On Tue, 18 Nov 1997, Tom wrote:
>
> On Tue, 18 Nov 1997, The UNIX Mighty! wrote:
>
> > Currently exim doesn't deliver mail to host with no mx record.
>
> Why does this keep coming up? Exim will do this with the default
> config. Most likely you broken a router and/or the smtp transport.
One of the reasons it keeps coming up is the following scenario:
. There is a wildcard MX record for *.a.b.c.
. There is a host called x.y.z that has an A record and no MX record.
. Somebody on a machine m.a.b.c domain tries to mail to user@???.
. Exim calls the DNS to look for an MX record for x.y.z.
. The DNS doesn't find any MX record. By default, it then tries
searching the current host's parent domain, so it looks for
x.y.z.a.b.c.
. This picks up the wildcard MX record, which of course isn't what you
want at all.
. Nasty things happen, depending on where the MX record points.
. The user asks what to do.
. I tell them to set no_search_parents on the lookuphost router.
. Things then work for them in that case. However, anybody on the
machine m.a.b.c who mails to user@??? (expecting it to go to
user@???) now has a problem.
Wildcard MX records are known to be evil, but it seems they are quite
widely used. I could make no_search_parents the default setting, but
this might cause trouble at sites who rely on it. In any case, I thought
it was the default setting, but on reading the man page for the
resolver on Solaris 2, I now can't find any statement one way or the
other. All it says about RES_DNSRCH (which is what search_parents maps
to) is:
RES_DNSRCH Search up local domain tree.
...
If the RES_DNSRCH
flag is enabled, res_search() will search up the local
domain tree until an answer has been retrieved or an unre-
coverable error has been encountered
However, in the man page for a different set of resolver functions (I
can't remember where this came from) there is a statement that it is set
by default, and it is also stated to be the default on IRIX (I haven't
checked any others) so I don't think changing the default is really the
answer.
--
Philip Hazel University Computing Service,
ph10@??? New Museums Site, Cambridge CB2 3QG,
P.Hazel@??? England. Phone: +44 1223 334714
--
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