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newbie firewalling questions

Hey all. I am running woody om an old p166 for routing purposes. Works quite 
well. Would like to add firewall functionality, so I have a few questions.
Also, I am planning on reinstalling with kernel 2.4 as opposed to the current 
config which uses 2.2.

I realize that I am asking a lot. But with a few examples, I could get a grasp
on some basic priciples that would allow me to get going.

Thanks in advace for anyone that provides assistance.

--Wishing all a wonderful summer 2004!

---If i recompile 2.4, what are the bare minimum requirements for future     
    firewall possibilities?
---How do I enable external requests for machines running network services 
    internally to be forwarded to their appropriate machines ( ie, ftp, apache
    and ssh as examples )?
---Like above, how do I allow/restrict internal requests, that would only
    be resolvable by being routed externally, possible/impossible?

ifup enables both nics from /etc/network/interfaces
eth1 is external and gets public ip from isp via dhcp
eth0 is internal and is statically assigned private ip and network info by me
all internal machines use static info
dns servers are those provided by isp 

Here is my existing proxy script:

echo "1" > /proc/sys/net/ipv4/ip_dynaddr
/sbin/ipchains -A input -j ACCEPT -i $EXTIF -s 0/0 67 -d 0/0 68 -p udp
EXTIP="`/sbin/ifconfig eth1 | grep 'inet addr' | awk '{print $2}' | sed -e 
/sbin/depmod -a
/sbin/modprobe ip_masq_ftp
/sbin/modprobe ip_masq_raudio
echo "1" > /proc/sys/net/ipv4/ip_forward
echo "1" > /proc/sys/net/ipv4/ip_always_defrag
/sbin/ipchains -M -S 7200 10 160
/sbin/ipchains -P input ACCEPT
/sbin/ipchains -P output ACCEPT
/sbin/ipchains -P forward REJECT
/sbin/ipchains -F input
/sbin/ipchains -F output
/sbin/ipchains -F forward
/sbin/ipchains -P forward DENY
/sbin/ipchains -A forward -i $EXTIF -s $INTLAN -j MASQ 

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