[Dnsmasq-discuss] [PATCH 1/1] forward.c: fix handling of truncated response

Tue Oct 1 22:30:17 UTC 2024

I think Simon has pointed out this is intentional. Partial reply is 
incomplete and for well behaving clients carries not useful information. 
It would use TCP anyway instead, therefore it adds just additional data.

I would consider clients not falling back to TCP as broken. TCP is not 
considered optional nowadays. Attempts to fix such clients by relying on 
incomplete responses instead are wrong.

It might make sense to allow enabling such behaviour by configuration, 
if you have broken software not able to workaround. But I would insist 
that software is broken and therefore sending incomplete responses 
should be only workaround for them, not something behaved by default.

Is that something using Alpine C library?

Cheers,
Petr

On 30/09/2024 06:39, Rahul Thakur via Dnsmasq-discuss wrote:
> Hi Simon,
>
> So what do you think of my reasoning for this patch? Do you agree?
>
> Best regards,
> Rahul Thakur
> ------------------------------------------------------------------------
> *From:* Rahul Thakur <rahul.thakur at genexis.eu>
> *Sent:* 25 September 2024 15:29
> *To:* Simon Kelley <simon at thekelleys.org.uk>; 
> dnsmasq-discuss at lists.thekelleys.org.uk 
> <dnsmasq-discuss at lists.thekelleys.org.uk>
> *Subject:* Re: [Dnsmasq-discuss] [PATCH 1/1] forward.c: fix handling 
> of truncated response
> Hi Simon,
>
> Thanks for responding to this patch, please find my justification for 
> this patch as follows:
>
> I think rfc 2181 is defining the behaviour for DNS server and not DNS 
> proxy.
>
> I am relying on and referring to rfc 5625 while making this change.
>
> In section 4.4 
> (https://datatracker.ietf.org/doc/html/rfc5625#section-4.4 
> <https://datatracker.ietf.org/doc/html/rfc5625#section-4.4>), the rfc 
> 5625 states,
>
>    If a proxy must unilaterally truncate a response, then the proxy MUST
>    set the TC bit.  Similarly, proxies MUST NOT remove the TC bit from
>    responses.
>
> Dnsmasq is ofcourse complying to this behaviour and not meddling with 
> the TC bit while setting the answers to 0. But, if I read further 
> section 4.4.1 
> (https://datatracker.ietf.org/doc/html/rfc5625#section-4.4.1 
> <https://datatracker.ietf.org/doc/html/rfc5625#section-4.4.1>),
>
>  Whilst TCP transport is not strictly mandatory, it is supported by
>    the vast majority of stub resolvers and recursive servers.
>
> So, this indicates that it is not mandatory that the client ignores 
> this truncated response. This is further supported by section 6.1.3.2 
> of rfc 1123 
> (https://datatracker.ietf.org/doc/html/rfc1123#section-6.1.3.2). In 
> paragraph 3 of the DISCUSSION in section 6.1.3.2, it states,
>
>                  Whether it is possible to use a truncated answer
>                  depends on the application.
>
> Hence, when dnsmasq explicitly deletes the answers, then it deprives 
> clients that do not fallback to TCP and are happy with the truncated 
> response to be able to resolve their queries.
Fix such client to fall back to TCP instead of making dnsmasq to provide 
it. Or ensure it uses EDNS0 with buffer big enough to receive whole message.
>
> To me, it sounds like a better strategy to forward the truncated 
> response as is to the client and let the client decide what it wants 
> to do rather than forcefully dropping the answers.
>
> Best regards,
> Rahul Thakur
>
>
>
>
>
> ------------------------------------------------------------------------
> *From:* Dnsmasq-discuss 
> <dnsmasq-discuss-bounces at lists.thekelleys.org.uk> on behalf of Simon 
> Kelley <simon at thekelleys.org.uk>
> *Sent:* 25 September 2024 13:39
> *To:* dnsmasq-discuss at lists.thekelleys.org.uk 
> <dnsmasq-discuss at lists.thekelleys.org.uk>
> *Subject:* Re: [Dnsmasq-discuss] [PATCH 1/1] forward.c: fix handling 
> of truncated response
> I think that this is legitimate behaviour. RFC 2181 para 9 says
>
>     Where TC is set, the partial RRSet that would not completely fit may
>     be left in the response.  When a DNS client receives a reply with TC
>     set, it should ignore that response, and query again, using a
>     mechanism, such as a TCP connection, that will permit larger replies.
>
> Which means the contents (or lack of them) of the answer, auth and
> additional sections has to be ignored by the client anyway.
>
> Do you have a standards reference which says otherwise? Test suites can
> tell you either that behaviour has changed over releases or that
> behaviour differs from other implementations. They cant tell you that
> behaviour is correct.
>
> There is a subtle reason for the code being as it is. Dnsmasq
> has various functions which change the contents of a packet being
> returned, and these can't reliably be applied to a truncated packet, so
> data in a truncated packet may (for instance) disclose DNS data which
> should be blocked.
>
> The patch is, in any case, broken because it gratuitously removes the
> call to the logging code.
>
>
> Cheers,
>
> Simon.
>
> On 24/09/2024 11:01, Rahul Thakur via Dnsmasq-discuss wrote:
> > From: Rahul Thakur <rahul.thakur at iopsys.eu>
> >
> > the handling of truncated reponse is broken in 2.90. The answers
> > are removed before forwarding in case TC bit is set, which
> > seems incorrect.
> >
> > test details-
> > the regression was caught by a CDrouter run and this change fixes
> > the regression.
> > ---
> >   src/forward.c | 7 -------
> >   1 file changed, 7 deletions(-)
> >
> > diff --git a/src/forward.c b/src/forward.c
> > index 10e7496..c893d84 100644
> > --- a/src/forward.c
> > +++ b/src/forward.c
> > @@ -782,13 +782,6 @@ static size_t process_reply(struct dns_header 
> *header, time_t now, struct server
> >        server->flags |= SERV_WARNED_RECURSIVE;
> >       }
> >
> > -  if (header->hb3 & HB3_TC)
> > -    {
> > -      log_query(F_UPSTREAM, NULL, NULL, "truncated", 0);
> > -      header->ancount = htons(0);
> > -      header->nscount = htons(0);
> > -      header->arcount = htons(0);
> > -    }
> >
> >     if  (!(header->hb3 & HB3_TC) && (!bogusanswer || (header->hb4 & 
> HB4_CD)))
> >       {
>
-- 
Petr Menšík
Software Engineer, RHEL
Red Hat,https://www.redhat.com/
PGP: DFCF908DB7C87E8E529925BC4931CA5B6C9FC5CB
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