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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.comc) 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.
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