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4. Configuring Masquerading

All right! The preliminaries are over, this is where the magic begins. IP masquerading is one of the truly magical services Linux provides. There are commercial products for Windows which do the same thing, but not nearly as efficiently: an ancient 386 can merrily provide IP masquerading services to a whole medium sized office, but cannot even run Windows 95, let alone the add on masquerading package. (As an addendum, I read in some recent reviews that Windows 2000 will support "connection sharing" without addon software. It looks like the companies which sold connection sharing software have been "embraced and extended" by MicroSoft. However, I wouldn't recommend you try the Windows 2000 solution on a 386.)

Linux has an extremely versatile firewalling capability, and we are going to be using it in the simplest and crudest possible manner. If you want to learn how to do firewalling like an expert, you should read both the Firewalling HOWTO for an understanding of the theory and the IPChains HOWTO for instructions on the new ipchains firewalling tool which ships with the Linux 2.2.X kernel (and by extension Red Hat 6.X). There is also now a very good IP Masquerading HOWTO available which has more details on masquerading tweaks.

Configuring simple masquerading is very very easy once your internal and external networking is operational. Edit the /etc/rc.d/rc.local file and add the following lines to the bottom:

# 1) Flush the rule tables. 
/sbin/ipchains -F input 
/sbin/ipchains -F forward 
/sbin/ipchains -F output 
# 2) Set the MASQ timings and allow packets in for DHCP configuration. 
/sbin/ipchains -M -S 7200 10 60 
/sbin/ipchains -A input -j ACCEPT -i eth0 -s 0/0 68 -d 0/0 67 -p udp 
# 3) Deny all forwarding packets except those from local network. 
#    Masquerage those. 
/sbin/ipchains -P forward DENY 
/sbin/ipchains -A forward -s -j MASQ 
# 4) Load forwarding modules for special services. 
/sbin/modprobe ip_masq_ftp 
/sbin/modprobe ip_masq_raudio

The last two lines insert kernel modules which allow FTP and RealAudio to work for computers on the inside network. There are other modules for special services which you can tack on if you need them:

Now you're ready to try masquerading! Run the rc.local script with the command /etc/rc.d/rc.local and you are ready to go! Sit down at one of your other computers and try some web surfing. With any luck, everything is now hunky dory.

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