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ARP(7) Linux Programmer's Manual ARP(7)

NAME

     arp - Linux ARP kernel module.

DESCRIPTION

     This  kernel protocol module implements the Address Resolution Protocol
     defined in RFC 826.  It is used  to  convert  between  Layer2  hardware
     addresses  and  IPv4 protocol addresses on directly connected networks.
     The user normally doesn't interact directly with this module except  to
     configure  it; instead it provides a service for other protocols in the
     kernel.

     A user process can receive ARP  packets  by  using  packet(7)  sockets.
     There  is  also a mechanism for managing the ARP cache in user-space by
     using netlink(7) sockets.  The ARP table can  also  be  controlled  via
     ioctl(2) on any AF_INET socket.

     The ARP module maintains a cache of mappings between hardware addresses
     and protocol addresses.  The cache has a limited size so old  and  less
     frequently  used  entries  are  garbage-collected.   Entries  which are
     marked as permanent are never deleted by  the  garbage-collector.   The
     cache can be directly manipulated by the use of ioctls and its behavior
     can be tuned by the /proc interfaces described below.

     When there is no positive feedback for an existing mapping  after  some
     time  (see  the /proc interfaces below), a neighbor cache entry is con-
     sidered stale.  Positive feedback can be gotten from  a  higher  layer;
     for example from a successful TCP ACK.  Other protocols can signal for-
     ward progress using the MSG_CONFIRM flag to sendmsg(2).  When there  is
     no  forward  progress,  ARP  tries to reprobe.  It first tries to ask a
     local arp daemon app_solicit times for an updated MAC address.  If that
     fails  and  an  old  MAC  address  is  known,  a  unicast probe is sent
     ucast_solicit times.  If that fails too, it will broadcast  a  new  ARP
     request  to  the  network.   Requests  are sent only when there is data
     queued for sending.

     Linux will automatically add a nonpermanent proxy  arp  entry  when  it
     receives  a  request  for  an  address  it forwards to and proxy arp is
     enabled on the receiving interface.  When there is a reject  route  for
     the target, no proxy arp entry is added.

 Ioctls
     Three ioctls are available on all AF_INET sockets.  They take a pointer
     to a struct arpreq as their argument.

         struct arpreq {
             struct sockaddr arp_pa;      /* protocol address */
             struct sockaddr arp_ha;      /* hardware address */
             int             arp_flags;   /* flags */
             struct sockaddr arp_netmask; /* netmask of protocol address */
             char            arp_dev[16]; };

     SIOCSARP, SIOCDARP and SIOCGARP respectively set, delete and get an ARP
     mapping.   Setting  and deleting ARP maps are privileged operations and
     may be performed only by a process with the CAP_NET_ADMIN capability or
     an effective UID of 0.

     arp_pa must be an AF_INET address and arp_ha must have the same type as
     the device which is specified in arp_dev.  arp_dev is a zero-terminated
     string which names a device.

            +-------------------------------------+
            |             arp_flags               |
            +----------------+--------------------+
            |flag            | meaning            |
            +----------------+--------------------+
            |ATF_COM         | Lookup complete    |
            +----------------+--------------------+
            |ATF_PERM        | Permanent entry    |
            +----------------+--------------------+
            |ATF_PUBL        | Publish entry      |
            +----------------+--------------------+
            |ATF_USETRAILERS | Trailers requested |
            +----------------+--------------------+
            |ATF_NETMASK     | Use a netmask      |
            +----------------+--------------------+
            |ATF_DONTPUB     | Don't answer       |
            +----------------+--------------------+
     If  the  ATF_NETMASK  flag  is  set,  then arp_netmask should be valid.
     Linux 2.2 does not support proxy network ARP entries, so this should be
     set  to  0xffffffff,  or  0  to  remove  an  existing  proxy arp entry.
     ATF_USETRAILERS is obsolete and should not be used.

 /proc interfaces
     ARP supports a range of /proc interfaces to configure parameters  on  a
     global or per-interface basis.  The interfaces can be accessed by read-
     ing or writing the /proc/sys/net/ipv4/neigh/*/* files.  Each  interface
     in  the system has its own directory in /proc/sys/net/ipv4/neigh/.  The
     setting in the "default"  directory  is  used  for  all  newly  created
     devices.  Unless otherwise specified, time-related interfaces are spec-
     ified in seconds.

     anycast_delay (since Linux 2.2)
            The maximum number of jiffies to delay before replying to a IPv6
            neighbor  solicitation  message.   Anycast  support  is  not yet
            implemented.  Defaults to 1 second.

     app_solicit (since Linux 2.2)
            The maximum number of probes to send to the user space ARP  dae-
            mon  via  netlink  before dropping back to multicast probes (see
            mcast_solicit).  Defaults to 0.

     base_reachable_time (since Linux 2.2)
            Once a neighbor has been found, the entry is  considered  to  be
            valid  for at least a random value between base_reachable_time/2
            and  3*base_reachable_time/2.   An  entry's  validity  will   be
            extended if it receives positive feedback from higher level pro-
            tocols.  Defaults to 30 seconds.  This file is now  obsolete  in
            favor of base_reachable_time_ms.

     base_reachable_time_ms (since Linux 2.6.12)
            As  for  base_reachable_time, but measures time in milliseconds.
            Defaults to 30000 milliseconds.

     delay_first_probe_time (since Linux 2.2)
            Delay before first probe after it has been decided that a neigh-
            bor is stale.  Defaults to 5 seconds.

     gc_interval (since Linux 2.2)
            How frequently the garbage collector for neighbor entries should
            attempt to run.  Defaults to 30 seconds.

     gc_stale_time (since Linux 2.2)
            Determines how often to check for stale neighbor entries.   When
            a  neighbor  entry  is  considered  stale,  it is resolved again
            before sending data to it.  Defaults to 60 seconds.

     gc_thresh1 (since Linux 2.2)
            The minimum number of entries to keep in  the  ARP  cache.   The
            garbage collector will not run if there are fewer than this num-
            ber of entries in the cache.  Defaults to 128.

     gc_thresh2 (since Linux 2.2)
            The soft maximum number of entries to keep  in  the  ARP  cache.
            The garbage collector will allow the number of entries to exceed
            this  for  5  seconds  before  collection  will  be   performed.
            Defaults to 512.

     gc_thresh3 (since Linux 2.2)
            The  hard  maximum  number  of entries to keep in the ARP cache.
            The garbage collector will always run if  there  are  more  than
            this number of entries in the cache.  Defaults to 1024.

     locktime (since Linux 2.2)
            The minimum number of jiffies to keep an ARP entry in the cache.
            This prevents ARP cache thrashing if  there  is  more  than  one
            potential  mapping  (generally due to network misconfiguration).
            Defaults to 1 second.

     mcast_solicit (since Linux 2.2)
            The maximum number of attempts to resolve an address  by  multi-
            cast/broadcast   before   marking   the  entry  as  unreachable.
            Defaults to 3.

     proxy_delay (since Linux 2.2)
            When an ARP request for a known proxy-ARP address  is  received,
            delay  up  to proxy_delay jiffies before replying.  This is used
            to prevent network flooding in some cases.  Defaults to 0.8 sec-
            onds.

     proxy_qlen (since Linux 2.2)
            The  maximum  number of packets which may be queued to proxy-ARP
            addresses.  Defaults to 64.

     retrans_time (since Linux 2.2)
            The number of jiffies to delay before retransmitting a  request.
            Defaults  to  1  second.   This file is now obsolete in favor of
            retrans_time_ms.

     retrans_time_ms (since Linux 2.6.12)
            The number of milliseconds  to  delay  before  retransmitting  a
            request.  Defaults to 1000 milliseconds.

     ucast_solicit (since Linux 2.2)
            The  maximum  number  of  attempts to send unicast probes before
            asking the ARP daemon (see app_solicit).  Defaults to 3.

     unres_qlen (since Linux 2.2)
            The maximum number of packets which may be queued for each unre-
            solved address by other network layers.  Defaults to 3.

VERSIONS

     The  struct  arpreq  changed in Linux 2.0 to include the arp_dev member
     and the ioctl numbers changed at the same time.  Support  for  the  old
     ioctls was dropped in Linux 2.2.

     Support   for  proxy  arp  entries  for  networks  (netmask  not  equal
     0xffffffff) was dropped in Linux 2.2.   It  is  replaced  by  automatic
     proxy  arp  setup by the kernel for all reachable hosts on other inter-
     faces (when forwarding and proxy arp is enabled for the interface).

     The neigh/* interfaces did not exist before Linux 2.2.

BUGS

     Some timer settings are specified in jiffies,  which  is  architecture-
     and kernel version-dependent; see time(7).

     There  is  no  way  to  signal positive feedback from user space.  This
     means connection-oriented protocols implemented in user space will gen-
     erate  excessive  ARP traffic, because ndisc will regularly reprobe the
     MAC address.  The same problem applies for some kernel protocols (e.g.,
     NFS over UDP).

     This  man page mashes together functionality that is IPv4-specific with
     functionality that is shared between IPv4 and IPv6.