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

[Simon Kelley]

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)))
>       {