Egress filtering for a healthier Internet.

[InfoSec News <isn () c4i org>]

+------------------------------------------------------------------+
|  Linux Security: Tips, Tricks, and Hackery                       |
|  Published by Onsight, Inc.                                      |
|                                                                  |
|  13-February-2003                                                |
|  http://www.hackinglinuxexposed.com/articles/20030213.html       |
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Egress filtering for a healthier Internet.
By Brian Hatch

Summary: Security is not just protecting yourself from others, you
must protect others from yourself. Egress filtering is an important
part of any firewall setup.

The Slammer/Sapphire/etc worm really ruined my Internet experience
for a few days. Yes, patches were available. Yes, people should have
blocked inbound access to their windows boxes when not needed. These
and many other explanations about what could have been done were
discussed all over. However one thing that seems to have been
overlooked in many sources is that it was the outbound packets that
were killing us, and we shouldn't have been subject to them at all.

We are used to thinking of security as a defensive battle: keep the
bad guys out of our systems. We set up our firewalls to block known
attacks, block any inbound packets that don't go to permitted ports,
restrict inbound packets from untrusted sources, keep spam out by
blocking connections from machines on DNSBLs[1]. These are all
defensive barriers we erect in front of our servers to keep the bad
guys out.

The problem is that sooner or later there will be a breach of those
bariers. Perhaps a bug exists in one of the services that you do need
to permit. Perhaps your firewall rules were too weak. The problem is
that once you are cracked, the cracker is on the inside of the
firewall, and is uninhibited by your inbound rulesets.

Blocking inapropriate inbound access is a very good and necessary
thing. However people need to start blocking inappropriate outbound
access too. If your machine is just a mail server, then it needs to
be able to send out packets from and to port 25, but it doesn't need
to be able to make an HTTP connection. If you run a DNS server, it
doesn't need to be able to support outbound FTP. And for goodness
sake, if your network is w.x.y.z, packets from other addresses should
never leave your boarders.

When your computer is compromised, you are no longer the innocent
party trying to defend yourself, to other machines you have become
the attacker. You owe it to others to make outbound attacks more
difficult to the cracker or worms that have managed to get onto your
machine.

Outbound packet security is called egress filtering. It's no harder
to implement than the ingress filtering rules you should already
have. Had the Slammer hosts been behind firewalls with proper egress
filters, the outbound attacks would never have been dropped at their
firewall/router.

As always, the best rule when creating firewalls/access lists is to
list those packets which should be allowed and deny the rest. Be very
restrictive! Better to break a few applications during testing than
to leave holes open. If you have a completely new network, it is easy
to block everything and open ports as needed. if you have an existing
network, you should run a sniffer, logging the type of network
activity you experience currently, and then create your rules to
match them.[2]

So, let's take an example of the following server. We'll create both
ingress and egress filters such that it does exactly what it needs,
and no more.

Our sample machine runs Apache, Postfix, and allows inbound SSH
access from the local network only. It needs to be able to look up
DNS entries for it's Apache logfiles and to support a few DNSBL lists
for preventing inbound spam. Here are the iptables rules you'd create
on this host to support this configuration:


  #!/bin/sh

  # Define a few variables for ease of use
  MYIP=123.456.789.012
  MYNET=123.456.789.0/24

  ETHERNET="eth0"

  # This machine makes all DNS requests to a single
  # DNS server, rather than asking the roots directly.
  DNSSERVER=210.987.654.321

  # Drop any packets which do not match our rules.
  iptables -P INPUT DROP
  iptables -P OUTPUT DROP
  
  # Flush all tables.
  iptables -F
  iptables -F INPUT
  iptables -F OUTPUT
  
  # Let's allow all packets on the local (127.1) network interface.
  iptables -A INPUT -i lo -j ACCEPT
  iptables -A OUTPUT -o lo -j ACCEPT
  
  # Drop all inbound packets that claim to be from us..
  iptables -A INPUT -i $ETHERNET -s $MYIP -j DROP
  
  # Drop all outbound packets that claim not to be from us.
  iptables -A OUTPUT -o $ETHERNET -s ! $MYIP -j DROP
  
  # BLock inbound from RFC1918 networks
  iptables -A INPUT -i $ETHERNET -s 10.0.0.0/8 -j DROP
  
  # Add other such rules here to block 172.16/12, 192.168./16
  # and other networks, multicast, and flag settings that you
  # should not expect from legitimate traffic.
  #
  # Remember, this example is to show egress filtering, not a
  # full firewall implementation.

  # Allow inbound HTTP from everywhere
  iptables -A INPUT  -i $ETHERNET -p tcp -d $MYIP --dport 80 -j ACCEPT
  iptables -A OUTPUT -o $ETHERNET -p tcp -s $MYIP --sport 80 ! --syn -j ACCEPT
  
  # Allow inbound SSH from local network
  iptables -A INPUT  -i $ETHERNET -p tcp -d $MYIP --dport 22 -s $MYNET -j ACCEPT
  iptables -A OUTPUT -o $ETHERNET -p tcp -s $MYIP --sport 22 -d $MYNET ! --syn -j ACCEPT
  

  # Allow outbound DNS to our nameserver[3]

  iptables -A OUTPUT -o $ETHERNET -p udp -s $MYIP --dport 53 -d $DNSSERVER -j ACCEPT
  iptables -A INPUT  -i $ETHERNET -p udp -d $MYIP --sport 53 -s $DNSSERVER -j ACCEPT
  
  # Log any rejects to syslog so we can debug
  iptables -A INPUT -j LOG
  iptables -A OUTPUT -j LOG


Many firewalls simply create rules on the INPUT chain. The OUTPUT
rules above comprise our egress filters. As you can see, an egress
filter will usually look pretty close to an ingress filter, so
generating them is pretty simple, just reverse the -d / --dport
(destination address / destination port) and -s / --sport (source
address / source port) arguments. I like to include ! --syn where
appropriate to prevent the initial packet of the TCP three way
handshake, just to be verbose.

The above code is by no means a complete firewall. You'll want to
include changes to some /proc entries[4], drop source routed packets
and packets from the non-routable RFC-1918 networks, include some NAT
or connection tracking modules, etc. I heartily recommend "Linux
Firewalls, Second Edition" by Bob Ziegler - it has excellent iptables
and ipchains info and sample configurations.

You can implement egress filtering on both the host itself, and on
your firewalls. To protect those Windows boxes on your network, the
firewall may be the only appropriate place. If a cracker gets onto
your machine, rather than an automated worm, he/she would be able to
undo your iptables rules quite easily, so it's always a good idea to
include rules on the firewall as well.

The one thing I'd like you to take away from this article is that
egress filtering is a good thing. Here are just a few reasons:

  * You won't be a source of attacks, and will thus be less likely to
    be blackholed as a bad netizen, or break the AUP of your ISP.
   
  * You're less likely to need to admit you were broken into. (Image
    is important for big companies.)
   
  * Most attacks use TCP, and if you drop the outgoing SYN packet,
    you'll never establish the session, and the drain on your network
    will be minimal.
   
  * You're more likely to find when an attacker is calling out to get
    his/her tools, or prevent them altogether.
   
  * You're more likely to see when an upgrade of your software
    changes it's network requirements in a way you don't like.
   
  * You'll find when internal users are acting differently than you
    expect.
   
  * You'll keep spyware and other software from 'calling home'

Many of the firewall-creation scripts you find on the Internet do
include egress filtering as well. If you are using one that does not,
you can modify it to do so fairly easily. Or you can look for one
that was designed with more paranoia from the beginning.

NOTES:

[1] A DNSBL is a DNS-based black hole list. Many of these exist,
listing machines that list open relays, known spam sources, etc.

[2] Be sure to validate the existing activity -- if there is
unnecessary or unaccounted for traffic, see what's causing it at the
source.

[3] Yes, this does not handle TCP responses. Some days I subscribe to
the DJB school of DNS.

[4] You may want to see a previous article about /proc at http://
www.hackinglinuxexposed.com/articles/20021015.html for example.

                            -------------                            
Brian Hatch is Chief Hacker at Onsight, Inc and author of Hacking
Linux Exposed and Building Linux VPNs. He loves to watch as
unscrupulous software on his fiancee's laptop tries to do things we
wouldn't like. He should really limit the hours of the day when the
log monitoring software pages him about it though. Brian can be
reached at brian () hackinglinuxexposed com.

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Copyright 2003, Brian Hatch.



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