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ip-sysctl.txt
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/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 - BOOLEAN
Disable Path MTU Discovery.
default FALSE
min_pmtu - INTEGER
default 562 - minimum discovered Path MTU
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_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.
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.
rt_cache_rebuild_count - INTEGER
The per net-namespace route cache emergency rebuild threshold.
Any net-namespace having its route cache rebuilt due to
a hash bucket chain being too long more than this many times
will have its route caching disabled
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.
inet_peer_gc_mintime - INTEGER
Minimum interval between garbage collection passes. This interval is
in effect under high memory pressure on the pool.
Measured in seconds.
inet_peer_gc_maxtime - INTEGER
Minimum interval between garbage collection passes. This interval is
in effect under low (or absent) memory pressure on the pool.
Measured in 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_abc - INTEGER
Controls Appropriate Byte Count (ABC) defined in RFC3465.
ABC is a way of increasing congestion window (cwnd) more slowly
in response to partial acknowledgments.
Possible values are:
0 increase cwnd once per acknowledgment (no ABC)
1 increase cwnd once per acknowledgment of full sized segment
2 allow increase cwnd by two if acknowledgment is
of two segments to compensate for delayed acknowledgments.
Default: 0 (off)
tcp_abort_on_overflow - BOOLEAN
If listening service is too slow to accept new connections,
reset them. Default state is FALSE. It means that if overflow
occurred due to a burst, connection will recover. Enable this
option _only_ if you are really sure that listening daemon
cannot be tuned to accept connections faster. Enabling this
option can harm clients of your server.
tcp_adv_win_scale - INTEGER
Count buffering overhead as bytes/2^tcp_adv_win_scale
(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
if it is <= 0.
Possible values are [-31, 31], inclusive.
Default: 2
tcp_allowed_congestion_control - STRING
Show/set the congestion control choices available to non-privileged
processes. The list is a subset of those listed in
tcp_available_congestion_control.
Default is "reno" and the default setting (tcp_congestion_control).
tcp_app_win - INTEGER
Reserve max(window/2^tcp_app_win, mss) of window for application
buffer. Value 0 is special, it means that nothing is reserved.
Default: 31
tcp_available_congestion_control - STRING
Shows the available congestion control choices that are registered.
More congestion control algorithms may be available as modules,
but not loaded.
tcp_base_mss - INTEGER
The initial value of search_low to be used by 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.
tcp_cookie_size - INTEGER
Default size of TCP Cookie Transactions (TCPCT) option, that may be
overridden on a per socket basis by the TCPCT socket option.
Values greater than the maximum (16) are interpreted as the maximum.
Values greater than zero and less than the minimum (8) are interpreted
as the minimum. Odd values are interpreted as the next even value.
Default: 0 (off).
tcp_dsack - BOOLEAN
Allows TCP to send "duplicate" SACKs.
tcp_ecn - INTEGER
Enable Explicit Congestion Notification (ECN) in TCP. ECN is only
used when both ends of the TCP flow support it. It is useful to
avoid losses due to congestion (when the bottleneck router supports
ECN).
Possible values are:
0 disable ECN
1 ECN enabled
2 Only server-side ECN enabled. If the other end does
not support ECN, behavior is like with ECN disabled.
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
Time to hold socket in state FIN-WAIT-2, if it was closed
by our side. Peer can be broken and never close its side,
or even died unexpectedly. Default value is 60sec.
Usual value used in 2.2 was 180 seconds, you may restore
it, but remember that if your machine is even underloaded WEB server,
you risk to overflow memory with kilotons of dead sockets,
FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1,
because they eat maximum 1.5K of memory, but they tend
to live longer. Cf. tcp_max_orphans.
tcp_frto - INTEGER
Enables Forward RTO-Recovery (F-RTO) defined in RFC4138.
F-RTO is an enhanced recovery algorithm for TCP retransmission
timeouts. It is particularly beneficial in wireless environments
where packet loss is typically due to random radio interference
rather than intermediate router congestion. F-RTO is sender-side
only modification. Therefore it does not require any support from
the peer.
If set to 1, basic version is enabled. 2 enables SACK enhanced
F-RTO if flow uses SACK. The basic version can be used also when
SACK is in use though scenario(s) with it exists where F-RTO
interacts badly with the packet counting of the SACK enabled TCP
flow.
tcp_frto_response - INTEGER
When F-RTO has detected that a TCP retransmission timeout was
spurious (i.e, the timeout would have been avoided had TCP set a
longer retransmission timeout), TCP has several options what to do
next. Possible values are:
0 Rate halving based; a smooth and conservative response,
results in halved cwnd and ssthresh after one RTT
1 Very conservative response; not recommended because even
though being valid, it interacts poorly with the rest of
Linux TCP, halves cwnd and ssthresh immediately
2 Aggressive response; undoes congestion control measures
that are now known to be unnecessary (ignoring the
possibility of a lost retransmission that would require
TCP to be more cautious), cwnd and ssthresh are restored
to the values prior timeout
Default: 0 (rate halving based)
tcp_keepalive_time - INTEGER
How often TCP sends out keepalive messages when keepalive is enabled.
Default: 2hours.
tcp_keepalive_probes - INTEGER
How many keepalive probes TCP sends out, until it decides that the
connection is broken. Default value: 9.
tcp_keepalive_intvl - INTEGER
How frequently the probes are send out. Multiplied by
tcp_keepalive_probes it is time to kill not responding connection,
after probes started. Default value: 75sec i.e. connection
will be aborted after ~11 minutes of retries.
tcp_low_latency - BOOLEAN
If set, the TCP stack makes decisions that prefer lower
latency as opposed to higher throughput. By default, this
option is not set meaning that higher throughput is preferred.
An example of an application where this default should be
changed would be a Beowulf compute cluster.
Default: 0
tcp_max_orphans - INTEGER
Maximal number of TCP sockets not attached to any user file handle,
held by system. If this number is exceeded orphaned connections are
reset immediately and warning is printed. This limit exists
only to prevent simple DoS attacks, you _must_ not rely on this
or lower the limit artificially, but rather increase it
(probably, after increasing installed memory),
if network conditions require more than default value,
and tune network services to linger and kill such states
more aggressively. Let me to remind again: each orphan eats
up to ~64K of unswappable memory.
tcp_max_ssthresh - INTEGER
Limited Slow-Start for TCP with large congestion windows (cwnd) defined in
RFC3742. Limited slow-start is a mechanism to limit growth of the cwnd
on the region where cwnd is larger than tcp_max_ssthresh. TCP increases cwnd
by at most tcp_max_ssthresh segments, and by at least tcp_max_ssthresh/2
segments per RTT when the cwnd is above tcp_max_ssthresh.
If TCP connection increased cwnd to thousands (or tens of thousands) segments,
and thousands of packets were being dropped during slow-start, you can set
tcp_max_ssthresh to improve performance for new TCP connection.
Default: 0 (off)
tcp_max_syn_backlog - INTEGER
Maximal number of remembered connection requests, which are
still did not receive an acknowledgment from connecting client.
Default value is 1024 for systems with more than 128Mb of memory,
and 128 for low memory machines. If server suffers of overload,
try to increase this number.
tcp_max_tw_buckets - INTEGER
Maximal number of timewait sockets held by system simultaneously.
If this number is exceeded time-wait socket is immediately destroyed
and warning is printed. This limit exists only to prevent
simple DoS attacks, you _must_ not lower the limit artificially,
but rather increase it (probably, after increasing installed memory),
if network conditions require more than default value.
tcp_mem - vector of 3 INTEGERs: min, pressure, max
min: below this number of pages TCP is not bothered about its
memory appetite.
pressure: when amount of memory allocated by TCP exceeds this number
of pages, TCP moderates its memory consumption and enters memory
pressure mode, which is exited when memory consumption falls
under "min".
max: number of pages allowed for queueing by all TCP sockets.
Defaults are calculated at boot time from amount of available
memory.
tcp_moderate_rcvbuf - BOOLEAN
If set, TCP performs receive buffer 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 7.
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: 8K
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 4MB, 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 ~180seconds.
tcp_syncookies - BOOLEAN
Only valid when the kernel was compiled with CONFIG_SYNCOOKIES
Send out syncookies when the syn backlog queue of a socket
overflows. This is to prevent against the common 'SYN flood attack'
Default: FALSE
Note, that syncookies is fallback facility.
It MUST NOT be used to help highly loaded servers to stand
against legal connection rate. If you see 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.
tcp_syn_retries - INTEGER
Number of times initial SYNs for an active TCP connection attempt
will be retransmitted. Should not be higher than 255. Default value
is 5, which corresponds to ~180seconds.
tcp_timestamps - BOOLEAN
Enable timestamps as defined in RFC1323.
tcp_tso_win_divisor - INTEGER
This allows control over what percentage of the congestion window
can be consumed by a single TSO frame.
The setting of this parameter is a choice between burstiness and
building larger TSO frames.
Default: 3
tcp_tw_recycle - BOOLEAN
Enable fast recycling TIME-WAIT sockets. Default value is 0.
It should not be changed without advice/request of technical
experts.
tcp_tw_reuse - BOOLEAN
Allow to reuse TIME-WAIT sockets for new connections when it is
safe from protocol viewpoint. Default value is 0.
It should not be changed without advice/request of technical
experts.
tcp_window_scaling - BOOLEAN
Enable window scaling as defined in RFC1323.
tcp_wmem - vector of 3 INTEGERs: min, default, max
min: Amount of memory reserved for send buffers for TCP sockets.
Each TCP socket has rights to use it due to fact of its birth.
Default: 4K
default: initial size of send buffer used by TCP sockets. This
value overrides net.core.wmem_default used by other protocols.
It is usually lower than net.core.wmem_default.
Default: 16K
max: Maximal amount of memory allowed for automatically tuned
send buffers for TCP sockets. This value does not override
net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables
automatic tuning of that socket's send buffer size, in which case
this value is ignored.
Default: between 64K and 4MB, depending on RAM size.
tcp_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
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: 4096
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: 4096
CIPSOv4 Variables:
cipso_cache_enable - BOOLEAN
If set, enable additions to and lookups from the CIPSO label mapping
cache. If unset, additions are ignored and lookups always result in a
miss. However, regardless of the setting the cache is still
invalidated when required when means you can safely toggle this on and
off and the cache will always be "safe".
Default: 1
cipso_cache_bucket_size - INTEGER
The CIPSO label cache consists of a fixed size hash table with each
hash bucket containing a number of cache entries. This variable limits
the number of entries in each hash bucket; the larger the value the
more CIPSO label mappings that can be cached. When the number of
entries in a given hash bucket reaches this limit adding new entries
causes the oldest entry in the bucket to be removed to make room.
Default: 10
cipso_rbm_optfmt - BOOLEAN
Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
the CIPSO draft specification (see Documentation/netlabel for details).
This means that when set the CIPSO tag will be padded with empty
categories in order to make the packet data 32-bit aligned.
Default: 0
cipso_rbm_structvalid - BOOLEAN
If set, do a very strict check of the CIPSO option when
ip_options_compile() is called. If unset, relax the checks done during
ip_options_compile(). Either way is "safe" as errors are caught else
where in the CIPSO processing code but setting this to 0 (False) should
result in less work (i.e. it should be faster) but could cause problems
with other implementations that require strict checking.
Default: 0
IP Variables:
ip_local_port_range - 2 INTEGERS
Defines the local port range that is used by TCP and UDP to
choose the local port. The first number is the first, the
second the last local port number. Default value depends on
amount of memory available on the system:
> 128Mb 32768-61000
< 128Mb 1024-4999 or even less.
This number defines number of active connections, which this
system can issue simultaneously to systems not supporting
TCP extensions (timestamps). With tcp_tw_recycle enabled
(i.e. by default) range 1024-4999 is enough to issue up to
2000 connections per second to systems supporting timestamps.
ip_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
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: FALSE
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.
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
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