Reverse DNS and mail

[Mike DiChiappari]

I am thinking about (finally) setting up reverse DNS.  Occassionally someone
reports the error " (reason: 501 Domain must resolve)" when trying to send
mail to us.  I simply want our mail servers, which serves mail for several
domains, to work properly with reverse DNS.

How does one setup reverse dns when a server supports multiple (virtual)
hosts?

For example, our server hosts foo.com and bar.com.  The /etc/named.conf
contains this:

zone "foo.com" IN {
        type master;
        file "foo.com.zone";
        allow-update { none; };
};
zone "bar.com" IN {
        type master;
        file "bar.com.zone";
        allow-update { none; };
};

The typical zone file looks like this:

$TTL 86400
bar.com.      IN      SOA     dns.foo.com.  me.foo.com (
                        4 ; serial
                        28800 ; refresh
                        7200 ; retry
                        604800 ; expire
                        86400 ; ttl
                        )
                NS      dns.foo.com.
                A       w.x.y.z
                MX      5 mail
localhost       IN      A       127.0.0.1
mail                    A       w.x.y.z
dns                     A       w.x.y.z
www             IN      A       a.b.c.d
fred               IN      A     m.n.o.p
harry             IN      A     m.n.o.p

NOTE: foo.com and bar.com on the same IP.  Also the IP addresses for mail
(w.x.y.z) are the same for foo.com and bar.com.  The web sites for both are
on the same web server (virtually hosted).

The question is how do I add reverse lookup to named.conf?

I would want to add something like this:

zone "w.x.w-in-addr.arpa" IN {
    type master;
    file ####.com.rr.zone
    allow-update { none };
};

What goes at ####?  Do I put foo or bar?  Or do I create two of these
sections, each with the exact same first line (since they both share the
same IP address)?

Another question: In the zone files for the reversed zones, is it okay to
have address listed more than once by a PTR when a machine is known by more
than one name (see fred and harry above)?  For example,

bar.com.      IN      SOA     dns.foo.com.  me.foo.com (
                        4 ; serial
                        28800 ; refresh
                        7200 ; retry
                        604800 ; expire
                        86400 ; ttl
                        )

p              IN PTR        fred.bar.com               ; NOTE: This line
and the next both reference "p"
p              IN PTR        harry.bar.com

Thanks,
Mike