Best Way to Control Relaying?

[Alex Neuman van der Hans]

Muhammad Nauman wrote:
> Hi
>
> I m using Sendmail 8.13.5
> and i m relaying my User - using the access featur in sendmail -with 
> file : /etc/mail/access
>
1. Remove the "xxx.xxx.xxx.xxx RELAY" option that you have in that file 
so that only AUTHENTICATED users can relay - that way you can have 
people accountable for what they do. You may need to run the saslauthd 
service for this to work.
2. Remember to change /usr/share/sendmail-cf/m4/cfhead.m4 to read
    _REC_FULL_AUTH_$?{auth_ssf} bits=${auth_ssf}$.)
instead of
    _REC_AUTH_$?{auth_ssf} bits=${auth_ssf}$.)

... so you can know *who* sent what. You have to "m4 < 
/etc/mail/sendmail.mc > /etc/mail/sendmail.cf after the change and 
restart MailScanner.
3. Throttle your users. Use a milter or some of sendmail's built-in 
features to throttle the connection so that users can send a reasonable 
amount of mail (say, 5 per minute, for example). You can always add 
exceptions for bigger customers. Check http://technoids.org/dossed.html 
for more info.

If you limit the amount of emails-per-minute and connections-per-second 
your server accepts, you can make it impractical for (ab)users to use 
you as a spam relay.

You can also look into 
http://www.five-ten-sg.com/syslog2iptables/rn01re01.html in order to 
temporarily firewall those who repeatedly try to send mail through your 
server and aren't authorized.