Linux TCP/IP parameters reference
ip-sysctl.txt reference for IP networking parameters based on the 2.4 kernel
2008-02-03 (updated: 2009-11-09) by Philip
Tags: Linux, tweak, TCP/IP, TCP Window, IPv4, IPv6
This is a reference of IP networking parameters that are configurable as described in our linux tweaking article -here-.
/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.
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.
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.
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.
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.
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_interval - 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 suspection 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 1.
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 acknowledgement 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. Warning! If you make it greater
than 1024, it would be better to change TCP_SYNQ_HSIZE in
include/net/tcp.h to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog
and to recompile kernel.
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 restransmission.
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 consumtion 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
asassination.
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 adresses,
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 - BOOLEAN
icmp_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_destunreach_rate - INTEGER
icmp_paramprob_rate - INTEGER
icmp_timeexceed_rate - INTEGER
icmp_echoreply_rate - INTEGER (not enabled per default)
Limit the maximal rates for sending ICMP packets to specific targets.
0 to disable any limiting, otherwise the maximal rate in jiffies(1)
See the source for more information.
icmp_ignore_bogus_error_responses - BOOLEAN
Some routers violate RFC 1122 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
(1) Jiffie: internal timeunit for the kernel. On the i386 1/100s, on the
Alpha 1/1024s. See the HZ define in /usr/include/asm/param.h for the exact
value on your system.
igmp_max_memberships - INTEGER
Change the maximum number of multicast groups we can subscribe to.
Default: 20
conf/interface/*:
conf/all/* is special and changes the settings for all interfaces.
Change special settings per interface.
log_martians - BOOLEAN
Log packets with impossible addresses to kernel log.
accept_redirects - BOOLEAN
Accept ICMP redirect messages.
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.
proxy_arp - BOOLEAN
Do proxy arp.
shared_media - BOOLEAN
Send(router) or accept(host) RFC1620 shared media redirects.
Overrides ip_secure_redirects.
default TRUE
secure_redirects - BOOLEAN
Accept ICMP redirect messages only for gateways,
listed in default gateway list.
default TRUE
send_redirects - BOOLEAN
Send redirects, if router. 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.
default FALSE
Not Implemented Yet.
accept_source_route - BOOLEAN
Accept packets with SRR option.
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.
Default value is 0. Note that some distributions enable it
in startip scripts.
Alexey Kuznetsov.
kuznet@ms2.inr.ac.ru
Updated by:
Andi Kleen
ak@muc.de
/proc/sys/net/ipv6/* Variables:
IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
apply to IPv6 [XXX?].
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
IPv6 Update by:
Pekka Savola
pekkas@netcore.fi
$Id: ip-sysctl.txt,v 1.1.1.1 2002-08-19 13:34:26 blueflux Exp $
