1 /proc/sys/net/ipv4/* Variables:
7 Forward Packets between interfaces.
9 This variable is special, its change resets all configuration
10 parameters to their default state (RFC1122 for hosts, RFC1812
13 ip_default_ttl - INTEGER
16 ip_no_pmtu_disc - BOOLEAN
17 Disable Path MTU Discovery.
22 ipfrag_high_thresh - INTEGER
23 Maximum memory used to reassemble IP fragments. When
24 ipfrag_high_thresh bytes of memory is allocated for this purpose,
25 the fragment handler will toss packets until ipfrag_low_thresh
28 ipfrag_low_thresh - INTEGER
29 See ipfrag_high_thresh
32 Time in seconds to keep an IP fragment in memory.
36 inet_peer_threshold - INTEGER
37 The approximate size of the storage. Starting from this threshold
38 entries will be thrown aggressively. This threshold also determines
39 entries' time-to-live and time intervals between garbage collection
40 passes. More entries, less time-to-live, less GC interval.
42 inet_peer_minttl - INTEGER
43 Minimum time-to-live of entries. Should be enough to cover fragment
44 time-to-live on the reassembling side. This minimum time-to-live is
45 guaranteed if the pool size is less than inet_peer_threshold.
46 Measured in jiffies(1).
48 inet_peer_maxttl - INTEGER
49 Maximum time-to-live of entries. Unused entries will expire after
50 this period of time if there is no memory pressure on the pool (i.e.
51 when the number of entries in the pool is very small).
52 Measured in jiffies(1).
54 inet_peer_gc_mintime - INTEGER
55 Minimum interval between garbage collection passes. This interval is
56 in effect under high memory pressure on the pool.
57 Measured in jiffies(1).
59 inet_peer_gc_maxtime - INTEGER
60 Minimum interval between garbage collection passes. This interval is
61 in effect under low (or absent) memory pressure on the pool.
62 Measured in jiffies(1).
66 tcp_syn_retries - INTEGER
67 Number of times initial SYNs for an active TCP connection attempt
68 will be retransmitted. Should not be higher than 255. Default value
69 is 5, which corresponds to ~180seconds.
71 tcp_synack_retries - INTEGER
72 Number of times SYNACKs for a passive TCP connection attempt will
73 be retransmitted. Should not be higher than 255. Default value
74 is 5, which corresponds to ~180seconds.
76 tcp_keepalive_time - INTEGER
77 How often TCP sends out keepalive messages when keepalive is enabled.
80 tcp_keepalive_probes - INTEGER
81 How many keepalive probes TCP sends out, until it decides that the
82 connection is broken. Default value: 9.
84 tcp_keepalive_intvl - INTEGER
85 How frequently the probes are send out. Multiplied by
86 tcp_keepalive_probes it is time to kill not responding connection,
87 after probes started. Default value: 75sec i.e. connection
88 will be aborted after ~11 minutes of retries.
90 tcp_retries1 - INTEGER
91 How many times to retry before deciding that something is wrong
92 and it is necessary to report this suspection to network layer.
93 Minimal RFC value is 3, it is default, which corresponds
94 to ~3sec-8min depending on RTO.
96 tcp_retries2 - INTEGER
97 How may times to retry before killing alive TCP connection.
98 RFC1122 says that the limit should be longer than 100 sec.
99 It is too small number. Default value 15 corresponds to ~13-30min
102 tcp_orphan_retries - INTEGER
103 How may times to retry before killing TCP connection, closed
104 by our side. Default value 7 corresponds to ~50sec-16min
105 depending on RTO. If you machine is loaded WEB server,
106 you should think about lowering this value, such sockets
107 may consume significant resources. Cf. tcp_max_orphans.
109 tcp_fin_timeout - INTEGER
110 Time to hold socket in state FIN-WAIT-2, if it was closed
111 by our side. Peer can be broken and never close its side,
112 or even died unexpectedly. Default value is 60sec.
113 Usual value used in 2.2 was 180 seconds, you may restore
114 it, but remember that if your machine is even underloaded WEB server,
115 you risk to overflow memory with kilotons of dead sockets,
116 FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1,
117 because they eat maximum 1.5K of memory, but they tend
118 to live longer. Cf. tcp_max_orphans.
120 tcp_max_tw_buckets - INTEGER
121 Maximal number of timewait sockets held by system simultaneously.
122 If this number is exceeded time-wait socket is immediately destroyed
123 and warning is printed. This limit exists only to prevent
124 simple DoS attacks, you _must_ not lower the limit artificially,
125 but rather increase it (probably, after increasing installed memory),
126 if network conditions require more than default value.
128 tcp_tw_recycle - BOOLEAN
129 Enable fast recycling TIME-WAIT sockets. Default value is 0.
130 It should not be changed without advice/request of technical
133 tcp_tw_reuse - BOOLEAN
134 Allow to reuse TIME-WAIT sockets for new connections when it is
135 safe from protocol viewpoint. Default value is 0.
136 It should not be changed without advice/request of technical
139 tcp_max_orphans - INTEGER
140 Maximal number of TCP sockets not attached to any user file handle,
141 held by system. If this number is exceeded orphaned connections are
142 reset immediately and warning is printed. This limit exists
143 only to prevent simple DoS attacks, you _must_ not rely on this
144 or lower the limit artificially, but rather increase it
145 (probably, after increasing installed memory),
146 if network conditions require more than default value,
147 and tune network services to linger and kill such states
148 more aggressively. Let me to remind again: each orphan eats
149 up to ~64K of unswappable memory.
151 tcp_abort_on_overflow - BOOLEAN
152 If listening service is too slow to accept new connections,
153 reset them. Default state is FALSE. It means that if overflow
154 occurred due to a burst, connection will recover. Enable this
155 option _only_ if you are really sure that listening daemon
156 cannot be tuned to accept connections faster. Enabling this
157 option can harm clients of your server.
159 tcp_syncookies - BOOLEAN
160 Only valid when the kernel was compiled with CONFIG_SYNCOOKIES
161 Send out syncookies when the syn backlog queue of a socket
162 overflows. This is to prevent against the common 'syn flood attack'
165 Note, that syncookies is fallback facility.
166 It MUST NOT be used to help highly loaded servers to stand
167 against legal connection rate. If you see synflood warnings
168 in your logs, but investigation shows that they occur
169 because of overload with legal connections, you should tune
170 another parameters until this warning disappear.
171 See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
173 syncookies seriously violate TCP protocol, do not allow
174 to use TCP extensions, can result in serious degradation
175 of some services (f.e. SMTP relaying), visible not by you,
176 but your clients and relays, contacting you. While you see
177 synflood warnings in logs not being really flooded, your server
178 is seriously misconfigured.
181 Use the Host requirements interpretation of the TCP urg pointer field.
182 Most hosts use the older BSD interpretation, so if you turn this on
183 Linux might not communicate correctly with them.
186 tcp_max_syn_backlog - INTEGER
187 Maximal number of remembered connection requests, which are
188 still did not receive an acknowledgement from connecting client.
189 Default value is 1024 for systems with more than 128Mb of memory,
190 and 128 for low memory machines. If server suffers of overload,
191 try to increase this number.
193 tcp_window_scaling - BOOLEAN
194 Enable window scaling as defined in RFC1323.
196 tcp_timestamps - BOOLEAN
197 Enable timestamps as defined in RFC1323.
200 Enable select acknowledgments (SACKS).
203 Enable FACK congestion avoidance and fast restransmission.
204 The value is not used, if tcp_sack is not enabled.
207 Allows TCP to send "duplicate" SACKs.
210 Enable Explicit Congestion Notification in TCP.
212 tcp_reordering - INTEGER
213 Maximal reordering of packets in a TCP stream.
216 tcp_retrans_collapse - BOOLEAN
217 Bug-to-bug compatibility with some broken printers.
218 On retransmit try to send bigger packets to work around bugs in
221 tcp_wmem - vector of 3 INTEGERs: min, default, max
222 min: Amount of memory reserved for send buffers for TCP socket.
223 Each TCP socket has rights to use it due to fact of its birth.
226 default: Amount of memory allowed for send buffers for TCP socket
227 by default. This value overrides net.core.wmem_default used
228 by other protocols, it is usually lower than net.core.wmem_default.
231 max: Maximal amount of memory allowed for automatically selected
232 send buffers for TCP socket. This value does not override
233 net.core.wmem_max, "static" selection via SO_SNDBUF does not use this.
236 tcp_rmem - vector of 3 INTEGERs: min, default, max
237 min: Minimal size of receive buffer used by TCP sockets.
238 It is guaranteed to each TCP socket, even under moderate memory
242 default: default size of receive buffer used by TCP sockets.
243 This value overrides net.core.rmem_default used by other protocols.
244 Default: 87380 bytes. This value results in window of 65535 with
245 default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
246 less for default tcp_app_win. See below about these variables.
248 max: maximal size of receive buffer allowed for automatically
249 selected receiver buffers for TCP socket. This value does not override
250 net.core.rmem_max, "static" selection via SO_RCVBUF does not use this.
251 Default: 87380*2 bytes.
253 tcp_mem - vector of 3 INTEGERs: min, pressure, max
254 low: below this number of pages TCP is not bothered about its
257 pressure: when amount of memory allocated by TCP exceeds this number
258 of pages, TCP moderates its memory consumption and enters memory
259 pressure mode, which is exited when memory consumtion falls
262 high: number of pages allowed for queueing by all TCP sockets.
264 Defaults are calculated at boot time from amount of available
267 tcp_app_win - INTEGER
268 Reserve max(window/2^tcp_app_win, mss) of window for application
269 buffer. Value 0 is special, it means that nothing is reserved.
272 tcp_adv_win_scale - INTEGER
273 Count buffering overhead as bytes/2^tcp_adv_win_scale
274 (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
278 tcp_rfc1337 - BOOLEAN
279 If set, the TCP stack behaves conforming to RFC1337. If unset,
280 we are not conforming to RFC, but prevent TCP TIME_WAIT
284 tcp_low_latency - BOOLEAN
285 If set, the TCP stack makes decisions that prefer lower
286 latency as opposed to higher throughput. By default, this
287 option is not set meaning that higher throughput is preferred.
288 An example of an application where this default should be
289 changed would be a Beowulf compute cluster.
292 ip_local_port_range - 2 INTEGERS
293 Defines the local port range that is used by TCP and UDP to
294 choose the local port. The first number is the first, the
295 second the last local port number. Default value depends on
296 amount of memory available on the system:
298 < 128Mb 1024-4999 or even less.
299 This number defines number of active connections, which this
300 system can issue simultaneously to systems not supporting
301 TCP extensions (timestamps). With tcp_tw_recycle enabled
302 (i.e. by default) range 1024-4999 is enough to issue up to
303 2000 connections per second to systems supporting timestamps.
305 ip_nonlocal_bind - BOOLEAN
306 If set, allows processes to bind() to non-local IP adresses,
307 which can be quite useful - but may break some applications.
311 If set non-zero, enables support for dynamic addresses.
312 If set to a non-zero value larger than 1, a kernel log
313 message will be printed when dynamic address rewriting
317 icmp_echo_ignore_all - BOOLEAN
318 icmp_echo_ignore_broadcasts - BOOLEAN
319 If either is set to true, then the kernel will ignore either all
320 ICMP ECHO requests sent to it or just those to broadcast/multicast
321 addresses, respectively.
323 icmp_ratelimit - INTEGER
324 Limit the maximal rates for sending ICMP packets whose type matches
325 icmp_ratemask (see below) to specific targets.
326 0 to disable any limiting, otherwise the maximal rate in jiffies(1)
329 icmp_ratemask - INTEGER
330 Mask made of ICMP types for which rates are being limited.
331 Significant bits: IHGFEDCBA9876543210
332 Default mask: 0000001100000011000 (6168)
334 Bit definitions (see include/linux/icmp.h):
336 3 Destination Unreachable *
341 C Parameter Problem *
346 H Address Mask Request
349 * These are rate limited by default (see default mask above)
351 icmp_ignore_bogus_error_responses - BOOLEAN
352 Some routers violate RFC1122 by sending bogus responses to broadcast
353 frames. Such violations are normally logged via a kernel warning.
354 If this is set to TRUE, the kernel will not give such warnings, which
355 will avoid log file clutter.
358 igmp_max_memberships - INTEGER
359 Change the maximum number of multicast groups we can subscribe to.
362 conf/interface/* changes special settings per interface (where "interface" is
363 the name of your network interface)
364 conf/all/* is special, changes the settings for all interfaces
367 log_martians - BOOLEAN
368 Log packets with impossible addresses to kernel log.
369 log_martians for the interface will be enabled if at least one of
370 conf/{all,interface}/log_martians is set to TRUE,
371 it will be disabled otherwise
373 accept_redirects - BOOLEAN
374 Accept ICMP redirect messages.
375 accept_redirects for the interface will be enabled if:
376 - both conf/{all,interface}/accept_redirects are TRUE in the case forwarding
377 for the interface is enabled
379 - at least one of conf/{all,interface}/accept_redirects is TRUE in the case
380 forwarding for the interface is disabled
381 accept_redirects for the interface will be disabled otherwise
386 Enable IP forwarding on this interface.
388 mc_forwarding - BOOLEAN
389 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
390 and a multicast routing daemon is required.
391 conf/all/mc_forwarding must also be set to TRUE to enable multicast routing
395 Integer value used to differentiate the devices by the medium they
396 are attached to. Two devices can have different id values when
397 the broadcast packets are received only on one of them.
398 The default value 0 means that the device is the only interface
399 to its medium, value of -1 means that medium is not known.
401 Currently, it is used to change the proxy_arp behavior:
402 the proxy_arp feature is enabled for packets forwarded between
403 two devices attached to different media.
407 proxy_arp for the interface will be enabled if at least one of
408 conf/{all,interface}/proxy_arp is set to TRUE,
409 it will be disabled otherwise
411 shared_media - BOOLEAN
412 Send(router) or accept(host) RFC1620 shared media redirects.
413 Overrides ip_secure_redirects.
414 shared_media for the interface will be enabled if at least one of
415 conf/{all,interface}/shared_media is set to TRUE,
416 it will be disabled otherwise
419 secure_redirects - BOOLEAN
420 Accept ICMP redirect messages only for gateways,
421 listed in default gateway list.
422 secure_redirects for the interface will be enabled if at least one of
423 conf/{all,interface}/secure_redirects is set to TRUE,
424 it will be disabled otherwise
427 send_redirects - BOOLEAN
428 Send redirects, if router.
429 send_redirects for the interface will be enabled if at least one of
430 conf/{all,interface}/send_redirects is set to TRUE,
431 it will be disabled otherwise
434 bootp_relay - BOOLEAN
435 Accept packets with source address 0.b.c.d destined
436 not to this host as local ones. It is supposed, that
437 BOOTP relay daemon will catch and forward such packets.
438 conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
443 accept_source_route - BOOLEAN
444 Accept packets with SRR option.
445 conf/all/accept_source_route must also be set to TRUE to accept packets
446 with SRR option on the interface
447 default TRUE (router)
451 1 - do source validation by reversed path, as specified in RFC1812
452 Recommended option for single homed hosts and stub network
453 routers. Could cause troubles for complicated (not loop free)
454 networks running a slow unreliable protocol (sort of RIP),
455 or using static routes.
457 0 - No source validation.
459 conf/all/rp_filter must also be set to TRUE to do source validation
462 Default value is 0. Note that some distributions enable it
466 1 - Allows you to have multiple network interfaces on the same
467 subnet, and have the ARPs for each interface be answered
468 based on whether or not the kernel would route a packet from
469 the ARP'd IP out that interface (therefore you must use source
470 based routing for this to work). In other words it allows control
471 of which cards (usually 1) will respond to an arp request.
473 0 - (default) The kernel can respond to arp requests with addresses
474 from other interfaces. This may seem wrong but it usually makes
475 sense, because it increases the chance of successful communication.
476 IP addresses are owned by the complete host on Linux, not by
477 particular interfaces. Only for more complex setups like load-
478 balancing, does this behaviour cause problems.
480 arp_filter for the interface will be enabled if at least one of
481 conf/{all,interface}/arp_filter is set to TRUE,
482 it will be disabled otherwise
485 Allows you to write a number, which can be used as required.
488 (1) Jiffie: internal timeunit for the kernel. On the i386 1/100s, on the
489 Alpha 1/1024s. See the HZ define in /usr/include/asm/param.h for the exact
490 value on your system.
499 delon.nicolas@wanadoo.fr
504 /proc/sys/net/ipv6/* Variables:
506 IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
507 apply to IPv6 [XXX?].
510 Default value for IPV6_V6ONLY socket option,
511 which restricts use of the IPv6 socket to IPv6 communication
513 TRUE: disable IPv4-mapped address feature
514 FALSE: enable IPv4-mapped address feature
516 Default: FALSE (as specified in RFC2553bis)
519 Change the interface-specific default settings.
523 Change all the interface-specific settings.
525 [XXX: Other special features than forwarding?]
527 conf/all/forwarding - BOOLEAN
528 Enable global IPv6 forwarding between all interfaces.
530 IPv4 and IPv6 work differently here; e.g. netfilter must be used
531 to control which interfaces may forward packets and which not.
533 This also sets all interfaces' Host/Router setting
534 'forwarding' to the specified value. See below for details.
536 This referred to as global forwarding.
539 Change special settings per interface.
541 The functional behaviour for certain settings is different
542 depending on whether local forwarding is enabled or not.
545 Accept Router Advertisements; autoconfigure using them.
547 Functional default: enabled if local forwarding is disabled.
548 disabled if local forwarding is enabled.
550 accept_redirects - BOOLEAN
553 Functional default: enabled if local forwarding is disabled.
554 disabled if local forwarding is enabled.
557 Configure link-local addresses using L2 hardware addresses.
561 dad_transmits - INTEGER
562 The amount of Duplicate Address Detection probes to send.
566 Configure interface-specific Host/Router behaviour.
568 Note: It is recommended to have the same setting on all
569 interfaces; mixed router/host scenarios are rather uncommon.
573 By default, Host behaviour is assumed. This means:
575 1. IsRouter flag is not set in Neighbour Advertisements.
576 2. Router Solicitations are being sent when necessary.
577 3. If accept_ra is TRUE (default), accept Router
578 Advertisements (and do autoconfiguration).
579 4. If accept_redirects is TRUE (default), accept Redirects.
583 If local forwarding is enabled, Router behaviour is assumed.
584 This means exactly the reverse from the above:
586 1. IsRouter flag is set in Neighbour Advertisements.
587 2. Router Solicitations are not sent.
588 3. Router Advertisements are ignored.
589 4. Redirects are ignored.
591 Default: FALSE if global forwarding is disabled (default),
595 Default Hop Limit to set.
599 Default Maximum Transfer Unit
600 Default: 1280 (IPv6 required minimum)
602 router_solicitation_delay - INTEGER
603 Number of seconds to wait after interface is brought up
604 before sending Router Solicitations.
607 router_solicitation_interval - INTEGER
608 Number of seconds to wait between Router Solicitations.
611 router_solicitations - INTEGER
612 Number of Router Solicitations to send until assuming no
618 Limit the maximal rates for sending ICMPv6 packets.
619 0 to disable any limiting, otherwise the maximal rate in jiffies(1)
623 Pekka Savola <pekkas@netcore.fi>
624 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
626 $Id: ip-sysctl.txt,v 1.1.1.1 2005/04/11 02:49:56 jack Exp $