[Date Prev][Date Next] [Chronological] [Thread] [Top]

Re: Make SASL hostname canonicalization optional (RFC on patch approach)

Joel Johnson wrote:

1) As an ISP service provider I may have several DNS service names that
happen to be on the same server, yet I want to maintain separate identities
to make the services nearly transparent to migrate to dedicated hardware in
the future if the need arises. Suppose that I have directory.example.com and
imap.example.com on the same box, whose hostname is lucy.example.com. I
would have three A records (or one A record and two CNAMES) for the same
box, but since I can only have one PTR record,

First off, you don't ever want to use CNAME aliases for mail services. That's going to seriously screw you up right there. Have everything directly resolve to the same IP address. That will solve a lot of canonicalization problems.

Secondly, there's absolutely nothing in the DNS that says you can't have multiple PTR records. However, application behaviour in the face of multiple PTR records is not defined by the standard, and different applications will handle this in different ways. You need to find out how your applications handle it.

Thirdly, you could always have the same services on the same server, but with multiple IP aliases bound to the same machine. Each service could get a separate IP address, and there would be one and only one PTR record for that IP address. If/when you decide to move that service to another machine, just move the IP address and leave everything else alone.

I've worked at both large & smaller scale ISPs and these problems are well-known, and their solutions are also well-known.

2) On my home network I wish to use OpenLDAP, but my local server is on a
DSL connection and I have no control over the DNS PTR records, and as such
the records are effectively meaningless to the operation of my system. I do
impose the requirement of mandatory TLS (via security ssf=128) which in and
of itself provides stonger server authentication than name canonicalization
via reverse DNS.

I've seen a lot of cases where the certificate is more screwed up than the reverse DNS. Scratching this itch might help your one particular case where you can guarantee the reverse, but this isn't likely to be of much use in the general populace where such a feature would just give them more ammo to use in blowing off their feet with their own thermo-nuclear shotgun.

I propose that this would be a very valuable option, especially since there
are cases where name canonicalization is infeasible if not impossible, as
well as the fact that combined with TLS stronger server authentication is
available. To use GSSAPI with SASL, it should also be noted that the more
recent Kerberos RFCs have specifically required that reverse DNS *not* be
used [3]. I'm looking for comments on the viability of such a patch being
included in the base software, as well as comments on the patch itself.

That's a good question. If it were me, I might accept the patch into the tracking system, but it wouldn't be a high priority item to fix. But then I'm not one of the core developers.

Brad Knowles <b.knowles@its.utexas.edu>
Sr. System Administrator, UT Austin ITS-Unix
COM 24  |  5-9342