Preventing Spam White Papers | Key Insights and Strategies
Manual Prepared by GalaxyVisions Customer Care Team
» The problem
» How to prevent spammers from using your server
» How to track down a spammer if you get one on your server
In 2004, the California Legislature released a study that showed spam costs U.S. organizations more than $10 billion a year.
Spam is known to be more than just annoying to the end use; it is known to carry viruses, Trojans, and spyware.
Spam causes additional load on mail servers
» Extra network usage
» Extra processing power needed
» Additional disk space for storing the spam
Methods to Stop Spam
» Securing your server
» Blocking outgoing spam
» Filtering spam
» Tightening your SMTP server
Securing your Server
» Make sure that you do not have scripts that are vulnerable to spammers
» Verify that the user “nobody” cannot send email
» Make sure that you are not an open relay
Watch the Scripts that Users Install
» Keep an eye on the scripts that your users install
» Make sure they keep them up to date!
Verify that “nobody” Cannot Send Out Email
» Enable suExec
» Enable phpSuExec
» Prevent the user “nobody” from being able to send email
» Adding X-source headers
» Include a list of POP before SMTP headers when relaying email
» By default suExec is installed on all cPanel® servers
» suExec makes it so that all perl scripts (cgi/pl) run as the user on the account as opposed to the nobody user
» This is enabled in WHM under Service Configuration -> Enable/Disable SuExec
» phpSuExec makes it so that all PHP scripts run as the user on the account as opposed to the nobody user
» You can enable this under WHM -> Software -> Apache Update
Adding X-Source Headers
You can track the origin of all email that is sent through your server by adding X- source headers
» Enable this in WHM -> Server Configuration -> Tweak Settings
Adding X-Pop Before SMTP Headers
This causes all email that is relayed through your server using SMTP to show you who was authenticated via POP first
» Enable in WHM -> Server Configuration ->Tweak Settings
What Exactly is an Open Relay
» An open relay is a server that allows anyone to send an email through it.
How Do I Tell if I am an Open Relay
» Remember that cPanel by default makes it so that you are not an open relay.
»» Custom configurations can cause you to open up Exim to become an open relay
»» There are many websites available to test and see if your server is an open relay
What if I Become an Open Relay
» Start by removing the modifications that you have made to Exim
» Searching for Spammers on your Servers
» You have locked down your server and you are sure that you are not an open relay; however, you are still getting abuse reports about spam coming from you server.
» What do you do now
Make Sure It was Sent from or Relayed from Your Server!
» Check the headers
»» Is your hostname in there
»» Is your IP in there
»» Use the extra headers you enabled to help track down the spammer for you!
Let's Start with the Logs
» Lets start by knowing where Exim keeps it logs
»» Linux: /var/log/exim_mainlog
»» FreeBSD: /var/log/exim/mainlog
» Enabling Extended Logging
In the Exim Configuration add – log_selector = +arguments + subject
What Exactly Do Extended Logs Do for Me but Take Up Space?
It makes looking through the logs easier
» Take a look at the following example. You can see exactly where the email was sent from.
2006-05-08 17:24 cwd=/home/user/public_html/phpBB 5 args: / usr/sbin/exim -Mc
So the Spammer Did Not Send the Email from localhost
In the following example the user was email@example.com
» 2006-05-22 17:32:21 1MV7FQ-03022-P1 << firstname.lastname@example.org H=ispexample.com ([192.168.1.1]) [126.96.36.199] P=esmtpa A=fixed_plain:email@example.com S=231 id=ABCDEFG T=”Buy me!”
The key here is
Spammers Also Connect to the Server via SMTP from the Server
The best way to check this is to run
» netstat -cen 2>/dev/null | grep 127.0.0.1:25
Look for the number that will report after established
grep through /etc/passwd for this # to get the username
Blocking / Filtering Spam from Coming into my Server
There are two major methods for helping prevent spam from getting to your users
» Blocking spam using things such as Box Trapper and RBLs
» Filtering spam using things such as spamassassin
We will also go over some additional ACLs that will help you prevent spam
Why Would I Want to Block Spam?
Blocking spam is one method used to prevent spam.
When blocking spam you are less likely to get spam through; however, you are more likely to block ham (non-spam) as spam.
Using BoxTrapper to Block Spam
BoxTrapper is included with cPanel
It uses the challenge response method to block email
» All email that is not on the white list is held in a queue
All email held in the queue will get a response back asking the owner of the address to verify it
» Once verified it will be released and the email address added to the white list
» Email addresses with no real owners (such as registration emails) will get blocked.
Using RBLs to Block Spam
An RBL is a Realtime Black List
» An RBL is a list of IPs that are known to allegedly house spammers on them or be owned by someone who allegedly harbors spammers
» All emails coming from these IP's are blocked immediately
Why Would I Want to Filter Spam?
Filtering email is the other method used to help prevent spam
When filtering email you are more likely to let spam get through; however, you are less likely to stop ham completely.
SpamAssassin is a program used for email filtering, which is an e-mail spam filtering program based on content-matching rules
» You can enable SpamAssassin in WHM -> Server Configuration -> Tweak Settings
SpamAssassin: Vipul's Razor
A checksum-based, collaborative, distributed spam-detection and filtering network. Spam is submitted by user contribution. Detection is done with statistical and randomized signatures that efficiently spot mutating spam content.
» More information on this is available at http://razor.sourceforge.net
SpamAssassin Rules Euporium (SARE)
» These are several sets of SpamAssassin Rules that are not included with the default distribution. These rules are constantly updated, improved, generalized and cleansed as the spammers modify their devious method to send mail.
More information is available at http://www.rulesemporium.com
SpamAssassin: SARE - RulesDuJour
A bash script is available to automatically download and install new rule sets as they become available. More information on this is available at http://exit0.us/index.phppagename=RulesDuJour
Exim Dictionary Attack ACL
Additional ACLs: Dictionary Attack ACL
A dictionary attack is when a SMTP connection attempts to send email from a spam source to a random set of names on a particular domain. They hope with this that it will reach a legitimate user. This is a common method spammers use.
» This ACL is available at http://configserver.com/free/eximdeny.html
Additional ACLs:HELO ACL
When a remote system connects to your mail server, it is supposed to say “HELO domain.com”. A spammer might try to HELO with your own IP address. This ACL will prevent this.
» This ACL is available at http://vamoswentworth.org/exim-tricks.html
Additional ACLs:Data ACL
A valid email address should always contain a message id. There is spamming software (and viruses too) that often do not. This ACL will refuse them.
» This ACL is available at http://vamoswentworth.org/exim-tricks.html
Additional ACLs PLEASE REMEMBER
Please remember that when installing an ACL into Exim you could potentially break Exim or even worse make your server an open relay. Please take the time to research what you add and fully test it once it's added!