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Re: exim, local resolver, host name lookups and IPv6

On Fri, 11 Apr 2008 17:48:19 +0000 (UTC), Robert Edmonds
<edmonds@debian.org> wrote:
>Yes, there is a much better way: do not perform name resolution to
>determine the host's FQDN.  It is wrong.

This is what exim does to determine the local host name:
|This variable contains the value set by primary_hostname in the
|    configuration file, or read by the uname() function. If uname() returns a
|    single-component name, Exim calls gethostbyname() (or getipnodebyname()
|    where available) in an attempt to acquire a fully qualified host name. See
|    also $smtp_active_hostname.

Is this broken?

But this documentation is kind of incorrect in the first place, since
the AAAA lookup I see is caused by a call to gethostbyname_2_, which
is not mentioned int he docs at all. Thankfully, gethostbyname2 is
used in exim's source code only twice (with one of the occurrences
being inside an if( primary_hostname == NULL ) which doesn't apply if
primary_hostname is set in configuration, which is the case if exim is
configured with the minimaldns option. So, the AAAA lookup must be
triggered by the gethostbyname2 call in host.c line 1969, which I not
yet have fully understood. Can some more experienced C programmer
comment on this part of the code?

>  The MTA needs to know the
>"mail name" or FQDN of the system, and it may need to know specific IPv4
>or IPv6 addresses to bind to if it is running an SMTP server, but it
>does not need to know any particular mapping between the two.

Where can I obtain the FQDN of the system instead?

Don't I need the particular mapping between IP addresses and host
names to generate a proper HELO? But I wouldn't expect these lookups
to be made at startup, but only when an outgoing message is sent.

>It looks like there are functions in src/host.c for performing DNS-based
>determination of the system's FQDN.  I don't know exactly under which
>circumstances these functions are invoked, but policy 11.6 implies that
>they are superfluous in the presence of the /etc/mailname file.

/etc/mailname is unfortunately unclearly defined in Policy, and IIRC
the policy editors refused to clarify when asked years ago. The exim 4
maintainers have then created http://wiki.debian.org/EtcMailName and
asked all MTA maintainers to comment how they use /etc/mailname, but
only a fraction of them bothered to comment.

Exim 4 only uses /etc/mailname to qualify unqualified recipient
addresses and for some rewriting tricks. This has been the cause of
unspeakable grief in the past so I do prefer to avoid touching this
particular part of the system.

>I don't see how this issue is analogous to the hack.  From
>reading the archived discussion[0], the problem is applications which
>use a sequence of legacy gethostname(), gethostbyname(), etc. calls to
>construct an FQDN, and avoiding accidentally using 'localhost' or
>'localhost.localdomain' as the system hostname.  If you're using the
>newer getipnode* functions, it's possible that you'll get an AI_V4MAPPED
>address even when asking for an AF_INET6 address.

It looks to me that the getipnode* functions are not available in
current Debian based on glibc 2.7.

>The analogous IPv6 hack, btw, would be something atrocious in /etc/hosts
>::ffff: hostname.domainname

I'll try that.

>> Any hints will be appreciated.
>IME, nullmailer and postfix seem to get along fine without generating
>spurious DNS traffic, so
>#include <flame/default/mta.h>

So please make postfix the default MTA for lenny and have exim
removed. It obviously sucks as badly as its maintainer. I'm _soooo_
sick of that.


-------------------------------------- !! No courtesy copies, please !! -----
Marc Haber         |   " Questions are the         | Mailadresse im Header
Mannheim, Germany  |     Beginning of Wisdom "     | http://www.zugschlus.de/
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