ip-sysctl.txt

/proc/sys/net/ipv4/* Variables:

ip_forward - BOOLEAN 0 - disabled (default) not 0 - enabled

Forward Packets between interfaces.

This variable is special, its change resets all configuration parameters to their default state (RFC1122 for hosts, RFC1812 for routers)

ip_default_ttl - INTEGER default 64

ip_no_pmtu_disc - BOOLEAN Disable Path MTU Discovery. default FALSE

IP Fragmentation:

ipfrag_high_thresh - INTEGER Maximum memory used to reassemble IP fragments. When ipfrag_high_thresh bytes of memory is allocated for this purpose, the fragment handler will toss packets until ipfrag_low_thresh is reached.

ipfrag_low_thresh - INTEGER See ipfrag_high_thresh

ipfrag_time - INTEGER Time in seconds to keep an IP fragment in memory.

ipfrag_secret_interval - INTEGER Regeneration interval (in seconds) of the hash secret (or lifetime for the hash secret) for IP fragments. Default: 600

INET peer storage:

inet_peer_threshold - INTEGER The approximate size of the storage. Starting from this threshold entries will be thrown aggressively. This threshold also determines entries' time-to-live and time intervals between garbage collection passes. More entries, less time-to-live, less GC interval.

inet_peer_minttl - INTEGER Minimum time-to-live of entries. Should be enough to cover fragment time-to-live on the reassembling side. This minimum time-to-live is guaranteed if the pool size is less than inet_peer_threshold. Measured in jiffies(1).

inet_peer_maxttl - INTEGER Maximum time-to-live of entries. Unused entries will expire after this period of time if there is no memory pressure on the pool (i.e. when the number of entries in the pool is very small). Measured in jiffies(1).

inet_peer_gc_mintime - INTEGER Minimum interval between garbage collection passes. This interval is in effect under high memory pressure on the pool. Measured in jiffies(1).

inet_peer_gc_maxtime - INTEGER Minimum interval between garbage collection passes. This interval is in effect under low (or absent) memory pressure on the pool. Measured in jiffies(1).

TCP variables:

tcp_syn_retries - INTEGER Number of times initial SYNs for an active TCP connection attempt will be retransmitted. Should not be higher than 255. Default value is 5, which corresponds to ~180seconds.

tcp_synack_retries - INTEGER Number of times SYNACKs for a passive TCP connection attempt will be retransmitted. Should not be higher than 255. Default value is 5, which corresponds to ~180seconds.

tcp_keepalive_time - INTEGER How often TCP sends out keepalive messages when keepalive is enabled. Default: 2hours.

tcp_keepalive_probes - INTEGER How many keepalive probes TCP sends out, until it decides that the connection is broken. Default value: 9.

tcp_keepalive_intvl - INTEGER How frequently the probes are send out. Multiplied by tcp_keepalive_probes it is time to kill not responding connection, after probes started. Default value: 75sec i.e. connection will be aborted after ~11 minutes of retries.

tcp_retries1 - INTEGER How many times to retry before deciding that something is wrong and it is necessary to report this suspicion to network layer. Minimal RFC value is 3, it is default, which corresponds to ~3sec-8min depending on RTO.

tcp_retries2 - INTEGER How may times to retry before killing alive TCP connection. RFC1122 says that the limit should be longer than 100 sec. It is too small number. Default value 15 corresponds to ~13-30min depending on RTO.

tcp_orphan_retries - INTEGER How may times to retry before killing TCP connection, closed by our side. Default value 7 corresponds to ~50sec-16min depending on RTO. If you machine is loaded WEB server, you should think about lowering this value, such sockets may consume significant resources. Cf. tcp_max_orphans.

tcp_fin_timeout - INTEGER Time to hold socket in state FIN-WAIT-2, if it was closed by our side. Peer can be broken and never close its side, or even died unexpectedly. Default value is 60sec. Usual value used in 2.2 was 180 seconds, you may restore it, but remember that if your machine is even underloaded WEB server, you risk to overflow memory with kilotons of dead sockets, FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1, because they eat maximum 1.5K of memory, but they tend to live longer. Cf. tcp_max_orphans.

tcp_max_tw_buckets - INTEGER Maximal number of timewait sockets held by system simultaneously. If this number is exceeded time-wait socket is immediately destroyed and warning is printed. This limit exists only to prevent simple DoS attacks, you _must_ not lower the limit artificially, but rather increase it (probably, after increasing installed memory), if network conditions require more than default value.

tcp_tw_recycle - BOOLEAN Enable fast recycling TIME-WAIT sockets. Default value is 0. It should not be changed without advice/request of technical experts.

tcp_tw_reuse - BOOLEAN Allow to reuse TIME-WAIT sockets for new connections when it is safe from protocol viewpoint. Default value is 0. It should not be changed without advice/request of technical experts.

tcp_max_orphans - INTEGER Maximal number of TCP sockets not attached to any user file handle, held by system. If this number is exceeded orphaned connections are reset immediately and warning is printed. This limit exists only to prevent simple DoS attacks, you _must_ not rely on this or lower the limit artificially, but rather increase it (probably, after increasing installed memory), if network conditions require more than default value, and tune network services to linger and kill such states more aggressively. Let me to remind again: each orphan eats up to ~64K of unswappable memory.

tcp_abort_on_overflow - BOOLEAN If listening service is too slow to accept new connections, reset them. Default state is FALSE. It means that if overflow occurred due to a burst, connection will recover. Enable this option _only_ if you are really sure that listening daemon cannot be tuned to accept connections faster. Enabling this option can harm clients of your server.

tcp_syncookies - BOOLEAN Only valid when the kernel was compiled with CONFIG_SYNCOOKIES Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common 'syn flood attack' Default: FALSE

Note, that syncookies is fallback facility. It MUST NOT be used to help highly loaded servers to stand against legal connection rate. If you see synflood warnings in your logs, but investigation shows that they occur because of overload with legal connections, you should tune another parameters until this warning disappear. See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

syncookies seriously violate TCP protocol, do not allow to use TCP extensions, can result in serious degradation of some services (f.e. SMTP relaying), visible not by you, but your clients and relays, contacting you. While you see synflood warnings in logs not being really flooded, your server is seriously misconfigured.

tcp_stdurg - BOOLEAN Use the Host requirements interpretation of the TCP urg pointer field. Most hosts use the older BSD interpretation, so if you turn this on Linux might not communicate correctly with them. Default: FALSE

tcp_max_syn_backlog - INTEGER Maximal number of remembered connection requests, which are still did not receive an acknowledgment from connecting client. Default value is 1024 for systems with more than 128Mb of memory, and 128 for low memory machines. If server suffers of overload, try to increase this number.

tcp_window_scaling - BOOLEAN Enable window scaling as defined in RFC1323.

tcp_timestamps - BOOLEAN Enable timestamps as defined in RFC1323.

tcp_sack - BOOLEAN Enable select acknowledgments (SACKS).

tcp_fack - BOOLEAN Enable FACK congestion avoidance and fast retransmission. The value is not used, if tcp_sack is not enabled.

tcp_dsack - BOOLEAN Allows TCP to send “duplicate” SACKs.

tcp_ecn - BOOLEAN Enable Explicit Congestion Notification in TCP.

tcp_reordering - INTEGER Maximal reordering of packets in a TCP stream. Default: 3

tcp_retrans_collapse - BOOLEAN Bug-to-bug compatibility with some broken printers. On retransmit try to send bigger packets to work around bugs in certain TCP stacks.

tcp_wmem - vector of 3 INTEGERs: min, default, max min: Amount of memory reserved for send buffers for TCP socket. Each TCP socket has rights to use it due to fact of its birth. Default: 4K

default: Amount of memory allowed for send buffers for TCP socket by default. This value overrides net.core.wmem_default used by other protocols, it is usually lower than net.core.wmem_default. Default: 16K

max: Maximal amount of memory allowed for automatically selected send buffers for TCP socket. This value does not override net.core.wmem_max, “static” selection via SO_SNDBUF does not use this. Default: 128K

tcp_rmem - vector of 3 INTEGERs: min, default, max min: Minimal size of receive buffer used by TCP sockets. It is guaranteed to each TCP socket, even under moderate memory pressure. Default: 8K

default: default size of receive buffer used by TCP sockets. This value overrides net.core.rmem_default used by other protocols. Default: 87380 bytes. This value results in window of 65535 with default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit less for default tcp_app_win. See below about these variables.

max: maximal size of receive buffer allowed for automatically selected receiver buffers for TCP socket. This value does not override net.core.rmem_max, “static” selection via SO_RCVBUF does not use this. Default: 87380*2 bytes.

tcp_mem - vector of 3 INTEGERs: min, pressure, max low: below this number of pages TCP is not bothered about its memory appetite.

pressure: when amount of memory allocated by TCP exceeds this number of pages, TCP moderates its memory consumption and enters memory pressure mode, which is exited when memory consumption falls under “low”.

high: number of pages allowed for queueing by all TCP sockets.

Defaults are calculated at boot time from amount of available memory.

tcp_app_win - INTEGER Reserve max(window/2^tcp_app_win, mss) of window for application buffer. Value 0 is special, it means that nothing is reserved. Default: 31

tcp_adv_win_scale - INTEGER Count buffering overhead as bytes/2^tcp_adv_win_scale (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale), if it is <= 0. Default: 2

tcp_rfc1337 - BOOLEAN If set, the TCP stack behaves conforming to RFC1337. If unset, we are not conforming to RFC, but prevent TCP TIME_WAIT assassination. Default: 0

tcp_low_latency - BOOLEAN If set, the TCP stack makes decisions that prefer lower latency as opposed to higher throughput. By default, this option is not set meaning that higher throughput is preferred. An example of an application where this default should be changed would be a Beowulf compute cluster. Default: 0

ip_local_port_range - 2 INTEGERS Defines the local port range that is used by TCP and UDP to choose the local port. The first number is the first, the second the last local port number. Default value depends on amount of memory available on the system: > 128Mb 32768-61000 < 128Mb 1024-4999 or even less. This number defines number of active connections, which this system can issue simultaneously to systems not supporting TCP extensions (timestamps). With tcp_tw_recycle enabled (i.e. by default) range 1024-4999 is enough to issue up to 2000 connections per second to systems supporting timestamps.

ip_nonlocal_bind - BOOLEAN If set, allows processes to bind() to non-local IP addresses, which can be quite useful - but may break some applications. Default: 0

ip_dynaddr - BOOLEAN If set non-zero, enables support for dynamic addresses. If set to a non-zero value larger than 1, a kernel log message will be printed when dynamic address rewriting occurs. Default: 0

icmp_echo_ignore_all - BOOLEANicmp_echo_ignore_broadcasts - BOOLEAN If either is set to true, then the kernel will ignore either all ICMP ECHO requests sent to it or just those to broadcast/multicast addresses, respectively.

icmp_ratelimit - INTEGER Limit the maximal rates for sending ICMP packets whose type matches icmp_ratemask (see below) to specific targets. 0 to disable any limiting, otherwise the maximal rate in jiffies(1) Default: 100

icmp_ratemask - INTEGER Mask made of ICMP types for which rates are being limited. Significant bits: IHGFEDCBA9876543210 Default mask: 0000001100000011000 (6168)

Bit definitions (see include/linux/icmp.h): 0 Echo Reply 3 Destination Unreachable * 4 Source Quench * 5 Redirect 8 Echo Request B Time Exceeded * C Parameter Problem * D Timestamp Request E Timestamp Reply F Info Request G Info Reply H Address Mask Request I Address Mask Reply

* These are rate limited by default (see default mask above)

icmp_ignore_bogus_error_responses - BOOLEAN Some routers violate RFC1122 by sending bogus responses to broadcast frames. Such violations are normally logged via a kernel warning. If this is set to TRUE, the kernel will not give such warnings, which will avoid log file clutter. Default: FALSE

igmp_max_memberships - INTEGER Change the maximum number of multicast groups we can subscribe to. Default: 20

conf/interface/* changes special settings per interface (where “interface” is the name of your network interface)conf/all/* is special, changes the settings for all interfaces

log_martians - BOOLEAN Log packets with impossible addresses to kernel log. log_martians for the interface will be enabled if at least one of conf/{all,interface}/log_martians is set to TRUE, it will be disabled otherwise

accept_redirects - BOOLEAN Accept ICMP redirect messages. accept_redirects for the interface will be enabled if: - both conf/{all,interface}/accept_redirects are TRUE in the case forwarding for the interface is enabled or - at least one of conf/{all,interface}/accept_redirects is TRUE in the case forwarding for the interface is disabled accept_redirects for the interface will be disabled otherwise default TRUE (host) FALSE (router)

forwarding - BOOLEAN Enable IP forwarding on this interface.

mc_forwarding - BOOLEAN Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE and a multicast routing daemon is required. conf/all/mc_forwarding must also be set to TRUE to enable multicast routing for the interface

medium_id - INTEGER Integer value used to differentiate the devices by the medium they are attached to. Two devices can have different id values when the broadcast packets are received only on one of them. The default value 0 means that the device is the only interface to its medium, value of -1 means that medium is not known.

Currently, it is used to change the proxy_arp behavior: the proxy_arp feature is enabled for packets forwarded between two devices attached to different media.

proxy_arp - BOOLEAN Do proxy arp. proxy_arp for the interface will be enabled if at least one of conf/{all,interface}/proxy_arp is set to TRUE, it will be disabled otherwise

shared_media - BOOLEAN Send(router) or accept(host) RFC1620 shared media redirects. Overrides ip_secure_redirects. shared_media for the interface will be enabled if at least one of conf/{all,interface}/shared_media is set to TRUE, it will be disabled otherwise default TRUE

secure_redirects - BOOLEAN Accept ICMP redirect messages only for gateways, listed in default gateway list. secure_redirects for the interface will be enabled if at least one of conf/{all,interface}/secure_redirects is set to TRUE, it will be disabled otherwise default TRUE

send_redirects - BOOLEAN Send redirects, if router. send_redirects for the interface will be enabled if at least one of conf/{all,interface}/send_redirects is set to TRUE, it will be disabled otherwise Default: TRUE

bootp_relay - BOOLEAN Accept packets with source address 0.b.c.d destined not to this host as local ones. It is supposed, that BOOTP relay daemon will catch and forward such packets. conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay for the interface default FALSE Not Implemented Yet.

accept_source_route - BOOLEAN Accept packets with SRR option. conf/all/accept_source_route must also be set to TRUE to accept packets with SRR option on the interface default TRUE (router) FALSE (host)

rp_filter - BOOLEAN 1 - do source validation by reversed path, as specified in RFC1812 Recommended option for single homed hosts and stub network routers. Could cause troubles for complicated (not loop free) networks running a slow unreliable protocol (sort of RIP), or using static routes.

0 - No source validation.

conf/all/rp_filter must also be set to TRUE to do source validation on the interface

Default value is 0. Note that some distributions enable it in startup scripts.

arp_filter - BOOLEAN 1 - Allows you to have multiple network interfaces on the same subnet, and have the ARPs for each interface be answered based on whether or not the kernel would route a packet from the ARP'd IP out that interface (therefore you must use source based routing for this to work). In other words it allows control of which cards (usually 1) will respond to an arp request.

0 - (default) The kernel can respond to arp requests with addresses from other interfaces. This may seem wrong but it usually makes sense, because it increases the chance of successful communication. IP addresses are owned by the complete host on Linux, not by particular interfaces. Only for more complex setups like load- balancing, does this behaviour cause problems.

arp_filter for the interface will be enabled if at least one of conf/{all,interface}/arp_filter is set to TRUE, it will be disabled otherwise

tag - INTEGER Allows you to write a number, which can be used as required. Default value is 0.

(1) Jiffie: internal timeunit for the kernel. On the i386 1/100s, on theAlpha 1/1024s. See the HZ define in /usr/include/asm/param.h for the exactvalue on your system.

Alexey Kuznetsov.kuznet-AT-ms2.inr.ac-DOT-ru

Updated by:Andi Kleenak-AT-muc-DOT-deNicolas Delondelon.nicolas-AT-wanadoo-DOT-fr

/proc/sys/net/ipv6/* Variables:

IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ alsoapply to IPv6 [XXX?].

bindv6only - BOOLEAN Default value for IPV6_V6ONLY socket option, which restricts use of the IPv6 socket to IPv6 communication only. TRUE: disable IPv4-mapped address feature FALSE: enable IPv4-mapped address feature

Default: FALSE (as specified in RFC2553bis)

IPv6 Fragmentation:

ip6frag_high_thresh - INTEGER Maximum memory used to reassemble IPv6 fragments. When ip6frag_high_thresh bytes of memory is allocated for this purpose, the fragment handler will toss packets until ip6frag_low_thresh is reached.

ip6frag_low_thresh - INTEGER See ip6frag_high_thresh

ip6frag_time - INTEGER Time in seconds to keep an IPv6 fragment in memory.

ip6frag_secret_interval - INTEGER Regeneration interval (in seconds) of the hash secret (or lifetime for the hash secret) for IPv6 fragments. Default: 600

conf/default/*: Change the interface-specific default settings.

conf/all/*: Change all the interface-specific settings.

[XXX: Other special features than forwarding?]

conf/all/forwarding - BOOLEAN Enable global IPv6 forwarding between all interfaces.

IPv4 and IPv6 work differently here; e.g. netfilter must be used to control which interfaces may forward packets and which not.

This also sets all interfaces' Host/Router setting 'forwarding' to the specified value. See below for details.

This referred to as global forwarding.

conf/interface/*: Change special settings per interface.

The functional behaviour for certain settings is different depending on whether local forwarding is enabled or not.

accept_ra - BOOLEAN Accept Router Advertisements; autoconfigure using them.

Functional default: enabled if local forwarding is disabled. disabled if local forwarding is enabled.

accept_redirects - BOOLEAN Accept Redirects.

Functional default: enabled if local forwarding is disabled. disabled if local forwarding is enabled.

autoconf - BOOLEAN Configure link-local addresses using L2 hardware addresses.

Default: TRUE

dad_transmits - INTEGER The amount of Duplicate Address Detection probes to send. Default: 1

forwarding - BOOLEAN Configure interface-specific Host/Router behaviour.

Note: It is recommended to have the same setting on all interfaces; mixed router/host scenarios are rather uncommon.

FALSE:

By default, Host behaviour is assumed. This means:

1. IsRouter flag is not set in Neighbour Advertisements. 2. Router Solicitations are being sent when necessary. 3. If accept_ra is TRUE (default), accept Router Advertisements (and do autoconfiguration). 4. If accept_redirects is TRUE (default), accept Redirects.

TRUE:

If local forwarding is enabled, Router behaviour is assumed. This means exactly the reverse from the above:

1. IsRouter flag is set in Neighbour Advertisements. 2. Router Solicitations are not sent. 3. Router Advertisements are ignored. 4. Redirects are ignored.

Default: FALSE if global forwarding is disabled (default), otherwise TRUE.

hop_limit - INTEGER Default Hop Limit to set. Default: 64

mtu - INTEGER Default Maximum Transfer Unit Default: 1280 (IPv6 required minimum)

router_solicitation_delay - INTEGER Number of seconds to wait after interface is brought up before sending Router Solicitations. Default: 1

router_solicitation_interval - INTEGER Number of seconds to wait between Router Solicitations. Default: 4

router_solicitations - INTEGER Number of Router Solicitations to send until assuming no routers are present. Default: 3

use_tempaddr - INTEGER Preference for Privacy Extensions (RFC3041). <= 0 : disable Privacy Extensions == 1 : enable Privacy Extensions, but prefer public addresses over temporary addresses. > 1 : enable Privacy Extensions and prefer temporary addresses over public addresses. Default: 0 (for most devices) -1 (for point-to-point devices and loopback devices)

temp_valid_lft - INTEGER valid lifetime (in seconds) for temporary addresses. Default: 604800 (7 days)

temp_prefered_lft - INTEGER Preferred lifetime (in seconds) for temorary addresses. Default: 86400 (1 day)

max_desync_factor - INTEGER Maximum value for DESYNC_FACTOR, which is a random value that ensures that clients don't synchronize with each other and generage new addresses at exactly the same time. value is in seconds. Default: 600

regen_max_retry - INTEGER Number of attempts before give up attempting to generate valid temporary addresses. Default: 5

icmp/*:ratelimit - INTEGER Limit the maximal rates for sending ICMPv6 packets. 0 to disable any limiting, otherwise the maximal rate in jiffies(1) Default: 100

IPv6 Update by:Pekka Savola <pekkas-AT-netcore-DOT-fi>YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>

$Id: ip-sysctl.txt,v 1.20 2001/12/13 09:00:18 davem Exp $