arp(9) [netbsd man page]

ARP(9)							   BSD Kernel Developer's Manual						    ARP(9)

NAME

     arp, arp_ifinit, arpresolve, arpintr -- externally visible ARP functions

SYNOPSIS

     #include <netinet/if_inarp.h>

     void
     arp_ifinit(struct ifnet *ifp, struct ifaddr *ifa);

     int
     arpresolve(struct ifnet *ifp, struct rtentry *rt, struct mbuf *m, struct sockaddr *dst, u_char *desten);

     void
     arpintr();

DESCRIPTION

     The arp functions provide the interface between the arp module and the network drivers which need arp functionality.  Such drivers must
     request the arp attribute in their "files" declaration.

     arp_ifinit()  Sets up the arp specific fields in ifa.  Additionally, it sends out a gratuitous arp request on ifp, so that other machines are
		   warned that we have a (new) address and duplicate addresses can be detected.

		   You must call this in your drivers' ioctl function when you get a SIOCSIFADDR request with an AF_INET address family.

     arpresolve()  is called by network output functions to resolve an IPv4 address.  If no rt is given, a new one is looked up or created.  If
		   the passed or found rt does not contain a valid gateway link level address, a pointer to the packet in m is stored in the route
		   entry, possibly replacing older stored packets, and an arp request is sent instead.	When an arp reply is received, the last
		   held packet is send.  Otherwise, the looked up address is returned and written into the storage desten points to.  arpresolve()
		   returns 1, if a valid address was stored to desten, and the packet can be sent immediately.	Else a 0 is returned.

     arpintr()	   When an arp packet is received, the network driver (class) input interrupt handler queues the packet on the arpintrq queue, and
		   requests an arpintr() soft interrupt callback.  arpintr() dequeues the packets, performs sanity checks and calls (for IPv4 arp
		   packets, which are the only ones supported currently) the in_arpinput() function.  in_arpinput() either generates a reply to
		   request packets, and adds the sender address translation to the routing table, if a matching route entry is found.  If the
		   route entry contained a pointer to a held packet, that packet is sent.

SEE ALSO

     ether_ifattach(9)

     Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.

STANDARDS

     RFC 826

HISTORY

     Rewritten to support other than Ethernet link level addresses in NetBSD 1.3.

AUTHORS

     UCB CSRG (original implementation)
     Ignatios Souvatzis (support for non-Ethernet)

CODE REFERENCES

     The ARP code is implemented in sys/net/if_arp.h, sys/netinet/if_inarp.h and sys/netinet/if_arp.c.

BSD
								   March 3, 1997							       BSD

ARP(4)							   BSD Kernel Interfaces Manual 						    ARP(4)

NAME

     arp -- Address Resolution Protocol

SYNOPSIS

     #include <netinet/if_ether.h>

DESCRIPTION

     The Address Resolution Protocol (ARP) is a protocol used to dynamically map between Internet host addresses and Ethernet addresses.  It is
     used by all the Ethernet interface drivers.  It is not specific to Internet protocols or to Ethernet, but this implementation currently sup-
     ports only that combination.

     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 mapping 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 con-
     sidered 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-created host routes.  The route to a directly-attached Ethernet network is
     installed 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; entries 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 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 encapsulation.  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).

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 ALSO

     inet(4), route(4), arp(8), ifconfig(8), route(8)

     Plummer, D., "RFC 826", An Ethernet Address Resolution Protocol.

     Leffler, S.J.  and Karels, M.J., "RFC 893", Trailer Encapsulations.

BSD
								  April 18, 1994							       BSD