On 02/10/2024 17:00, Dominik Derigs via Dnsmasq-discuss wrote:
Hey all,
having this configurable makes sense particularly thinking about the
multitude of IoT devices often receiving not all that much attention
from their manufacturers. A particular example is the camera Tapo C-310
which seems to be widely used and has often been reported in the Pi-hole
forums to cause a huge amount of queries due to exactly this truncation
method we are talking about here (when dnsmasq is used with DNSSEC, the
query A time.nist.gov results in truncation).
Said Tapo camera is not behaving nicely and does not retry over TCP.
Instead, it retries the same query roughly one second later, resulting
in a whooping 86,400 identical queries, all being truncated. A
workaround is querying this domain periodically manually over TCP but it
is a rather ugly workaround.
If there would be an option to allow truncated content to remain, I
would indeed find it very useful for situations like the one mentioned
above. I confirmed that the camera could successfully reads the returned
CNAME/A record. This is what I got when simply applying the submitted
patch (but keeping the logging):
Oct 2 17:46:53 dnsmasq[1861633]: 25 127.0.0.1/45027 query[A]
time.nist.gov from 127.0.0.1
Oct 2 17:46:53 dnsmasq[1861633]: 25 127.0.0.1/45027 forwarded
time.nist.gov to 127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 26 dnssec-query[DS] gov to 127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 26 reply gov is DS for keytag 2536,
algo 13, digest 2
Oct 2 17:46:53 dnsmasq[1861633]: 27 dnssec-query[DS] nist.gov to
127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 28 dnssec-query[DNSKEY] gov to
127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 28 reply gov is DNSKEY keytag 2536,
algo 13
Oct 2 17:46:53 dnsmasq[1861633]: 28 reply gov is DNSKEY keytag 35496,
algo 13
Oct 2 17:46:53 dnsmasq[1861633]: 27 reply nist.gov is DS for keytag
33751, algo 8, digest 2
Oct 2 17:46:53 dnsmasq[1861633]: 29 dnssec-query[DNSKEY] nist.gov to
127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 29 reply nist.gov is DNSKEY keytag
18303, algo 8
Oct 2 17:46:53 dnsmasq[1861633]: 29 reply nist.gov is DNSKEY keytag
33751, algo 8
Oct 2 17:46:53 dnsmasq[1861633]: 30 dnssec-query[DS] glb.nist.gov to
127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 30 reply glb.nist.gov is DS for keytag
56235, algo 7, digest 1
Oct 2 17:46:53 dnsmasq[1861633]: 30 reply glb.nist.gov is DS for keytag
4395, algo 7, digest 1
Oct 2 17:46:53 dnsmasq[1861633]: 31 dnssec-query[DNSKEY] glb.nist.gov
to 127.0.0.1#5335
Oct 2 17:46:53 dnsmasq[1861633]: 31 reply glb.nist.gov is truncated[DNSKEY]
Oct 2 17:46:53 dnsmasq[1861633]: 25 127.0.0.1/45027 validation result
is TRUNCATED
Oct 2 17:46:53 dnsmasq[1861633]: 25 127.0.0.1/45027 reply is truncated
For testing, "dig +ignore + notcp time.nist.gov" is handy as it is
guaranteed to be truncated with dnsmasq and DNSSEC enabled if the domain
isn't already in your local cache through a preceding TCP query.
I'm in the process of testing some code that fixes this problem. What's
happening here is not that the answer to the original query is too large
and truncated, it's that the answer to a subsidiary query needed to
validate the original answer is too large (in this case DNSKEY
glb.nist.gov) All existing dnsmasq releases return the answer to the
original query with the truncated bit set, even if the answer in fact
fits, just to cause the client to retry with TCP so that the subsidiary
query can be done over TCP and avoid truncation.
The new code swaps to TCP for subsidiary DNSSEC queries without having
to return an artificially truncated answer to the original query and,
once the validation is complete, returns the validated answer over UDP.
This is much cleaner, saves a round trip to the client, and avoids
breakage with clients which fail to do TCP properly.
It's not the whole solution, however. If the answer to a query exceeds
the truncation limit, then no client without TCP support can get an good
answer. DNSSEC adds one more possible state. That's when the answer
without DNSSEC records will fit untruncated, but the addition of
signatures makes the answer too large. Dnsmasq passes on the query with
the DO bit set to make upstream add the signature RRs and if that causes
truncation then the truncated answer goes back to client. In theory,
dnsmasq could switch to TCP, and get the validated answer. It then
strips the RRSIG records from the answer, and if it then fits, returns
it via UDP. Only if the answer still doesn't fit (or the client set the
DO bit, and wants RRSIGS) is TCP fallback by the client required.
I'm minded to aee if the code I have can be extended to handle this state.
Simon.
Best,
Dominik
Am 10/2/24 um 12:30 AM schrieb Petr Menšík:
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 <[email protected]>
*Sent:* 25 September 2024 15:29
*To:* Simon Kelley <[email protected]>; dnsmasq-
[email protected] <[email protected]>
*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-
[email protected]> on behalf of Simon Kelley
<[email protected]>
*Sent:* 25 September 2024 13:39
*To:* [email protected] <dnsmasq-
[email protected]>
*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 <[email protected]>
>
> 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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