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From: Rainer Weikusat <rweikusat@talktalk.net>
Newsgroups: comp.unix.programmer
Subject: Re: Resolver API question
Date: Wed, 30 Mar 2016 15:54:16 +0100
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spud@potato.field writes:
> On Tue, 29 Mar 2016 11:35:00 -0400
> Barry Margolin <barmar@alum.mit.edu> wrote:
>>In article <nddn6b$eqk$1@gioia.aioe.org>, spud@potato.field wrote:
>>
>>> I'm trying to decode the rdata field returned in a resource record by the
>>> ns_parserr() function. eg:
>>> 
>>> ns_parserr(&msg,ns_s_an,i,&r_rec);
>>> rdata = ns_rr_rdata(r_rec);
>>> 
>>> However the ns_name_uncompress() function only seems to work with rdata in
>>  NS  records

[...]

>>There's no name in the rdata of an A record, it just has an IP address, 
>>which is in binary. And for an MX record you have to skip over the 
>>priority field to get to the server name.
>
> I'm presuming that priority field is a subfield of the rdata field in the
> resource record because its not a field defined in the ns_rr structure?

DNS answers contain so-called 'resource records' of various types and
the resource record data (rdata) format depends on the type, eg, for MX
records, RFC1035 says

,----
| 3.3.9. MX RDATA format
| 
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     |                  PREFERENCE                   |
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|     /                   EXCHANGE                    /
|     /                                               /
|     +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| 
| where:
| 
| PREFERENCE      A 16 bit integer which specifies the preference given to
|                 this RR among others at the same owner.  Lower values
|                 are preferred.
| 
| EXCHANGE        A <domain-name> which specifies a host willing to act as
|                 a mail exchange for the owner name.
`----
https://tools.ietf.org/html/rfc1035