ARP(4) BSD Programmer's Manual ARP(4)NAMEarp - Address Resolution Protocol
SYNOPSIS
pseudo-device ether
DESCRIPTION
The Address Resolution Protocol (ARP) is used to dynamically map between
Internet host addresses and Ethernet addresses. It is used by all of the
Ethernet interface drivers. It is not specific to Internet protocols or
to Ethernet, but this implementation currently supports only that combi-
nation.
ARP caches Internet-Ethernet address mappings. When an interface requests
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 map-
ping is cached and any pending message is transmitted. ARP will queue at
most one packet while waiting for a response to a mapping request; only
the most recently transmitted packet is kept. If the target host does not
respond after several requests, 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), causing
routes to individual hosts on that network to be created on demand. These
routes time out periodically (normally 20 minutes after validated; en-
tries are not validated when not in use). 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, static 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. A static entry will
not time out, but may be overwritten by network traffic, while a per-
manent entry will not time out and can not be overwritten.
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 ad-
dress).
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 at-
tempting to use the same Internet address.
arp info overwritten for %x!! by %x:%x:%x:%x:%x:%x on %x An existing
route has been overwritten with a new Ethernet address, for example when
the other host has changed Ethernet cards. If the route previously was
static/non-expiring, the new route will expire normally.
arp: attempt to overwrite permanent entry for %x!! by %x:%x:%x:%x:%x:%x
on %x As above, but the existing route had been manually set up as per-
manent. The routing information is not modified.
arp: attempt to overwrite entry for %x!! on %x by %x:%x:%x:%x:%x:%x on
%x ARP has noticed an attempt to overwrite a host's routing entry on one
interface with a routing entry for a different interface. The routing in-
formation is not modified.
arp: received reply to broadcast or multicast address ARP received a
response which is a broadcast or multicast address. This might indicate
an ARP spoofing attempt.
arp: ether address is broadcast for IP address %s! ARP requested infor-
mation for a host, and received an answer indicating that the host's Eth-
ernet address is the Ethernet broadcast address. This indicates a miscon-
figured or broken device.
arp: ether address is multicast for IP address %s! ARP requested infor-
mation for a host, and received an answer indicating that the host's Eth-
ernet address is the Ethernet multicast address. This indicates a miscon-
figured or broken device.
arp: attempt to add entry for %s on %s by %s on %s This usually indi-
cates there is more than one interface connected to the same hub, or that
the networks have somehow been short-circuited (e.g. IPs that should have
been present on interface one are present on interface two).
arplookup: unable to enter address for %s An IP received on the inter-
face does not match the network/netmask of the interface. This indicates
a netmask problem.
SEE ALSOinet(4), route(4), arp(8), ifconfig(8), route(8)
Plummer, D., "RFC 826", An Ethernet Address Resolution Protocol.
Karels, M.J. and Leffler, S.J., "RFC 893", Trailer Encapsulations.
MirOS BSD #10-current April 18, 1994 1
