[PATCH] Remove CNAME check as it breaks with DNS responses > 512B

Remove an ancient misfeature that was introduced for compatibility with
misguided setups that don't exist anymore. Not doing an ANY query (to
check for CNAMEs) avoids getting large amounts of data we have no
interest in and that may overflow our response buffer. This is becoming
an increasingly widespread problem in practice due to DNSSEC.

With this change, qmail will only break for (i.e. be unable to deliver
to) domains that have enough _MX_ records to overflow the fixed buffer
size.

This approach was suggested [1] by DJB:

> Hi Bhasker,
>
> Back in the 1990s there were many sites relying on the following feature
> of the SMTP infrastructure:
>
> If you set up www.your.site with a CNAME for your.site, mail to
> www.your.site will automatically be accepted by your.site's mailer.
>
> This feature was implemented by SMTP clients: the client would see the
> CNAME record for www.your.site and rewrite www.your.site as your.site in
> SMTP. This wasn't in the RFCs---I'm pretty sure that it started with
> Eric Allman misinterpreting a stupid side comment in the RFCs---but new
> clients such as qmail had to do the same thing for interoperability.
>
> Implementors discussing this in the late 1990s agreed that it would be
> good to drop this feature, eliminating all special knowledge of CNAMEs
> from clients and telling servers to take care of themselves. I hate to
> break the mail system, so I advocated a two-step transition with a gap
> in time between
>
> (1) warning clients to stop relying on the feature and
> (2) turning the feature off.
>
> Other people---including the RFC 2821 author---advocated simply turning
> the feature off, mail delivery be damned.
>
> I don't know who was the first to actually turn the feature off. I'm
> sure that there aren't any sites relying on the feature now. It's safe
> to simply skip the CNAME lookup: i.e., have dns_cname simply return 0.
>
> ---Dan

Click to expand...

[1] Re: CNAME lookup failure started

Signed-off-by: Sascha Silbe <sascha-pgp@silbe.org>
---
dns.c | 27 +--------------------------
1 file changed, 1 insertion(+), 26 deletions(-)

diff --git a/dns.c b/dns.c
index e9faad7..f00c16d 100644
--- a/dns.c
+++ b/dns.c
@@ -187,32 +187,7 @@ int flagsearch;
int dns_cname(sa)
stralloc *sa;
{
- int r;
- int loop;
- for (loop = 0;loop < 10;++loop)
- {
- if (!sa->len) return loop;
- if (sa->s[sa->len - 1] == ']') return loop;
- if (sa->s[sa->len - 1] == '.') { --sa->len; continue; }
- switch(resolve(sa,T_ANY))
- {
- case DNS_MEM: return DNS_MEM;
- case DNS_SOFT: return DNS_SOFT;
- case DNS_HARD: return loop;
- default:
- while ((r = findname(T_CNAME)) != 2)
- {
- if (r == DNS_SOFT) return DNS_SOFT;
- if (r == 1)
- {
- if (!stralloc_copys(sa,name)) return DNS_MEM;
- break;
- }
- }
- if (r == 2) return loop;
- }
- }
- return DNS_HARD; /* alias loop */
+ return 0;
}

#define FMT_IAA 40