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arp(7) Miscellaneous Information Manual arp(7) NAME arp - Address Resolution Protocol SYNOPSIS pseudo-device ether DESCRIPTION The ARP protocol is used to map dynamically between DARPA Internet and 10Mb/s Ethernet addresses. It is used by all the 10Mb/s Ethernet in- terface drivers. The ARP protocol 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 re- sponse is provided, the new mapping is cached and any pending messages are transmitted. The ARP protocol queues only the most recently ``transmitted'' packet while waiting for a mapping request to be re- sponded to. To enable communications with systems which do not use ARP, ioctls are provided to enter and delete entries in the Internet-to-Ethernet ta- bles. The usage is: #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 en- try. These ioctls may be applied to any socket descriptor s, but only by the superuser. 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 2 /* completed entry (arp_ha valid) */ #define ATF_PERM 4 /* permanent entry */ #define ATF_PUBL 8 /* pub- lish (respond for other 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 that can be written are ATF_PERM and ATF_PUBL. ATF_PERM causes the entry to be permanent if the ioctl call succeeds. The ioctl may fail if more than four permanent Internet host addresses hash to the same slot. ATF_PUBL specifies that the ARP code should respond to ARP requests for the in- dicated host coming from other machines. This lets a SUN act as an ARP server, which can be used to make an ARP-only machine talk to a non-ARP machine. The ARP protocol watches passively for a host that responds to an ARP mapping request for the local host's address. RESTRICTIONS ARP packets on the Ethernet use only 42 bytes of data. The smallest legal Ethernet packet is 60 bytes, however, not including CRC. Some systems may not enforce the minimum packet size. ERRORS arp: local IP address nn.nn.nn.nn in use by hardware address %x:%x:%x:%x:%x%x ARP has discovered another host on the local network that responds to mapping requests for its own Internet address. RELATED INFORMATION inet(7), arp(8), ifconfig(8) delim off arp(7)