Is this a safe way to multi-home a mail server?

Doug Lee dgl at
Sat Jun 25 11:38:23 GMT 2005

I have a machine on two DSL networks: a /29 and a /28 provided by
different ISPS (why is a long story).  The machine acts as a mail
server (sendmail) as well as a NAT server for an internal network.
Both DSL nets arrive at one interface card, and the LAN is on the
other card.  I have added one of the DSL nets as the main net for the
external interface and the other DSL net as an alias via ifconfig.

Two questions:

1.  Can I have both host IPs (one from each DSL net) as A records in
DNS for the mail server's name--e.g.,	IN A	IN A

and expect mail to arrive at the machine regardless of which network
is working at any given time?  (Part of the "long story" is that we're
having serious trouble with one or the other network at various times
and are trying, temporarily at least, to stay afloat by using
whichever is better at the moment.)  Both host IPs have correct
(identical) reverse DNS.

2.  Is there a way, via routed or other means, to cause the machine to
figure out automatically which net to use for "default" traffic?  It
would be wonderful if natd could keep up with this too, but there I
suspect I'm asking for the moon...

Thanks much for any responses.  Please Cc me.

Doug Lee           dgl at
BART Group         doug at
"I before E, except after C, or when sounded like A, as in neighbor
and weigh, except for when weird foreign concierges seize neither
leisure nor science from the height of society."

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