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.
273 inet_peer_threshold - INTEGER
274 The approximate size of the storage. Starting from this threshold
275 entries will be thrown aggressively. This threshold also determines
276 entries' time-to-live and time intervals between garbage collection
277 passes. More entries, less time-to-live, less GC interval.
279 inet_peer_minttl - INTEGER
280 Minimum time-to-live of entries. Should be enough to cover fragment
281 time-to-live on the reassembling side. This minimum time-to-live is
282 guaranteed if the pool size is less than inet_peer_threshold.
285 inet_peer_maxttl - INTEGER
286 Maximum time-to-live of entries. Unused entries will expire after
287 this period of time if there is no memory pressure on the pool (i.e.
288 when the number of entries in the pool is very small).
295 Limit of socket listen() backlog, known in userspace as SOMAXCONN.
296 Defaults to 4096. (Was 128 before linux-5.4)
297 See also tcp_max_syn_backlog for additional tuning for TCP sockets.
299 tcp_abort_on_overflow - BOOLEAN
300 If listening service is too slow to accept new connections,
301 reset them. Default state is FALSE. It means that if overflow
302 occurred due to a burst, connection will recover. Enable this
303 option _only_ if you are really sure that listening daemon
304 cannot be tuned to accept connections faster. Enabling this
305 option can harm clients of your server.
307 tcp_adv_win_scale - INTEGER
308 Count buffering overhead as bytes/2^tcp_adv_win_scale
309 (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
312 Possible values are [-31, 31], inclusive.
316 tcp_allowed_congestion_control - STRING
317 Show/set the congestion control choices available to non-privileged
318 processes. The list is a subset of those listed in
319 tcp_available_congestion_control.
321 Default is "reno" and the default setting (tcp_congestion_control).
323 tcp_app_win - INTEGER
324 Reserve max(window/2^tcp_app_win, mss) of window for application
325 buffer. Value 0 is special, it means that nothing is reserved.
329 tcp_autocorking - BOOLEAN
330 Enable TCP auto corking :
331 When applications do consecutive small write()/sendmsg() system calls,
332 we try to coalesce these small writes as much as possible, to lower
333 total amount of sent packets. This is done if at least one prior
334 packet for the flow is waiting in Qdisc queues or device transmit
335 queue. Applications can still use TCP_CORK for optimal behavior
336 when they know how/when to uncork their sockets.
340 tcp_available_congestion_control - STRING
341 Shows the available congestion control choices that are registered.
342 More congestion control algorithms may be available as modules,
345 tcp_base_mss - INTEGER
346 The initial value of search_low to be used by the packetization layer
347 Path MTU discovery (MTU probing). If MTU probing is enabled,
348 this is the initial MSS used by the connection.
350 tcp_mtu_probe_floor - INTEGER
351 If MTU probing is enabled this caps the minimum MSS used for search_low
356 tcp_min_snd_mss - INTEGER
357 TCP SYN and SYNACK messages usually advertise an ADVMSS option,
358 as described in RFC 1122 and RFC 6691.
360 If this ADVMSS option is smaller than tcp_min_snd_mss,
361 it is silently capped to tcp_min_snd_mss.
363 Default : 48 (at least 8 bytes of payload per segment)
365 tcp_congestion_control - STRING
366 Set the congestion control algorithm to be used for new
367 connections. The algorithm "reno" is always available, but
368 additional choices may be available based on kernel configuration.
369 Default is set as part of kernel configuration.
370 For passive connections, the listener congestion control choice
373 [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
376 Allows TCP to send "duplicate" SACKs.
378 tcp_early_retrans - INTEGER
379 Tail loss probe (TLP) converts RTOs occurring due to tail
380 losses into fast recovery (draft-ietf-tcpm-rack). Note that
381 TLP requires RACK to function properly (see tcp_recovery below)
391 Control use of Explicit Congestion Notification (ECN) by TCP.
392 ECN is used only when both ends of the TCP connection indicate
393 support for it. This feature is useful in avoiding losses due
394 to congestion by allowing supporting routers to signal
395 congestion before having to drop packets.
399 = =====================================================
400 0 Disable ECN. Neither initiate nor accept ECN.
401 1 Enable ECN when requested by incoming connections and
402 also request ECN on outgoing connection attempts.
403 2 Enable ECN when requested by incoming connections
404 but do not request ECN on outgoing connections.
405 = =====================================================
409 tcp_ecn_fallback - BOOLEAN
410 If the kernel detects that ECN connection misbehaves, enable fall
411 back to non-ECN. Currently, this knob implements the fallback
412 from RFC3168, section 6.1.1.1., but we reserve that in future,
413 additional detection mechanisms could be implemented under this
414 knob. The value is not used, if tcp_ecn or per route (or congestion
415 control) ECN settings are disabled.
417 Default: 1 (fallback enabled)
420 This is a legacy option, it has no effect anymore.
422 tcp_fin_timeout - INTEGER
423 The length of time an orphaned (no longer referenced by any
424 application) connection will remain in the FIN_WAIT_2 state
425 before it is aborted at the local end. While a perfectly
426 valid "receive only" state for an un-orphaned connection, an
427 orphaned connection in FIN_WAIT_2 state could otherwise wait
428 forever for the remote to close its end of the connection.
435 Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
436 F-RTO is an enhanced recovery algorithm for TCP retransmission
437 timeouts. It is particularly beneficial in networks where the
438 RTT fluctuates (e.g., wireless). F-RTO is sender-side only
439 modification. It does not require any support from the peer.
441 By default it's enabled with a non-zero value. 0 disables F-RTO.
443 tcp_fwmark_accept - BOOLEAN
444 If set, incoming connections to listening sockets that do not have a
445 socket mark will set the mark of the accepting socket to the fwmark of
446 the incoming SYN packet. This will cause all packets on that connection
447 (starting from the first SYNACK) to be sent with that fwmark. The
448 listening socket's mark is unchanged. Listening sockets that already
449 have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
454 tcp_invalid_ratelimit - INTEGER
455 Limit the maximal rate for sending duplicate acknowledgments
456 in response to incoming TCP packets that are for an existing
457 connection but that are invalid due to any of these reasons:
459 (a) out-of-window sequence number,
460 (b) out-of-window acknowledgment number, or
461 (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
463 This can help mitigate simple "ack loop" DoS attacks, wherein
464 a buggy or malicious middlebox or man-in-the-middle can
465 rewrite TCP header fields in manner that causes each endpoint
466 to think that the other is sending invalid TCP segments, thus
467 causing each side to send an unterminating stream of duplicate
468 acknowledgments for invalid segments.
470 Using 0 disables rate-limiting of dupacks in response to
471 invalid segments; otherwise this value specifies the minimal
472 space between sending such dupacks, in milliseconds.
474 Default: 500 (milliseconds).
476 tcp_keepalive_time - INTEGER
477 How often TCP sends out keepalive messages when keepalive is enabled.
480 tcp_keepalive_probes - INTEGER
481 How many keepalive probes TCP sends out, until it decides that the
482 connection is broken. Default value: 9.
484 tcp_keepalive_intvl - INTEGER
485 How frequently the probes are send out. Multiplied by
486 tcp_keepalive_probes it is time to kill not responding connection,
487 after probes started. Default value: 75sec i.e. connection
488 will be aborted after ~11 minutes of retries.
490 tcp_l3mdev_accept - BOOLEAN
491 Enables child sockets to inherit the L3 master device index.
492 Enabling this option allows a "global" listen socket to work
493 across L3 master domains (e.g., VRFs) with connected sockets
494 derived from the listen socket to be bound to the L3 domain in
495 which the packets originated. Only valid when the kernel was
496 compiled with CONFIG_NET_L3_MASTER_DEV.
498 Default: 0 (disabled)
500 tcp_low_latency - BOOLEAN
501 This is a legacy option, it has no effect anymore.
503 tcp_max_orphans - INTEGER
504 Maximal number of TCP sockets not attached to any user file handle,
505 held by system. If this number is exceeded orphaned connections are
506 reset immediately and warning is printed. This limit exists
507 only to prevent simple DoS attacks, you _must_ not rely on this
508 or lower the limit artificially, but rather increase it
509 (probably, after increasing installed memory),
510 if network conditions require more than default value,
511 and tune network services to linger and kill such states
512 more aggressively. Let me to remind again: each orphan eats
513 up to ~64K of unswappable memory.
515 tcp_max_syn_backlog - INTEGER
516 Maximal number of remembered connection requests (SYN_RECV),
517 which have not received an acknowledgment from connecting client.
519 This is a per-listener limit.
521 The minimal value is 128 for low memory machines, and it will
522 increase in proportion to the memory of machine.
524 If server suffers from overload, try increasing this number.
526 Remember to also check /proc/sys/net/core/somaxconn
527 A SYN_RECV request socket consumes about 304 bytes of memory.
529 tcp_max_tw_buckets - INTEGER
530 Maximal number of timewait sockets held by system simultaneously.
531 If this number is exceeded time-wait socket is immediately destroyed
532 and warning is printed. This limit exists only to prevent
533 simple DoS attacks, you _must_ not lower the limit artificially,
534 but rather increase it (probably, after increasing installed memory),
535 if network conditions require more than default value.
537 tcp_mem - vector of 3 INTEGERs: min, pressure, max
538 min: below this number of pages TCP is not bothered about its
541 pressure: when amount of memory allocated by TCP exceeds this number
542 of pages, TCP moderates its memory consumption and enters memory
543 pressure mode, which is exited when memory consumption falls
546 max: number of pages allowed for queueing by all TCP sockets.
548 Defaults are calculated at boot time from amount of available
551 tcp_min_rtt_wlen - INTEGER
552 The window length of the windowed min filter to track the minimum RTT.
553 A shorter window lets a flow more quickly pick up new (higher)
554 minimum RTT when it is moved to a longer path (e.g., due to traffic
555 engineering). A longer window makes the filter more resistant to RTT
556 inflations such as transient congestion. The unit is seconds.
558 Possible values: 0 - 86400 (1 day)
562 tcp_moderate_rcvbuf - BOOLEAN
563 If set, TCP performs receive buffer auto-tuning, attempting to
564 automatically size the buffer (no greater than tcp_rmem[2]) to
565 match the size required by the path for full throughput. Enabled by
568 tcp_mtu_probing - INTEGER
569 Controls TCP Packetization-Layer Path MTU Discovery. Takes three
573 - 1 - Disabled by default, enabled when an ICMP black hole detected
574 - 2 - Always enabled, use initial MSS of tcp_base_mss.
576 tcp_probe_interval - UNSIGNED INTEGER
577 Controls how often to start TCP Packetization-Layer Path MTU
578 Discovery reprobe. The default is reprobing every 10 minutes as
581 tcp_probe_threshold - INTEGER
582 Controls when TCP Packetization-Layer Path MTU Discovery probing
583 will stop in respect to the width of search range in bytes. Default
586 tcp_no_metrics_save - BOOLEAN
587 By default, TCP saves various connection metrics in the route cache
588 when the connection closes, so that connections established in the
589 near future can use these to set initial conditions. Usually, this
590 increases overall performance, but may sometimes cause performance
591 degradation. If set, TCP will not cache metrics on closing
594 tcp_no_ssthresh_metrics_save - BOOLEAN
595 Controls whether TCP saves ssthresh metrics in the route cache.
597 Default is 1, which disables ssthresh metrics.
599 tcp_orphan_retries - INTEGER
600 This value influences the timeout of a locally closed TCP connection,
601 when RTO retransmissions remain unacknowledged.
602 See tcp_retries2 for more details.
604 The default value is 8.
606 If your machine is a loaded WEB server,
607 you should think about lowering this value, such sockets
608 may consume significant resources. Cf. tcp_max_orphans.
610 tcp_recovery - INTEGER
611 This value is a bitmap to enable various experimental loss recovery
614 ========= =============================================================
615 RACK: 0x1 enables the RACK loss detection for fast detection of lost
616 retransmissions and tail drops. It also subsumes and disables
617 RFC6675 recovery for SACK connections.
619 RACK: 0x2 makes RACK's reordering window static (min_rtt/4).
621 RACK: 0x4 disables RACK's DUPACK threshold heuristic
622 ========= =============================================================
626 tcp_reordering - INTEGER
627 Initial reordering level of packets in a TCP stream.
628 TCP stack can then dynamically adjust flow reordering level
629 between this initial value and tcp_max_reordering
633 tcp_max_reordering - INTEGER
634 Maximal reordering level of packets in a TCP stream.
635 300 is a fairly conservative value, but you might increase it
636 if paths are using per packet load balancing (like bonding rr mode)
640 tcp_retrans_collapse - BOOLEAN
641 Bug-to-bug compatibility with some broken printers.
642 On retransmit try to send bigger packets to work around bugs in
645 tcp_retries1 - INTEGER
646 This value influences the time, after which TCP decides, that
647 something is wrong due to unacknowledged RTO retransmissions,
648 and reports this suspicion to the network layer.
649 See tcp_retries2 for more details.
651 RFC 1122 recommends at least 3 retransmissions, which is the
654 tcp_retries2 - INTEGER
655 This value influences the timeout of an alive TCP connection,
656 when RTO retransmissions remain unacknowledged.
657 Given a value of N, a hypothetical TCP connection following
658 exponential backoff with an initial RTO of TCP_RTO_MIN would
659 retransmit N times before killing the connection at the (N+1)th RTO.
661 The default value of 15 yields a hypothetical timeout of 924.6
662 seconds and is a lower bound for the effective timeout.
663 TCP will effectively time out at the first RTO which exceeds the
664 hypothetical timeout.
666 RFC 1122 recommends at least 100 seconds for the timeout,
667 which corresponds to a value of at least 8.
669 tcp_rfc1337 - BOOLEAN
670 If set, the TCP stack behaves conforming to RFC1337. If unset,
671 we are not conforming to RFC, but prevent TCP TIME_WAIT
676 tcp_rmem - vector of 3 INTEGERs: min, default, max
677 min: Minimal size of receive buffer used by TCP sockets.
678 It is guaranteed to each TCP socket, even under moderate memory
683 default: initial size of receive buffer used by TCP sockets.
684 This value overrides net.core.rmem_default used by other protocols.
685 Default: 131072 bytes.
686 This value results in initial window of 65535.
688 max: maximal size of receive buffer allowed for automatically
689 selected receiver buffers for TCP socket. This value does not override
690 net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables
691 automatic tuning of that socket's receive buffer size, in which
692 case this value is ignored.
693 Default: between 131072 and 6MB, depending on RAM size.
696 Enable select acknowledgments (SACKS).
698 tcp_comp_sack_delay_ns - LONG INTEGER
699 TCP tries to reduce number of SACK sent, using a timer
700 based on 5% of SRTT, capped by this sysctl, in nano seconds.
701 The default is 1ms, based on TSO autosizing period.
703 Default : 1,000,000 ns (1 ms)
705 tcp_comp_sack_slack_ns - LONG INTEGER
706 This sysctl control the slack used when arming the
707 timer used by SACK compression. This gives extra time
708 for small RTT flows, and reduces system overhead by allowing
709 opportunistic reduction of timer interrupts.
711 Default : 100,000 ns (100 us)
713 tcp_comp_sack_nr - INTEGER
714 Max number of SACK that can be compressed.
715 Using 0 disables SACK compression.
719 tcp_slow_start_after_idle - BOOLEAN
720 If set, provide RFC2861 behavior and time out the congestion
721 window after an idle period. An idle period is defined at
722 the current RTO. If unset, the congestion window will not
723 be timed out after an idle period.
728 Use the Host requirements interpretation of the TCP urgent pointer field.
729 Most hosts use the older BSD interpretation, so if you turn this on
730 Linux might not communicate correctly with them.
734 tcp_synack_retries - INTEGER
735 Number of times SYNACKs for a passive TCP connection attempt will
736 be retransmitted. Should not be higher than 255. Default value
737 is 5, which corresponds to 31seconds till the last retransmission
738 with the current initial RTO of 1second. With this the final timeout
739 for a passive TCP connection will happen after 63seconds.
741 tcp_syncookies - INTEGER
742 Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
743 Send out syncookies when the syn backlog queue of a socket
744 overflows. This is to prevent against the common 'SYN flood attack'
747 Note, that syncookies is fallback facility.
748 It MUST NOT be used to help highly loaded servers to stand
749 against legal connection rate. If you see SYN flood warnings
750 in your logs, but investigation shows that they occur
751 because of overload with legal connections, you should tune
752 another parameters until this warning disappear.
753 See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
755 syncookies seriously violate TCP protocol, do not allow
756 to use TCP extensions, can result in serious degradation
757 of some services (f.e. SMTP relaying), visible not by you,
758 but your clients and relays, contacting you. While you see
759 SYN flood warnings in logs not being really flooded, your server
760 is seriously misconfigured.
762 If you want to test which effects syncookies have to your
763 network connections you can set this knob to 2 to enable
764 unconditionally generation of syncookies.
766 tcp_migrate_req - BOOLEAN
767 The incoming connection is tied to a specific listening socket when
768 the initial SYN packet is received during the three-way handshake.
769 When a listener is closed, in-flight request sockets during the
770 handshake and established sockets in the accept queue are aborted.
772 If the listener has SO_REUSEPORT enabled, other listeners on the
773 same port should have been able to accept such connections. This
774 option makes it possible to migrate such child sockets to another
775 listener after close() or shutdown().
777 The BPF_SK_REUSEPORT_SELECT_OR_MIGRATE type of eBPF program should
778 usually be used to define the policy to pick an alive listener.
779 Otherwise, the kernel will randomly pick an alive listener only if
780 this option is enabled.
782 Note that migration between listeners with different settings may
783 crash applications. Let's say migration happens from listener A to
784 B, and only B has TCP_SAVE_SYN enabled. B cannot read SYN data from
785 the requests migrated from A. To avoid such a situation, cancel
786 migration by returning SK_DROP in the type of eBPF program, or
791 tcp_fastopen - INTEGER
792 Enable TCP Fast Open (RFC7413) to send and accept data in the opening
795 The client support is enabled by flag 0x1 (on by default). The client
796 then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
797 rather than connect() to send data in SYN.
799 The server support is enabled by flag 0x2 (off by default). Then
800 either enable for all listeners with another flag (0x400) or
801 enable individual listeners via TCP_FASTOPEN socket option with
802 the option value being the length of the syn-data backlog.
804 The values (bitmap) are
806 ===== ======== ======================================================
807 0x1 (client) enables sending data in the opening SYN on the client.
808 0x2 (server) enables the server support, i.e., allowing data in
809 a SYN packet to be accepted and passed to the
810 application before 3-way handshake finishes.
811 0x4 (client) send data in the opening SYN regardless of cookie
812 availability and without a cookie option.
813 0x200 (server) accept data-in-SYN w/o any cookie option present.
814 0x400 (server) enable all listeners to support Fast Open by
815 default without explicit TCP_FASTOPEN socket option.
816 ===== ======== ======================================================
820 Note that additional client or server features are only
821 effective if the basic support (0x1 and 0x2) are enabled respectively.
823 tcp_fastopen_blackhole_timeout_sec - INTEGER
824 Initial time period in second to disable Fastopen on active TCP sockets
825 when a TFO firewall blackhole issue happens.
826 This time period will grow exponentially when more blackhole issues
827 get detected right after Fastopen is re-enabled and will reset to
828 initial value when the blackhole issue goes away.
829 0 to disable the blackhole detection.
831 By default, it is set to 0 (feature is disabled).
833 tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
834 The list consists of a primary key and an optional backup key. The
835 primary key is used for both creating and validating cookies, while the
836 optional backup key is only used for validating cookies. The purpose of
837 the backup key is to maximize TFO validation when keys are rotated.
839 A randomly chosen primary key may be configured by the kernel if
840 the tcp_fastopen sysctl is set to 0x400 (see above), or if the
841 TCP_FASTOPEN setsockopt() optname is set and a key has not been
842 previously configured via sysctl. If keys are configured via
843 setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
844 per-socket keys will be used instead of any keys that are specified via
847 A key is specified as 4 8-digit hexadecimal integers which are separated
848 by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
849 omitted. A primary and a backup key may be specified by separating them
850 by a comma. If only one key is specified, it becomes the primary key and
851 any previously configured backup keys are removed.
853 tcp_syn_retries - INTEGER
854 Number of times initial SYNs for an active TCP connection attempt
855 will be retransmitted. Should not be higher than 127. Default value
856 is 6, which corresponds to 63seconds till the last retransmission
857 with the current initial RTO of 1second. With this the final timeout
858 for an active TCP connection attempt will happen after 127seconds.
860 tcp_timestamps - INTEGER
861 Enable timestamps as defined in RFC1323.
864 - 1: Enable timestamps as defined in RFC1323 and use random offset for
865 each connection rather than only using the current time.
866 - 2: Like 1, but without random offsets.
870 tcp_min_tso_segs - INTEGER
871 Minimal number of segments per TSO frame.
873 Since linux-3.12, TCP does an automatic sizing of TSO frames,
874 depending on flow rate, instead of filling 64Kbytes packets.
875 For specific usages, it's possible to force TCP to build big
876 TSO frames. Note that TCP stack might split too big TSO packets
877 if available window is too small.
881 tcp_tso_rtt_log - INTEGER
882 Adjustment of TSO packet sizes based on min_rtt
884 Starting from linux-5.18, TCP autosizing can be tweaked
885 for flows having small RTT.
887 Old autosizing was splitting the pacing budget to send 1024 TSO
890 tso_packet_size = sk->sk_pacing_rate / 1024;
892 With the new mechanism, we increase this TSO sizing using:
894 distance = min_rtt_usec / (2^tcp_tso_rtt_log)
895 tso_packet_size += gso_max_size >> distance;
897 This means that flows between very close hosts can use bigger
898 TSO packets, reducing their cpu costs.
900 If you want to use the old autosizing, set this sysctl to 0.
902 Default: 9 (2^9 = 512 usec)
904 tcp_pacing_ss_ratio - INTEGER
905 sk->sk_pacing_rate is set by TCP stack using a ratio applied
906 to current rate. (current_rate = cwnd * mss / srtt)
907 If TCP is in slow start, tcp_pacing_ss_ratio is applied
908 to let TCP probe for bigger speeds, assuming cwnd can be
909 doubled every other RTT.
913 tcp_pacing_ca_ratio - INTEGER
914 sk->sk_pacing_rate is set by TCP stack using a ratio applied
915 to current rate. (current_rate = cwnd * mss / srtt)
916 If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
917 is applied to conservatively probe for bigger throughput.
921 tcp_tso_win_divisor - INTEGER
922 This allows control over what percentage of the congestion window
923 can be consumed by a single TSO frame.
924 The setting of this parameter is a choice between burstiness and
925 building larger TSO frames.
929 tcp_tw_reuse - INTEGER
930 Enable reuse of TIME-WAIT sockets for new connections when it is
931 safe from protocol viewpoint.
935 - 2 - enable for loopback traffic only
937 It should not be changed without advice/request of technical
942 tcp_window_scaling - BOOLEAN
943 Enable window scaling as defined in RFC1323.
945 tcp_wmem - vector of 3 INTEGERs: min, default, max
946 min: Amount of memory reserved for send buffers for TCP sockets.
947 Each TCP socket has rights to use it due to fact of its birth.
951 default: initial size of send buffer used by TCP sockets. This
952 value overrides net.core.wmem_default used by other protocols.
954 It is usually lower than net.core.wmem_default.
958 max: Maximal amount of memory allowed for automatically tuned
959 send buffers for TCP sockets. This value does not override
960 net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables
961 automatic tuning of that socket's send buffer size, in which case
962 this value is ignored.
964 Default: between 64K and 4MB, depending on RAM size.
966 tcp_notsent_lowat - UNSIGNED INTEGER
967 A TCP socket can control the amount of unsent bytes in its write queue,
968 thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
969 reports POLLOUT events if the amount of unsent bytes is below a per
970 socket value, and if the write queue is not full. sendmsg() will
971 also not add new buffers if the limit is hit.
973 This global variable controls the amount of unsent data for
974 sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
975 to the global variable has immediate effect.
977 Default: UINT_MAX (0xFFFFFFFF)
979 tcp_workaround_signed_windows - BOOLEAN
980 If set, assume no receipt of a window scaling option means the
981 remote TCP is broken and treats the window as a signed quantity.
982 If unset, assume the remote TCP is not broken even if we do
983 not receive a window scaling option from them.
987 tcp_thin_linear_timeouts - BOOLEAN
988 Enable dynamic triggering of linear timeouts for thin streams.
989 If set, a check is performed upon retransmission by timeout to
990 determine if the stream is thin (less than 4 packets in flight).
991 As long as the stream is found to be thin, up to 6 linear
992 timeouts may be performed before exponential backoff mode is
993 initiated. This improves retransmission latency for
994 non-aggressive thin streams, often found to be time-dependent.
995 For more information on thin streams, see
996 Documentation/networking/tcp-thin.rst
1000 tcp_limit_output_bytes - INTEGER
1001 Controls TCP Small Queue limit per tcp socket.
1002 TCP bulk sender tends to increase packets in flight until it
1003 gets losses notifications. With SNDBUF autotuning, this can
1004 result in a large amount of packets queued on the local machine
1005 (e.g.: qdiscs, CPU backlog, or device) hurting latency of other
1006 flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes
1007 limits the number of bytes on qdisc or device to reduce artificial
1008 RTT/cwnd and reduce bufferbloat.
1010 Default: 1048576 (16 * 65536)
1012 tcp_challenge_ack_limit - INTEGER
1013 Limits number of Challenge ACK sent per second, as recommended
1014 in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
1020 udp_l3mdev_accept - BOOLEAN
1021 Enabling this option allows a "global" bound socket to work
1022 across L3 master domains (e.g., VRFs) with packets capable of
1023 being received regardless of the L3 domain in which they
1024 originated. Only valid when the kernel was compiled with
1025 CONFIG_NET_L3_MASTER_DEV.
1027 Default: 0 (disabled)
1029 udp_mem - vector of 3 INTEGERs: min, pressure, max
1030 Number of pages allowed for queueing by all UDP sockets.
1032 min: Number of pages allowed for queueing by all UDP sockets.
1034 pressure: This value was introduced to follow format of tcp_mem.
1036 max: This value was introduced to follow format of tcp_mem.
1038 Default is calculated at boot time from amount of available memory.
1040 udp_rmem_min - INTEGER
1041 Minimal size of receive buffer used by UDP sockets in moderation.
1042 Each UDP socket is able to use the size for receiving data, even if
1043 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
1047 udp_wmem_min - INTEGER
1048 Minimal size of send buffer used by UDP sockets in moderation.
1049 Each UDP socket is able to use the size for sending data, even if
1050 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
1057 raw_l3mdev_accept - BOOLEAN
1058 Enabling this option allows a "global" bound socket to work
1059 across L3 master domains (e.g., VRFs) with packets capable of
1060 being received regardless of the L3 domain in which they
1061 originated. Only valid when the kernel was compiled with
1062 CONFIG_NET_L3_MASTER_DEV.
1064 Default: 1 (enabled)
1069 cipso_cache_enable - BOOLEAN
1070 If set, enable additions to and lookups from the CIPSO label mapping
1071 cache. If unset, additions are ignored and lookups always result in a
1072 miss. However, regardless of the setting the cache is still
1073 invalidated when required when means you can safely toggle this on and
1074 off and the cache will always be "safe".
1078 cipso_cache_bucket_size - INTEGER
1079 The CIPSO label cache consists of a fixed size hash table with each
1080 hash bucket containing a number of cache entries. This variable limits
1081 the number of entries in each hash bucket; the larger the value the
1082 more CIPSO label mappings that can be cached. When the number of
1083 entries in a given hash bucket reaches this limit adding new entries
1084 causes the oldest entry in the bucket to be removed to make room.
1088 cipso_rbm_optfmt - BOOLEAN
1089 Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
1090 the CIPSO draft specification (see Documentation/netlabel for details).
1091 This means that when set the CIPSO tag will be padded with empty
1092 categories in order to make the packet data 32-bit aligned.
1096 cipso_rbm_structvalid - BOOLEAN
1097 If set, do a very strict check of the CIPSO option when
1098 ip_options_compile() is called. If unset, relax the checks done during
1099 ip_options_compile(). Either way is "safe" as errors are caught else
1100 where in the CIPSO processing code but setting this to 0 (False) should
1101 result in less work (i.e. it should be faster) but could cause problems
1102 with other implementations that require strict checking.
1109 ip_local_port_range - 2 INTEGERS
1110 Defines the local port range that is used by TCP and UDP to
1111 choose the local port. The first number is the first, the
1112 second the last local port number.
1113 If possible, it is better these numbers have different parity
1114 (one even and one odd value).
1115 Must be greater than or equal to ip_unprivileged_port_start.
1116 The default values are 32768 and 60999 respectively.
1118 ip_local_reserved_ports - list of comma separated ranges
1119 Specify the ports which are reserved for known third-party
1120 applications. These ports will not be used by automatic port
1121 assignments (e.g. when calling connect() or bind() with port
1122 number 0). Explicit port allocation behavior is unchanged.
1124 The format used for both input and output is a comma separated
1125 list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
1126 10). Writing to the file will clear all previously reserved
1127 ports and update the current list with the one given in the
1130 Note that ip_local_port_range and ip_local_reserved_ports
1131 settings are independent and both are considered by the kernel
1132 when determining which ports are available for automatic port
1135 You can reserve ports which are not in the current
1136 ip_local_port_range, e.g.::
1138 $ cat /proc/sys/net/ipv4/ip_local_port_range
1140 $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
1143 although this is redundant. However such a setting is useful
1144 if later the port range is changed to a value that will
1145 include the reserved ports. Also keep in mind, that overlapping
1146 of these ranges may affect probability of selecting ephemeral
1147 ports which are right after block of reserved ports.
1151 ip_unprivileged_port_start - INTEGER
1152 This is a per-namespace sysctl. It defines the first
1153 unprivileged port in the network namespace. Privileged ports
1154 require root or CAP_NET_BIND_SERVICE in order to bind to them.
1155 To disable all privileged ports, set this to 0. They must not
1156 overlap with the ip_local_port_range.
1160 ip_nonlocal_bind - BOOLEAN
1161 If set, allows processes to bind() to non-local IP addresses,
1162 which can be quite useful - but may break some applications.
1166 ip_autobind_reuse - BOOLEAN
1167 By default, bind() does not select the ports automatically even if
1168 the new socket and all sockets bound to the port have SO_REUSEADDR.
1169 ip_autobind_reuse allows bind() to reuse the port and this is useful
1170 when you use bind()+connect(), but may break some applications.
1171 The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
1172 option should only be set by experts.
1175 ip_dynaddr - BOOLEAN
1176 If set non-zero, enables support for dynamic addresses.
1177 If set to a non-zero value larger than 1, a kernel log
1178 message will be printed when dynamic address rewriting
1183 ip_early_demux - BOOLEAN
1184 Optimize input packet processing down to one demux for
1185 certain kinds of local sockets. Currently we only do this
1186 for established TCP and connected UDP sockets.
1188 It may add an additional cost for pure routing workloads that
1189 reduces overall throughput, in such case you should disable it.
1193 ping_group_range - 2 INTEGERS
1194 Restrict ICMP_PROTO datagram sockets to users in the group range.
1195 The default is "1 0", meaning, that nobody (not even root) may
1196 create ping sockets. Setting it to "100 100" would grant permissions
1197 to the single group. "0 4294967295" would enable it for the world, "100
1198 4294967295" would enable it for the users, but not daemons.
1200 tcp_early_demux - BOOLEAN
1201 Enable early demux for established TCP sockets.
1205 udp_early_demux - BOOLEAN
1206 Enable early demux for connected UDP sockets. Disable this if
1207 your system could experience more unconnected load.
1211 icmp_echo_ignore_all - BOOLEAN
1212 If set non-zero, then the kernel will ignore all ICMP ECHO
1213 requests sent to it.
1217 icmp_echo_enable_probe - BOOLEAN
1218 If set to one, then the kernel will respond to RFC 8335 PROBE
1219 requests sent to it.
1223 icmp_echo_ignore_broadcasts - BOOLEAN
1224 If set non-zero, then the kernel will ignore all ICMP ECHO and
1225 TIMESTAMP requests sent to it via broadcast/multicast.
1229 icmp_ratelimit - INTEGER
1230 Limit the maximal rates for sending ICMP packets whose type matches
1231 icmp_ratemask (see below) to specific targets.
1232 0 to disable any limiting,
1233 otherwise the minimal space between responses in milliseconds.
1234 Note that another sysctl, icmp_msgs_per_sec limits the number
1235 of ICMP packets sent on all targets.
1239 icmp_msgs_per_sec - INTEGER
1240 Limit maximal number of ICMP packets sent per second from this host.
1241 Only messages whose type matches icmp_ratemask (see below) are
1242 controlled by this limit. For security reasons, the precise count
1243 of messages per second is randomized.
1247 icmp_msgs_burst - INTEGER
1248 icmp_msgs_per_sec controls number of ICMP packets sent per second,
1249 while icmp_msgs_burst controls the burst size of these packets.
1250 For security reasons, the precise burst size is randomized.
1254 icmp_ratemask - INTEGER
1255 Mask made of ICMP types for which rates are being limited.
1257 Significant bits: IHGFEDCBA9876543210
1259 Default mask: 0000001100000011000 (6168)
1261 Bit definitions (see include/linux/icmp.h):
1263 = =========================
1265 3 Destination Unreachable [1]_
1266 4 Source Quench [1]_
1269 B Time Exceeded [1]_
1270 C Parameter Problem [1]_
1275 H Address Mask Request
1276 I Address Mask Reply
1277 = =========================
1279 .. [1] These are rate limited by default (see default mask above)
1281 icmp_ignore_bogus_error_responses - BOOLEAN
1282 Some routers violate RFC1122 by sending bogus responses to broadcast
1283 frames. Such violations are normally logged via a kernel warning.
1284 If this is set to TRUE, the kernel will not give such warnings, which
1285 will avoid log file clutter.
1289 icmp_errors_use_inbound_ifaddr - BOOLEAN
1291 If zero, icmp error messages are sent with the primary address of
1292 the exiting interface.
1294 If non-zero, the message will be sent with the primary address of
1295 the interface that received the packet that caused the icmp error.
1296 This is the behaviour many network administrators will expect from
1297 a router. And it can make debugging complicated network layouts
1300 Note that if no primary address exists for the interface selected,
1301 then the primary address of the first non-loopback interface that
1302 has one will be used regardless of this setting.
1306 igmp_max_memberships - INTEGER
1307 Change the maximum number of multicast groups we can subscribe to.
1310 Theoretical maximum value is bounded by having to send a membership
1311 report in a single datagram (i.e. the report can't span multiple
1312 datagrams, or risk confusing the switch and leaving groups you don't
1315 The number of supported groups 'M' is bounded by the number of group
1316 report entries you can fit into a single datagram of 65535 bytes.
1318 M = 65536-sizeof (ip header)/(sizeof(Group record))
1320 Group records are variable length, with a minimum of 12 bytes.
1321 So net.ipv4.igmp_max_memberships should not be set higher than:
1323 (65536-24) / 12 = 5459
1325 The value 5459 assumes no IP header options, so in practice
1326 this number may be lower.
1328 igmp_max_msf - INTEGER
1329 Maximum number of addresses allowed in the source filter list for a
1335 Controls the IGMP query robustness variable (see RFC2236 8.1).
1337 Default: 2 (as specified by RFC2236 8.1)
1339 Minimum: 1 (as specified by RFC6636 4.5)
1341 force_igmp_version - INTEGER
1342 - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
1343 allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
1344 Present timer expires.
1345 - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
1346 receive IGMPv2/v3 query.
1347 - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
1348 IGMPv1 query message. Will reply report if receive IGMPv3 query.
1349 - 3 - Enforce to use IGMP version 3. The same react with default 0.
1353 this is not the same with force_mld_version because IGMPv3 RFC3376
1354 Security Considerations does not have clear description that we could
1355 ignore other version messages completely as MLDv2 RFC3810. So make
1356 this value as default 0 is recommended.
1358 ``conf/interface/*``
1359 changes special settings per interface (where
1360 interface" is the name of your network interface)
1363 is special, changes the settings for all interfaces
1365 log_martians - BOOLEAN
1366 Log packets with impossible addresses to kernel log.
1367 log_martians for the interface will be enabled if at least one of
1368 conf/{all,interface}/log_martians is set to TRUE,
1369 it will be disabled otherwise
1371 accept_redirects - BOOLEAN
1372 Accept ICMP redirect messages.
1373 accept_redirects for the interface will be enabled if:
1375 - both conf/{all,interface}/accept_redirects are TRUE in the case
1376 forwarding for the interface is enabled
1380 - at least one of conf/{all,interface}/accept_redirects is TRUE in the
1381 case forwarding for the interface is disabled
1383 accept_redirects for the interface will be disabled otherwise
1390 forwarding - BOOLEAN
1391 Enable IP forwarding on this interface. This controls whether packets
1392 received _on_ this interface can be forwarded.
1394 mc_forwarding - BOOLEAN
1395 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
1396 and a multicast routing daemon is required.
1397 conf/all/mc_forwarding must also be set to TRUE to enable multicast
1398 routing for the interface
1401 Integer value used to differentiate the devices by the medium they
1402 are attached to. Two devices can have different id values when
1403 the broadcast packets are received only on one of them.
1404 The default value 0 means that the device is the only interface
1405 to its medium, value of -1 means that medium is not known.
1407 Currently, it is used to change the proxy_arp behavior:
1408 the proxy_arp feature is enabled for packets forwarded between
1409 two devices attached to different media.
1414 proxy_arp for the interface will be enabled if at least one of
1415 conf/{all,interface}/proxy_arp is set to TRUE,
1416 it will be disabled otherwise
1418 proxy_arp_pvlan - BOOLEAN
1419 Private VLAN proxy arp.
1421 Basically allow proxy arp replies back to the same interface
1422 (from which the ARP request/solicitation was received).
1424 This is done to support (ethernet) switch features, like RFC
1425 3069, where the individual ports are NOT allowed to
1426 communicate with each other, but they are allowed to talk to
1427 the upstream router. As described in RFC 3069, it is possible
1428 to allow these hosts to communicate through the upstream
1429 router by proxy_arp'ing. Don't need to be used together with
1432 This technology is known by different names:
1434 In RFC 3069 it is called VLAN Aggregation.
1435 Cisco and Allied Telesyn call it Private VLAN.
1436 Hewlett-Packard call it Source-Port filtering or port-isolation.
1437 Ericsson call it MAC-Forced Forwarding (RFC Draft).
1439 shared_media - BOOLEAN
1440 Send(router) or accept(host) RFC1620 shared media redirects.
1441 Overrides secure_redirects.
1443 shared_media for the interface will be enabled if at least one of
1444 conf/{all,interface}/shared_media is set to TRUE,
1445 it will be disabled otherwise
1449 secure_redirects - BOOLEAN
1450 Accept ICMP redirect messages only to gateways listed in the
1451 interface's current gateway list. Even if disabled, RFC1122 redirect
1454 Overridden by shared_media.
1456 secure_redirects for the interface will be enabled if at least one of
1457 conf/{all,interface}/secure_redirects is set to TRUE,
1458 it will be disabled otherwise
1462 send_redirects - BOOLEAN
1463 Send redirects, if router.
1465 send_redirects for the interface will be enabled if at least one of
1466 conf/{all,interface}/send_redirects is set to TRUE,
1467 it will be disabled otherwise
1471 bootp_relay - BOOLEAN
1472 Accept packets with source address 0.b.c.d destined
1473 not to this host as local ones. It is supposed, that
1474 BOOTP relay daemon will catch and forward such packets.
1475 conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
1480 Not Implemented Yet.
1482 accept_source_route - BOOLEAN
1483 Accept packets with SRR option.
1484 conf/all/accept_source_route must also be set to TRUE to accept packets
1485 with SRR option on the interface
1492 accept_local - BOOLEAN
1493 Accept packets with local source addresses. In combination with
1494 suitable routing, this can be used to direct packets between two
1495 local interfaces over the wire and have them accepted properly.
1498 route_localnet - BOOLEAN
1499 Do not consider loopback addresses as martian source or destination
1500 while routing. This enables the use of 127/8 for local routing purposes.
1505 - 0 - No source validation.
1506 - 1 - Strict mode as defined in RFC3704 Strict Reverse Path
1507 Each incoming packet is tested against the FIB and if the interface
1508 is not the best reverse path the packet check will fail.
1509 By default failed packets are discarded.
1510 - 2 - Loose mode as defined in RFC3704 Loose Reverse Path
1511 Each incoming packet's source address is also tested against the FIB
1512 and if the source address is not reachable via any interface
1513 the packet check will fail.
1515 Current recommended practice in RFC3704 is to enable strict mode
1516 to prevent IP spoofing from DDos attacks. If using asymmetric routing
1517 or other complicated routing, then loose mode is recommended.
1519 The max value from conf/{all,interface}/rp_filter is used
1520 when doing source validation on the {interface}.
1522 Default value is 0. Note that some distributions enable it
1525 src_valid_mark - BOOLEAN
1526 - 0 - The fwmark of the packet is not included in reverse path
1527 route lookup. This allows for asymmetric routing configurations
1528 utilizing the fwmark in only one direction, e.g., transparent
1531 - 1 - The fwmark of the packet is included in reverse path route
1532 lookup. This permits rp_filter to function when the fwmark is
1533 used for routing traffic in both directions.
1535 This setting also affects the utilization of fmwark when
1536 performing source address selection for ICMP replies, or
1537 determining addresses stored for the IPOPT_TS_TSANDADDR and
1538 IPOPT_RR IP options.
1540 The max value from conf/{all,interface}/src_valid_mark is used.
1544 arp_filter - BOOLEAN
1545 - 1 - Allows you to have multiple network interfaces on the same
1546 subnet, and have the ARPs for each interface be answered
1547 based on whether or not the kernel would route a packet from
1548 the ARP'd IP out that interface (therefore you must use source
1549 based routing for this to work). In other words it allows control
1550 of which cards (usually 1) will respond to an arp request.
1552 - 0 - (default) The kernel can respond to arp requests with addresses
1553 from other interfaces. This may seem wrong but it usually makes
1554 sense, because it increases the chance of successful communication.
1555 IP addresses are owned by the complete host on Linux, not by
1556 particular interfaces. Only for more complex setups like load-
1557 balancing, does this behaviour cause problems.
1559 arp_filter for the interface will be enabled if at least one of
1560 conf/{all,interface}/arp_filter is set to TRUE,
1561 it will be disabled otherwise
1563 arp_announce - INTEGER
1564 Define different restriction levels for announcing the local
1565 source IP address from IP packets in ARP requests sent on
1568 - 0 - (default) Use any local address, configured on any interface
1569 - 1 - Try to avoid local addresses that are not in the target's
1570 subnet for this interface. This mode is useful when target
1571 hosts reachable via this interface require the source IP
1572 address in ARP requests to be part of their logical network
1573 configured on the receiving interface. When we generate the
1574 request we will check all our subnets that include the
1575 target IP and will preserve the source address if it is from
1576 such subnet. If there is no such subnet we select source
1577 address according to the rules for level 2.
1578 - 2 - Always use the best local address for this target.
1579 In this mode we ignore the source address in the IP packet
1580 and try to select local address that we prefer for talks with
1581 the target host. Such local address is selected by looking
1582 for primary IP addresses on all our subnets on the outgoing
1583 interface that include the target IP address. If no suitable
1584 local address is found we select the first local address
1585 we have on the outgoing interface or on all other interfaces,
1586 with the hope we will receive reply for our request and
1587 even sometimes no matter the source IP address we announce.
1589 The max value from conf/{all,interface}/arp_announce is used.
1591 Increasing the restriction level gives more chance for
1592 receiving answer from the resolved target while decreasing
1593 the level announces more valid sender's information.
1595 arp_ignore - INTEGER
1596 Define different modes for sending replies in response to
1597 received ARP requests that resolve local target IP addresses:
1599 - 0 - (default): reply for any local target IP address, configured
1601 - 1 - reply only if the target IP address is local address
1602 configured on the incoming interface
1603 - 2 - reply only if the target IP address is local address
1604 configured on the incoming interface and both with the
1605 sender's IP address are part from same subnet on this interface
1606 - 3 - do not reply for local addresses configured with scope host,
1607 only resolutions for global and link addresses are replied
1609 - 8 - do not reply for all local addresses
1611 The max value from conf/{all,interface}/arp_ignore is used
1612 when ARP request is received on the {interface}
1614 arp_notify - BOOLEAN
1615 Define mode for notification of address and device changes.
1617 == ==========================================================
1618 0 (default): do nothing
1619 1 Generate gratuitous arp requests when device is brought up
1620 or hardware address changes.
1621 == ==========================================================
1623 arp_accept - BOOLEAN
1624 Define behavior for gratuitous ARP frames who's IP is not
1625 already present in the ARP table:
1627 - 0 - don't create new entries in the ARP table
1628 - 1 - create new entries in the ARP table
1630 Both replies and requests type gratuitous arp will trigger the
1631 ARP table to be updated, if this setting is on.
1633 If the ARP table already contains the IP address of the
1634 gratuitous arp frame, the arp table will be updated regardless
1635 if this setting is on or off.
1637 arp_evict_nocarrier - BOOLEAN
1638 Clears the ARP cache on NOCARRIER events. This option is important for
1639 wireless devices where the ARP cache should not be cleared when roaming
1640 between access points on the same network. In most cases this should
1641 remain as the default (1).
1643 - 1 - (default): Clear the ARP cache on NOCARRIER events
1644 - 0 - Do not clear ARP cache on NOCARRIER events
1646 mcast_solicit - INTEGER
1647 The maximum number of multicast probes in INCOMPLETE state,
1648 when the associated hardware address is unknown. Defaults
1651 ucast_solicit - INTEGER
1652 The maximum number of unicast probes in PROBE state, when
1653 the hardware address is being reconfirmed. Defaults to 3.
1655 app_solicit - INTEGER
1656 The maximum number of probes to send to the user space ARP daemon
1657 via netlink before dropping back to multicast probes (see
1658 mcast_resolicit). Defaults to 0.
1660 mcast_resolicit - INTEGER
1661 The maximum number of multicast probes after unicast and
1662 app probes in PROBE state. Defaults to 0.
1664 disable_policy - BOOLEAN
1665 Disable IPSEC policy (SPD) for this interface
1667 disable_xfrm - BOOLEAN
1668 Disable IPSEC encryption on this interface, whatever the policy
1670 igmpv2_unsolicited_report_interval - INTEGER
1671 The interval in milliseconds in which the next unsolicited
1672 IGMPv1 or IGMPv2 report retransmit will take place.
1674 Default: 10000 (10 seconds)
1676 igmpv3_unsolicited_report_interval - INTEGER
1677 The interval in milliseconds in which the next unsolicited
1678 IGMPv3 report retransmit will take place.
1680 Default: 1000 (1 seconds)
1682 ignore_routes_with_linkdown - BOOLEAN
1683 Ignore routes whose link is down when performing a FIB lookup.
1685 promote_secondaries - BOOLEAN
1686 When a primary IP address is removed from this interface
1687 promote a corresponding secondary IP address instead of
1688 removing all the corresponding secondary IP addresses.
1690 drop_unicast_in_l2_multicast - BOOLEAN
1691 Drop any unicast IP packets that are received in link-layer
1692 multicast (or broadcast) frames.
1694 This behavior (for multicast) is actually a SHOULD in RFC
1695 1122, but is disabled by default for compatibility reasons.
1699 drop_gratuitous_arp - BOOLEAN
1700 Drop all gratuitous ARP frames, for example if there's a known
1701 good ARP proxy on the network and such frames need not be used
1702 (or in the case of 802.11, must not be used to prevent attacks.)
1708 Allows you to write a number, which can be used as required.
1712 xfrm4_gc_thresh - INTEGER
1713 (Obsolete since linux-4.14)
1714 The threshold at which we will start garbage collecting for IPv4
1715 destination cache entries. At twice this value the system will
1716 refuse new allocations.
1718 igmp_link_local_mcast_reports - BOOLEAN
1719 Enable IGMP reports for link local multicast groups in the
1725 kuznet@ms2.inr.ac.ru
1732 delon.nicolas@wanadoo.fr
1737 /proc/sys/net/ipv6/* Variables
1738 ==============================
1740 IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
1741 apply to IPv6 [XXX?].
1743 bindv6only - BOOLEAN
1744 Default value for IPV6_V6ONLY socket option,
1745 which restricts use of the IPv6 socket to IPv6 communication
1748 - TRUE: disable IPv4-mapped address feature
1749 - FALSE: enable IPv4-mapped address feature
1751 Default: FALSE (as specified in RFC3493)
1753 flowlabel_consistency - BOOLEAN
1754 Protect the consistency (and unicity) of flow label.
1755 You have to disable it to use IPV6_FL_F_REFLECT flag on the
1763 auto_flowlabels - INTEGER
1764 Automatically generate flow labels based on a flow hash of the
1765 packet. This allows intermediate devices, such as routers, to
1766 identify packet flows for mechanisms like Equal Cost Multipath
1767 Routing (see RFC 6438).
1769 = ===========================================================
1770 0 automatic flow labels are completely disabled
1771 1 automatic flow labels are enabled by default, they can be
1772 disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
1774 2 automatic flow labels are allowed, they may be enabled on a
1775 per socket basis using the IPV6_AUTOFLOWLABEL socket option
1776 3 automatic flow labels are enabled and enforced, they cannot
1777 be disabled by the socket option
1778 = ===========================================================
1782 flowlabel_state_ranges - BOOLEAN
1783 Split the flow label number space into two ranges. 0-0x7FFFF is
1784 reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
1785 is reserved for stateless flow labels as described in RFC6437.
1792 flowlabel_reflect - INTEGER
1793 Control flow label reflection. Needed for Path MTU
1794 Discovery to work with Equal Cost Multipath Routing in anycast
1795 environments. See RFC 7690 and:
1796 https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
1800 - 1: enabled for established flows
1802 Note that this prevents automatic flowlabel changes, as done
1803 in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
1804 and "tcp: Change txhash on every SYN and RTO retransmit"
1806 - 2: enabled for TCP RESET packets (no active listener)
1807 If set, a RST packet sent in response to a SYN packet on a closed
1808 port will reflect the incoming flow label.
1810 - 4: enabled for ICMPv6 echo reply messages.
1814 fib_multipath_hash_policy - INTEGER
1815 Controls which hash policy to use for multipath routes.
1817 Default: 0 (Layer 3)
1821 - 0 - Layer 3 (source and destination addresses plus flow label)
1822 - 1 - Layer 4 (standard 5-tuple)
1823 - 2 - Layer 3 or inner Layer 3 if present
1824 - 3 - Custom multipath hash. Fields used for multipath hash calculation
1825 are determined by fib_multipath_hash_fields sysctl
1827 fib_multipath_hash_fields - UNSIGNED INTEGER
1828 When fib_multipath_hash_policy is set to 3 (custom multipath hash), the
1829 fields used for multipath hash calculation are determined by this
1832 This value is a bitmask which enables various fields for multipath hash
1835 Possible fields are:
1837 ====== ============================
1838 0x0001 Source IP address
1839 0x0002 Destination IP address
1843 0x0020 Destination port
1844 0x0040 Inner source IP address
1845 0x0080 Inner destination IP address
1846 0x0100 Inner IP protocol
1847 0x0200 Inner Flow Label
1848 0x0400 Inner source port
1849 0x0800 Inner destination port
1850 ====== ============================
1852 Default: 0x0007 (source IP, destination IP and IP protocol)
1854 anycast_src_echo_reply - BOOLEAN
1855 Controls the use of anycast addresses as source addresses for ICMPv6
1863 idgen_delay - INTEGER
1864 Controls the delay in seconds after which time to retry
1865 privacy stable address generation if a DAD conflict is
1868 Default: 1 (as specified in RFC7217)
1870 idgen_retries - INTEGER
1871 Controls the number of retries to generate a stable privacy
1872 address if a DAD conflict is detected.
1874 Default: 3 (as specified in RFC7217)
1877 Controls the MLD query robustness variable (see RFC3810 9.1).
1879 Default: 2 (as specified by RFC3810 9.1)
1881 Minimum: 1 (as specified by RFC6636 4.5)
1883 max_dst_opts_number - INTEGER
1884 Maximum number of non-padding TLVs allowed in a Destination
1885 options extension header. If this value is less than zero
1886 then unknown options are disallowed and the number of known
1887 TLVs allowed is the absolute value of this number.
1891 max_hbh_opts_number - INTEGER
1892 Maximum number of non-padding TLVs allowed in a Hop-by-Hop
1893 options extension header. If this value is less than zero
1894 then unknown options are disallowed and the number of known
1895 TLVs allowed is the absolute value of this number.
1899 max_dst_opts_length - INTEGER
1900 Maximum length allowed for a Destination options extension
1903 Default: INT_MAX (unlimited)
1905 max_hbh_length - INTEGER
1906 Maximum length allowed for a Hop-by-Hop options extension
1909 Default: INT_MAX (unlimited)
1911 skip_notify_on_dev_down - BOOLEAN
1912 Controls whether an RTM_DELROUTE message is generated for routes
1913 removed when a device is taken down or deleted. IPv4 does not
1914 generate this message; IPv6 does by default. Setting this sysctl
1915 to true skips the message, making IPv4 and IPv6 on par in relying
1916 on userspace caches to track link events and evict routes.
1918 Default: false (generate message)
1920 nexthop_compat_mode - BOOLEAN
1921 New nexthop API provides a means for managing nexthops independent of
1922 prefixes. Backwards compatibilty with old route format is enabled by
1923 default which means route dumps and notifications contain the new
1924 nexthop attribute but also the full, expanded nexthop definition.
1925 Further, updates or deletes of a nexthop configuration generate route
1926 notifications for each fib entry using the nexthop. Once a system
1927 understands the new API, this sysctl can be disabled to achieve full
1928 performance benefits of the new API by disabling the nexthop expansion
1929 and extraneous notifications.
1930 Default: true (backward compat mode)
1932 fib_notify_on_flag_change - INTEGER
1933 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
1934 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
1936 After installing a route to the kernel, user space receives an
1937 acknowledgment, which means the route was installed in the kernel,
1938 but not necessarily in hardware.
1939 It is also possible for a route already installed in hardware to change
1940 its action and therefore its flags. For example, a host route that is
1941 trapping packets can be "promoted" to perform decapsulation following
1942 the installation of an IPinIP/VXLAN tunnel.
1943 The notifications will indicate to user-space the state of the route.
1945 Default: 0 (Do not emit notifications.)
1949 - 0 - Do not emit notifications.
1950 - 1 - Emit notifications.
1951 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
1954 Define the IOAM id of this node. Uses only 24 bits out of 32 in total.
1961 ioam6_id_wide - LONG INTEGER
1962 Define the wide IOAM id of this node. Uses only 56 bits out of 64 in
1963 total. Can be different from ioam6_id.
1966 Max: 0xFFFFFFFFFFFFFF
1968 Default: 0xFFFFFFFFFFFFFF
1972 ip6frag_high_thresh - INTEGER
1973 Maximum memory used to reassemble IPv6 fragments. When
1974 ip6frag_high_thresh bytes of memory is allocated for this purpose,
1975 the fragment handler will toss packets until ip6frag_low_thresh
1978 ip6frag_low_thresh - INTEGER
1979 See ip6frag_high_thresh
1981 ip6frag_time - INTEGER
1982 Time in seconds to keep an IPv6 fragment in memory.
1985 Change the interface-specific default settings.
1987 These settings would be used during creating new interfaces.
1991 Change all the interface-specific settings.
1993 [XXX: Other special features than forwarding?]
1995 conf/all/disable_ipv6 - BOOLEAN
1996 Changing this value is same as changing ``conf/default/disable_ipv6``
1997 setting and also all per-interface ``disable_ipv6`` settings to the same
2000 Reading this value does not have any particular meaning. It does not say
2001 whether IPv6 support is enabled or disabled. Returned value can be 1
2002 also in the case when some interface has ``disable_ipv6`` set to 0 and
2003 has configured IPv6 addresses.
2005 conf/all/forwarding - BOOLEAN
2006 Enable global IPv6 forwarding between all interfaces.
2008 IPv4 and IPv6 work differently here; e.g. netfilter must be used
2009 to control which interfaces may forward packets and which not.
2011 This also sets all interfaces' Host/Router setting
2012 'forwarding' to the specified value. See below for details.
2014 This referred to as global forwarding.
2019 fwmark_reflect - BOOLEAN
2020 Controls the fwmark of kernel-generated IPv6 reply packets that are not
2021 associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
2022 If unset, these packets have a fwmark of zero. If set, they have the
2023 fwmark of the packet they are replying to.
2027 ``conf/interface/*``:
2028 Change special settings per interface.
2030 The functional behaviour for certain settings is different
2031 depending on whether local forwarding is enabled or not.
2034 Accept Router Advertisements; autoconfigure using them.
2036 It also determines whether or not to transmit Router
2037 Solicitations. If and only if the functional setting is to
2038 accept Router Advertisements, Router Solicitations will be
2041 Possible values are:
2043 == ===========================================================
2044 0 Do not accept Router Advertisements.
2045 1 Accept Router Advertisements if forwarding is disabled.
2046 2 Overrule forwarding behaviour. Accept Router Advertisements
2047 even if forwarding is enabled.
2048 == ===========================================================
2052 - enabled if local forwarding is disabled.
2053 - disabled if local forwarding is enabled.
2055 accept_ra_defrtr - BOOLEAN
2056 Learn default router in Router Advertisement.
2060 - enabled if accept_ra is enabled.
2061 - disabled if accept_ra is disabled.
2063 ra_defrtr_metric - UNSIGNED INTEGER
2064 Route metric for default route learned in Router Advertisement. This value
2065 will be assigned as metric for the default route learned via IPv6 Router
2066 Advertisement. Takes affect only if accept_ra_defrtr is enabled.
2071 Default: IP6_RT_PRIO_USER i.e. 1024.
2073 accept_ra_from_local - BOOLEAN
2074 Accept RA with source-address that is found on local machine
2075 if the RA is otherwise proper and able to be accepted.
2077 Default is to NOT accept these as it may be an un-intended
2082 - enabled if accept_ra_from_local is enabled
2083 on a specific interface.
2084 - disabled if accept_ra_from_local is disabled
2085 on a specific interface.
2087 accept_ra_min_hop_limit - INTEGER
2088 Minimum hop limit Information in Router Advertisement.
2090 Hop limit Information in Router Advertisement less than this
2091 variable shall be ignored.
2095 accept_ra_pinfo - BOOLEAN
2096 Learn Prefix Information in Router Advertisement.
2100 - enabled if accept_ra is enabled.
2101 - disabled if accept_ra is disabled.
2103 accept_ra_rt_info_min_plen - INTEGER
2104 Minimum prefix length of Route Information in RA.
2106 Route Information w/ prefix smaller than this variable shall
2111 * 0 if accept_ra_rtr_pref is enabled.
2112 * -1 if accept_ra_rtr_pref is disabled.
2114 accept_ra_rt_info_max_plen - INTEGER
2115 Maximum prefix length of Route Information in RA.
2117 Route Information w/ prefix larger than this variable shall
2122 * 0 if accept_ra_rtr_pref is enabled.
2123 * -1 if accept_ra_rtr_pref is disabled.
2125 accept_ra_rtr_pref - BOOLEAN
2126 Accept Router Preference in RA.
2130 - enabled if accept_ra is enabled.
2131 - disabled if accept_ra is disabled.
2133 accept_ra_mtu - BOOLEAN
2134 Apply the MTU value specified in RA option 5 (RFC4861). If
2135 disabled, the MTU specified in the RA will be ignored.
2139 - enabled if accept_ra is enabled.
2140 - disabled if accept_ra is disabled.
2142 accept_redirects - BOOLEAN
2147 - enabled if local forwarding is disabled.
2148 - disabled if local forwarding is enabled.
2150 accept_source_route - INTEGER
2151 Accept source routing (routing extension header).
2153 - >= 0: Accept only routing header type 2.
2154 - < 0: Do not accept routing header.
2159 Autoconfigure addresses using Prefix Information in Router
2164 - enabled if accept_ra_pinfo is enabled.
2165 - disabled if accept_ra_pinfo is disabled.
2167 dad_transmits - INTEGER
2168 The amount of Duplicate Address Detection probes to send.
2172 forwarding - INTEGER
2173 Configure interface-specific Host/Router behaviour.
2177 It is recommended to have the same setting on all
2178 interfaces; mixed router/host scenarios are rather uncommon.
2180 Possible values are:
2182 - 0 Forwarding disabled
2183 - 1 Forwarding enabled
2187 By default, Host behaviour is assumed. This means:
2189 1. IsRouter flag is not set in Neighbour Advertisements.
2190 2. If accept_ra is TRUE (default), transmit Router
2192 3. If accept_ra is TRUE (default), accept Router
2193 Advertisements (and do autoconfiguration).
2194 4. If accept_redirects is TRUE (default), accept Redirects.
2198 If local forwarding is enabled, Router behaviour is assumed.
2199 This means exactly the reverse from the above:
2201 1. IsRouter flag is set in Neighbour Advertisements.
2202 2. Router Solicitations are not sent unless accept_ra is 2.
2203 3. Router Advertisements are ignored unless accept_ra is 2.
2204 4. Redirects are ignored.
2206 Default: 0 (disabled) if global forwarding is disabled (default),
2207 otherwise 1 (enabled).
2210 Default Hop Limit to set.
2215 Default Maximum Transfer Unit
2217 Default: 1280 (IPv6 required minimum)
2219 ip_nonlocal_bind - BOOLEAN
2220 If set, allows processes to bind() to non-local IPv6 addresses,
2221 which can be quite useful - but may break some applications.
2225 router_probe_interval - INTEGER
2226 Minimum interval (in seconds) between Router Probing described
2231 router_solicitation_delay - INTEGER
2232 Number of seconds to wait after interface is brought up
2233 before sending Router Solicitations.
2237 router_solicitation_interval - INTEGER
2238 Number of seconds to wait between Router Solicitations.
2242 router_solicitations - INTEGER
2243 Number of Router Solicitations to send until assuming no
2244 routers are present.
2248 use_oif_addrs_only - BOOLEAN
2249 When enabled, the candidate source addresses for destinations
2250 routed via this interface are restricted to the set of addresses
2251 configured on this interface (vis. RFC 6724, section 4).
2255 use_tempaddr - INTEGER
2256 Preference for Privacy Extensions (RFC3041).
2258 * <= 0 : disable Privacy Extensions
2259 * == 1 : enable Privacy Extensions, but prefer public
2260 addresses over temporary addresses.
2261 * > 1 : enable Privacy Extensions and prefer temporary
2262 addresses over public addresses.
2266 * 0 (for most devices)
2267 * -1 (for point-to-point devices and loopback devices)
2269 temp_valid_lft - INTEGER
2270 valid lifetime (in seconds) for temporary addresses.
2272 Default: 172800 (2 days)
2274 temp_prefered_lft - INTEGER
2275 Preferred lifetime (in seconds) for temporary addresses.
2277 Default: 86400 (1 day)
2279 keep_addr_on_down - INTEGER
2280 Keep all IPv6 addresses on an interface down event. If set static
2281 global addresses with no expiration time are not flushed.
2284 * 0 : system default
2287 Default: 0 (addresses are removed)
2289 max_desync_factor - INTEGER
2290 Maximum value for DESYNC_FACTOR, which is a random value
2291 that ensures that clients don't synchronize with each
2292 other and generate new addresses at exactly the same time.
2293 value is in seconds.
2297 regen_max_retry - INTEGER
2298 Number of attempts before give up attempting to generate
2299 valid temporary addresses.
2303 max_addresses - INTEGER
2304 Maximum number of autoconfigured addresses per interface. Setting
2305 to zero disables the limitation. It is not recommended to set this
2306 value too large (or to zero) because it would be an easy way to
2307 crash the kernel by allowing too many addresses to be created.
2311 disable_ipv6 - BOOLEAN
2312 Disable IPv6 operation. If accept_dad is set to 2, this value
2313 will be dynamically set to TRUE if DAD fails for the link-local
2316 Default: FALSE (enable IPv6 operation)
2318 When this value is changed from 1 to 0 (IPv6 is being enabled),
2319 it will dynamically create a link-local address on the given
2320 interface and start Duplicate Address Detection, if necessary.
2322 When this value is changed from 0 to 1 (IPv6 is being disabled),
2323 it will dynamically delete all addresses and routes on the given
2324 interface. From now on it will not possible to add addresses/routes
2325 to the selected interface.
2327 accept_dad - INTEGER
2328 Whether to accept DAD (Duplicate Address Detection).
2330 == ==============================================================
2332 1 Enable DAD (default)
2333 2 Enable DAD, and disable IPv6 operation if MAC-based duplicate
2334 link-local address has been found.
2335 == ==============================================================
2337 DAD operation and mode on a given interface will be selected according
2338 to the maximum value of conf/{all,interface}/accept_dad.
2340 force_tllao - BOOLEAN
2341 Enable sending the target link-layer address option even when
2342 responding to a unicast neighbor solicitation.
2346 Quoting from RFC 2461, section 4.4, Target link-layer address:
2348 "The option MUST be included for multicast solicitations in order to
2349 avoid infinite Neighbor Solicitation "recursion" when the peer node
2350 does not have a cache entry to return a Neighbor Advertisements
2351 message. When responding to unicast solicitations, the option can be
2352 omitted since the sender of the solicitation has the correct link-
2353 layer address; otherwise it would not have be able to send the unicast
2354 solicitation in the first place. However, including the link-layer
2355 address in this case adds little overhead and eliminates a potential
2356 race condition where the sender deletes the cached link-layer address
2357 prior to receiving a response to a previous solicitation."
2359 ndisc_notify - BOOLEAN
2360 Define mode for notification of address and device changes.
2362 * 0 - (default): do nothing
2363 * 1 - Generate unsolicited neighbour advertisements when device is brought
2364 up or hardware address changes.
2366 ndisc_tclass - INTEGER
2367 The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
2368 Discovery (Router Solicitation, Router Advertisement, Neighbor
2369 Solicitation, Neighbor Advertisement, Redirect) messages.
2370 These 8 bits can be interpreted as 6 high order bits holding the DSCP
2371 value and 2 low order bits representing ECN (which you probably want
2376 ndisc_evict_nocarrier - BOOLEAN
2377 Clears the neighbor discovery table on NOCARRIER events. This option is
2378 important for wireless devices where the neighbor discovery cache should
2379 not be cleared when roaming between access points on the same network.
2380 In most cases this should remain as the default (1).
2382 - 1 - (default): Clear neighbor discover cache on NOCARRIER events.
2383 - 0 - Do not clear neighbor discovery cache on NOCARRIER events.
2385 mldv1_unsolicited_report_interval - INTEGER
2386 The interval in milliseconds in which the next unsolicited
2387 MLDv1 report retransmit will take place.
2389 Default: 10000 (10 seconds)
2391 mldv2_unsolicited_report_interval - INTEGER
2392 The interval in milliseconds in which the next unsolicited
2393 MLDv2 report retransmit will take place.
2395 Default: 1000 (1 second)
2397 force_mld_version - INTEGER
2398 * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
2399 * 1 - Enforce to use MLD version 1
2400 * 2 - Enforce to use MLD version 2
2402 suppress_frag_ndisc - INTEGER
2403 Control RFC 6980 (Security Implications of IPv6 Fragmentation
2404 with IPv6 Neighbor Discovery) behavior:
2406 * 1 - (default) discard fragmented neighbor discovery packets
2407 * 0 - allow fragmented neighbor discovery packets
2409 optimistic_dad - BOOLEAN
2410 Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
2412 * 0: disabled (default)
2415 Optimistic Duplicate Address Detection for the interface will be enabled
2416 if at least one of conf/{all,interface}/optimistic_dad is set to 1,
2417 it will be disabled otherwise.
2419 use_optimistic - BOOLEAN
2420 If enabled, do not classify optimistic addresses as deprecated during
2421 source address selection. Preferred addresses will still be chosen
2422 before optimistic addresses, subject to other ranking in the source
2423 address selection algorithm.
2425 * 0: disabled (default)
2428 This will be enabled if at least one of
2429 conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
2431 stable_secret - IPv6 address
2432 This IPv6 address will be used as a secret to generate IPv6
2433 addresses for link-local addresses and autoconfigured
2434 ones. All addresses generated after setting this secret will
2435 be stable privacy ones by default. This can be changed via the
2436 addrgenmode ip-link. conf/default/stable_secret is used as the
2437 secret for the namespace, the interface specific ones can
2438 overwrite that. Writes to conf/all/stable_secret are refused.
2440 It is recommended to generate this secret during installation
2441 of a system and keep it stable after that.
2443 By default the stable secret is unset.
2445 addr_gen_mode - INTEGER
2446 Defines how link-local and autoconf addresses are generated.
2448 = =================================================================
2449 0 generate address based on EUI64 (default)
2450 1 do no generate a link-local address, use EUI64 for addresses
2451 generated from autoconf
2452 2 generate stable privacy addresses, using the secret from
2453 stable_secret (RFC7217)
2454 3 generate stable privacy addresses, using a random secret if unset
2455 = =================================================================
2457 drop_unicast_in_l2_multicast - BOOLEAN
2458 Drop any unicast IPv6 packets that are received in link-layer
2459 multicast (or broadcast) frames.
2461 By default this is turned off.
2463 drop_unsolicited_na - BOOLEAN
2464 Drop all unsolicited neighbor advertisements, for example if there's
2465 a known good NA proxy on the network and such frames need not be used
2466 (or in the case of 802.11, must not be used to prevent attacks.)
2468 By default this is turned off.
2470 enhanced_dad - BOOLEAN
2471 Include a nonce option in the IPv6 neighbor solicitation messages used for
2472 duplicate address detection per RFC7527. A received DAD NS will only signal
2473 a duplicate address if the nonce is different. This avoids any false
2474 detection of duplicates due to loopback of the NS messages that we send.
2475 The nonce option will be sent on an interface unless both of
2476 conf/{all,interface}/enhanced_dad are set to FALSE.
2484 Limit the maximal rates for sending ICMPv6 messages.
2486 0 to disable any limiting,
2487 otherwise the minimal space between responses in milliseconds.
2491 ratemask - list of comma separated ranges
2492 For ICMPv6 message types matching the ranges in the ratemask, limit
2493 the sending of the message according to ratelimit parameter.
2495 The format used for both input and output is a comma separated
2496 list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
2497 129). Writing to the file will clear all previous ranges of ICMPv6
2498 message types and update the current list with the input.
2500 Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
2501 for numerical values of ICMPv6 message types, e.g. echo request is 128
2502 and echo reply is 129.
2504 Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)
2506 echo_ignore_all - BOOLEAN
2507 If set non-zero, then the kernel will ignore all ICMP ECHO
2508 requests sent to it over the IPv6 protocol.
2512 echo_ignore_multicast - BOOLEAN
2513 If set non-zero, then the kernel will ignore all ICMP ECHO
2514 requests sent to it over the IPv6 protocol via multicast.
2518 echo_ignore_anycast - BOOLEAN
2519 If set non-zero, then the kernel will ignore all ICMP ECHO
2520 requests sent to it over the IPv6 protocol destined to anycast address.
2524 xfrm6_gc_thresh - INTEGER
2525 (Obsolete since linux-4.14)
2526 The threshold at which we will start garbage collecting for IPv6
2527 destination cache entries. At twice this value the system will
2528 refuse new allocations.
2532 Pekka Savola <pekkas@netcore.fi>
2533 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
2536 /proc/sys/net/bridge/* Variables:
2537 =================================
2539 bridge-nf-call-arptables - BOOLEAN
2540 - 1 : pass bridged ARP traffic to arptables' FORWARD chain.
2545 bridge-nf-call-iptables - BOOLEAN
2546 - 1 : pass bridged IPv4 traffic to iptables' chains.
2551 bridge-nf-call-ip6tables - BOOLEAN
2552 - 1 : pass bridged IPv6 traffic to ip6tables' chains.
2557 bridge-nf-filter-vlan-tagged - BOOLEAN
2558 - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
2563 bridge-nf-filter-pppoe-tagged - BOOLEAN
2564 - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
2569 bridge-nf-pass-vlan-input-dev - BOOLEAN
2570 - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
2571 interface on the bridge and set the netfilter input device to the
2572 vlan. This allows use of e.g. "iptables -i br0.1" and makes the
2573 REDIRECT target work with vlan-on-top-of-bridge interfaces. When no
2574 matching vlan interface is found, or this switch is off, the input
2575 device is set to the bridge interface.
2577 - 0: disable bridge netfilter vlan interface lookup.
2581 ``proc/sys/net/sctp/*`` Variables:
2582 ==================================
2584 addip_enable - BOOLEAN
2585 Enable or disable extension of Dynamic Address Reconfiguration
2586 (ADD-IP) functionality specified in RFC5061. This extension provides
2587 the ability to dynamically add and remove new addresses for the SCTP
2590 1: Enable extension.
2592 0: Disable extension.
2597 Enable or disable pf (pf is short for potentially failed) state. A value
2598 of pf_retrans > path_max_retrans also disables pf state. That is, one of
2599 both pf_enable and pf_retrans > path_max_retrans can disable pf state.
2600 Since pf_retrans and path_max_retrans can be changed by userspace
2601 application, sometimes user expects to disable pf state by the value of
2602 pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
2603 or path_max_retrans is changed by the user application, this pf state is
2604 enabled. As such, it is necessary to add this to dynamically enable
2605 and disable pf state. See:
2606 https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
2616 Unset or enable/disable pf (pf is short for potentially failed) state
2617 exposure. Applications can control the exposure of the PF path state
2618 in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
2619 sockopt. When it's unset, no SCTP_PEER_ADDR_CHANGE event with
2620 SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
2621 can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it's enabled,
2622 a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
2623 SCTP_PF state and a SCTP_PF-state transport info can be got via
2624 SCTP_GET_PEER_ADDR_INFO sockopt; When it's diabled, no
2625 SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
2626 trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
2629 0: Unset pf state exposure, Compatible with old applications.
2631 1: Disable pf state exposure.
2633 2: Enable pf state exposure.
2637 addip_noauth_enable - BOOLEAN
2638 Dynamic Address Reconfiguration (ADD-IP) requires the use of
2639 authentication to protect the operations of adding or removing new
2640 addresses. This requirement is mandated so that unauthorized hosts
2641 would not be able to hijack associations. However, older
2642 implementations may not have implemented this requirement while
2643 allowing the ADD-IP extension. For reasons of interoperability,
2644 we provide this variable to control the enforcement of the
2645 authentication requirement.
2647 == ===============================================================
2648 1 Allow ADD-IP extension to be used without authentication. This
2649 should only be set in a closed environment for interoperability
2650 with older implementations.
2652 0 Enforce the authentication requirement
2653 == ===============================================================
2657 auth_enable - BOOLEAN
2658 Enable or disable Authenticated Chunks extension. This extension
2659 provides the ability to send and receive authenticated chunks and is
2660 required for secure operation of Dynamic Address Reconfiguration
2663 - 1: Enable this extension.
2664 - 0: Disable this extension.
2668 prsctp_enable - BOOLEAN
2669 Enable or disable the Partial Reliability extension (RFC3758) which
2670 is used to notify peers that a given DATA should no longer be expected.
2672 - 1: Enable extension
2678 The limit of the number of new packets that can be initially sent. It
2679 controls how bursty the generated traffic can be.
2683 association_max_retrans - INTEGER
2684 Set the maximum number for retransmissions that an association can
2685 attempt deciding that the remote end is unreachable. If this value
2686 is exceeded, the association is terminated.
2690 max_init_retransmits - INTEGER
2691 The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
2692 that an association will attempt before declaring the destination
2693 unreachable and terminating.
2697 path_max_retrans - INTEGER
2698 The maximum number of retransmissions that will be attempted on a given
2699 path. Once this threshold is exceeded, the path is considered
2700 unreachable, and new traffic will use a different path when the
2701 association is multihomed.
2705 pf_retrans - INTEGER
2706 The number of retransmissions that will be attempted on a given path
2707 before traffic is redirected to an alternate transport (should one
2708 exist). Note this is distinct from path_max_retrans, as a path that
2709 passes the pf_retrans threshold can still be used. Its only
2710 deprioritized when a transmission path is selected by the stack. This
2711 setting is primarily used to enable fast failover mechanisms without
2712 having to reduce path_max_retrans to a very low value. See:
2713 http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
2714 for details. Note also that a value of pf_retrans > path_max_retrans
2715 disables this feature. Since both pf_retrans and path_max_retrans can
2716 be changed by userspace application, a variable pf_enable is used to
2721 ps_retrans - INTEGER
2722 Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
2723 from section-5 "Primary Path Switchover" in rfc7829. The primary path
2724 will be changed to another active path when the path error counter on
2725 the old primary path exceeds PSMR, so that "the SCTP sender is allowed
2726 to continue data transmission on a new working path even when the old
2727 primary destination address becomes active again". Note this feature
2728 is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
2729 and its value can't be less than 'pf_retrans' when changing by sysctl.
2733 rto_initial - INTEGER
2734 The initial round trip timeout value in milliseconds that will be used
2735 in calculating round trip times. This is the initial time interval
2736 for retransmissions.
2741 The maximum value (in milliseconds) of the round trip timeout. This
2742 is the largest time interval that can elapse between retransmissions.
2747 The minimum value (in milliseconds) of the round trip timeout. This
2748 is the smallest time interval the can elapse between retransmissions.
2752 hb_interval - INTEGER
2753 The interval (in milliseconds) between HEARTBEAT chunks. These chunks
2754 are sent at the specified interval on idle paths to probe the state of
2755 a given path between 2 associations.
2759 sack_timeout - INTEGER
2760 The amount of time (in milliseconds) that the implementation will wait
2765 valid_cookie_life - INTEGER
2766 The default lifetime of the SCTP cookie (in milliseconds). The cookie
2767 is used during association establishment.
2771 cookie_preserve_enable - BOOLEAN
2772 Enable or disable the ability to extend the lifetime of the SCTP cookie
2773 that is used during the establishment phase of SCTP association
2775 - 1: Enable cookie lifetime extension.
2780 cookie_hmac_alg - STRING
2781 Select the hmac algorithm used when generating the cookie value sent by
2782 a listening sctp socket to a connecting client in the INIT-ACK chunk.
2789 Ability to assign md5 or sha1 as the selected alg is predicated on the
2790 configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
2791 CONFIG_CRYPTO_SHA1).
2793 Default: Dependent on configuration. MD5 if available, else SHA1 if
2794 available, else none.
2796 rcvbuf_policy - INTEGER
2797 Determines if the receive buffer is attributed to the socket or to
2798 association. SCTP supports the capability to create multiple
2799 associations on a single socket. When using this capability, it is
2800 possible that a single stalled association that's buffering a lot
2801 of data may block other associations from delivering their data by
2802 consuming all of the receive buffer space. To work around this,
2803 the rcvbuf_policy could be set to attribute the receiver buffer space
2804 to each association instead of the socket. This prevents the described
2807 - 1: rcvbuf space is per association
2808 - 0: rcvbuf space is per socket
2812 sndbuf_policy - INTEGER
2813 Similar to rcvbuf_policy above, this applies to send buffer space.
2815 - 1: Send buffer is tracked per association
2816 - 0: Send buffer is tracked per socket.
2820 sctp_mem - vector of 3 INTEGERs: min, pressure, max
2821 Number of pages allowed for queueing by all SCTP sockets.
2823 min: Below this number of pages SCTP is not bothered about its
2824 memory appetite. When amount of memory allocated by SCTP exceeds
2825 this number, SCTP starts to moderate memory usage.
2827 pressure: This value was introduced to follow format of tcp_mem.
2829 max: Number of pages allowed for queueing by all SCTP sockets.
2831 Default is calculated at boot time from amount of available memory.
2833 sctp_rmem - vector of 3 INTEGERs: min, default, max
2834 Only the first value ("min") is used, "default" and "max" are
2837 min: Minimal size of receive buffer used by SCTP socket.
2838 It is guaranteed to each SCTP socket (but not association) even
2839 under moderate memory pressure.
2843 sctp_wmem - vector of 3 INTEGERs: min, default, max
2844 Currently this tunable has no effect.
2846 addr_scope_policy - INTEGER
2847 Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
2849 - 0 - Disable IPv4 address scoping
2850 - 1 - Enable IPv4 address scoping
2851 - 2 - Follow draft but allow IPv4 private addresses
2852 - 3 - Follow draft but allow IPv4 link local addresses
2857 The listening port for the local UDP tunneling sock. Normally it's
2858 using the IANA-assigned UDP port number 9899 (sctp-tunneling).
2860 This UDP sock is used for processing the incoming UDP-encapsulated
2861 SCTP packets (from RFC6951), and shared by all applications in the
2862 same net namespace. This UDP sock will be closed when the value is
2865 The value will also be used to set the src port of the UDP header
2866 for the outgoing UDP-encapsulated SCTP packets. For the dest port,
2867 please refer to 'encap_port' below.
2871 encap_port - INTEGER
2872 The default remote UDP encapsulation port.
2874 This value is used to set the dest port of the UDP header for the
2875 outgoing UDP-encapsulated SCTP packets by default. Users can also
2876 change the value for each sock/asoc/transport by using setsockopt.
2877 For further information, please refer to RFC6951.
2879 Note that when connecting to a remote server, the client should set
2880 this to the port that the UDP tunneling sock on the peer server is
2881 listening to and the local UDP tunneling sock on the client also
2882 must be started. On the server, it would get the encap_port from
2883 the incoming packet's source port.
2887 plpmtud_probe_interval - INTEGER
2888 The time interval (in milliseconds) for the PLPMTUD probe timer,
2889 which is configured to expire after this period to receive an
2890 acknowledgment to a probe packet. This is also the time interval
2891 between the probes for the current pmtu when the probe search
2894 PLPMTUD will be disabled when 0 is set, and other values for it
2900 ``/proc/sys/net/core/*``
2901 ========================
2903 Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
2906 ``/proc/sys/net/unix/*``
2907 ========================
2909 max_dgram_qlen - INTEGER
2910 The maximum length of dgram socket receive queue