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Poor Man’s Firewall . A firewall that can be setup and implemented with a minimum amount of time and money. Why do I need one?. A Windows server can not be secured as it stands. Don’t believe anyone who tells you otherwise. MSSQL server should never be placed directly on the Internet.
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Poor Man’s Firewall • A firewall that can be setup and implemented with a minimum amount of time and money.
Why do I need one? • A Windows server can not be secured as it stands. Don’t believe anyone who tells you otherwise. • MSSQL server should never be placed directly on the Internet. • And yes, some people do have too much time on their hands. Anyone remember the Blaster worm?
OSI Model Lower Layers • Lower layers provide more primitive network-specific functions like routing, addressing, and flow control. • Layer II - (Data Link Layer) of the OSI Model • Layer III - (Network Layer) of the OSI Model
Switch/Hub (Layer II) • Switches and Hubs are used to connect various devices to a network. • Switches are intelligent, they look at the source and destination of each packet and route them to the appropriate switch port. • Hubs are dumb devices that present a copy of each packet that is seen to every other port on the device.
Bridge (Layer II) • A device that can be used to segment Local Area Networks (LANs). • They can be used to control the traffic going between two network segments based on Ethernet addresses. • They are essentially transparent devices. They can be replaced with a cross-over cable.
Router (Layer III) • A network device used for connecting different networks together. • They are responsible for intelligently routing packets based on IP address.
Firewall • A firewall filters packets based on a set of filter rules. • Packets that pass the rule set are forwarded through the firewall from one network interface to another. Packets that don’t, are dropped. • Firewalls can be either Software or Hardware based.
Bridging Mode Firewalls • A bridge that allows you to filter the packets that pass through its interfaces. • Can be placed anywhere in an existing network without disrupting existing services. • Transparent to your servers.
Linux – Bridging Mode Firewall • A software based firewall that uses Linux as the operating system. • The software is free. • Relatively easy to setup. • Can run on old hardware.
Software Needed • Iptables – Software that filters IP based traffic based on a set of rules. • Ebtables – Software that allows Iptables to see the packets as they go through the Bridge interface. • Bridge-Utils – Software that allows you to create the bridge.
Hardware Needed • Any old Pentium based computer • 128MB of RAM • ~1GB Harddrive • 2 - Network Cards (Minimum)
Example Bridge Script #!/bin/bash # /etc/rc.d/init.d/bridge BRCTL=/usr/sbin/brctl IFCONFIG=/sbin/ifconfig return=$rc_done case "$1" in start) echo "Starting service bridge br0" # Create bridge interface $BRCTL addbr br0 || return=$rc_failed # Turn Spanning Tree Protocall off $BRCTL stp br0 off || return=$rc_failed # Add interfaces to bridge $BRCTL addif br0 eth1 || return=$rc_failed $BRCTL addif br0 eth2 || return=$rc_failed # Reset to clean state $IFCONFIG eth1 down || return=$rc_failed $IFCONFIG eth2 down || return=$rc_failed # Set interfaces to Promiscuous Mode $IFCONFIG eth1 0.0.0.0 promisc || return=$rc_failed $IFCONFIG eth2 0.0.0.0 promisc || return=$rc_failed #Bring bridge interface up $IFCONFIG br0 promisc up || return=$rc_failed $BRCTL show echo -e "$return" ;; stop) echo "Shutting down service bridge br0" $IFCONFIG br0 down || return=$rc_failed $BRCTL delif br0 eth1 || return=$rc_failed $BRCTL delif br0 eth2 || return=$rc_failed $BRCTL delbr br0 || return=$rc_failed echo -e "$return" ;; status) $IFCONFIG br0 $BRCTL show ;; restart) $0 stop && $0 start || return=$rc_failed ;; *) echo "Usage: $0 {start|stop|status|restart}" exit 1 esac test "$return" = "$rc_done" || exit 1 exit 0
Example Filter Rules #!/bin/bash # Example Firewall Script IPTABLES="/sbin/iptables -v" # Any Subnet ANY=0.0.0.0/0 # ILLIAD Server ILLIAD=128.193.123.456 #### Flush all rules $IPTABLES -F # Delete all user created chains $IPTABLES -X # Zero all byte counters $IPTABLES -Z # Drop all packets without a rule $IPTABLES -P FORWARD DROP # loopback interface $IPTABLES -A FORWARD -i lo -j ACCEPT # Syn-flood protection: $IPTABLES -A FORWARD -p tcp --syn -m limit --limit 1/s -j ACCEPT # Ping of death: $IPTABLES -A FORWARD -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT # HTTP $IPTABLES -A FORWARD -s $ILLIAD -d $ANY -p tcp --dport 80 -m state --state NEW -j ACCEPT $IPTABLES -A FORWARD -s $ANY -d $ILLIAD -p tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
Useful Application • Ethereal – A powerful network protocol/packet analyzer that can be used to aid in the development of your filter rules.
Resources • Linux bridging how-to http://bridge.sourceforge.net • Ebtables http://ebtables.sourceforge.net • Ethereal http://www.ethereal.com/
