/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 hardend 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
  
  route/max_size - INTEGER
  	Maximum number of routes allowed in the kernel.  Increase
  	this when using large numbers of interfaces and/or routes.
  
  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_thresh3 - INTEGER
  	Maximum number of 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: 65536 Bytes(64KB)
  
  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: 31
  
  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 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 128.  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_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
  	Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold
  	for triggering fast retransmit when the amount of outstanding data is
  	small and when no previously unsent data can be transmitted (such
  	that limited transmit could be used). Also controls the use of
  	Tail loss probe (TLP) that converts RTOs occurring due to tail
  	losses into fast recovery (draft-dukkipati-tcpm-tcp-loss-probe-01).
  	Possible values:
  		0 disables ER
  		1 enables ER
  		2 enables ER but delays fast recovery and fast retransmit
  		  by a fourth of RTT. This mitigates connection falsely
  		  recovers when network has a small degree of reordering
  		  (less than 3 packets).
  		3 enables delayed ER and TLP.
  		4 enables TLP only.
  	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_fack - BOOLEAN
  	Enable FACK congestion avoidance and fast retransmission.
  	The value is not used, if tcp_sack is not enabled.
  
  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_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 have not
  	received an acknowledgment from connecting client.
  	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.
  
  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 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_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
  	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_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
  	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: 1 page
  
  	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_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 - BOOLEAN
  	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 feature (draft-ietf-tcpm-fastopen) to send data
  	in the opening SYN packet. To use this feature, the client application
  	must use sendmsg() or sendto() with MSG_FASTOPEN flag rather than
  	connect() to perform a TCP handshake automatically.
  
  	The values (bitmap) are
  	1: Enables sending data in the opening SYN on the client w/ MSG_FASTOPEN.
  	2: Enables TCP Fast Open on the server side, i.e., allowing data in
  	   a SYN packet to be accepted and passed to the application before
  	   3-way hand shake finishes.
  	4: Send data in the opening SYN regardless of cookie availability and
  	   without a cookie option.
  	0x100: Accept SYN data w/o validating the cookie.
  	0x200: Accept data-in-SYN w/o any cookie option present.
  	0x400/0x800: Enable Fast Open on all listeners regardless of the
  	   TCP_FASTOPEN socket option. The two different flags designate two
  	   different ways of setting max_qlen without the TCP_FASTOPEN socket
  	   option.
  
  	Default: 1
  
  	Note that the client & server side Fast Open flags (1 and 2
  	respectively) must be also enabled before the rest of flags can take
  	effect.
  
  	See include/net/tcp.h and the code for more details.
  
  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 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 - BOOLEAN
  	Enable timestamps as defined in RFC1323.
  
  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_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 sockets.
  	Each TCP socket has rights to use it due to fact of its birth.
  	Default: 1 page
  
  	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_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
  
  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.txt
  	Default: 0
  
  tcp_thin_dupack - BOOLEAN
  	Enable dynamic triggering of retransmissions after one dupACK
  	for thin streams. If set, a check is performed upon reception
  	of a dupACK to determine if the stream is thin (less than 4
  	packets in flight). As long as the stream is found to be thin,
  	data is retransmitted on the first received dupACK. 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.txt
  	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 in qdisc/device
  	on the local machine, 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: 131072
  
  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: 100
  
  UDP variables:
  
  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: 1 page
  
  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: 1 page
  
  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. The default values are
  	32768 and 61000 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	61000
  	$ 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_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
  
  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 sockets.
  
  	It may add an additional cost for pure routing workloads that
  	reduces overall throughput, in such case you should disable it.
  	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.
  	Default: 1000
  
  icmp_ratemask - INTEGER
  	Mask made of ICMP types for which rates are being limited.
  	Significant bits: IHGFEDCBA9876543210
  	Default mask:     0000001100000011000 (6168)
  
  	Bit definitions (see include/linux/icmp.h):
  		0 Echo Reply
  		3 Destination Unreachable *
  		4 Source Quench *
  		5 Redirect
  		8 Echo Request
  		B Time Exceeded *
  		C Parameter Problem *
  		D Timestamp Request
  		E Timestamp Reply
  		F Info Request
  		G Info Reply
  		H Address Mask Request
  		I Address Mask Reply
  
  	* These are rate limited by default (see default mask above)
  
  icmp_ignore_bogus_error_responses - BOOLEAN
  	Some routers violate RFC1122 by sending bogus responses to broadcast
  	frames.  Such violations are normally logged via a kernel warning.
  	If this is set to TRUE, the kernel will not give such warnings, which
  	will avoid log file clutter.
  	Default: 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.
  
  	conf/interface/*  changes special settings per interface (where
  	"interface" is the name of your network interface)
  
  	conf/all/*	  is special, changes the settings for all interfaces
  
  log_martians - BOOLEAN
  	Log packets with impossible addresses to kernel log.
  	log_martians for the interface will be enabled if at least one of
  	conf/{all,interface}/log_martians is set to TRUE,
  	it will be disabled otherwise
  
  accept_redirects - BOOLEAN
  	Accept ICMP redirect messages.
  	accept_redirects for the interface will be enabled if:
  	- both conf/{all,interface}/accept_redirects are TRUE in the case
  	  forwarding for the interface is enabled
  	or
  	- at least one of conf/{all,interface}/accept_redirects is TRUE in the
  	  case forwarding for the interface is disabled
  	accept_redirects for the interface will be disabled otherwise
  	default TRUE (host)
  		FALSE (router)
  
  forwarding - BOOLEAN
  	Enable IP forwarding on this interface.
  
  mc_forwarding - BOOLEAN
  	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
  	and a multicast routing daemon is required.
  	conf/all/mc_forwarding must also be set to TRUE to enable multicast
  	routing	for the interface
  
  medium_id - INTEGER
  	Integer value used to differentiate the devices by the medium they
  	are attached to. Two devices can have different id values when
  	the broadcast packets are received only on one of them.
  	The default value 0 means that the device is the only interface
  	to its medium, value of -1 means that medium is not known.
  
  	Currently, it is used to change the proxy_arp behavior:
  	the proxy_arp feature is enabled for packets forwarded between
  	two devices attached to different media.
  
  proxy_arp - BOOLEAN
  	Do proxy arp.
  	proxy_arp for the interface will be enabled if at least one of
  	conf/{all,interface}/proxy_arp is set to TRUE,
  	it will be disabled otherwise
  
  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 ip_secure_redirects.
  	shared_media for the interface will be enabled if at least one of
  	conf/{all,interface}/shared_media is set to TRUE,
  	it will be disabled otherwise
  	default TRUE
  
  secure_redirects - BOOLEAN
  	Accept ICMP redirect messages only for gateways,
  	listed in default gateway list.
  	secure_redirects for the interface will be enabled if at least one of
  	conf/{all,interface}/secure_redirects is set to TRUE,
  	it will be disabled otherwise
  	default TRUE
  
  send_redirects - BOOLEAN
  	Send redirects, if router.
  	send_redirects for the interface will be enabled if at least one of
  	conf/{all,interface}/send_redirects is set to TRUE,
  	it will be disabled otherwise
  	Default: TRUE
  
  bootp_relay - BOOLEAN
  	Accept packets with source address 0.b.c.d destined
  	not to this host as local ones. It is supposed, that
  	BOOTP relay daemon will catch and forward such packets.
  	conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
  	for the interface
  	default FALSE
  	Not Implemented Yet.
  
  accept_source_route - BOOLEAN
  	Accept packets with SRR option.
  	conf/all/accept_source_route must also be set to TRUE to accept packets
  	with SRR option on the interface
  	default TRUE (router)
  		FALSE (host)
  
  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.
  
  	rp_filter must be set to a non-zero value in order for
  	accept_local to have an effect.
  
  	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.
  
  
  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_solicit).  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.
  
  
  tag - INTEGER
  	Allows you to write a number, which can be used as required.
  	Default value is 0.
  
  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
  
  anycast_src_echo_reply - BOOLEAN
  	Controls the use of anycast addresses as source addresses for ICMPv6
  	echo reply
  	TRUE:  enabled
  	FALSE: disabled
  	Default: FALSE
  
  IPv6 Fragmentation:
  
  ip6frag_high_thresh - INTEGER
  	Maximum memory used to reassemble IPv6 fragments. When
  	ip6frag_high_thresh bytes of memory is allocated for this purpose,
  	the fragment handler will toss packets until ip6frag_low_thresh
  	is reached.
  
  ip6frag_low_thresh - INTEGER
  	See ip6frag_high_thresh
  
  ip6frag_time - INTEGER
  	Time in seconds to keep an IPv6 fragment in memory.
  
  ip6frag_secret_interval - INTEGER
  	Regeneration interval (in seconds) of the hash secret (or lifetime
  	for the hash secret) for IPv6 fragments.
  	Default: 600
  
  conf/default/*:
  	Change the interface-specific default settings.
  
  
  conf/all/*:
  	Change all the interface-specific settings.
  
  	[XXX:  Other special features than forwarding?]
  
  conf/all/forwarding - BOOLEAN
  	Enable global IPv6 forwarding between all interfaces.
  
  	IPv4 and IPv6 work differently here; e.g. netfilter must be used
  	to control which interfaces may forward packets and which not.
  
  	This also sets all interfaces' Host/Router setting
  	'forwarding' to the specified value.  See below for details.
  
  	This referred to as global forwarding.
  
  proxy_ndp - BOOLEAN
  	Do proxy ndp.
  
  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 - INTEGER
  	Accept Router Advertisements; autoconfigure using them.
  
  	It also determines whether or not to transmit Router
  	Solicitations. If and only if the functional setting is to
  	accept Router Advertisements, Router Solicitations will be
  	transmitted.
  
  	Possible values are:
  		0 Do not accept Router Advertisements.
  		1 Accept Router Advertisements if forwarding is disabled.
  		2 Overrule forwarding behaviour. Accept Router Advertisements
  		  even if forwarding is enabled.
  
  	Functional default: enabled if local forwarding is disabled.
  			    disabled if local forwarding is enabled.
  
  accept_ra_defrtr - BOOLEAN
  	Learn default router in Router Advertisement.
  
  	Functional default: enabled if accept_ra is enabled.
  			    disabled if accept_ra is disabled.
  
  accept_ra_pinfo - BOOLEAN
  	Learn Prefix Information in Router Advertisement.
  
  	Functional default: enabled if accept_ra is enabled.
  			    disabled if accept_ra is disabled.
  
  accept_ra_rt_info_max_plen - INTEGER
  	Maximum prefix length of Route Information in RA.
  
  	Route Information w/ prefix larger than or equal to this
  	variable shall be ignored.
  
  	Functional default: 0 if accept_ra_rtr_pref is enabled.
  			    -1 if accept_ra_rtr_pref is disabled.
  
  accept_ra_rtr_pref - BOOLEAN
  	Accept Router Preference in RA.
  
  	Functional default: enabled if accept_ra is enabled.
  			    disabled if accept_ra is disabled.
  
  accept_redirects - BOOLEAN
  	Accept Redirects.
  
  	Functional default: enabled if local forwarding is disabled.
  			    disabled if local forwarding is enabled.
  
  accept_source_route - INTEGER
  	Accept source routing (routing extension header).
  
  	>= 0: Accept only routing header type 2.
  	< 0: Do not accept routing header.
  
  	Default: 0
  
  autoconf - BOOLEAN
  	Autoconfigure addresses using Prefix Information in Router
  	Advertisements.
  
  	Functional default: enabled if accept_ra_pinfo is enabled.
  			    disabled if accept_ra_pinfo is disabled.
  
  dad_transmits - INTEGER
  	The amount of Duplicate Address Detection probes to send.
  	Default: 1
  
  forwarding - INTEGER
  	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.
  
  	Possible values are:
  		0 Forwarding disabled
  		1 Forwarding enabled
  
  	FALSE (0):
  
  	By default, Host behaviour is assumed.  This means:
  
  	1. IsRouter flag is not set in Neighbour Advertisements.
  	2. If accept_ra is TRUE (default), transmit Router
  	   Solicitations.
  	3. If accept_ra is TRUE (default), accept Router
  	   Advertisements (and do autoconfiguration).
  	4. If accept_redirects is TRUE (default), accept Redirects.
  
  	TRUE (1):
  
  	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 unless accept_ra is 2.
  	3. Router Advertisements are ignored unless accept_ra is 2.
  	4. Redirects are ignored.
  
  	Default: 0 (disabled) if global forwarding is disabled (default),
  		 otherwise 1 (enabled).
  
  hop_limit - INTEGER
  	Default Hop Limit to set.
  	Default: 64
  
  mtu - INTEGER
  	Default Maximum Transfer Unit
  	Default: 1280 (IPv6 required minimum)
  
  router_probe_interval - INTEGER
  	Minimum interval (in seconds) between Router Probing described
  	in RFC4191.
  
  	Default: 60
  
  router_solicitation_delay - INTEGER
  	Number of seconds to wait after interface is brought up
  	before sending Router Solicitations.
  	Default: 1
  
  router_solicitation_interval - INTEGER
  	Number of seconds to wait between Router Solicitations.
  	Default: 4
  
  router_solicitations - INTEGER
  	Number of Router Solicitations to send until assuming no
  	routers are present.
  	Default: 3
  
  use_tempaddr - INTEGER
  	Preference for Privacy Extensions (RFC3041).
  	  <= 0 : disable Privacy Extensions
  	  == 1 : enable Privacy Extensions, but prefer public
  	         addresses over temporary addresses.
  	  >  1 : enable Privacy Extensions and prefer temporary
  	         addresses over public addresses.
  	Default:  0 (for most devices)
  		 -1 (for point-to-point devices and loopback devices)
  
  temp_valid_lft - INTEGER
  	valid lifetime (in seconds) for temporary addresses.
  	Default: 604800 (7 days)
  
  temp_prefered_lft - INTEGER
  	Preferred lifetime (in seconds) for temporary addresses.
  	Default: 86400 (1 day)
  
  max_desync_factor - INTEGER
  	Maximum value for DESYNC_FACTOR, which is a random value
  	that ensures that clients don't synchronize with each
  	other and generate new addresses at exactly the same time.
  	value is in seconds.
  	Default: 600
  
  regen_max_retry - INTEGER
  	Number of attempts before give up attempting to generate
  	valid temporary addresses.
  	Default: 5
  
  max_addresses - INTEGER
  	Maximum number of autoconfigured addresses per interface.  Setting
  	to zero disables the limitation.  It is not recommended to set this
  	value too large (or to zero) because it would be an easy way to
  	crash the kernel by allowing too many addresses to be created.
  	Default: 16
  
  disable_ipv6 - BOOLEAN
  	Disable IPv6 operation.  If accept_dad is set to 2, this value
  	will be dynamically set to TRUE if DAD fails for the link-local
  	address.
  	Default: FALSE (enable IPv6 operation)
  
  	When this value is changed from 1 to 0 (IPv6 is being enabled),
  	it will dynamically create a link-local address on the given
  	interface and start Duplicate Address Detection, if necessary.
  
  	When this value is changed from 0 to 1 (IPv6 is being disabled),
  	it will dynamically delete all address on the given interface.
  
  accept_dad - INTEGER
  	Whether to accept DAD (Duplicate Address Detection).
  	0: Disable DAD
  	1: Enable DAD (default)
  	2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
  	   link-local address has been found.
  
  force_tllao - BOOLEAN
  	Enable sending the target link-layer address option even when
  	responding to a unicast neighbor solicitation.
  	Default: FALSE
  
  	Quoting from RFC 2461, section 4.4, Target link-layer address:
  
  	"The option MUST be included for multicast solicitations in order to
  	avoid infinite Neighbor Solicitation "recursion" when the peer node
  	does not have a cache entry to return a Neighbor Advertisements
  	message.  When responding to unicast solicitations, the option can be
  	omitted since the sender of the solicitation has the correct link-
  	layer address; otherwise it would not have be able to send the unicast
  	solicitation in the first place. However, including the link-layer
  	address in this case adds little overhead and eliminates a potential
  	race condition where the sender deletes the cached link-layer address
  	prior to receiving a response to a previous solicitation."
  
  ndisc_notify - BOOLEAN
  	Define mode for notification of address and device changes.
  	0 - (default): do nothing
  	1 - Generate unsolicited neighbour advertisements when device is brought
  	    up or hardware address changes.
  
  mldv1_unsolicited_report_interval - INTEGER
  	The interval in milliseconds in which the next unsolicited
  	MLDv1 report retransmit will take place.
  	Default: 10000 (10 seconds)
  
  mldv2_unsolicited_report_interval - INTEGER
  	The interval in milliseconds in which the next unsolicited
  	MLDv2 report retransmit will take place.
  	Default: 1000 (1 second)
  
  force_mld_version - INTEGER
  	0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
  	1 - Enforce to use MLD version 1
  	2 - Enforce to use MLD version 2
  
  suppress_frag_ndisc - INTEGER
  	Control RFC 6980 (Security Implications of IPv6 Fragmentation
  	with IPv6 Neighbor Discovery) behavior:
  	1 - (default) discard fragmented neighbor discovery packets
  	0 - allow fragmented neighbor discovery packets
  
  icmp/*:
  ratelimit - INTEGER
  	Limit the maximal rates for sending ICMPv6 packets.
  	0 to disable any limiting,
  	otherwise the minimal space between responses in milliseconds.
  	Default: 1000
  
  
  IPv6 Update by:
  Pekka Savola <pekkas@netcore.fi>
  YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
  
  
  /proc/sys/net/bridge/* Variables:
  
  bridge-nf-call-arptables - BOOLEAN
  	1 : pass bridged ARP traffic to arptables' FORWARD chain.
  	0 : disable this.
  	Default: 1
  
  bridge-nf-call-iptables - BOOLEAN
  	1 : pass bridged IPv4 traffic to iptables' chains.
  	0 : disable this.
  	Default: 1
  
  bridge-nf-call-ip6tables - BOOLEAN
  	1 : pass bridged IPv6 traffic to ip6tables' chains.
  	0 : disable this.
  	Default: 1
  
  bridge-nf-filter-vlan-tagged - BOOLEAN
  	1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
  	0 : disable this.
  	Default: 0
  
  bridge-nf-filter-pppoe-tagged - BOOLEAN
  	1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
  	0 : disable this.
  	Default: 0
  
  bridge-nf-pass-vlan-input-dev - BOOLEAN
  	1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
  	interface on the bridge and set the netfilter input device to the vlan.
  	This allows use of e.g. "iptables -i br0.1" and makes the REDIRECT
  	target work with vlan-on-top-of-bridge interfaces.  When no matching
  	vlan interface is found, or this switch is off, the input device is
  	set to the bridge interface.
  	0: disable bridge netfilter vlan interface lookup.
  	Default: 0
  
  proc/sys/net/sctp/* Variables:
  
  addip_enable - BOOLEAN
  	Enable or disable extension of  Dynamic Address Reconfiguration
  	(ADD-IP) functionality specified in RFC5061.  This extension provides
  	the ability to dynamically add and remove new addresses for the SCTP
  	associations.
  
  	1: Enable extension.
  
  	0: Disable extension.
  
  	Default: 0
  
  addip_noauth_enable - BOOLEAN
  	Dynamic Address Reconfiguration (ADD-IP) requires the use of
  	authentication to protect the operations of adding or removing new
  	addresses.  This requirement is mandated so that unauthorized hosts
  	would not be able to hijack associations.  However, older
  	implementations may not have implemented this requirement while
  	allowing the ADD-IP extension.  For reasons of interoperability,
  	we provide this variable to control the enforcement of the
  	authentication requirement.
  
  	1: Allow ADD-IP extension to be used without authentication.  This
  	   should only be set in a closed environment for interoperability
  	   with older implementations.
  
  	0: Enforce the authentication requirement
  
  	Default: 0
  
  auth_enable - BOOLEAN
  	Enable or disable Authenticated Chunks extension.  This extension
  	provides the ability to send and receive authenticated chunks and is
  	required for secure operation of Dynamic Address Reconfiguration
  	(ADD-IP) extension.
  
  	1: Enable this extension.
  	0: Disable this extension.
  
  	Default: 0
  
  prsctp_enable - BOOLEAN
  	Enable or disable the Partial Reliability extension (RFC3758) which
  	is used to notify peers that a given DATA should no longer be expected.
  
  	1: Enable extension
  	0: Disable
  
  	Default: 1
  
  max_burst - INTEGER
  	The limit of the number of new packets that can be initially sent.  It
  	controls how bursty the generated traffic can be.
  
  	Default: 4
  
  association_max_retrans - INTEGER
  	Set the maximum number for retransmissions that an association can
  	attempt deciding that the remote end is unreachable.  If this value
  	is exceeded, the association is terminated.
  
  	Default: 10
  
  max_init_retransmits - INTEGER
  	The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
  	that an association will attempt before declaring the destination
  	unreachable and terminating.
  
  	Default: 8
  
  path_max_retrans - INTEGER
  	The maximum number of retransmissions that will be attempted on a given
  	path.  Once this threshold is exceeded, the path is considered
  	unreachable, and new traffic will use a different path when the
  	association is multihomed.
  
  	Default: 5
  
  pf_retrans - INTEGER
  	The number of retransmissions that will be attempted on a given path
  	before traffic is redirected to an alternate transport (should one
  	exist).  Note this is distinct from path_max_retrans, as a path that
  	passes the pf_retrans threshold can still be used.  Its only
  	deprioritized when a transmission path is selected by the stack.  This
  	setting is primarily used to enable fast failover mechanisms without
  	having to reduce path_max_retrans to a very low value.  See:
  	http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
  	for details.  Note also that a value of pf_retrans > path_max_retrans
  	disables this feature
  
  	Default: 0
  
  rto_initial - INTEGER
  	The initial round trip timeout value in milliseconds that will be used
  	in calculating round trip times.  This is the initial time interval
  	for retransmissions.
  
  	Default: 3000
  
  rto_max - INTEGER
  	The maximum value (in milliseconds) of the round trip timeout.  This
  	is the largest time interval that can elapse between retransmissions.
  
  	Default: 60000
  
  rto_min - INTEGER
  	The minimum value (in milliseconds) of the round trip timeout.  This
  	is the smallest time interval the can elapse between retransmissions.
  
  	Default: 1000
  
  hb_interval - INTEGER
  	The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
  	are sent at the specified interval on idle paths to probe the state of
  	a given path between 2 associations.
  
  	Default: 30000
  
  sack_timeout - INTEGER
  	The amount of time (in milliseconds) that the implementation will wait
  	to send a SACK.
  
  	Default: 200
  
  valid_cookie_life - INTEGER
  	The default lifetime of the SCTP cookie (in milliseconds).  The cookie
  	is used during association establishment.
  
  	Default: 60000
  
  cookie_preserve_enable - BOOLEAN
  	Enable or disable the ability to extend the lifetime of the SCTP cookie
  	that is used during the establishment phase of SCTP association
  
  	1: Enable cookie lifetime extension.
  	0: Disable
  
  	Default: 1
  
  cookie_hmac_alg - STRING
  	Select the hmac algorithm used when generating the cookie value sent by
  	a listening sctp socket to a connecting client in the INIT-ACK chunk.
  	Valid values are:
  	* md5
  	* sha1
  	* none
  	Ability to assign md5 or sha1 as the selected alg is predicated on the
  	configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
  	CONFIG_CRYPTO_SHA1).
  
  	Default: Dependent on configuration.  MD5 if available, else SHA1 if
  	available, else none.
  
  rcvbuf_policy - INTEGER
  	Determines if the receive buffer is attributed to the socket or to
  	association.   SCTP supports the capability to create multiple
  	associations on a single socket.  When using this capability, it is
  	possible that a single stalled association that's buffering a lot
  	of data may block other associations from delivering their data by
  	consuming all of the receive buffer space.  To work around this,
  	the rcvbuf_policy could be set to attribute the receiver buffer space
  	to each association instead of the socket.  This prevents the described
  	blocking.
  
  	1: rcvbuf space is per association
  	0: rcvbuf space is per socket
  
  	Default: 0
  
  sndbuf_policy - INTEGER
  	Similar to rcvbuf_policy above, this applies to send buffer space.
  
  	1: Send buffer is tracked per association
  	0: Send buffer is tracked per socket.
  
  	Default: 0
  
  sctp_mem - vector of 3 INTEGERs: min, pressure, max
  	Number of pages allowed for queueing by all SCTP sockets.
  
  	min: Below this number of pages SCTP is not bothered about its
  	memory appetite. When amount of memory allocated by SCTP exceeds
  	this number, SCTP 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 SCTP sockets.
  
  	Default is calculated at boot time from amount of available memory.
  
  sctp_rmem - vector of 3 INTEGERs: min, default, max
  	Only the first value ("min") is used, "default" and "max" are
  	ignored.
  
  	min: Minimal size of receive buffer used by SCTP socket.
  	It is guaranteed to each SCTP socket (but not association) even
  	under moderate memory pressure.
  
  	Default: 1 page
  
  sctp_wmem  - vector of 3 INTEGERs: min, default, max
  	Currently this tunable has no effect.
  
  addr_scope_policy - INTEGER
  	Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
  
  	0   - Disable IPv4 address scoping
  	1   - Enable IPv4 address scoping
  	2   - Follow draft but allow IPv4 private addresses
  	3   - Follow draft but allow IPv4 link local addresses
  
  	Default: 1
  
  
  /proc/sys/net/core/*
  	Please see: Documentation/sysctl/net.txt for descriptions of these entries.
  
  
  /proc/sys/net/unix/*
  max_dgram_qlen - INTEGER
  	The maximum length of dgram socket receive queue
  
  	Default: 10
  
  
  UNDOCUMENTED:
  
  /proc/sys/net/irda/*
  	fast_poll_increase FIXME
  	warn_noreply_time FIXME
  	discovery_slots FIXME
  	slot_timeout FIXME
  	max_baud_rate FIXME
  	discovery_timeout FIXME
  	lap_keepalive_time FIXME
  	max_noreply_time FIXME
  	max_tx_data_size FIXME
  	max_tx_window FIXME
  	min_tx_turn_time FIXME