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Full Discussion: ARP Cache
Operating Systems Solaris ARP Cache Post 302124435 by Neo on Saturday 30th of June 2007 10:30:53 PM
Old 06-30-2007
Quote:
Originally Posted by earlysame55
Thanks Neo,
So when i lookup the dns, get the ip. Meaning when the application runs a gethostbyname(), it updates the dns cache. How and when is the server's arp cache get updated. I thought it's after the ipaddress is returned and when a transmission happens to that ip, it's stored in the arp cache(after the llokup procedure for the mac address). Correct me if i'm wrong.

TIA

No, that is not actually correct.

If the IP address of the destination is on the same LAN segment as the origin, then when you send the first packet to that designation, the ARP tables on that LAN segment will update.

However, if your destination is 2 hops away, then there will be no ARP entry for the destination address.

ARP is for mapping IP address to MAC addresses for LAN traffic, not for IP routing.

Just doing a DNS lookup will not update an ARP table.

For example, right now you sit down and you want to visit www.unix.com .
There is no ARP entry on your server for www.unix.com because your computer is not on the same LAN segment as www.unix.com.

Is this clear now?
 

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ARP(8C) 																   ARP(8C)

NAME
arp - address resolution display and control SYNOPSIS
arp hostname arp -a [ vmunix ] [ kmem ] arp -d hostname arp -s hostname ether_addr [ temp ] [ pub ] [ trail ] arp -f filename DESCRIPTION
The arp program displays and modifies the Internet-to-Ethernet address translation tables used by the address resolution protocol (arp(4p)). 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. With the -a flag, the program displays all of the current ARP entries by reading the table from the file kmem (default /dev/kmem) based on the kernel file vmunix (default /vmunix). With the -d flag, a super-user may delete an entry for the host called hostname. The -s flag is given to 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. The word trail indicates that trailer encapsulations may be sent to this host. The -f flag 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 ] [ trail ] with argument meanings as given above. SEE ALSO
inet(3N), arp(4P), ifconfig(8C) 4.3 Berkeley Distribution May 20, 1986 ARP(8C)
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