unknow user????

slack66 · 11-22-2004, 07:48 AM

hi!
For the past few days, my logwatch entries for Sendmail are full of attempts
to send mail to unknown users no less than two times per. It looks like this
person is working through the alphabet, and all of these are coming from
multiple IP's. I'm pretty sure it is one person as these unknown users
appear to be sequential. Is there anyway I can stop this?

I tried Google'ing, I have searched through FAQ's, but to be honest, I just
don't know what to look for.

rjlee · 11-22-2004, 08:06 AM

If they are all coming from the same IP address, then you can block it from the firewall.

Also, check that you are not running an open mail relay.

You might also look at filtering software like SpamAssassin.

slack66 · 11-22-2004, 08:23 AM

i tried to block the ip address thru iptables

but it keeps coming with a different ip address??? iam sure that my server is not an open relay server cuz i saw in my maillog that someone try to relay to me and my mailserver reject it

i try also enable dnsl's in sendmail.cf it work in some spammer site like verizon.net but not this one

i dont want too install a 3rd party software...
cuz iam afraid to misconfigure my mailserver...can sendmail and iptables do the job???

peacebwitchu · 11-22-2004, 03:15 PM

People are going to try this as long as you have a email server accessible to the internet. All you need to do is make surethat you aren't an open relay and don't worry about it. I get thousands of attempts daily but there is really nothing you can do. Just as if you have a webserver open to the inet you will see thousands of hits from infected boxes looking for .dlls.

slack66 · 11-23-2004, 01:01 AM

thk guys! now i know iam not alone to this attack

one more question? if i can not do anything to prevent the attack is there a way to hardening my linux box??? iam using slackware 10.0 thks again!

peacebwitchu · 11-23-2004, 09:00 AM

If you were running postfix you could very easily run it in a chroot environment, I have never tried to run sendmail in a chroot jail but i'm sure it is possible.

dmigh · 11-23-2004, 03:19 PM

Hi,
Don't use sendmail.
use exim4 or something else.
http://www.exim.org/

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sigsegv · 11-24-2004, 06:03 PM

The attack is called a "dictionary attack" and isn't used by anything but spammer 'n00bs" now. The solution you're looking for is tarpitting.

As an alternative, you could write a cron job that inspects the maillog for lots of attempts for invalid users from the same IP and then adds it to the list of DENY IPs in your firewall rules. Beware though, as this can block real (legit) mail in certain circumstances.

RBL checks would probably help too.