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Annotation of 43BSDTahoe/man/man4/arp.4, revision 1.1

1.1     ! root        1: .\" Copyright (c) 1983 Regents of the University of California.
        !             2: .\" All rights reserved.  The Berkeley software License Agreement
        !             3: .\" specifies the terms and conditions for redistribution.
        !             4: .\"
        !             5: .\"    @(#)arp.4p      6.2 (Berkeley) 5/15/86
        !             6: .\"
        !             7: .TH ARP 4P "May 15, 1986"
        !             8: .UC 5
        !             9: .SH NAME
        !            10: arp \- Address Resolution Protocol
        !            11: .SH SYNOPSIS
        !            12: .B "pseudo-device ether"
        !            13: .SH DESCRIPTION
        !            14: ARP is a protocol used to dynamically map between DARPA Internet
        !            15: and 10Mb/s Ethernet addresses.  It is
        !            16: used by all the 10Mb/s Ethernet interface drivers.
        !            17: It is not specific to Internet protocols or to 10Mb/s Ethernet,
        !            18: but this implementation currently supports only that combination.
        !            19: .PP
        !            20: ARP caches Internet-Ethernet address mappings.  When an interface
        !            21: requests a mapping for an address not in the cache, ARP queues the
        !            22: message which requires the mapping and broadcasts
        !            23: a message on the associated network requesting the address mapping.
        !            24: If a response is provided, the new mapping is cached and any pending
        !            25: message is transmitted.
        !            26: ARP will queue
        !            27: at most one packet while waiting for a mapping request to be responded to;
        !            28: only the most recently ``transmitted'' packet is kept.
        !            29: .PP
        !            30: To facilitate communications with systems which do not use ARP,
        !            31: .IR ioctl \^s
        !            32: are provided to enter and delete entries in the Internet-to-Ethernet tables.
        !            33: Usage:
        !            34: .LP
        !            35: .nf
        !            36: .ft B
        !            37:        #include <sys/ioctl.h>
        !            38:        #include <sys/socket.h>
        !            39:        #include <net/if.h>
        !            40:        struct arpreq arpreq;
        !            41: 
        !            42:        ioctl(s, SIOCSARP, (caddr_t)&arpreq);
        !            43:        ioctl(s, SIOCGARP, (caddr_t)&arpreq);
        !            44:        ioctl(s, SIOCDARP, (caddr_t)&arpreq);
        !            45: .fi
        !            46: .ft R
        !            47: Each ioctl takes the same structure as an argument.
        !            48: SIOCSARP sets an ARP entry, SIOCGARP gets an ARP entry, and SIOCDARP
        !            49: deletes an ARP entry.  These ioctls may be applied to any socket descriptor
        !            50: .I s,
        !            51: but only by the super-user.
        !            52: The
        !            53: .I arpreq
        !            54: structure contains:
        !            55: .LP
        !            56: .RS
        !            57: .ta \w'#define\ \ 'u +\w'ATF_USETRAILERS\ \ 'u +\w'0x08\ \ \ \  'u
        !            58: .nf
        !            59: /*
        !            60:  * ARP ioctl request
        !            61:  */
        !            62: struct arpreq {
        !            63:        struct sockaddr arp_pa;         /* protocol address */
        !            64:        struct sockaddr arp_ha;         /* hardware address */
        !            65:        int     arp_flags;              /* flags */
        !            66: };
        !            67: /*  arp_flags field values */
        !            68: #define ATF_COM                0x02    /* completed entry (arp_ha valid) */
        !            69: #define        ATF_PERM        0x04    /* permanent entry */
        !            70: #define        ATF_PUBL        0x08    /* publish (respond for other host) */
        !            71: #define        ATF_USETRAILERS 0x10    /* send trailer packets to host */
        !            72: .fi
        !            73: .RE
        !            74: .LP
        !            75: The address family for the
        !            76: .I arp_pa
        !            77: sockaddr must be AF_INET; for the 
        !            78: .I arp_ha
        !            79: sockaddr it must be AF_UNSPEC.
        !            80: The only flag bits which may be written are ATF_PERM, ATF_PUBL
        !            81: and ATF_USETRAILERS.
        !            82: ATF_PERM causes the entry to be permanent if the ioctl call succeeds.
        !            83: The peculiar nature of the ARP tables may cause the ioctl to fail if more
        !            84: than 8 (permanent) Internet host addresses hash to the same slot.
        !            85: ATF_PUBL specifies that the ARP code should respond to ARP requests for the
        !            86: indicated host coming from other machines.  This allows a host to act as an
        !            87: ``ARP server,'' which may be useful in convincing an ARP-only machine to talk
        !            88: to a non-ARP machine.
        !            89: .PP
        !            90: ARP is also used to negotiate the use of trailer IP encapsulations;
        !            91: trailers are an alternate encapsulation used to allow efficient packet
        !            92: alignment for large packets despite variable-sized headers.
        !            93: Hosts which wish to receive trailer encapsulations so indicate
        !            94: by sending gratuitous ARP translation replies along with replies
        !            95: to IP requests; they are also sent in reply to IP translation replies.
        !            96: The negotiation is thus fully symmetrical, in that either or both hosts
        !            97: may request trailers.
        !            98: The ATF_USETRAILERS flag is used to record the receipt of such a reply,
        !            99: and enables the transmission of trailer packets to that host.
        !           100: .PP
        !           101: ARP watches passively for hosts impersonating the local host (i.e. a host
        !           102: which responds to an ARP mapping request for the local host's address).
        !           103: .SH DIAGNOSTICS
        !           104: .B "duplicate IP address!! sent from ethernet address: %x:%x:%x:%x:%x:%x."
        !           105: ARP has discovered another host on the local network which responds to
        !           106: mapping requests for its own Internet address.
        !           107: .SH SEE ALSO
        !           108: ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C)
        !           109: .br
        !           110: ``An Ethernet Address Resolution Protocol,'' RFC826, Dave Plummer,
        !           111: Network Information Center, SRI.
        !           112: .br
        !           113: ``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J. Karels,
        !           114: Network Information Center, SRI.
        !           115: .SH BUGS
        !           116: ARP packets on the Ethernet use only 42 bytes of data; however, the smallest
        !           117: legal Ethernet packet is 60 bytes (not including CRC).
        !           118: Some systems may not enforce the minimum packet size, others will.

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