Limiting outbound spam
One trick ISPs can use to control spam is to limit customer access to SMTP and
NNTP servers. The router between your site and the Internet can be set to
block access by your customers to offsite servers. This forces mail to be
relayed through your mail servers, and disallows posting netnews via other
site's servers. An example of how you might accomplish this with a Cisco
router (assume your net is ppp.qqq.rrr.0 and your mail servers are
ppp.qqq.rrr.11 and ppp.qqq.rrr.12):
interface serial0
ip access-group 101 out
access-list 101 permit tcp host ppp.qqq.rrr.11 any eq smtp
access-list 101 permit tcp host ppp.qqq.rrr.12 any eq smtp
access-list 101 deny tcp ppp.qqq.rrr.0 0.0.0.255 any eq smtp
access-list 101 deny tcp ppp.qqq.rrr.0 0.0.0.255 any eq nntp
access-list 101 permit ip any any
Back to Tools and techniques
Scott Hazen Mueller /
scott@zorch.sf-bay.org