Sendmail on a LAN
Gordon Messmer
yinyang at eburg.com
Tue Aug 17 20:27:28 UTC 2010
On 08/17/2010 09:33 AM, JD wrote:
> Re: a.b.c.d ==> valid.host.name
> and valid.host.name ==> a.b.c.d
> does not seem to apply to the google smtp server I use for Thunderbird.
You did your test entirely backward. You did a forward lookup first,
and then checked the PTR of the IP which was returned. There is no
requirement for a PTR to match every hostname that resolves to its IP
address.
Let's finish your test:
$ host smtp.gmail.com
smtp.gmail.com is an alias for gmail-smtp-msa.l.google.com.
gmail-smtp-msa.l.google.com has address 74.125.155.109
The result of this test merely identifies an IP address. Now, let's
test to validate that the IP returns a PTR that resolves to the same IP:
$ host 74.125.155.109
109.155.125.74.in-addr.arpa domain name pointer px-in-f109.1e100.net.
$ host px-in-f109.1e100.net.
px-in-f109.1e100.net has address 74.125.155.109
Yep, totally valid. That IP address has a PTR record, and the hostname
contained in that PTR resolves back to the same IP address. This host
is properly configured.
> So, Thunderbird client does not seem to mind that
> reverse lookup does not match the name smtp.gmail.com
Clients rarely do. It's the servers to which you're going to try to
deliver mail that will mind.
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