path: root/Documentation/networking/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

min_pmtu - INTEGER
	default 562 - minimum discovered Path MTU

mtu_expires - INTEGER
	Time, in seconds, that cached PMTU information is kept.

min_adv_mss - INTEGER
	The advertised MSS depends on the first hop route MTU, but will
	never be lower than this setting.

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

ipfrag_max_dist - INTEGER
	ipfrag_max_dist is a non-negative integer value which defines the 
	maximum "disorder" which is allowed among fragments which share a 
	common IP source address. Note that reordering of packets is 
	not unusual, but if a large number of fragments arrive from a source 
	IP address while a particular fragment queue remains incomplete, it 
	probably indicates that one or more fragments belonging to that queue 
	have been lost. When ipfrag_max_dist is positive, an additional check 
	is done on fragments before they are added to a reassembly queue - if 
	ipfrag_max_dist (or more) fragments have arrived from a particular IP 
	address between additions to any IP fragment queue using that source 
	address, it's presumed that one or more fragments in the queue are 
	lost. The existing fragment queue will be dropped, and a new one 
	started. An ipfrag_max_dist value of zero disables this check.

	Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
	result in unnecessarily dropping fragment queues when normal
	reordering of packets occurs, which could lead to poor application 
	performance. Using a very large value, e.g. 50000, increases the 
	likelihood of incorrectly reassembling IP fragments that originate 
	from different IP datagrams, which could result in data corruption.
	Default: 64

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: 

somaxconn - INTEGER
	Limit of socket listen() backlog, known in userspace as SOMAXCONN.
	Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
	for TCP sockets.

tcp_abc - INTEGER
	Controls Appropriate Byte Count (ABC) defined in RFC3465.
	ABC is a way of increasing congestion window (cwnd) more slowly
	in response to partial acknowledgments.
	Possible values are:
		0 increase cwnd once per acknowledgment (no ABC)
		1 increase cwnd once per acknowledgment of full sized segment
		2 allow increase cwnd by two if acknowledgment is
		  of two segments to compensate for delayed acknowledgments.
	Default: 0 (off)

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_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_allowed_congestion_control - STRING
	Show/set the congestion control choices available to non-privileged
	processes. The list is a subset of those listed in
	tcp_available_congestion_control.
	Default is "reno" and the default setting (tcp_congestion_control).

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_available_congestion_control - STRING
	Shows the available congestion control choices that are registered.
	More congestion control algorithms may be available as modules,
	but not loaded.

tcp_base_mss - INTEGER
	The initial value of search_low to be used by Packetization Layer
	Path MTU Discovery (MTU probing).  If MTU probing is enabled,
	this is the inital MSS used by the connection.

tcp_congestion_control - STRING
	Set the congestion control algorithm to be used for new
	connections. The algorithm "reno" is always available, but
	additional choices may be available based on kernel configuration.
	Default is set as part of kernel configuration.

tcp_dsack - BOOLEAN
	Allows TCP to send "duplicate" SACKs.

tcp_ecn - BOOLEAN
	Enable Explicit Congestion Notification in TCP.

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

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_frto - INTEGER
	Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
	F-RTO is an enhanced recovery algorithm for TCP retransmission
	timeouts.  It is particularly beneficial in wireless environments
	where packet loss is typically due to random radio interference
	rather than intermediate router congestion.  F-RTO is sender-side
	only modification.  Therefore it does not require any support from
	the peer, but in a typical case, however, where wireless link is
	the local access link and most of the data flows downlink, the
	faraway servers should have F-RTO enabled to take advantage of it.
	If set to 1, basic version is enabled.  2 enables SACK enhanced
	F-RTO if flow uses SACK.  The basic version can be used also when
	SACK is in use though scenario(s) with it exists where F-RTO
	interacts badly with the packet counting of the SACK enabled TCP
	flow.

tcp_frto_response - INTEGER
	When F-RTO has detected that a TCP retransmission timeout was
	spurious (i.e, the timeout would have been avoided had TCP set a
	longer retransmission timeout), TCP has several options what to do
	next. Possible values are:
		0 Rate halving based; a smooth and conservative response,
		  results in halved cwnd and ssthresh after one RTT
		1 Very conservative response; not recommended because even
		  though being valid, it interacts poorly with the rest of
		  Linux TCP, halves cwnd and ssthresh immediately
		2 Aggressive response; undoes congestion control measures
		  that are now known to be unnecessary (ignoring the
		  possibility of a lost retransmission that would require
		  TCP to be more cautious), cwnd and ssthresh are restored
		  to the values prior timeout
	Default: 0 (rate halving based)

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_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

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_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_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_mem - vector of 3 INTEGERs: min, pressure, max
	min: 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 "min".

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

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

tcp_moderate_rcvbuf - BOOLEAN
	If set, TCP performs receive buffer autotuning, attempting to
	automatically size the buffer (no greater than tcp_rmem[2]) to
	match the size required by the path for full throughput.  Enabled by
	default.

tcp_mtu_probing - INTEGER
	Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
	values:
	  0 - Disabled
	  1 - Disabled by default, enabled when an ICMP black hole detected
	  2 - Always enabled, use initial MSS of tcp_base_mss.

tcp_no_metrics_save - BOOLEAN
	By default, TCP saves various connection metrics in the route cache
	when the connection closes, so that connections established in the
	near future can use these to set initial conditions.  Usually, this
	increases overall performance, but may sometimes cause performance
	degradation.  If set, TCP will not cache metrics on closing
	connections.

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_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_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_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_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_sack - BOOLEAN
	Enable select acknowledgments (SACKS).

tcp_slow_start_after_idle - BOOLEAN
	If set, provide RFC2861 behavior and time out the congestion
	window after an idle period.  An idle period is defined at
	the current RTO.  If unset, the congestion window will not
	be timed out after an idle period.
	Default: 1

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_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_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_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_timestamps - BOOLEAN
	Enable timestamps as defined in RFC1323.

tcp_tso_win_divisor - INTEGER
	This allows control over what percentage of the congestion window
	can be consumed by a single TSO frame.
	The setting of this parameter is a choice between burstiness and
	building larger TSO frames.
	Default: 3

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_window_scaling - BOOLEAN
	Enable window scaling as defined in RFC1323.

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_workaround_signed_windows - BOOLEAN
	If set, assume no receipt of a window scaling option means the
	remote TCP is broken and treats the window as a signed quantity.
	If unset, assume the remote TCP is not broken even if we do
	not receive a window scaling option from them.
	Default: 0

tcp_dma_copybreak - INTEGER
	Lower limit, in bytes, of the size of socket reads that will be
	offloaded to a DMA copy engine, if one is present in the system
	and CONFIG_NET_DMA is enabled.
	Default: 4096

CIPSOv4 Variables:

cipso_cache_enable - BOOLEAN
	If set, enable additions to and lookups from the CIPSO label mapping
	cache.  If unset, additions are ignored and lookups always result in a
	miss.  However, regardless of the setting the cache is still
	invalidated when required when means you can safely toggle this on and
	off and the cache will always be "safe".
	Default: 1

cipso_cache_bucket_size - INTEGER
	The CIPSO label cache consists of a fixed size hash table with each
	hash bucket containing a number of cache entries.  This variable limits
	the number of entries in each hash bucket; the larger the value the
	more CIPSO label mappings that can be cached.  When the number of
	entries in a given hash bucket reaches this limit adding new entries
	causes the oldest entry in the bucket to be removed to make room.
	Default: 10

cipso_rbm_optfmt - BOOLEAN
	Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
	the CIPSO draft specification (see Documentation/netlabel for details).
	This means that when set the CIPSO tag will be padded with empty
	categories in order to make the packet data 32-bit aligned.
	Default: 0

cipso_rbm_structvalid - BOOLEAN
	If set, do a very strict check of the CIPSO option when
	ip_options_compile() is called.  If unset, relax the checks done during
	ip_options_compile().  Either way is "safe" as errors are caught else
	where in the CIPSO processing code but setting this to 0 (False) should
	result in less work (i.e. it should be faster) but could cause problems
	with other implementations that require strict checking.
	Default: 0

IP Variables:

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 - BOOLEAN
	If set non-zero, th