# arp

## 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 considered stale.  Positive feedback can be gotten from a
           higher layer; for example from a successful TCP ACK.  Other
           protocols can signal forward 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 reading 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 specified 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
                  daemon 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 protocols.  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
                  neighbor 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 number 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
                  multicast/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 seconds.
    
           _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
                  unresolved 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
           interfaces (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
           generate 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.
    


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