ARP(4) BSD Programmer's Manual ARP(4)NAMEarp - Address Resolution Protocol
SYNOPSIS
pseudo-device ether
DESCRIPTION
The Address Resolution Protocol (ARP) is a protocol used to dynamically
map between Internet host addresses and 10Mb/s Ethernet addresses. It is
used by all the 10Mb/s Ethernet interface drivers. It is not specific to
Internet protocols or to 10Mb/s Ethernet, but this implementation cur-
rently supports only that combination.
ARP caches Internet-Ethernet address mappings. When an interface re-
quests a mapping for an address not in the cache, ARP queues the message
which requires the mapping and broadcasts a message on the associated
network requesting the address mapping. If a response is provided, the
new mapping is cached and any pending message is transmitted. ARP will
queue at most one packet while waiting for a response to a mapping re-
quest; only the most recently ``transmitted'' packet is kept. If the
target host does not respond after several requests (normally 5) within a
certain time period (normally 120 seconds), the host is considered to be
down for a short period (normally 20 seconds), allowing an error to be
returned to transmission attempts during this interval. The error is
EHOSTDOWN for a non-responding destination host, and EHOSTUNREACH for a
non-responding router.
The ARP cache is stored in the system routing table as dynamically-creat-
ed host routes. The route to a directly-attached Ethernet network is in-
stalled as a ``cloning'' route (one with the RTF_CLONING flag set), caus-
ing routes to individual hosts on that network to be created on demand.
Routes that have been resolved are retained for a period of time (normal-
ly 10 minutes), at which time the entry is marked as needing to be re-
freshed, and a refresh timer is set (normally 10 minutes). During the
refresh period the entry is still valid, but if the entry is used, ARP
will also send a query to the remote host to verify the Internet-Ethernet
addressing. If a response is provided by the remote host, the entry is
then retained for another 10 minutes, when it will go into the refresh
state again. An entry for a host which is not responding is a ``reject''
route (one with the RTF_REJECT flag set).
ARP entries may be added, deleted or changed with the arp(8) utility.
Manually-added entries may be temporary or permanent, and may be
``published'', in which case the system will respond to ARP requests for
that host as if it were the target of the request.
In the past, ARP was used to negotiate the use of a trailer encapsula-
tion. This is no longer supported.
ARP watches passively for hosts impersonating the local host (i.e. a host
which responds to an ARP mapping request for the local host's address).
SYSCTL VARIABLES
Some ARP options can be read or written via the sysctl(3) facility.
Variables specific to ARP are:
CTL_NET, PF_ROUTE, NET_ROUTE_ARP
These variables are used to get or set various global TCP options. The
changeable column shows whether a process with appropriate privilege may
change the value.
Fourth level name Type Changeable
ARPCTL_DOWN integer yes
ARPCTL_KEEP integer yes
ARPCTL_MAXTRIES integer yes
ARPCTL_PROBE integer yes
ARPCTL_REFRESH integer yes
ARPCTL_USELOOPBACK integer yes
ARPCTL_DOWN
The number of seconds for which a host is assumed to be down or
unreachable if it fails to respond to an ARP request after the
maximum number of requests.
ARPCTL_KEEP
The number of seconds that a resolved ARP entry will be consid-
ered to be valid without verification.
ARPCTL_MAXTRIES
The maximum number of ARP requests that are sent before giving
up. Requests are sent no more often than once per second.
ARPCTL_PROBE
The number of seconds for which an unresolved ARP entry will be
retained.
ARPCTL_REFRESH
After the ARPCTL_KEEP time, the number of seconds that a valid
ARP entry will be retained and used, but the value is revalidated
by sending a non-broadcast request if the entry is used.
ARPCTL_USELOOPBACK
If nonzero, packets sent to the local network interface are de-
livered via the loopback interface rather than attempting to send
via the local network hardware.
DIAGNOSTICS
duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x.
ARP has discovered another host on the local network which responds to
mapping requests for its own Internet address with a different Ethernet
address, generally indicating that two hosts are attempting to use the
same Internet address.
SEE ALSOsysctl(3), inet(4), route(4), arp(8), ifconfig(8), route(8)
Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.
Leffler, S.J., and Karels, M.J., "RFC893", Trailer Encapsulations.
4th Berkeley Distribution April 18, 1994 2
