.. SPDX-License-Identifier: GPL-2.0
  
  =========
  IP Sysctl
  =========
  
  /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 value of TTL field (Time To Live) for outgoing (but not
  	forwarded) IP packets. Should be between 1 and 255 inclusive.
  	Default: 64 (as recommended by RFC1700)

  ip_no_pmtu_disc - INTEGER
  	Disable Path MTU Discovery. If enabled in mode 1 and a

  	fragmentation-required ICMP is received, the PMTU to this
  	destination will be set to min_pmtu (see below). You will need
  	to raise min_pmtu to the smallest interface MTU on your system
  	manually if you want to avoid locally generated fragments.

  
  	In mode 2 incoming Path MTU Discovery messages will be
  	discarded. Outgoing frames are handled the same as in mode 1,
  	implicitly setting IP_PMTUDISC_DONT on every created socket.

  	Mode 3 is a hardened pmtu discover mode. The kernel will only

  	accept fragmentation-needed errors if the underlying protocol
  	can verify them besides a plain socket lookup. Current
  	protocols for which pmtu events will be honored are TCP, SCTP
  	and DCCP as they verify e.g. the sequence number or the
  	association. This mode should not be enabled globally but is
  	only intended to secure e.g. name servers in namespaces where
  	TCP path mtu must still work but path MTU information of other
  	protocols should be discarded. If enabled globally this mode
  	could break other protocols.
  
  	Possible values: 0-3

  	Default: FALSE

  
  min_pmtu - INTEGER

  	default 552 - minimum discovered Path MTU

  ip_forward_use_pmtu - BOOLEAN
  	By default we don't trust protocol path MTUs while forwarding
  	because they could be easily forged and can lead to unwanted
  	fragmentation by the router.
  	You only need to enable this if you have user-space software
  	which tries to discover path mtus by itself and depends on the
  	kernel honoring this information. This is normally not the
  	case.

  	Default: 0 (disabled)

  	Possible values:

  
  	- 0 - disabled
  	- 1 - enabled

  fwmark_reflect - BOOLEAN
  	Controls the fwmark of kernel-generated IPv4 reply packets that are not
  	associated with a socket for example, TCP RSTs or ICMP echo replies).
  	If unset, these packets have a fwmark of zero. If set, they have the
  	fwmark of the packet they are replying to.

  	Default: 0

  fib_multipath_use_neigh - BOOLEAN
  	Use status of existing neighbor entry when determining nexthop for
  	multipath routes. If disabled, neighbor information is not used and
  	packets could be directed to a failed nexthop. Only valid for kernels
  	built with CONFIG_IP_ROUTE_MULTIPATH enabled.

  	Default: 0 (disabled)

  	Possible values:

  
  	- 0 - disabled
  	- 1 - enabled

  fib_multipath_hash_policy - INTEGER
  	Controls which hash policy to use for multipath routes. Only valid
  	for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.

  	Default: 0 (Layer 3)

  	Possible values:

  
  	- 0 - Layer 3
  	- 1 - Layer 4
  	- 2 - Layer 3 or inner Layer 3 if present

  fib_sync_mem - UNSIGNED INTEGER
  	Amount of dirty memory from fib entries that can be backlogged before
  	synchronize_rcu is forced.

  
  	Default: 512kB   Minimum: 64kB   Maximum: 64MB

  ip_forward_update_priority - INTEGER
  	Whether to update SKB priority from "TOS" field in IPv4 header after it
  	is forwarded. The new SKB priority is mapped from TOS field value
  	according to an rt_tos2priority table (see e.g. man tc-prio).

  	Default: 1 (Update priority.)

  	Possible values:

  
  	- 0 - Do not update priority.
  	- 1 - Update priority.

  route/max_size - INTEGER
  	Maximum number of routes allowed in the kernel.  Increase
  	this when using large numbers of interfaces and/or routes.

  	From linux kernel 3.6 onwards, this is deprecated for ipv4
  	as route cache is no longer used.

  neigh/default/gc_thresh1 - INTEGER
  	Minimum number of entries to keep.  Garbage collector will not
  	purge entries if there are fewer than this number.

  	Default: 128

  neigh/default/gc_thresh2 - INTEGER
  	Threshold when garbage collector becomes more aggressive about
  	purging entries. Entries older than 5 seconds will be cleared
  	when over this number.

  	Default: 512

  neigh/default/gc_thresh3 - INTEGER

  	Maximum number of non-PERMANENT neighbor entries allowed.  Increase
  	this when using large numbers of interfaces and when communicating

  	with large numbers of directly-connected peers.

  	Default: 1024

  neigh/default/unres_qlen_bytes - INTEGER
  	The maximum number of bytes which may be used by packets
  	queued for each	unresolved address by other network layers.
  	(added in linux 3.3)

  	Setting negative value is meaningless and will return error.

  	Default: SK_WMEM_MAX, (same as net.core.wmem_default).

  		Exact value depends on architecture and kernel options,
  		but should be enough to allow queuing 256 packets
  		of medium size.

  
  neigh/default/unres_qlen - INTEGER
  	The maximum number of packets which may be queued for each
  	unresolved address by other network layers.

  	(deprecated in linux 3.3) : use unres_qlen_bytes instead.

  	Prior to linux 3.3, the default value is 3 which may cause

  	unexpected packet loss. The current default value is calculated

  	according to default value of unres_qlen_bytes and true size of
  	packet.

  	Default: 101

  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 - LONG INTEGER

  	Maximum memory used to reassemble IP fragments.

  ipfrag_low_thresh - LONG INTEGER

  	(Obsolete since linux-4.17)

  	Maximum memory used to reassemble IP fragments before the kernel
  	begins to remove incomplete fragment queues to free up resources.
  	The kernel still accepts new fragments for defragmentation.

  
  ipfrag_time - INTEGER

  	Time in seconds to keep an IP fragment in memory.

  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 seconds.

  
  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 seconds.

  TCP variables
  =============

  somaxconn - INTEGER
  	Limit of socket listen() backlog, known in userspace as SOMAXCONN.

  	Defaults to 4096. (Was 128 before linux-5.4)
  	See also tcp_max_syn_backlog for additional tuning for TCP sockets.

  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.

  	Possible values are [-31, 31], inclusive.

  	Default: 1

  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_autocorking - BOOLEAN
  	Enable TCP auto corking :
  	When applications do consecutive small write()/sendmsg() system calls,
  	we try to coalesce these small writes as much as possible, to lower
  	total amount of sent packets. This is done if at least one prior
  	packet for the flow is waiting in Qdisc queues or device transmit
  	queue. Applications can still use TCP_CORK for optimal behavior
  	when they know how/when to uncork their sockets.

  	Default : 1

  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 the packetization layer
  	Path MTU discovery (MTU probing).  If MTU probing is enabled,
  	this is the initial MSS used by the connection.

  tcp_mtu_probe_floor - INTEGER
  	If MTU probing is enabled this caps the minimum MSS used for search_low
  	for the connection.
  
  	Default : 48

  tcp_min_snd_mss - INTEGER
  	TCP SYN and SYNACK messages usually advertise an ADVMSS option,
  	as described in RFC 1122 and RFC 6691.

  	If this ADVMSS option is smaller than tcp_min_snd_mss,
  	it is silently capped to tcp_min_snd_mss.
  
  	Default : 48 (at least 8 bytes of payload per segment)

  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.

  	For passive connections, the listener congestion control choice
  	is inherited.

  	[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]

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

  tcp_early_retrans - INTEGER

  	Tail loss probe (TLP) converts RTOs occurring due to tail
  	losses into fast recovery (draft-ietf-tcpm-rack). Note that
  	TLP requires RACK to function properly (see tcp_recovery below)

  	Possible values:

  
  		- 0 disables TLP
  		- 3 or 4 enables TLP

  	Default: 3

  tcp_ecn - INTEGER

  	Control use of Explicit Congestion Notification (ECN) by TCP.
  	ECN is used only when both ends of the TCP connection indicate
  	support for it.  This feature is useful in avoiding losses due
  	to congestion by allowing supporting routers to signal
  	congestion before having to drop packets.

  	Possible values are:

  
  		=  =====================================================
  		0  Disable ECN.  Neither initiate nor accept ECN.
  		1  Enable ECN when requested by incoming connections and
  		   also request ECN on outgoing connection attempts.
  		2  Enable ECN when requested by incoming connections
  		   but do not request ECN on outgoing connections.
  		=  =====================================================

  	Default: 2

  tcp_ecn_fallback - BOOLEAN
  	If the kernel detects that ECN connection misbehaves, enable fall
  	back to non-ECN. Currently, this knob implements the fallback
  	from RFC3168, section 6.1.1.1., but we reserve that in future,
  	additional detection mechanisms could be implemented under this
  	knob. The value	is not used, if tcp_ecn or per route (or congestion
  	control) ECN settings are disabled.

  	Default: 1 (fallback enabled)

  tcp_fack - BOOLEAN

  	This is a legacy option, it has no effect anymore.

  
  tcp_fin_timeout - INTEGER

  	The length of time an orphaned (no longer referenced by any
  	application) connection will remain in the FIN_WAIT_2 state
  	before it is aborted at the local end.  While a perfectly
  	valid "receive only" state for an un-orphaned connection, an
  	orphaned connection in FIN_WAIT_2 state could otherwise wait
  	forever for the remote to close its end of the connection.

  	Cf. tcp_max_orphans

  	Default: 60 seconds

  tcp_frto - INTEGER

  	Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.

  	F-RTO is an enhanced recovery algorithm for TCP retransmission

  	timeouts.  It is particularly beneficial in networks where the
  	RTT fluctuates (e.g., wireless). F-RTO is sender-side only
  	modification. It does not require any support from the peer.
  
  	By default it's enabled with a non-zero value. 0 disables F-RTO.

  tcp_fwmark_accept - BOOLEAN
  	If set, incoming connections to listening sockets that do not have a
  	socket mark will set the mark of the accepting socket to the fwmark of
  	the incoming SYN packet. This will cause all packets on that connection
  	(starting from the first SYNACK) to be sent with that fwmark. The
  	listening socket's mark is unchanged. Listening sockets that already
  	have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
  	unaffected.
  
  	Default: 0

  tcp_invalid_ratelimit - INTEGER
  	Limit the maximal rate for sending duplicate acknowledgments
  	in response to incoming TCP packets that are for an existing
  	connection but that are invalid due to any of these reasons:
  
  	  (a) out-of-window sequence number,
  	  (b) out-of-window acknowledgment number, or
  	  (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
  
  	This can help mitigate simple "ack loop" DoS attacks, wherein
  	a buggy or malicious middlebox or man-in-the-middle can
  	rewrite TCP header fields in manner that causes each endpoint
  	to think that the other is sending invalid TCP segments, thus
  	causing each side to send an unterminating stream of duplicate
  	acknowledgments for invalid segments.
  
  	Using 0 disables rate-limiting of dupacks in response to
  	invalid segments; otherwise this value specifies the minimal
  	space between sending such dupacks, in milliseconds.
  
  	Default: 500 (milliseconds).

  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_l3mdev_accept - BOOLEAN
  	Enables child sockets to inherit the L3 master device index.
  	Enabling this option allows a "global" listen socket to work
  	across L3 master domains (e.g., VRFs) with connected sockets
  	derived from the listen socket to be bound to the L3 domain in
  	which the packets originated. Only valid when the kernel was
  	compiled with CONFIG_NET_L3_MASTER_DEV.

  
  	Default: 0 (disabled)

  tcp_low_latency - BOOLEAN

  	This is a legacy option, it has no effect anymore.

  
  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 (SYN_RECV),
  	which have not received an acknowledgment from connecting client.

  	This is a per-listener limit.

  	The minimal value is 128 for low memory machines, and it will
  	increase in proportion to the memory of machine.

  	If server suffers from overload, try increasing this number.

  	Remember to also check /proc/sys/net/core/somaxconn
  	A SYN_RECV request socket consumes about 304 bytes of memory.

  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_min_rtt_wlen - INTEGER
  	The window length of the windowed min filter to track the minimum RTT.
  	A shorter window lets a flow more quickly pick up new (higher)
  	minimum RTT when it is moved to a longer path (e.g., due to traffic
  	engineering). A longer window makes the filter more resistant to RTT
  	inflations such as transient congestion. The unit is seconds.

  	Possible values: 0 - 86400 (1 day)

  	Default: 300

  tcp_moderate_rcvbuf - BOOLEAN

  	If set, TCP performs receive buffer auto-tuning, 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_probe_interval - UNSIGNED INTEGER

  	Controls how often to start TCP Packetization-Layer Path MTU
  	Discovery reprobe. The default is reprobing every 10 minutes as
  	per RFC4821.
  
  tcp_probe_threshold - INTEGER
  	Controls when TCP Packetization-Layer Path MTU Discovery probing
  	will stop in respect to the width of search range in bytes. Default
  	is 8 bytes.

  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_no_ssthresh_metrics_save - BOOLEAN
  	Controls whether TCP saves ssthresh metrics in the route cache.

  	Default is 1, which disables ssthresh metrics.

  tcp_orphan_retries - INTEGER

  	This value influences the timeout of a locally closed TCP connection,
  	when RTO retransmissions remain unacknowledged.
  	See tcp_retries2 for more details.

  	The default value is 8.

  	If your machine is a loaded WEB server,

  	you should think about lowering this value, such sockets
  	may consume significant resources. Cf. tcp_max_orphans.

  tcp_recovery - INTEGER
  	This value is a bitmap to enable various experimental loss recovery
  	features.

  	=========   =============================================================
  	RACK: 0x1   enables the RACK loss detection for fast detection of lost
  		    retransmissions and tail drops. It also subsumes and disables
  		    RFC6675 recovery for SACK connections.
  
  	RACK: 0x2   makes RACK's reordering window static (min_rtt/4).
  
  	RACK: 0x4   disables RACK's DUPACK threshold heuristic
  	=========   =============================================================

  
  	Default: 0x1

  tcp_reordering - INTEGER

  	Initial reordering level of packets in a TCP stream.
  	TCP stack can then dynamically adjust flow reordering level
  	between this initial value and tcp_max_reordering

  	Default: 3

  tcp_max_reordering - INTEGER
  	Maximal reordering level of packets in a TCP stream.
  	300 is a fairly conservative value, but you might increase it
  	if paths are using per packet load balancing (like bonding rr mode)

  	Default: 300

  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

  	This value influences the time, after which TCP decides, that
  	something is wrong due to unacknowledged RTO retransmissions,
  	and reports this suspicion to the network layer.
  	See tcp_retries2 for more details.
  
  	RFC 1122 recommends at least 3 retransmissions, which is the
  	default.

  tcp_retries2 - INTEGER

  	This value influences the timeout of an alive TCP connection,
  	when RTO retransmissions remain unacknowledged.
  	Given a value of N, a hypothetical TCP connection following
  	exponential backoff with an initial RTO of TCP_RTO_MIN would
  	retransmit N times before killing the connection at the (N+1)th RTO.
  
  	The default value of 15 yields a hypothetical timeout of 924.6
  	seconds and is a lower bound for the effective timeout.
  	TCP will effectively time out at the first RTO which exceeds the
  	hypothetical timeout.
  
  	RFC 1122 recommends at least 100 seconds for the timeout,
  	which corresponds to a value of at least 8.

  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: 4K

  	default: initial 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.  Calling setsockopt() with SO_RCVBUF disables
  	automatic tuning of that socket's receive buffer size, in which
  	case this value is ignored.

  	Default: between 87380B and 6MB, depending on RAM size.

  tcp_sack - BOOLEAN
  	Enable select acknowledgments (SACKS).

  tcp_comp_sack_delay_ns - LONG INTEGER
  	TCP tries to reduce number of SACK sent, using a timer
  	based on 5% of SRTT, capped by this sysctl, in nano seconds.
  	The default is 1ms, based on TSO autosizing period.
  
  	Default : 1,000,000 ns (1 ms)

  tcp_comp_sack_slack_ns - LONG INTEGER
  	This sysctl control the slack used when arming the
  	timer used by SACK compression. This gives extra time
  	for small RTT flows, and reduces system overhead by allowing
  	opportunistic reduction of timer interrupts.
  
  	Default : 100,000 ns (100 us)

  tcp_comp_sack_nr - INTEGER

  	Max number of SACK that can be compressed.

  	Using 0 disables SACK compression.

  	Default : 44

  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 urgent 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 31seconds till the last retransmission
  	with the current initial RTO of 1second. With this the final timeout
  	for a passive TCP connection will happen after 63seconds.

  tcp_syncookies - INTEGER

  	Only valid when the kernel was compiled with CONFIG_SYN_COOKIES

  	Send out syncookies when the syn backlog queue of a socket

  	overflows. This is to prevent against the common 'SYN flood attack'

  	Default: 1

  	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 SYN flood 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

  	SYN flood warnings in logs not being really flooded, your server

  	is seriously misconfigured.

  	If you want to test which effects syncookies have to your
  	network connections you can set this knob to 2 to enable
  	unconditionally generation of syncookies.

  tcp_fastopen - INTEGER

  	Enable TCP Fast Open (RFC7413) to send and accept data in the opening
  	SYN packet.

  	The client support is enabled by flag 0x1 (on by default). The client
  	then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
  	rather than connect() to send data in SYN.

  	The server support is enabled by flag 0x2 (off by default). Then
  	either enable for all listeners with another flag (0x400) or
  	enable individual listeners via TCP_FASTOPEN socket option with
  	the option value being the length of the syn-data backlog.

  	The values (bitmap) are

  
  	=====  ======== ======================================================
  	  0x1  (client) enables sending data in the opening SYN on the client.
  	  0x2  (server) enables the server support, i.e., allowing data in

  			a SYN packet to be accepted and passed to the
  			application before 3-way handshake finishes.

  	  0x4  (client) send data in the opening SYN regardless of cookie

  			availability and without a cookie option.

  	0x200  (server) accept data-in-SYN w/o any cookie option present.
  	0x400  (server) enable all listeners to support Fast Open by

  			default without explicit TCP_FASTOPEN socket option.

  	=====  ======== ======================================================

  
  	Default: 0x1

  	Note that additional client or server features are only

  	effective if the basic support (0x1 and 0x2) are enabled respectively.

  tcp_fastopen_blackhole_timeout_sec - INTEGER
  	Initial time period in second to disable Fastopen on active TCP sockets
  	when a TFO firewall blackhole issue happens.
  	This time period will grow exponentially when more blackhole issues
  	get detected right after Fastopen is re-enabled and will reset to
  	initial value when the blackhole issue goes away.

  	0 to disable the blackhole detection.

  	By default, it is set to 1hr.

  tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
  	The list consists of a primary key and an optional backup key. The
  	primary key is used for both creating and validating cookies, while the
  	optional backup key is only used for validating cookies. The purpose of
  	the backup key is to maximize TFO validation when keys are rotated.
  
  	A randomly chosen primary key may be configured by the kernel if
  	the tcp_fastopen sysctl is set to 0x400 (see above), or if the
  	TCP_FASTOPEN setsockopt() optname is set and a key has not been
  	previously configured via sysctl. If keys are configured via
  	setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
  	per-socket keys will be used instead of any keys that are specified via
  	sysctl.
  
  	A key is specified as 4 8-digit hexadecimal integers which are separated
  	by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
  	omitted. A primary and a backup key may be specified by separating them
  	by a comma. If only one key is specified, it becomes the primary key and
  	any previously configured backup keys are removed.

  tcp_syn_retries - INTEGER
  	Number of times initial SYNs for an active TCP connection attempt

  	will be retransmitted. Should not be higher than 127. Default value

  	is 6, which corresponds to 63seconds till the last retransmission

  	with the current initial RTO of 1second. With this the final timeout
  	for an active TCP connection attempt will happen after 127seconds.

  tcp_timestamps - INTEGER

  	Enable timestamps as defined in RFC1323.
  
  	- 0: Disabled.
  	- 1: Enable timestamps as defined in RFC1323 and use random offset for
  	  each connection rather than only using the current time.
  	- 2: Like 1, but without random offsets.

  	Default: 1

  tcp_min_tso_segs - INTEGER
  	Minimal number of segments per TSO frame.

  	Since linux-3.12, TCP does an automatic sizing of TSO frames,
  	depending on flow rate, instead of filling 64Kbytes packets.
  	For specific usages, it's possible to force TCP to build big
  	TSO frames. Note that TCP stack might split too big TSO packets
  	if available window is too small.

  	Default: 2

  tcp_pacing_ss_ratio - INTEGER
  	sk->sk_pacing_rate is set by TCP stack using a ratio applied
  	to current rate. (current_rate = cwnd * mss / srtt)
  	If TCP is in slow start, tcp_pacing_ss_ratio is applied
  	to let TCP probe for bigger speeds, assuming cwnd can be
  	doubled every other RTT.

  	Default: 200
  
  tcp_pacing_ca_ratio - INTEGER
  	sk->sk_pacing_rate is set by TCP stack using a ratio applied
  	to current rate. (current_rate = cwnd * mss / srtt)
  	If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
  	is applied to conservatively probe for bigger throughput.

  	Default: 120

  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_reuse - INTEGER
  	Enable reuse of TIME-WAIT sockets for new connections when it is
  	safe from protocol viewpoint.

  
  	- 0 - disable
  	- 1 - global enable
  	- 2 - enable for loopback traffic only

  	It should not be changed without advice/request of technical
  	experts.

  	Default: 2

  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 sockets.

  	Each TCP socket has rights to use it due to fact of its birth.

  	Default: 4K

  	default: initial size of send buffer used by TCP sockets.  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 tuned
  	send buffers for TCP sockets. This value does not override
  	net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
  	automatic tuning of that socket's send buffer size, in which case
  	this value is ignored.

  	Default: between 64K and 4MB, depending on RAM size.

  tcp_notsent_lowat - UNSIGNED INTEGER
  	A TCP socket can control the amount of unsent bytes in its write queue,
  	thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
  	reports POLLOUT events if the amount of unsent bytes is below a per
  	socket value, and if the write queue is not full. sendmsg() will
  	also not add new buffers if the limit is hit.
  
  	This global variable controls the amount of unsent data for
  	sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
  	to the global variable has immediate effect.
  
  	Default: UINT_MAX (0xFFFFFFFF)

  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_thin_linear_timeouts - BOOLEAN
  	Enable dynamic triggering of linear timeouts for thin streams.
  	If set, a check is performed upon retransmission by timeout to
  	determine if the stream is thin (less than 4 packets in flight).
  	As long as the stream is found to be thin, up to 6 linear
  	timeouts may be performed before exponential backoff mode is
  	initiated. This improves retransmission latency for
  	non-aggressive thin streams, often found to be time-dependent.
  	For more information on thin streams, see

  	Documentation/networking/tcp-thin.rst

  	Default: 0

  tcp_limit_output_bytes - INTEGER
  	Controls TCP Small Queue limit per tcp socket.
  	TCP bulk sender tends to increase packets in flight until it
  	gets losses notifications. With SNDBUF autotuning, this can

  	result in a large amount of packets queued on the local machine
  	(e.g.: qdiscs, CPU backlog, or device) hurting latency of other
  	flows, for typical pfifo_fast qdiscs.  tcp_limit_output_bytes
  	limits the number of bytes on qdisc or device to reduce artificial
  	RTT/cwnd and reduce bufferbloat.

  	Default: 1048576 (16 * 65536)

  tcp_challenge_ack_limit - INTEGER
  	Limits number of Challenge ACK sent per second, as recommended
  	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)

  	Default: 1000

  tcp_rx_skb_cache - BOOLEAN
  	Controls a per TCP socket cache of one skb, that might help
  	performance of some workloads. This might be dangerous
  	on systems with a lot of TCP sockets, since it increases
  	memory usage.
  
  	Default: 0 (disabled)

  UDP variables
  =============

  udp_l3mdev_accept - BOOLEAN
  	Enabling this option allows a "global" bound socket to work
  	across L3 master domains (e.g., VRFs) with packets capable of
  	being received regardless of the L3 domain in which they
  	originated. Only valid when the kernel was compiled with
  	CONFIG_NET_L3_MASTER_DEV.

  
  	Default: 0 (disabled)

  udp_mem - vector of 3 INTEGERs: min, pressure, max
  	Number of pages allowed for queueing by all UDP sockets.
  
  	min: Below this number of pages UDP is not bothered about its
  	memory appetite. When amount of memory allocated by UDP exceeds
  	this number, UDP starts to moderate memory usage.
  
  	pressure: This value was introduced to follow format of tcp_mem.
  
  	max: Number of pages allowed for queueing by all UDP sockets.
  
  	Default is calculated at boot time from amount of available memory.
  
  udp_rmem_min - INTEGER
  	Minimal size of receive buffer used by UDP sockets in moderation.
  	Each UDP socket is able to use the size for receiving data, even if
  	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

  	Default: 4K

  
  udp_wmem_min - INTEGER
  	Minimal size of send buffer used by UDP sockets in moderation.
  	Each UDP socket is able to use the size for sending data, even if
  	total pages of UDP sockets exceed udp_mem pressure. The unit is byte.

  	Default: 4K

  RAW variables
  =============

  
  raw_l3mdev_accept - BOOLEAN
  	Enabling this option allows a "global" bound socket to work
  	across L3 master domains (e.g., VRFs) with packets capable of
  	being received regardless of the L3 domain in which they
  	originated. Only valid when the kernel was compiled with
  	CONFIG_NET_L3_MASTER_DEV.

  	Default: 1 (enabled)

  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.

  	If possible, it is better these numbers have different parity
  	(one even and one odd value).
  	Must be greater than or equal to ip_unprivileged_port_start.

  	The default values are 32768 and 60999 respectively.

  ip_local_reserved_ports - list of comma separated ranges
  	Specify the ports which are reserved for known third-party
  	applications. These ports will not be used by automatic port
  	assignments (e.g. when calling connect() or bind() with port
  	number 0). Explicit port allocation behavior is unchanged.
  
  	The format used for both input and output is a comma separated
  	list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
  	10). Writing to the file will clear all previously reserved
  	ports and update the current list with the one given in the
  	input.
  
  	Note that ip_local_port_range and ip_local_reserved_ports
  	settings are independent and both are considered by the kernel
  	when determining which ports are available for automatic port
  	assignments.
  
  	You can reserve ports which are not in the current

  	ip_local_port_range, e.g.::

  	    $ cat /proc/sys/net/ipv4/ip_local_port_range
  	    32000	60999
  	    $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
  	    8080,9148

  
  	although this is redundant. However such a setting is useful
  	if later the port range is changed to a value that will
  	include the reserved ports.
  
  	Default: Empty

  ip_unprivileged_port_start - INTEGER
  	This is a per-namespace sysctl.  It defines the first
  	unprivileged port in the network namespace.  Privileged ports
  	require root or CAP_NET_BIND_SERVICE in order to bind to them.

  	To disable all privileged ports, set this to 0.  They must not
  	overlap with the ip_local_port_range.

  
  	Default: 1024

  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_autobind_reuse - BOOLEAN
  	By default, bind() does not select the ports automatically even if
  	the new socket and all sockets bound to the port have SO_REUSEADDR.
  	ip_autobind_reuse allows bind() to reuse the port and this is useful
  	when you use bind()+connect(), but may break some applications.
  	The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
  	option should only be set by experts.
  	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

  ip_early_demux - BOOLEAN
  	Optimize input packet processing down to one demux for
  	certain kinds of local sockets.  Currently we only do this

  	for established TCP and connected UDP sockets.

  
  	It may add an additional cost for pure routing workloads that
  	reduces overall throughput, in such case you should disable it.

  	Default: 1

  ping_group_range - 2 INTEGERS
  	Restrict ICMP_PROTO datagram sockets to users in the group range.
  	The default is "1 0", meaning, that nobody (not even root) may
  	create ping sockets.  Setting it to "100 100" would grant permissions
  	to the single group. "0 4294967295" would enable it for the world, "100
  	4294967295" would enable it for the users, but not daemons.

  tcp_early_demux - BOOLEAN
  	Enable early demux for established TCP sockets.

  	Default: 1
  
  udp_early_demux - BOOLEAN
  	Enable early demux for connected UDP sockets. Disable this if
  	your system could experience more unconnected load.

  	Default: 1

  icmp_echo_ignore_all - BOOLEAN

  	If set non-zero, then the kernel will ignore all ICMP ECHO
  	requests sent to it.

  	Default: 0

  icmp_echo_ignore_broadcasts - BOOLEAN

  	If set non-zero, then the kernel will ignore all ICMP ECHO and
  	TIMESTAMP requests sent to it via broadcast/multicast.

  	Default: 1

  
  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 minimal space between responses in milliseconds.

  	Note that another sysctl, icmp_msgs_per_sec limits the number
  	of ICMP packets	sent on all targets.

  	Default: 1000

  icmp_msgs_per_sec - INTEGER
  	Limit maximal number of ICMP packets sent per second from this host.
  	Only messages whose type matches icmp_ratemask (see below) are

  	controlled by this limit. For security reasons, the precise count
  	of messages per second is randomized.

  	Default: 1000

  icmp_msgs_burst - INTEGER
  	icmp_msgs_per_sec controls number of ICMP packets sent per second,
  	while icmp_msgs_burst controls the burst size of these packets.

  	For security reasons, the precise burst size is randomized.

  	Default: 50

  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 [1]_
  		4 Source Quench [1]_

  		5 Redirect
  		8 Echo Request

  		B Time Exceeded [1]_
  		C Parameter Problem [1]_

  		D Timestamp Request
  		E Timestamp Reply
  		F Info Request
  		G Info Reply
  		H Address Mask Request
  		I Address Mask Reply

  		= =========================

  	.. [1] 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: 1

  icmp_errors_use_inbound_ifaddr - BOOLEAN

  	If zero, icmp error messages are sent with the primary address of
  	the exiting interface.

  	If non-zero, the message will be sent with the primary address of
  	the interface that received the packet that caused the icmp error.
  	This is the behaviour network many administrators will expect from
  	a router. And it can make debugging complicated network layouts

  	much easier.

  
  	Note that if no primary address exists for the interface selected,
  	then the primary address of the first non-loopback interface that

  	has one will be used regardless of this setting.

  
  	Default: 0

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

  	Theoretical maximum value is bounded by having to send a membership
  	report in a single datagram (i.e. the report can't span multiple
  	datagrams, or risk confusing the switch and leaving groups you don't
  	intend to).

  	The number of supported groups 'M' is bounded by the number of group
  	report entries you can fit into a single datagram of 65535 bytes.
  
  	M = 65536-sizeof (ip header)/(sizeof(Group record))
  
  	Group records are variable length, with a minimum of 12 bytes.
  	So net.ipv4.igmp_max_memberships should not be set higher than:
  
  	(65536-24) / 12 = 5459
  
  	The value 5459 assumes no IP header options, so in practice
  	this number may be lower.

  igmp_max_msf - INTEGER
  	Maximum number of addresses allowed in the source filter list for a
  	multicast group.

  	Default: 10

  igmp_qrv - INTEGER

  	Controls the IGMP query robustness variable (see RFC2236 8.1).

  	Default: 2 (as specified by RFC2236 8.1)

  	Minimum: 1 (as specified by RFC6636 4.5)

  force_igmp_version - INTEGER

  	- 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
  	  allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
  	  Present timer expires.
  	- 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
  	  receive IGMPv2/v3 query.
  	- 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
  	  IGMPv1 query message. Will reply report if receive IGMPv3 query.
  	- 3 - Enforce to use IGMP version 3. The same react with default 0.
  
  	.. note::

  	   this is not the same with force_mld_version because IGMPv3 RFC3376
  	   Security Considerations does not have clear description that we could
  	   ignore other version messages completely as MLDv2 RFC3810. So make
  	   this value as default 0 is recommended.

  ``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.  This controls whether packets
  	received _on_ this interface can be forwarded.

  
  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

  proxy_arp_pvlan - BOOLEAN
  	Private VLAN proxy arp.

  	Basically allow proxy arp replies back to the same interface
  	(from which the ARP request/solicitation was received).
  
  	This is done to support (ethernet) switch features, like RFC
  	3069, where the individual ports are NOT allowed to
  	communicate with each other, but they are allowed to talk to
  	the upstream router.  As described in RFC 3069, it is possible
  	to allow these hosts to communicate through the upstream
  	router by proxy_arp'ing. Don't need to be used together with
  	proxy_arp.
  
  	This technology is known by different names:

  	  In RFC 3069 it is called VLAN Aggregation.
  	  Cisco and Allied Telesyn call it Private VLAN.
  	  Hewlett-Packard call it Source-Port filtering or port-isolation.
  	  Ericsson call it MAC-Forced Forwarding (RFC Draft).

  shared_media - BOOLEAN
  	Send(router) or accept(host) RFC1620 shared media redirects.

  	Overrides 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 to gateways listed in the
  	interface's current gateway list. Even if disabled, RFC1122 redirect
  	rules still apply.

  	Overridden by shared_media.

  	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)

  accept_local - BOOLEAN

  	Accept packets with local source addresses. In combination with
  	suitable routing, this can be used to direct packets between two
  	local interfaces over the wire and have them accepted properly.

  	default FALSE

  route_localnet - BOOLEAN
  	Do not consider loopback addresses as martian source or destination
  	while routing. This enables the use of 127/8 for local routing purposes.

  	default FALSE

  rp_filter - INTEGER

  	- 0 - No source validation.
  	- 1 - Strict mode as defined in RFC3704 Strict Reverse Path
  	  Each incoming packet is tested against the FIB and if the interface
  	  is not the best reverse path the packet check will fail.
  	  By default failed packets are discarded.
  	- 2 - Loose mode as defined in RFC3704 Loose Reverse Path
  	  Each incoming packet's source address is also tested against the FIB
  	  and if the source address is not reachable via any interface
  	  the packet check will fail.

  	Current recommended practice in RFC3704 is to enable strict mode

  	to prevent IP spoofing from DDos attacks. If using asymmetric routing

  	or other complicated routing, then loose mode is recommended.

  	The max value from conf/{all,interface}/rp_filter is used
  	when doing 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
  
  arp_announce - INTEGER
  	Define different restriction levels for announcing the local
  	source IP address from IP packets in ARP requests sent on
  	interface:

  
  	- 0 - (default) Use any local address, configured on any interface
  	- 1 - Try to avoid local addresses that are not in the target's
  	  subnet for this interface. This mode is useful when target
  	  hosts reachable via this interface require the source IP
  	  address in ARP requests to be part of their logical network
  	  configured on the receiving interface. When we generate the
  	  request we will check all our subnets that include the
  	  target IP and will preserve the source address if it is from
  	  such subnet. If there is no such subnet we select source
  	  address according to the rules for level 2.
  	- 2 - Always use the best local address for this target.
  	  In this mode we ignore the source address in the IP packet
  	  and try to select local address that we prefer for talks with
  	  the target host. Such local address is selected by looking
  	  for primary IP addresses on all our subnets on the outgoing
  	  interface that include the target IP address. If no suitable
  	  local address is found we select the first local address
  	  we have on the outgoing interface or on all other interfaces,
  	  with the hope we will receive reply for our request and
  	  even sometimes no matter the source IP address we announce.

  
  	The max value from conf/{all,interface}/arp_announce is used.
  
  	Increasing the restriction level gives more chance for
  	receiving answer from the resolved target while decreasing
  	the level announces more valid sender's information.
  
  arp_ignore - INTEGER
  	Define different modes for sending replies in response to
  	received ARP requests that resolve local target IP addresses:

  
  	- 0 - (default): reply for any local target IP address, configured
  	  on any interface
  	- 1 - reply only if the target IP address is local address
  	  configured on the incoming interface
  	- 2 - reply only if the target IP address is local address
  	  configured on the incoming interface and both with the
  	  sender's IP address are part from same subnet on this interface
  	- 3 - do not reply for local addresses configured with scope host,
  	  only resolutions for global and link addresses are replied
  	- 4-7 - reserved
  	- 8 - do not reply for all local addresses

  
  	The max value from conf/{all,interface}/arp_ignore is used
  	when ARP request is received on the {interface}

  arp_notify - BOOLEAN
  	Define mode for notification of address and device changes.

  
  	 ==  ==========================================================
  	  0  (default): do nothing
  	  1  Generate gratuitous arp requests when device is brought up
  	     or hardware address changes.
  	 ==  ==========================================================

  arp_accept - BOOLEAN

  	Define behavior for gratuitous ARP frames who's IP is not
  	already present in the ARP table:

  
  	- 0 - don't create new entries in the ARP table
  	- 1 - create new entries in the ARP table

  
  	Both replies and requests type gratuitous arp will trigger the
  	ARP table to be updated, if this setting is on.
  
  	If the ARP table already contains the IP address of the
  	gratuitous arp frame, the arp table will be updated regardless
  	if this setting is on or off.

  mcast_solicit - INTEGER
  	The maximum number of multicast probes in INCOMPLETE state,
  	when the associated hardware address is unknown.  Defaults
  	to 3.
  
  ucast_solicit - INTEGER
  	The maximum number of unicast probes in PROBE state, when
  	the hardware address is being reconfirmed.  Defaults to 3.

  app_solicit - INTEGER
  	The maximum number of probes to send to the user space ARP daemon
  	via netlink before dropping back to multicast probes (see

  	mcast_resolicit).  Defaults to 0.
  
  mcast_resolicit - INTEGER
  	The maximum number of multicast probes after unicast and
  	app probes in PROBE state.  Defaults to 0.

  
  disable_policy - BOOLEAN
  	Disable IPSEC policy (SPD) for this interface
  
  disable_xfrm - BOOLEAN
  	Disable IPSEC encryption on this interface, whatever the policy

  igmpv2_unsolicited_report_interval - INTEGER
  	The interval in milliseconds in which the next unsolicited
  	IGMPv1 or IGMPv2 report retransmit will take place.

  	Default: 10000 (10 seconds)

  igmpv3_unsolicited_report_interval - INTEGER
  	The interval in milliseconds in which the next unsolicited
  	IGMPv3 report retransmit will take place.

  	Default: 1000 (1 seconds)

  promote_secondaries - BOOLEAN
  	When a primary IP address is removed from this interface
  	promote a corresponding secondary IP address instead of
  	removing all the corresponding secondary IP addresses.

  drop_unicast_in_l2_multicast - BOOLEAN
  	Drop any unicast IP packets that are received in link-layer
  	multicast (or broadcast) frames.

  	This behavior (for multicast) is actually a SHOULD in RFC
  	1122, but is disabled by default for compatibility reasons.

  	Default: off (0)

  drop_gratuitous_arp - BOOLEAN
  	Drop all gratuitous ARP frames, for example if there's a known
  	good ARP proxy on the network and such frames need not be used
  	(or in the case of 802.11, must not be used to prevent attacks.)

  	Default: off (0)

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

  	Default value is 0.

  xfrm4_gc_thresh - INTEGER

  	(Obsolete since linux-4.14)

  	The threshold at which we will start garbage collecting for IPv4
  	destination cache entries.  At twice this value the system will

  	refuse new allocations.

  igmp_link_local_mcast_reports - BOOLEAN
  	Enable IGMP reports for link local multicast groups in the
  	224.0.0.X range.

  	Default TRUE

  Alexey Kuznetsov.
  kuznet@ms2.inr.ac.ru
  
  Updated by:

  - Andi Kleen
    ak@muc.de
  - Nicolas Delon
    delon.nicolas@wanadoo.fr

  
  /proc/sys/net/ipv6/* Variables
  ==============================

  
  IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
  apply 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 RFC3493)

  flowlabel_consistency - BOOLEAN
  	Protect the consistency (and unicity) of flow label.
  	You have to disable it to use IPV6_FL_F_REFLECT flag on the
  	flow label manager.

  
  	- TRUE: enabled
  	- FALSE: disabled

  	Default: TRUE

  auto_flowlabels - INTEGER
  	Automatically generate flow labels based on a flow hash of the
  	packet. This allows intermediate devices, such as routers, to
  	identify packet flows for mechanisms like Equal Cost Multipath

  	Routing (see RFC 6438).

  
  	=  ===========================================================
  	0  automatic flow labels are completely disabled
  	1  automatic flow labels are enabled by default, they can be

  	   disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
  	   socket option

  	2  automatic flow labels are allowed, they may be enabled on a

  	   per socket basis using the IPV6_AUTOFLOWLABEL socket option

  	3  automatic flow labels are enabled and enforced, they cannot

  	   be disabled by the socket option

  	=  ===========================================================

  	Default: 1

  flowlabel_state_ranges - BOOLEAN
  	Split the flow label number space into two ranges. 0-0x7FFFF is
  	reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
  	is reserved for stateless flow labels as described in RFC6437.

  
  	- TRUE: enabled
  	- FALSE: disabled

  	Default: true

  flowlabel_reflect - INTEGER
  	Control flow label reflection. Needed for Path MTU

  	Discovery to work with Equal Cost Multipath Routing in anycast
  	environments. See RFC 7690 and:
  	https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01