arp(4) [netbsd man page]
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
Check Out this Related Man Page
ARP(4P) ARP(4P) NAME
arp - Address Resolution Protocol SYNOPSIS
/sys/conf/SYSTEM: NETHER 1 # ether pseudo-device DESCRIPTION
ARP is a protocol used to dynamically map between DARPA Internet 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 currently supports 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 mes- sage which requires the mapping and broadcasts a message on the associated network requesting the address mapping. If a response is pro- vided, the new mapping is cached and any pending message is transmitted. ARP will queue at most one packet while waiting for a mapping request to be responded to; only the most recently ``transmitted'' packet is kept. To facilitate communications with systems which do not use ARP, ioctls are provided to enter and delete entries in the Internet-to-Ethernet tables. Usage: #include <sys/ioctl.h> #include <sys/socket.h> #include <net/if.h> struct arpreq arpreq; ioctl(s, SIOCSARP, (caddr_t)&arpreq); ioctl(s, SIOCGARP, (caddr_t)&arpreq); ioctl(s, SIOCDARP, (caddr_t)&arpreq); Each ioctl takes the same structure as an argument. SIOCSARP sets an ARP entry, SIOCGARP gets an ARP entry, and SIOCDARP deletes an ARP entry. These ioctls may be applied to any socket descriptor s, but only by the super-user. The arpreq structure contains: /* * ARP ioctl request */ struct arpreq { struct sockaddr arp_pa; /* protocol address */ struct sockaddr arp_ha; /* hardware address */ int arp_flags;/* flags */ }; /* arp_flags field values */ #define ATF_COM 0x02/* completed entry (arp_ha valid) */ #define ATF_PERM 0x04 /* permanent entry */ #define ATF_PUBL 0x08 /* publish (respond for other host) */ #define ATF_USETRAILERS 0x10 /* send trailer packets to host */ The address family for the arp_pa sockaddr must be AF_INET; for the arp_ha sockaddr it must be AF_UNSPEC. The only flag bits which may be written are ATF_PERM, ATF_PUBL and ATF_USETRAILERS. ATF_PERM causes the entry to be permanent if the ioctl call succeeds. The peculiar nature of the ARP tables may cause the ioctl to fail if more than 8 (permanent) Internet host addresses hash to the same slot. ATF_PUBL specifies that the ARP code should respond to ARP requests for the indicated host coming from other machines. This allows a host to act as an ``ARP server,'' which may be useful in convincing an ARP-only machine to talk to a non-ARP machine. ARP is also used to negotiate the use of trailer IP encapsulations; trailers are an alternate encapsulation used to allow efficient packet alignment for large packets despite variable-sized headers. Hosts which wish to receive trailer encapsulations so indicate by sending gra- tuitous ARP translation replies along with replies to IP requests; they are also sent in reply to IP translation replies. The negotiation is thus fully symmetrical, in that either or both hosts may request trailers. The ATF_USETRAILERS flag is used to record the receipt of such a reply, and enables the transmission of trailer packets to that host. 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!! 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. SEE ALSO
ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C) ``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer, Network Information Center, SRI. ``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels, Network Information Center, SRI. BUGS
ARP packets on the Ethernet use only 42 bytes of data; however, the smallest legal Ethernet packet is 60 bytes (not including CRC). Some systems may not enforce the minimum packet size, others will. 3rd Berkeley Distribution August 1, 1987 ARP(4P)