[dns-operations] Nameserver responses from different IP than destination of request

Puneet Sood puneets at google.com
Mon Aug 31 16:40:28 UTC 2020

On Sat, Aug 29, 2020 at 12:18 AM Robert Edmonds <edmonds at mycre.ws> wrote:
> Puneet Sood via dns-operations wrote:
> > RFC 1035 section 7.3 (https://tools.ietf.org/html/rfc1035)
> >      Some name servers send their responses from different
> >      addresses than the one used to receive the query.  That is, a
> >      resolver cannot rely that a response will come from the same
> >      address which it sent the corresponding query to.  This name
> >      server bug is typically encountered in UNIX systems.
> >
> > RFC 2181 (https://tools.ietf.org/html/rfc2181#section-4)
> >    Most, if not all, DNS clients, expect the address from which a reply
> >    is received to be the same address as that to which the query
> >    eliciting the reply was sent.  This is true for servers acting as
> >    clients for the purposes of recursive query resolution, as well as
> >    simple resolver clients.  The address, along with the identifier (ID)
> >    in the reply is used for disambiguating replies, and filtering
> >    spurious responses.  This may, or may not, have been intended when
> >    the DNS was designed, but is now a fact of life.
> >
> >    Some multi-homed hosts running DNS servers generate a reply using a
> >    source address that is not the same as the destination address from
> >    the client's request packet.  Such replies will be discarded by the
> >    client because the source address of the reply does not match that of
> >    a host to which the client sent the original request.  That is, it
> >    appears to be an unsolicited response.
> See also RFC 5452 section 9.1
> (https://tools.ietf.org/html/rfc5452#section-9.1) which puts the
> clarification in RFC 2181 into mandatory RFC 2119 language.
>     9.1.  Query Matching Rules
>        A resolver implementation MUST match responses to all of the
>        following attributes of the query:
>        o  Source address against query destination address
>        o  Destination address against query source address
>        o  Destination port against query source port
>        o  Query ID
>        o  Query name
>        o  Query class and type
>        before applying DNS trustworthiness rules (see Section 5.4.1 of
>        [RFC2181]).
>        A mismatch and the response MUST be considered invalid.

Thanks Rob for that reference. It is quite clear. I didn't follow the
"Updated By" links from 1035 to this section :(

Is there an online tool that does mark up on RFCs to show which other
RFCs are referring to specific sections in it?

> --
> Robert Edmonds

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