Maintained by: NLnet Labs

Trying to fetch SRV data with libunbound / libldns

W.C.A. Wijngaards
Thu May 12 16:16:55 CEST 2016


Hi Rick,

On 12/05/16 15:51, Rick van Rein via Unbound-users wrote:
> Hello,
> 
> I'm trying to use libunbound (and possibly libldns) to pull out SRV
> records.  I am not certain how to proceed, based on the documentation
> that I could found in the form of man pages, tutorials and doxygen.
> 
> Am I correct that libunbound returns RDATA unmodified, and is basically
> concerned with the security chain but not the RDATA contents?  And so,
> that I have to turn to another tool?
> 
> I therefore looked at LDNS, but that does not seem to help either.  I
> don't mind following the RFC (though in the case of SRV it is not
> explicit on the wire format) to pull out data, but I would be surprised
> if I was to make assumptions about the structure of hosts, especially
> because it may point back into a DNS packet.  Is this really how it is
> done though?  Then, should I assume the data[i]/len[i] to be pointers
> into the frame, and compute offsets manually?

The data[i] elements contain self-contained, uncompressed, rdata
elements that do not point to a packet somewhere.

For SRV that means there is an uncompressed domainname in there.  In
wireformat.  LDNS can parse the rdata in data[i].

Best regards, Wouter

> 
> I can hardly believe that this hasn't been tried before :) but as I
> said, I can't find the documentation that answers this with clarity.  So
> any help is welcome, including pointers to documentation that I've
> overlooked!
> 
> Thanks,
>  -Rick
> 


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