Re: DNS Lookups

[web400@xxxxxxxxxxxxxxxx]

> I am using a small PHP program to perform validation on e-mail domain names
> using gethostbyname.

To perform a DNS lookup for an e-mail address, for example user@xxxxxxx 
you need to do this:

a) Look up the A record for "foo.com"
b) If (A) didn't work, look up the MX records for foo.com
c) Go through each MX record and look up the corresponding A record.  At 
least one of those MX records needs to resolve to an IP address.

You can't do this with gethostbyname().  Actually, you could use 
gethostbyname() for the A records, but I wouldn't because gethostbyname() 
will use the host table if available, and that's not technically legal for 
e-mail.

For the MX records, gethostbyname() definitely won't work. You need to use 
the lower-level resolver routines for that.  Namely, use res_search() to 
look up the MX records... the hard part is decoding the results from 
res_search which are in a compressed format.

I have samples of doing that, but I've only written those routines in C 
and RPG, never in PHP.

> I found that some domains (e.g. xyz.com) gave false fails, so I adapted the
> program to also try mail.xxx.xxx.  I'm still seeing failures on some
> addresses that are supposed to be good.  Any ideas on what else to check?

No, don't make up domain names.  If the e-mail address is user@xxxxxxx, 
don't look up mail.foo.com, that's not how e-mail works.