Ubuntu Manpage: arp - Linux ARP kernel module.

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.

NAME

DESCRIPTION

VERSIONS

BUGS

SEE ALSO