Build Your Own Spam FirewallUsing Postfix & SpamAssassin

Zach Levow, vp engineering

April 20, 2005 / SecureIT

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Agenda Introduction to Barracuda Networks (10 Min) Building a security appliance using open source

technologies (10 Min) Anti-Spam technologies (40 Min) System considerations (10 Min) Q/A

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Company Background Mission

Deliver easy to use and cost effective solutions for protecting email servers Founded December 2002

Research and development since 2001 Barracuda Spam Firewall Launch October 2003 Barracuda Spyware Firewall Launch April 2005 Headquarters in Cupertino, California

Offices in Europe (UK), China (Shanghai), Canada, Australia, India, Pakistan, United Arab Emirates (Dubai), and USA

100+ employees worldwide Experienced management & development team

Privately Funded Profitable

Market Leader 14,000 customers worldwide

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Barracuda Spam Firewall Comprehensive email protection

Blocks spam and virus Integrated hardware and software solution

Ease of use Plug-and-play No changes needed to email servers

Enterprise Features Reliable and Robust

Aggressively Priced No per user licensing fees

Market leading anti-spam appliance

Launched Oct. 13, 2003

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Barracuda Spam Firewall - Outbound Edition

Comprehensive MTA Includes Barracuda Spam Firewall Features

Easy to use and Configure (web interface) Secure Reporting and logging

Stops Virus Proliferation Enforces Corporate & Regulatory Policies

Foul language and security HIPAA, Sarbanes-Oxley

Prevents Spamming & Open Relay Function

Launched Jan. 17, 2005

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Barracuda Spyware Firewall Features Gateway appliance Powerful, easy to use & install

Intuitive user interface

Affordable Prices starting at $1,999

Available in five models: Spyware Firewall 210 ($1,999)

Spyware Firewall 310 ($3,299) Spyware Firewall 410 ($5,999)

Inline hardware appliance Complete scalability for growing organizations

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Customers

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Cardinal Rules of Spam Filtering No false positives! A false positive where the sender is not notified is even

worse Reject rather than bounce Don’t assume everyone’s mail looks like yours

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Open Source Technical Issues Immature products: One size does not fit all Mature products: Bloated codebase – hard to maintain Security issues

Pro: an active community will find and fix security issues. Con: an active community will introduce security flaws. Con: publishing your source does expose you to more exploits.

Hackers go for the lowest common denominator. Chroot, chroot, chroot – it’s always worth it.

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Open Source Business Issues Giving back to the community

Many changes aren’t for everyone Extra time to polish changes for contribution

Separating proprietary technology Configuration files are yours Absolutely no linking if you don’t want to share.

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Anti-spam Technologies Intent Analysis

Open alternative: SURBL – Bill Stearns’ URL Blacklist Real-time query performance issues

RBLs Spamhaus – only list with minimal false positives

SpamAssassin Rules Updates

SPF Rate Control/Throttling Virus scanning

Several fairly good open source solutions… No one solution catches all… Combine them

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Anti-Spam Technologies (Cont.) Bayesian

International Charsets IBM’s ICU library very efficient Token Chaining Crucial

Per-user Bayes very important Noise reduction very helpful Pro: most proactive anti-spam technique Con: Troubleshooting is usually a nightmare! Make user classification easy

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Controversial Anti-Spam Techniques Graylisting

Pro: Very effective at blocking spam Con: Potentially delays all messages from new senders by

several hours Con: Spammers know how to defeat it, but most don’t yet

Tarpitting Pro: effective at slowing down dictionary attacks Con: Will bury a busy system if a process or thread is required

per connection.

Challenge-response Increases internet chatter Unless linked to outbound SMTP, can lead to “Deadlock”

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DNS MX Records Example MX recordbarracudanetworks.com MX preference = 10, mail exchanger = barracuda2.barracudanetworks.com

barracudanetworks.com MX preference = 10, mail exchanger = barracuda.barracudanetworks.com

SMTP is great to load-balancing/failover Put as many systems as you like at the same

“Preference” and all known clients will round-robin until they find an available system

DON’T LEAVE YOUR MAIL SERVER AS A BACKUP MX FOR YOUR SPAM FILTER!! Spammers will attack it directly

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Phishing No link should ever say that it is HTTPS in a

message and then actually link to a non-HTTPS page

Relatively small list of known scams – fairly easy to keep up with if you have a good sample of email. It is worth the effort.

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Quarantine Effective tool for reducing “False Positives” while

increasing catch rate. Best if integrated with directory services so that a user

with multiple email addresses only has one quarantine box.

No perfect open-source solution: Need web interface Should send daily digest

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Per-User Settings Major reduction in administration if users can update

personal allow/block lists, passphrases, etc. Again, best when integrated with directory services. User interface issues.

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System Considerations Databases:

Most open source databases are great for low-volume, general purpose applications.

In high load situations they all break down – specialized databases become necessary.

High-availability Syncing of configurations (meta-data) Syncing of quarantine information (data)

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System Considerations (Cont.) Hard drives

Typical drives will last 6-12 months under a constant and steady mail load.

Use Raid Turn off write cache (hdparm)

Filesystems Use Journaling Filesystem

Ext3: slow, but robust XFS/ReiserFS: faster, but less robust Mount with synchronous I/O (sync)

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Fighting Spam Can Be Effective False positives are not acceptable or necessary. Keep your spam rules and virus definitions up to

date. Reduce your administration load and false

positives/negatives by giving control to your users through personal settings and quarantine.

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Q/A