[lug] Sending e-Mail under Linux

[Jeff Schroeder]

John:

> One final question - I've heard that spammers could use my computer
> as a mail-relay (something about telnetting via port 25?).  How can I
> detect and/or prevent that from happening?  Or is that even a concern
> for a home user?

Open relaying is a concern for *anyone* who's got a direct Internet 
connection, home users included.  Many spammers employ portscanners 
that sweep over a range of IP addresses and find servers whose SMTP 
port (25) is open.  Then they relay their mail off it.

If you have a static IP, you're a juicier target than someone whose IP 
changes every time they go online.

Three solutions present themselves, in increasing complexity:

1) Disable Sendmail.  This will close port 25, and force you to use your 
ISP to send your mail.  All ISPs provide a mail relay for their users, 
and AT&T should be no exception.

2) Use a firewall.  Block port 25 (and nearly everything else, in fact).  
This will allow you to still use Sendmail locally to send your 
messages-- although, as Warren pointed out, some mail servers will 
refuse mail from "localhost".

3) Use selective relaying, so you define which IPs can use port 25 to 
send mail.  Obviously you'd allow yourself (127.0.0.1), and potentially 
allow your friends (with their static IPs) to relay off you.  Not 
recommended, but possible.

HTH,
Jeff