Documentation for /proc/sys/net/*

  	(c) 1999		Terrehon Bowden <terrehon@pacbell.net>
  				Bodo Bauer <bb@ricochet.net>
  	(c) 2000		Jorge Nerin <comandante@zaralinux.com>
  	(c) 2009		Shen Feng <shen@cn.fujitsu.com>
  
  For general info and legal blurb, please look in README.
  
  ==============================================================
  
  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
   core      General parameter   appletalk  Appletalk protocol
   unix      Unix domain sockets netrom     NET/ROM
   802       E802 protocol       ax25       AX25
   ethernet  Ethernet protocol   rose       X.25 PLP layer
   ipv4      IP version 4        x25        X.25 protocol
   ipx       IPX                 token-ring IBM token ring
   bridge    Bridging            decnet     DEC net

   ipv6      IP version 6        tipc       TIPC

  ..............................................................................
  
  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
    - arm64

    - arm32

    - ppc64
    - sparc64
    - mips64

    - s390x

  And the older cBPF JIT supported on the following archs:

    - mips
    - ppc
    - 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

  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

  dev_weight
  --------------
  
  The maximum number of packets that kernel can handle on a NAPI interrupt,
  it's a Per-CPU variable.
  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)

  rmem_default
  ------------
  
  The default setting of the socket receive buffer in bytes.
  
  rmem_max
  --------
  
  The maximum receive socket buffer size in bytes.

  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.
  
  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.

  netdev_rss_key
  --------------
  
  RSS (Receive Side Scaling) enabled drivers use a 40 bytes 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: ... (52 bytes total)
  
  File contains nul bytes if no driver ever called netdev_rss_key_fill() function.
  Note:
  /proc/sys/net/core/netdev_rss_key contains 52 bytes of key,
  but most drivers only use 40 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.

  optmem_max
  ----------
  
  Maximum ancillary buffer size allowed per socket. Ancillary data is a sequence
  of struct cmsghdr structures with appended data.
  
  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.txt and ipvs-sysctl.txt for
  descriptions of these entries.
  
  
  4. Appletalk
  -------------------------------------------------------
  
  The /proc/sys/net/appletalk  directory  holds the Appletalk configuration data
  when Appletalk is loaded. The configurable parameters are:
  
  aarp-expiry-time
  ----------------
  
  The amount  of  time  we keep an ARP entry before expiring it. Used to age out
  old hosts.
  
  aarp-resolve-time
  -----------------
  
  The amount of time we will spend trying to resolve an Appletalk address.
  
  aarp-retransmit-limit
  ---------------------
  
  The number of times we will retransmit a query before giving up.
  
  aarp-tick-time
  --------------
  
  Controls the rate at which expires are checked.
  
  The directory  /proc/net/appletalk  holds the list of active Appletalk sockets
  on a machine.
  
  The fields  indicate  the DDP type, the local address (in network:node format)
  the remote  address,  the  size of the transmit pending queue, the size of the
  received queue  (bytes waiting for applications to read) the state and the uid
  owning the socket.
  
  /proc/net/atalk_iface lists  all  the  interfaces  configured for appletalk.It
  shows the  name  of the interface, its Appletalk address, the network range on
  that address  (or  network number for phase 1 networks), and the status of the
  interface.
  
  /proc/net/atalk_route lists  each  known  network  route.  It lists the target
  (network) that the route leads to, the router (may be directly connected), the
  route flags, and the device the route is using.
  
  
  5. IPX
  -------------------------------------------------------
  
  The IPX protocol has no tunable values in proc/sys/net.
  
  The IPX  protocol  does,  however,  provide  proc/net/ipx. This lists each IPX
  socket giving  the  local  and  remote  addresses  in  Novell  format (that is
  network:node:port). In  accordance  with  the  strange  Novell  tradition,
  everything but the port is in hex. Not_Connected is displayed for sockets that
  are not  tied to a specific remote address. The Tx and Rx queue sizes indicate
  the number  of  bytes  pending  for  transmission  and  reception.  The  state
  indicates the  state  the  socket  is  in and the uid is the owning uid of the
  socket.
  
  The /proc/net/ipx_interface  file lists all IPX interfaces. For each interface
  it gives  the network number, the node number, and indicates if the network is
  the primary  network.  It  also  indicates  which  device  it  is bound to (or
  Internal for  internal  networks)  and  the  Frame  Type if appropriate. Linux
  supports 802.3,  802.2,  802.2  SNAP  and DIX (Blue Book) ethernet framing for
  IPX.
  
  The /proc/net/ipx_route  table  holds  a list of IPX routes. For each route it
  gives the  destination  network, the router node (or Directly) and the network
  address of the router (or Connected) for internal networks.

  
  6. 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.