Re: Sendmail/Blacklists rejecting authenticated users

Bill Weiss <houdini+focus-linux@xxxxxxxxxxxx> writes:

> Scott Gifford(sgifford@xxxxxxxxxxxxxxxx)@Wed, Jan 11, 2006 at 04:47:05PM -0500:

[...]

>> Most blacklists put 127.0.0.2 on their blacklist for testing, so you
>> can do:
>>
>> telnet 127.0.0.2 25
>>
>> from the server to test.
>
> Uh... I don't think that's going to work. Even if a server uses a
> blacklist that lists 127.0.0.2, that's not always going to provide
> reverse DNS for that IP. Even if it does, telnet to that should just
> come back to the same machine (maybe) or nowhere (probably).

Did you try it? For the 2 DNSBL's I currently use, it works fine:

$ dig +short 2.0.0.127.bl.spamcop.net.
127.0.0.2
$ dig +short 1.0.0.127.bl.spamcop.net.
$ dig +short 2.0.0.127.relays.ordb.org
127.0.0.2
$ dig +short 1.0.0.127.relays.ordb.org
$

$ telnet 127.0.0.2 25
Trying 127.0.0.2...
Connected to 127.0.0.2.
Escape character is '^]'.
220 rblsmtpd.local
mail from:<blah>
250 rblsmtpd.local
rcpt to:<blah>
451 Listed by ORDB - for testing purposes only
Connection closed by foreign host.

$ telnet 127.0.0.1 25
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
220 accept.example.com ESMTP
mail from:<blah>
250 ok
rcpt to:<blah>
250 ok
quit
221 accept.example.com
Connection closed by foreign host.

As you can see, these RBLs list 127.0.0.2 but not 127.0.0.1, so when
you telnet to your own host using the address "127.0.0.2" it will act
as if you're blacklisted, while if you use "127.0.0.1" it will act as
normal. That's a good way to test whether your blacklist is
configured properly: if it's overzealous, you'll be blacklisted at
127.0.0.1, and if it's underzealous, you'll be allowed to send via
127.0.0.2

Not all DNSBLs support this, but for those that do it's quite
convenient. Some OS's require you to "ifconfig up" an interface alias
with the address 127.0.0.2 to perform this test; Linux will
automatically route 127/8 to the loopback adapter on standard
configurations.

----Scott.

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