.TH ARP 8 "5 Jan 1999" "net-tools" "Linux Programmer's Manual".SH NAMEarp \- manipulate the system ARP cache.SH SYNOPSIS.B arp .RB [ \-vn ] .RB [ "\-H type" ] .RB [ "-i if" ] .B -a .RB [ hostname ].PP.B arp .RB [ \-v ].RB [ "\-i if" ] .B "\-d hostname".RB [ pub ].PP.B arp .RB [ \-v ] .RB [ "\-H type" ] .RB [ "\-i if" ] .B -s hostname hw_addr.RB [ temp ] .PP.B arp .RB [ \-v ] .RB [ "\-H type" ] .RB [ "\-i if" ] .B -s hostname hw_addr.RB [ "netmask nm" ] .B pub.PP.B arp .RB [ \-v ] .RB [ "\-H type" ] .RB [ "\-i if" ] .B -Ds hostname ifa.RB [ "netmask nm" ] .B pub.PP.B arp .RB [ \-vnD ].RB [ "\-H type" ] .RB [ "-i if" ].B -f [filename].SH DESCRIPTION.B Arpmanipulates the kernel's ARP cache in various ways.  The primary optionsare clearing an address mapping entry and manually setting up one.  Fordebugging purposes, the.B arpprogram also allows a complete dump of the ARP cache..SH OPTIONS.TP.B "\-v, \-\-verbose"Tell the user what is going on by being verbose..TP.B "\-n, \-\-numeric"shows numerical addresses instead of trying to determine symbolic host, portor user names..TP.B "\-H type, \-\-hw-type type"When setting or reading the ARP cache, this optional parameter tells.B arpwhich class of entries it should check for.  The default value ofthis parameter is.B ether(i.e. hardware code 0x01 for IEEE 802.3 10Mbps Ethernet).Other values might include network technologies such as.RB "ARCnet (" arcnet ")",.RB "PROnet (" pronet ")",.RB "AX.25 (" ax25 ")"and.RB "NET/ROM (" netrom ").".TP.B "\-a [hostname], \-\-display [hostname]"Shows the entries of the specified hosts.  If the.B hostnameparameter is not used,.B allentries will be displayed..TP.B "\-d hostname, \-\-delete hostname"Remove any entry for the specified host.  This can be used if theindicated host is brought down, for example..TP.B "\-D, \-\-use-device"Use the interface.BR ifa "'s"hardware address..TP.B "\-i If, \-\-device If"Select an interface. When dumping the ARP cache only entries matchingthe specified interface will be printed. When setting a permanent or.B tempARP entry this interface will be associated with the entry; if thisoption is not used, the kernel will guess based on the routingtable. For.B pubentries the specified interface is the interface on which ARP requests willbe answered. .br.B NOTE:This has to be different from the interface to which the IPdatagrams will be routed..TP.B "\-s hostname hw_addr, \-\-set hostname"Manually create an ARP address mapping entry for host.B hostnamewith hardware address set to.B hw_addr.  The format of the hardware address is dependent on the hardwareclass, but for most classes one can assume that the usual presentationcan be used.  For the Ethernet class, this is 6 bytes in hexadecimal,separated by colons. When adding proxy arp entries (that is those withthe .BR pub lish flag set a .B netmask may be specified to proxy arp for entire subnets. This is not goodpractice, but is supported by older kernels because it can beuseful. If the.B tempflag is not supplied entries will be permanent stored into the ARPcache..br.B NOTE:As of kernel 2.2.0 it is no longer possible to set an ARP entry for an entire subnet. Linux instead does automagic proxy arp when a routeexists and it is forwarding. See .BR arp (7)for details..TP.B "\-f filename, \-\-file filename"Similar to the.B \-soption, only this time the address info is taken from file.B filename.  This can be used if ARP entries for a lot of hosts have to beset up.  The name of the data file is very often.IR /etc/ethers , but this is not official. If no filename is specified /etc/ethersis used as default..sp 1The format of the file is simple; itonly contains ASCII text lines with a hostname, and a hardwareaddress separated by whitespace. Additionally the .BR "pub" , " temp" " and" " netmask"flags can be used..LPIn all places where a.B hostnameis expected, one can also enter an.B "IP address"in dotted-decimal notation..PAs a special case for compatibility the order of the hostname and the hardware address can be exchanged..LP Each complete entry in the ARP cache will be marked with the.B Cflag. Permanent entries are marked with.B Mand published entries have the.B Pflag..SH FILES.I /proc/net/arp,.br.I /etc/networks.br.I /etc/hosts.br.I /etc/ethers.SH SEE ALSOrarp(8), route(8), ifconfig(8), netstat(8).SH AUTHORSFred N. van Kempen, <waltje@uwalt.nl.mugnet.org> with a lot of improvementsfrom net-tools Maintainer Bernd Eckenfels <net-tools@lina.inka.de>.