Simple script questions

[Chris Mulcahy]

I have this very simple script which I use to set up a very 
simple IPTables firewall with masquerading.  It does a great 
job and keeps my machine very well protected, I believe.  I'm 
not sure where I got it, maybe from the IPTables Tutorial.

Now, I would like to understand this a bit more.  Could 
someone help me to comment it and maybe make it a bit better?  
Specifically, I would like to be able to re-run it without the 
errors that occur because the chains already exist.  I would 
also like to know how I would allow a few specific ports to be 
visible on my external interface, specifically 25 and/or 80.

Thanks for the help!
Chris

--> SCRIPT BEGINS <--
iptables -N block
echo Moving to Add blocking where state is not extablished or 
related
iptables -A block -m state --state ESTABLISHED,RELATED -j 
ACCEPT
iptables -A block -m state --state NEW -i ! ppp0 -j ACCEPT
iptables -A block -j DROP

## Jump to that chain from INPUT and FORWARD chains.
iptables -A INPUT -j block
iptables -A FORWARD -j block

# Masquerade out ppp0
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

# Disallow NEW and INVALID incoming or forwarded packets from 
ppp0.
iptables -A INPUT -i ppp0 -m state --state NEW,INVALID -j DROP
iptables -A FORWARD -i ppp0 -m state --state NEW,INVALID -j 
DROP

# Turn on IP forwarding
echo 1 > /proc/sys/net/ipv4/ip_forward
--> SCRIPT ENDS <--