Originální popis anglicky:
arp - Linux ARP kernel module.
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
PF_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 behaviour can be tuned by the sysctls defined
below.
When there is no positive feedback for an existing mapping after some time (see
the sysctls below) a neighbour 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 an
unicast probe is send
ucast_solicit times. If that fails too it will
broadcast a new ARP request to the network. Requests are only send when there
is data queued for sending.
Linux will automatically add a non-permanent 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.
Three ioctls are available on all
PF_INET sockets. They take a pointer to
a
struct arpreq as their parameter.
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 & deleting ARP maps are privileged
operations and may only be performed by a process with the
CAP_NET_ADMIN capability or an effective UID of 0.
arp_pa must be an
AF_INET socket 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.
ARP supports a sysctl interface to configure parameters on a global or
per-interface basis. The sysctls can be accessed by reading or writing the
/proc/sys/net/ipv4/neigh/*/* files or with the
sysctl(2)
interface. 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 sysctls are
specified in seconds.
- anycast_delay
- The maximum number of jiffies to delay before replying to a
IPv6 neighbour solicitation message. Anycast support is not yet
implemented. Defaults to 1 second.
- app_solicit
- 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
- Once a neighbour 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.
- delay_first_probe_time
- Delay before first probe after it has been decided that a
neighbour is stale. Defaults to 5 seconds.
- gc_interval
- How frequently the garbage collector for neighbour entries
should attempt to run. Defaults to 30 seconds.
- gc_stale_time
- Determines how often to check for stale neighbour entries.
When a neighbour entry is considered stale it is resolved again before
sending data to it. Defaults to 60 seconds.
- gc_thresh1
- 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
- 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
- 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
- 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
- The maximum number of attempts to resolve an address by
multicast/broadcast before marking the entry as unreachable. Defaults to
3.
- proxy_delay
- 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
- The maximum number of packets which may be queued to
proxy-ARP addresses. Defaults to 64.
- retrans_time
- The number of jiffies to delay before retransmitting a
request. Defaults to 1 second.
- ucast_solicit
- The maximum number of attempts to send unicast probes
before asking the ARP daemon (see app_solicit). Defaults to 3.
- unres_qlen
- The maximum number of packets which may be queued for each
unresolved address by other network layers. Defaults to 3.
Some timer settings are specified in jiffies, which is architecture related. On
the Alpha a jiffy is 1/1024 of a second, on most other architectures it is
1/100s.
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 IPv4 specific and shared between IPv4 and IPv6
functionality together.
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/* sysctls did not exist before Linux 2.2.
capabilities(7),
ip(7)
RFC826 for a description of ARP.
RFC2461 for a description of IPv6 neighbour discovery and the base algorithms
used.
Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.
