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Full Discussion: arp questions
Top Forums UNIX for Advanced & Expert Users arp questions Post 302553058 by Corona688 on Tuesday 6th of September 2011 02:20:11 PM
Old 09-06-2011
Quote:
Originally Posted by COKEDUDE
Can someone please explain this output to me. Why doesn't ifconfig show the same info?
ifconfig just shows you information on your own network card. (which does have one bit of arp-related information -- the hwaddr). Entries in your ARP table are from other network cards -- other computers on the same subnet.

This is because to communicate with these addresses, it needs their MAC addresses, and must ask for them. On a local network, i.e. things on the same subnet as you, IP doesn't need to bother routing -- it can transmit directly to the destination network card and expect to be heard. But to talk to a card and not an IP, it needs the MAC address. If you're talking to something not on the local subnet, it doesn't care about the destination's MAC address, it just sends it to the appropriate gateway, which forwards it along. (You'd need the gateway's mac address, though!)

ARP is how it finds out MAC addresses. It works without IP or routing or any addressing at all -- it broadcasts requests and replies across an entire subnet. "arp who-has 10.71.0.1" would get answered with "arp reply 00:1b:21:2b:eb:0c has 10.71.0.1", so your computer would know to send packets for 10.71.0.1 directly to 00:1b:21:2b:eb:0c. It does all this without the programmer having to intervene at all, as far as IP can tell it's just 10.71.0.1 talking to 10.71.0.2 or whatever.

Since this bypasses all routing, I've seen hardwired ARP addresses abused to allow one IP address to talk across multiple subnets on the same wire, though I'm not convinced this would work everywhere.

Last edited by Corona688; 09-06-2011 at 03:27 PM..
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ARP(8)							    BSD System Manager's Manual 						    ARP(8)

NAME
arp -- address resolution display and control SYNOPSIS
arp [-n] hostname arp [-nv] -a arp [-v] -d -a arp [-v] -d hostname [proxy] arp -s hostname ether_addr [temp] [pub [proxy]] arp -f filename DESCRIPTION
The arp program displays and modifies the Internet-to-Ethernet address translation tables used by the address resolution protocol (arp(4)). With no flags, the program displays the current ARP entry for hostname. The host may be specified by name or by number, using Internet dot notation. Available options: -a The program displays all of the current ARP entries. -d A super-user may delete an entry for the host called hostname with the -d flag. If the proxy keyword is specified, only the pub- lished ``proxy only'' ARP entry for this host will be deleted. If used with -a instead of a hostname, it will delete all arp entries. -f Causes the file filename to be read and multiple entries to be set in the ARP tables. Entries in the file should be of the form hostname ether_addr [temp] [pub] with argument meanings as described below. -n Show network addresses as numbers (normally arp attempts to display addresses symbolically). -s hostname ether_addr Create an ARP entry for the host called hostname with the Ethernet address ether_addr. The Ethernet address is given as six hex bytes separated by colons. The entry will be permanent unless the word temp is given in the command. If the word pub is given, the entry will be "published"; i.e., this system will act as an ARP server, responding to requests for hostname even though the host address is not its own. If the word proxy is also given, the published entry will be a ``proxy only'' entry. -v Display verbose information when adding or deleting ARP entries. SEE ALSO
inet(3), arp(4), ifconfig(8) HISTORY
The arp command appeared in 4.3BSD. BSD
January 31, 2006 BSD
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