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Home > Archive > Unix Programming > May 2005 > Hostname Lookup, part 3.
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Hostname Lookup, part 3.
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| Daniel Rudy 2005-05-12, 7:56 am |
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Hopefully, this is a simple question, but how does one do a hostname
lookup to a specific server instead of the configured DNS servers?
I've looked at the source for host, dig, and nslookup, but It's not
really clear as to what it is doing.
--
Daniel Rudy
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| Gordon Burditt 2005-05-12, 5:52 pm |
| >Hopefully, this is a simple question, but how does one do a hostname
>lookup to a specific server instead of the configured DNS servers?
>
>I've looked at the source for host, dig, and nslookup, but It's not
>really clear as to what it is doing.
It is possible (and I've made considerable use of it, for a program
that traces down domains from the root and checks things like all
of the servers for a domain return the same serial number and the
same set of nameservers) to go into the _res structure, change
_res.nscount (probably to 1) and _res.nsaddr_list[] (a struct
sockaddr_in) to point at the nameserver(s) you want to query, after
calling res_init(). You might want to grab the default nameservers
out of _res.nsaddr_list[] first in case there are some queries you
want to use the default list for, like obtaining the IP address of
the server (given as a hostname) you want to query.
When changing _res.nsaddr_list[], be sure to fill in the whole
struct, including the port number.
This probably isn't structly portable, but it works on FreeBSD,
MacOS X, and I think I even got it working under Ultrix. All ran
some version of BIND.
Gordon L. Burditt
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