arp questions Post: 302553058

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Top Forums UNIX for Advanced & Expert Users arp questions Post 302553058 by Corona688 on Tuesday 6th of September 2011 02:20:11 PM

09-06-2011

Registered User

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

arp(7P) 																   arp(7P)

NAME

       arp - Address Resolution Protocol

DESCRIPTION

       ARP is a protocol used to dynamically map between DARPA Internet and hardware station addresses.  It is used by all LAN drivers.

       ARP caches Internet-to-hardware station address mappings.  When an interface requests a mapping for an address not in the cache, ARP queues
       the message that requires the mapping, and broadcasts a message on the associated network requesting the address mapping if the	encapsula-
       tion  method has been enabled for the interface.  If a response is provided, the new mapping is cached and any pending message is transmit-
       ted.  ARP queues 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 that do not use ARP, calls are provided to enter and delete entries in the Internet-to-hardware
       station address tables.

   Application Usage:
       Each call takes the same structure as an argument.  sets an ARP entry, gets an ARP entry, and deletes an ARP entry.   These  calls  can	be
       applied to any socket descriptor s, but only by the super-user.	The structure contains:

       The  address  family for the must be for the it must be The only flag bits that can be written are and Fibre Channel hosts only support the
       flag.  causes the entry to be permanent.  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 watches passively for hosts impersonating the local host (i.e., a host that responds to an ARP mapping request  for	the  local  host's
       address).

DIAGNOSTICS

       This message printed on the console screen means that
	    ARP has discovered another host on the local network that responds to mapping requests for its own Internet address.

WARNINGS

       To enable the encapsulation method, use the command (see ifconfig(1M)).

AUTHOR

       ARP was developed by the University of California, Berkeley.

SEE ALSO

       ifconfig(1M), inet(3N), lan(7), arp(1M).

       RFC826, Dave Plummer, Network Information Center, SRI.

																	   arp(7P)