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.tha...@genexis.eu>
*Sent:* 25 September 2024 15:29
*To:* Simon Kelley <si...@thekelleys.org.uk>;
dnsmasq-discuss@lists.thekelleys.org.uk
<dnsmasq-discuss@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-boun...@lists.thekelleys.org.uk> on behalf of Simon
Kelley <si...@thekelleys.org.uk>
*Sent:* 25 September 2024 13:39
*To:* dnsmasq-discuss@lists.thekelleys.org.uk
<dnsmasq-discuss@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.tha...@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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