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

On 2008-04-11, Marc Haber <mh+debian-devel@zugschlus.de> wrote:
> Hi,
> Exim has the habit of trying to find out about its host names and IP
> addresses when it starts up. This has, in the past, been an issue for
> the Debian packages, since a Debian system might be on a
> dial-on-demand modem line with expensive costs and thus should not do
> unnecessary DNS lookup when the MTA is started. 
> A very similiar mail has been on Debian Planet two weeks ago, but I
> haven't received responses that got me clued up. Therefore, I am using
> the last resort, debian-devel.
> I'd like to solicit opinions from people who are more experienced than
> me with Unix, the local resolver library including /etc/hosts and
> /etc/nsswitch.conf, DNS, and - especially - the customs that apply on
> a system running IPv6. 
> To avoid the extra DNS lookups, the Exim packages have a Debconf
> option to configure exim for "minimal DNS usage", which hardcodes the
> hostname into Exim's configuration at package configuration time. This
> was necessary since - without this option - exim looks up its own host
> name in the DNS even when a completely local operation is invoked.
> In some cases, exim still looks up its IP address when a listening
> daemon starts up. This is why the Debian installer configures
> (not for the local hostname on installation,
> yielding /etc/hosts files like 
>       localhost
>       myfoo.localdomain   myfoo
> # The following lines are desirable for IPv6 capable hosts
>::1     ip6-localhost ip6-loopback
> fe00::0 ip6-localnet
> ff00::0 ip6-mcastprefix
> ff02::1 ip6-allnodes
> ff02::2 ip6-allrouters
> ff02::3 ip6-allhosts
> However, in the last few weeks I have heard a few cases where exim
> does IPv6 AAAA lookups when a listening daemon starts up. An strace
> shows a gethostbyname2 call for AF_INET6, and if we want to continue
> the line we went in the past, we'd need an IPv6 address for
> myfoo.localdomain in /etc/hosts as well. 

Hi, Marc:

Why is exim using gethost* calls?  If you look in the
exim-4.69/src/host.c file, you'll see implementation details like:

    hostdata = getipnodebyname(CS host->name, af, 0, &error_num);
    hostdata = gethostbyname2(CS host->name, af);
    error_num = h_errno;


  hosts = getipnodebyaddr(CS &addr6, sizeof(addr6), AF_INET6, &h_errno);
  hosts = gethostbyaddr(CS &addr6, sizeof(addr6), AF_INET6);

glibc has had these new getipnode* calls for quite some time; there's no
reason not to use them.  Why doesn't exim-4.69/OS/os.h-Linux define
these feature macros?

> I am now wondering how this could be implemented. In IPv4, we have
> available for the local host and could arbitrarily choose
> to configure the local host name on. In IPv6, there is only
>::1, which is a single address. Would it be possible to choose an
> arbitrary "link local" address on lo, the loopback interface? Or is
> there any better way? 

Yes, there is a much better way: do not perform name resolution to
determine the host's FQDN.  It is wrong.  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.

SMTP clients (in this case, MTAs which do not necessarily have an SMTP
server running, but which may initiate SMTP connections to remote
servers) need something plausible for the HELO verb of the SMTP
transaction, and they also need a plausible domain name to use in the
right hand side of the email address given in the MAIL FROM stage.

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.

> This being said, I consider the entire business a horrible
> hack which is one of the most ugly things I have ever seen. Do we have
> a chance to implement this in a more cleaner way, or is it still the
> way to go for the distribution, where we don't know zilch about the
> environment where an installed system is going to be used?
> This issue leads to people adding their local host name to ::1 in
> /etc/hosts, which might re-introduce other issues that we experienced
> in a phase when we did the same for, eventually ending up
> with, or to disabling IPv6 altogether, which is a bad thing
> in a time where IPv6 should be enabled, not disabled. So I'd like to
> find a clean solution which could then be implemented in whatever part
> of Debian might be responsible. 
> I feel that the IPv6 issue is the same that led us to invoke the
> hack for IPv4, and if the answer to the IPv6 issue is "fix
> exim", then _how_ should exim be fixed, and why wasn't the answer to
> the IPv4 version of the issue "fix exim"?

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.

The analogous IPv6 hack, btw, would be something atrocious in /etc/hosts

::ffff: hostname.domainname

> 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>

> Greetings
> Marc

[0] http://lists.debian.org/debian-boot/2005/06/msg00938.html

Robert Edmonds

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