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Unformatted text preview: m a failed server by issuing a gratuitous ARP request with the backup's
hardware address and the failed server's IP address. This causes all packets destined for the failed server
to be sent to the backup instead, without the client applications being aware that the original server has file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/arp_addr.htm (10 of 12) [12/09/2001 14.46.39] Chapter 4. ARP: Address Resolution Protocol failed.
Unfortunately the authors then decided against this approach, since it depends on the correct
implementation of ARP on all types of clients. They obviously encountered client implementations that
did not implement ARP according to its specification.
Monitoring all the systems on the author's subnet shows that SunOS 4.1.3 and 4.4BSD both issue
gratuitous ARPs when bootstrapping, but SVR4 does not. 4.8 arp Command
We've used this command with the -a flag to display all the entries in the ARP cache. Other options are
The superuser can specify the -d option to delete an entry from the ARP cache. (This was used before running a
few of the examples, to let us see the ARP exchange.)
Entries can also be added using the -s option. It requires a hostname and an Ethernet address: the IP address
corresponding to the hostname, and the Ethernet address are added to the cache. This entry is made permanent
(i.e., it won't time out from the cache) unless the keyword temp appears at the end of the command line.
The keyword pub at the end of a command line with the -s option causes the system to act as an ARP agent for
that host. The system will answer ARP requests for the IP address corresponding to the hostname, replying with
the specified Ethernet address. If the advertised address is the system's own, then this system is acting as a proxy
ARP agent for the specified hostname. 4.9 Summary
ARP is a basic protocol in almost every TCP/IP implementation, but it normally does its work without the
application or the system...
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