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

此文内容从网络收集，介绍了Linux系统 /proc/sys/net/ipv4/ 目录下所有内核参数的含义，由于简书每篇文章字数的限制，此文被分成了两个部分，这是第一部分。

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

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

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 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 - 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. This also serves as a maximum limit to namespaces
different from the initial one.

ipfrag_low_thresh - INTEGER

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 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

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_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.

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, 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_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.
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 - 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_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.
RACK: 0x2 makes RACK's reordering window static (min_rtt/4).

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: 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 (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 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.
By default, it is set to 1hr.

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 - 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_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_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: 262144

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_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.

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.
If possible, it is better these numbers have different parity.
(one even and one odd values)
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.  It may not
overlap with the ip_local_reserved_ports 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_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

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.
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.
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 *
    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