linux/Documentation/admin-guide/sysctl/net.rst

================================
Documentation for /proc/sys/net/
================================

Copyright

Copyright (c) 1999

	- Terrehon Bowden <terrehon@pacbell.net>
	- Bodo Bauer <bb@ricochet.net>

Copyright (c) 2000

	- Jorge Nerin <comandante@zaralinux.com>

Copyright (c) 2009

	- Shen Feng <shen@cn.fujitsu.com>

For general info and legal blurb, please look in index.rst.

------------------------------------------------------------------------------

This file contains the documentation for the sysctl files in
/proc/sys/net

The interface  to  the  networking  parts  of  the  kernel  is  located  in
/proc/sys/net. The following table shows all possible subdirectories.  You may
see only some of them, depending on your kernel's configuration.


Table : Subdirectories in /proc/sys/net

 ========= =================== = ========== ===================
 Directory Content               Directory  Content
 ========= =================== = ========== ===================
 802       E802 protocol         mptcp      Multipath TCP
 appletalk Appletalk protocol    netfilter  Network Filter
 bridge    Bridging              tipc       TIPC
 core      General parameter     unix       Unix domain sockets
 ethernet  Ethernet protocol     vsock      VSOCK sockets
 ipv4      IP version 4          x25        X.25 protocol
 ipv6      IP version 6
 ========= =================== = ========== ===================

1. /proc/sys/net/core - Network core options
============================================

bpf_jit_enable
--------------

This enables the BPF Just in Time (JIT) compiler. BPF is a flexible
and efficient infrastructure allowing to execute bytecode at various
hook points. It is used in a number of Linux kernel subsystems such
as networking (e.g. XDP, tc), tracing (e.g. kprobes, uprobes, tracepoints)
and security (e.g. seccomp). LLVM has a BPF back end that can compile
restricted C into a sequence of BPF instructions. After program load
through bpf(2) and passing a verifier in the kernel, a JIT will then
translate these BPF proglets into native CPU instructions. There are
two flavors of JITs, the newer eBPF JIT currently supported on:

  - x86_64
  - x86_32
  - arm64
  - arm32
  - ppc64
  - ppc32
  - sparc64
  - mips64
  - s390x
  - riscv64
  - riscv32
  - loongarch64
  - arc

And the older cBPF JIT supported on the following archs:

  - mips
  - sparc

eBPF JITs are a superset of cBPF JITs, meaning the kernel will
migrate cBPF instructions into eBPF instructions and then JIT
compile them transparently. Older cBPF JITs can only translate
tcpdump filters, seccomp rules, etc, but not mentioned eBPF
programs loaded through bpf(2).

Values:

	- 0 - disable the JIT (default value)
	- 1 - enable the JIT
	- 2 - enable the JIT and ask the compiler to emit traces on kernel log.

bpf_jit_harden
--------------

This enables hardening for the BPF JIT compiler. Supported are eBPF
JIT backends. Enabling hardening trades off performance, but can
mitigate JIT spraying.

Values:

	- 0 - disable JIT hardening (default value)
	- 1 - enable JIT hardening for unprivileged users only
	- 2 - enable JIT hardening for all users

where "privileged user" in this context means a process having
CAP_BPF or CAP_SYS_ADMIN in the root user name space.

bpf_jit_kallsyms
----------------

When BPF JIT compiler is enabled, then compiled images are unknown
addresses to the kernel, meaning they neither show up in traces nor
in /proc/kallsyms. This enables export of these addresses, which can
be used for debugging/tracing. If bpf_jit_harden is enabled, this
feature is disabled.

Values :

	- 0 - disable JIT kallsyms export (default value)
	- 1 - enable JIT kallsyms export for privileged users only

bpf_jit_limit
-------------

This enforces a global limit for memory allocations to the BPF JIT
compiler in order to reject unprivileged JIT requests once it has
been surpassed. bpf_jit_limit contains the value of the global limit
in bytes.

dev_weight
----------

The maximum number of packets that kernel can handle on a NAPI interrupt,
it's a Per-CPU variable. For drivers that support LRO or GRO_HW, a hardware
aggregated packet is counted as one packet in this context.

Default: 64

dev_weight_rx_bias
------------------

RPS (e.g. RFS, aRFS) processing is competing with the registered NAPI poll function
of the driver for the per softirq cycle netdev_budget. This parameter influences
the proportion of the configured netdev_budget that is spent on RPS based packet
processing during RX softirq cycles. It is further meant for making current
dev_weight adaptable for asymmetric CPU needs on RX/TX side of the network stack.
(see dev_weight_tx_bias) It is effective on a per CPU basis. Determination is based
on dev_weight and is calculated multiplicative (dev_weight * dev_weight_rx_bias).

Default: 1

dev_weight_tx_bias
------------------

Scales the maximum number of packets that can be processed during a TX softirq cycle.
Effective on a per CPU basis. Allows scaling of current dev_weight for asymmetric
net stack processing needs. Be careful to avoid making TX softirq processing a CPU hog.

Calculation is based on dev_weight (dev_weight * dev_weight_tx_bias).

Default: 1

default_qdisc
-------------

The default queuing discipline to use for network devices. This allows
overriding the default of pfifo_fast with an alternative. Since the default
queuing discipline is created without additional parameters so is best suited
to queuing disciplines that work well without configuration like stochastic
fair queue (sfq), CoDel (codel) or fair queue CoDel (fq_codel). Don't use
queuing disciplines like Hierarchical Token Bucket or Deficit Round Robin
which require setting up classes and bandwidths. Note that physical multiqueue
interfaces still use mq as root qdisc, which in turn uses this default for its
leaves. Virtual devices (like e.g. lo or veth) ignore this setting and instead
default to noqueue.

Default: pfifo_fast

busy_read
---------

Low latency busy poll timeout for socket reads. (needs CONFIG_NET_RX_BUSY_POLL)
Approximate time in us to busy loop waiting for packets on the device queue.
This sets the default value of the SO_BUSY_POLL socket option.
Can be set or overridden per socket by setting socket option SO_BUSY_POLL,
which is the preferred method of enabling. If you need to enable the feature
globally via sysctl, a value of 50 is recommended.

Will increase power usage.

Default: 0 (off)

busy_poll
----------------
Low latency busy poll timeout for poll and select. (needs CONFIG_NET_RX_BUSY_POLL)
Approximate time in us to busy loop waiting for events.
Recommended value depends on the number of sockets you poll on.
For several sockets 50, for several hundreds 100.
For more than that you probably want to use epoll.
Note that only sockets with SO_BUSY_POLL set will be busy polled,
so you want to either selectively set SO_BUSY_POLL on those sockets or set
sysctl.net.busy_read globally.

Will increase power usage.

Default: 0 (off)

mem_pcpu_rsv
------------

Per-cpu reserved forward alloc cache size in page units. Default 1MB per CPU.

bypass_prot_mem
---------------

Skip charging socket buffers to the global per-protocol memory
accounting controlled by net.ipv4.tcp_mem, net.ipv4.udp_mem, etc.

Default: 0 (off)

rmem_default
------------

The default setting of the socket receive buffer in bytes.

rmem_max
--------

The maximum receive socket buffer size in bytes.

Default: 4194304

rps_default_mask
----------------

The default RPS CPU mask used on newly created network devices. An empty
mask means RPS disabled by default.

tstamp_allow_data
-----------------
Allow processes to receive tx timestamps looped together with the original
packet contents. If disabled, transmit timestamp requests from unprivileged
processes are dropped unless socket option SOF_TIMESTAMPING_OPT_TSONLY is set.

Default: 1 (on)


wmem_default
------------

The default setting (in bytes) of the socket send buffer.

wmem_max
--------

The maximum send socket buffer size in bytes.

Default: 4194304

message_burst and message_cost
------------------------------

These parameters  are used to limit the warning messages written to the kernel
log from  the  networking  code.  They  enforce  a  rate  limit  to  make  a
denial-of-service attack  impossible. A higher message_cost factor, results in
fewer messages that will be written. Message_burst controls when messages will
be dropped.  The  default  settings  limit  warning messages to one every five
seconds.

warnings
--------

This sysctl is now unused.

This was used to control console messages from the networking stack that
occur because of problems on the network like duplicate address or bad
checksums.

These messages are now emitted at KERN_DEBUG and can generally be enabled
and controlled by the dynamic_debug facility.

netdev_budget
-------------

Maximum number of packets taken from all interfaces in one polling cycle (NAPI
poll). In one polling cycle interfaces which are registered to polling are
probed in a round-robin manner. Also, a polling cycle may not exceed
netdev_budget_usecs microseconds, even if netdev_budget has not been
exhausted.

netdev_budget_usecs
---------------------

Maximum number of microseconds in one NAPI polling cycle. Polling
will exit when either netdev_budget_usecs have elapsed during the
poll cycle or the number of packets processed reaches netdev_budget.

netdev_max_backlog
------------------

Maximum number of packets, queued on the INPUT side, when the interface
receives packets faster than kernel can process them.

qdisc_max_burst
------------------

Maximum number of packets that can be temporarily stored before
reaching qdisc.

Default: 1000

netdev_rss_key
--------------

RSS (Receive Side Scaling) enabled drivers use a host key that
is randomly generated.
Some user space might need to gather its content even if drivers do not
provide ethtool -x support yet.

::

  myhost:~# cat /proc/sys/net/core/netdev_rss_key
  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8: ... (256 bytes total)

File contains all nul bytes if no driver ever called netdev_rss_key_fill()
function.

Note:
  /proc/sys/net/core/netdev_rss_key contains 256 bytes of key,
  but many drivers only use 40 or 52 bytes of it.

::

  myhost:~# ethtool -x eth0
  RX flow hash indirection table for eth0 with 8 RX ring(s):
      0:    0     1     2     3     4     5     6     7
  RSS hash key:
  84:50:f4:00:a8:15:d1:a7:e9:7f:1d:60:35:c7:47:25:42:97:74:ca:56:bb:b6:a1:d8:43:e3:c9:0c:fd:17:55:c2:3a:4d:69:ed:f1:42:89

netdev_tstamp_prequeue
----------------------

If set to 0, RX packet timestamps can be sampled after RPS processing, when
the target CPU processes packets. It might give some delay on timestamps, but
permit to distribute the load on several cpus.

If set to 1 (default), timestamps are sampled as soon as possible, before
queueing.

netdev_unregister_timeout_secs
------------------------------

Unregister network device timeout in seconds.
This option controls the timeout (in seconds) used to issue a warning while
waiting for a network device refcount to drop to 0 during device
unregistration. A lower value may be useful during bisection to detect
a leaked reference faster. A larger value may be useful to prevent false
warnings on slow/loaded systems.
Default value is 10, minimum 1, maximum 3600.

skb_defer_max
-------------

Max size (in skbs) of the per-cpu list of skbs being freed
by the cpu which allocated them.

Default: 128

optmem_max
----------

Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
of struct cmsghdr structures with appended data. TCP tx zerocopy also uses
optmem_max as a limit for its internal structures.

Default : 128 KB

fb_tunnels_only_for_init_net
----------------------------

Controls if fallback tunnels (like tunl0, gre0, gretap0, erspan0,
sit0, ip6tnl0, ip6gre0) are automatically created. There are 3 possibilities
(a) value = 0; respective fallback tunnels are created when module is
loaded in every net namespaces (backward compatible behavior).
(b) value = 1; [kcmd value: initns] respective fallback tunnels are
created only in init net namespace and every other net namespace will
not have them.
(c) value = 2; [kcmd value: none] fallback tunnels are not created
when a module is loaded in any of the net namespace. Setting value to
"2" is pointless after boot if these modules are built-in, so there is
a kernel command-line option that can change this default. Please refer to
Documentation/admin-guide/kernel-parameters.txt for additional details.

Not creating fallback tunnels gives control to userspace to create
whatever is needed only and avoid creating devices which are redundant.

Default : 0  (for compatibility reasons)

devconf_inherit_init_net
------------------------

Controls if a new network namespace should inherit all current
settings under /proc/sys/net/{ipv4,ipv6}/conf/{all,default}/. By
default, we keep the current behavior: for IPv4 we inherit all current
settings from init_net and for IPv6 we reset all settings to default.

If set to 1, both IPv4 and IPv6 settings are forced to inherit from
current ones in init_net. If set to 2, both IPv4 and IPv6 settings are
forced to reset to their default values. If set to 3, both IPv4 and IPv6
settings are forced to inherit from current ones in the netns where this
new netns has been created.

Default : 0  (for compatibility reasons)

txrehash
--------

Controls default hash rethink behaviour on socket when SO_TXREHASH option is set
to SOCK_TXREHASH_DEFAULT (i. e. not overridden by setsockopt).

If set to 1 (default), hash rethink is performed on listening socket.
If set to 0, hash rethink is not performed.

txq_reselection_ms
------------------

Controls how often (in ms) a busy connected flow can select another tx queue.

A resection is desirable when/if user thread has migrated and XPS
would select a different queue. Same can occur without XPS
if the flow hash has changed.

But switching txq can introduce reorders, especially if the
old queue is under high pressure. Modern TCP stacks deal
well with reorders if they happen not too often.

To disable this feature, set the value to 0.

Default : 1000

gro_normal_batch
----------------

Maximum number of the segments to batch up on output of GRO. When a packet
exits GRO, either as a coalesced superframe or as an original packet which
GRO has decided not to coalesce, it is placed on a per-NAPI list. This
list is then passed to the stack when the number of segments reaches the
gro_normal_batch limit.

high_order_alloc_disable
------------------------

By default the allocator for page frags tries to use high order pages (order-3
on x86). While the default behavior gives good results in most cases, some users
might have hit a contention in page allocations/freeing. This was especially
true on older kernels (< 5.14) when high-order pages were not stored on per-cpu
lists. This allows to opt-in for order-0 allocation instead but is now mostly of
historical importance.

Default: 0

2. /proc/sys/net/unix - Parameters for Unix domain sockets
----------------------------------------------------------

There is only one file in this directory.
unix_dgram_qlen limits the max number of datagrams queued in Unix domain
socket's buffer. It will not take effect unless PF_UNIX flag is specified.


3. /proc/sys/net/ipv4 - IPV4 settings
-------------------------------------
Please see: Documentation/networking/ip-sysctl.rst and
Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.


4. TIPC
-------

tipc_rmem
---------

The TIPC protocol now has a tunable for the receive memory, similar to the
tcp_rmem - i.e. a vector of 3 INTEGERs: (min, default, max)

::

    # cat /proc/sys/net/tipc/tipc_rmem
    4252725 34021800        68043600
    #

The max value is set to CONN_OVERLOAD_LIMIT, and the default and min values
are scaled (shifted) versions of that same value.  Note that the min value
is not at this point in time used in any meaningful way, but the triplet is
preserved in order to be consistent with things like tcp_rmem.

named_timeout
-------------

TIPC name table updates are distributed asynchronously in a cluster, without
any form of transaction handling. This means that different race scenarios are
possible. One such is that a name withdrawal sent out by one node and received
by another node may arrive after a second, overlapping name publication already
has been accepted from a third node, although the conflicting updates
originally may have been issued in the correct sequential order.
If named_timeout is nonzero, failed topology updates will be placed on a defer
queue until another event arrives that clears the error, or until the timeout
expires. Value is in milliseconds.

6. /proc/sys/net/vsock - VSOCK sockets
--------------------------------------

VSOCK sockets (AF_VSOCK) provide communication between virtual machines and
their hosts. The behavior of VSOCK sockets in a network namespace is determined
by the namespace's mode (``global`` or ``local``), which controls how CIDs
(Context IDs) are allocated and how sockets interact across namespaces.

ns_mode
-------

Read-only. Reports the current namespace's mode, set at namespace creation
and immutable thereafter.

Values:

	- ``global`` - the namespace shares system-wide CID allocation and
	  its sockets can reach any VM or socket in any global namespace.
	  Sockets in this namespace cannot reach sockets in local
	  namespaces.
	- ``local`` - the namespace has private CID allocation and its
	  sockets can only connect to VMs or sockets within the same
	  namespace.

The init_net mode is always ``global``.

child_ns_mode
-------------

Controls what mode newly created child namespaces will inherit. At namespace
creation, ``ns_mode`` is inherited from the parent's ``child_ns_mode``. The
initial value matches the namespace's own ``ns_mode``.

Values:

	- ``global`` - child namespaces will share system-wide CID allocation
	  and their sockets will be able to reach any VM or socket in any
	  global namespace.
	- ``local`` - child namespaces will have private CID allocation and
	  their sockets will only be able to connect within their own
	  namespace.

The first write to ``child_ns_mode`` locks its value. Subsequent writes of the
same value succeed, but writing a different value returns ``-EBUSY``.

Changing ``child_ns_mode`` only affects namespaces created after the change;
it does not modify the current namespace or any existing children.

A namespace with ``ns_mode`` set to ``local`` cannot change
``child_ns_mode`` to ``global`` (returns ``-EPERM``).

g2h_fallback
------------

Controls whether connections to CIDs not owned by the host-to-guest (H2G)
transport automatically fall back to the guest-to-host (G2H) transport.

When enabled, if a connect targets a CID that the H2G transport (e.g.
vhost-vsock) does not serve, or if no H2G transport is loaded at all, the
connection is routed via the G2H transport (e.g. virtio-vsock) instead. This
allows a host running both nested VMs (via vhost-vsock) and sibling VMs
reachable through the hypervisor (e.g. Nitro Enclaves) to address both using
a single CID space, without requiring applications to set
``VMADDR_FLAG_TO_HOST``.

When the fallback is taken, ``VMADDR_FLAG_TO_HOST`` is automatically set on
the remote address so that userspace can determine the path via
``getpeername()``.

Note: With this sysctl enabled, user space that attempts to talk to a guest
CID which is not implemented by the H2G transport will create host vsock
traffic. Environments that rely on H2G-only isolation should set it to 0.

Values:

	- 0 - Connections to CIDs <= 2 or with VMADDR_FLAG_TO_HOST use G2H;
	  all others use H2G (or fail with ENODEV if H2G is not loaded).
	- 1 - Connections to CIDs not owned by H2G fall back to G2H. (default)