dns resolution: nslookup/other command

[Barry Margolin]

In article <9i7imk$l9f at pub3.rc.vix.com>,
G. Roderick Singleton <gsingleton at home.com> wrote:
>> The DNS query is sent by the client, a correct answer is replied by the
>> DNS server but the client replies by a "ICMP Destination unreachable"
>> From snoop:
>> 172.16.56.4 -> lithium      DNS C oxygen.domain.com. Internet Addr ?lithium ->
>> 172.16.56.4    DNS R oxygen.domain.com. Internet Addr 172.16.48.16
>> 172.16.56.4 -> lithium      ICMP Destination unreachable (UDP port 33228
>> unreachable)
>> 
>> Thanks.
>> 
>> Vi.
>
>Off hand I'd guess you have a routing problem or a firewall getting in
>the way. e.g. packet goes out, answer comes back, packet doesn't get
>to target. 

That's not what snoop says.  It shows the packet being received by the
client and the client sending an ICMP error when that happens.  Also notice
that it's a *port* unreachable error, not a host unreachable.

This can happen if the server takes too long to reply to the query.  If the
client has timed out, it will no longer be listening on the port that the
answer is going to, so a Port Unreachable error will result.

-- 
Barry Margolin, barmar at genuity.net
Genuity, Burlington, MA
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