consuming all of the receive buffer space. To work around this,
2832 the rcvbuf_policy could be set to attribute the receiver buffer space
2833 to each association instead of the socket. This prevents the described
2836 - 1: rcvbuf space is per association
2837 - 0: rcvbuf space is per socket
2841 sndbuf_policy - INTEGER
2842 Similar to rcvbuf_policy above, this applies to send buffer space.
2844 - 1: Send buffer is tracked per association
2845 - 0: Send buffer is tracked per socket.
2849 sctp_mem - vector of 3 INTEGERs: min, pressure, max
2850 Number of pages allowed for queueing by all SCTP sockets.
2852 min: Below this number of pages SCTP is not bothered about its
2853 memory appetite. When amount of memory allocated by SCTP exceeds
2854 this number, SCTP starts to moderate memory usage.
2856 pressure: This value was introduced to follow format of tcp_mem.
2858 max: Number of pages allowed for queueing by all SCTP sockets.
2860 Default is calculated at boot time from amount of available memory.
2862 sctp_rmem - vector of 3 INTEGERs: min, default, max
2863 Only the first value ("min") is used, "default" and "max" are
2866 min: Minimal size of receive buffer used by SCTP socket.
2867 It is guaranteed to each SCTP socket (but not association) even
2868 under moderate memory pressure.
2872 sctp_wmem - vector of 3 INTEGERs: min, default, max
2873 Currently this tunable has no effect.
2875 addr_scope_policy - INTEGER
2876 Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
2878 - 0 - Disable IPv4 address scoping
2879 - 1 - Enable IPv4 address scoping
2880 - 2 - Follow draft but allow IPv4 private addresses
2881 - 3 - Follow draft but allow IPv4 link local addresses
2886 The listening port for the local UDP tunneling sock. Normally it's
2887 using the IANA-assigned UDP port number 9899 (sctp-tunneling).
2889 This UDP sock is used for processing the incoming UDP-encapsulated
2890 SCTP packets (from RFC6951), and shared by all applications in the
2891 same net namespace. This UDP sock will be closed when the value is
2894 The value will also be used to set the src port of the UDP header
2895 for the outgoing UDP-encapsulated SCTP packets. For the dest port,
2896 please refer to 'encap_port' below.
2900 encap_port - INTEGER
2901 The default remote UDP encapsulation port.
2903 This value is used to set the dest port of the UDP header for the
2904 outgoing UDP-encapsulated SCTP packets by default. Users can also
2905 change the value for each sock/asoc/transport by using setsockopt.
2906 For further information, please refer to RFC6951.
2908 Note that when connecting to a remote server, the client should set
2909 this to the port that the UDP tunneling sock on the peer server is
2910 listening to and the local UDP tunneling sock on the client also
2911 must be started. On the server, it would get the encap_port from
2912 the incoming packet's source port.
2916 plpmtud_probe_interval - INTEGER
2917 The time interval (in milliseconds) for the PLPMTUD probe timer,
2918 which is configured to expire after this period to receive an
2919 acknowledgment to a probe packet. This is also the time interval
2920 between the probes for the current pmtu when the probe search
2923 PLPMTUD will be disabled when 0 is set, and other values for it
2928 reconf_enable - BOOLEAN
2929 Enable or disable extension of Stream Reconfiguration functionality
2930 specified in RFC6525. This extension provides the ability to "reset"
2931 a stream, and it includes the Parameters of "Outgoing/Incoming SSN
2932 Reset", "SSN/TSN Reset" and "Add Outgoing/Incoming Streams".
2934 - 1: Enable extension.
2935 - 0: Disable extension.
2939 intl_enable - BOOLEAN
2940 Enable or disable extension of User Message Interleaving functionality
2941 specified in RFC8260. This extension allows the interleaving of user
2942 messages sent on different streams. With this feature enabled, I-DATA
2943 chunk will replace DATA chunk to carry user messages if also supported
2944 by the peer. Note that to use this feature, one needs to set this option
2945 to 1 and also needs to set socket options SCTP_FRAGMENT_INTERLEAVE to 2
2946 and SCTP_INTERLEAVING_SUPPORTED to 1.
2948 - 1: Enable extension.
2949 - 0: Disable extension.
2953 ecn_enable - BOOLEAN
2954 Control use of Explicit Congestion Notification (ECN) by SCTP.
2955 Like in TCP, ECN is used only when both ends of the SCTP connection
2956 indicate support for it. This feature is useful in avoiding losses
2957 due to congestion by allowing supporting routers to signal congestion
2958 before having to drop packets.
2966 ``/proc/sys/net/core/*``
2967 ========================
2969 Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
2972 ``/proc/sys/net/unix/*``
2973 ========================
2975 max_dgram_qlen - INTEGER
2976 The maximum length of dgram socket receive queue
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