On Tue, 11 Jan 2000, John Horne wrote:
> My understanding is that when exim performs a DNS lookup of a host (for mail
> delivery) it looks for the MX records. (Assuming it gets some) it checks to
> see if its own name is listed (again, are aliases checked here or is it
> strictly a check of the list names?).
Not only its own name; also any host whose IP address is that of one of
its own interfaces.
> At this point it may have
> several hosts remaining which it can try. It makes a connection to the
> lowest preference host. Now, I would have thought that if the connection
> fails then it would try the next host, but that doesn't seem to happen.
That is all true if it has never tried these hosts before. However, if
there have been prior failures there may be retry times set for the
hosts. They may be different for the different hosts. So while it does
go through the list in order, it may skip one or more hosts if a retry
time has not been reached. If you have
a.b.c MX 5 x.y.z
MX 6 p.q.r
the first time it will try x.y.z, and if it's down, it will try p.q.r.
However, if another message comes along immediately (before x.y.z's
retry time) it won't bother trying x.y.z, but go straight to p.q.r
(assuming it worked last time). Only when x.y.z's retry time is reached
will it try it again.
--
Philip Hazel University of Cambridge Computing Service,
ph10@??? Cambridge, England. Phone: +44 1223 334714.
