I'm trying to figure out how NAT and iptables work. While I'm in the trial-and-error phase of learning about it, I found two somewhat conflicting howtos.
One howto uses a script to call
iptables rules one after another. The script seems to be named and stored such that it is executed early during system boot, and I think a problem may be that other scripts may be called after it and undo its intentions. I even think I did this once by accident when I saved and renamed the original script (00-firewall) using a backup (00-firewall-old). The example script form the howto is:
#!/bin/sh PATH=/usr/sbin:/sbin:/bin:/usr/bin # # delete all existing rules. # iptables -F iptables -t nat -F iptables -t mangle -F iptables -X # Always accept loopback traffic iptables -A INPUT -i lo -j ACCEPT # Allow established connections, and those not coming from the outside iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -m state --state NEW -i ! eth1 -j ACCEPT iptables -A FORWARD -i eth1 -o eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT # Allow outgoing connections from the LAN side. iptables -A FORWARD -i eth0 -o eth1 -j ACCEPT # Masquerade. iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE # Don't forward from the outside to the inside. iptables -A FORWARD -i eth1 -o eth1 -j REJECT # Enable routing. echo 1 > /proc/sys/net/ipv4/ip_forward
Another howto does not use a script but a file where some filter rules are defined. It looks like this:
*filter # Allows all loopback (lo0) traffic and drop all traffic to 127/8 that doesn't use lo0 -A INPUT -i lo -j ACCEPT -A INPUT ! -i lo -d 127.0.0.0/8 -j REJECT # Accepts all established inbound connections -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT # Allows all outbound traffic # You could modify this to only allow certain traffic -A OUTPUT -j ACCEPT # Allows HTTP and HTTPS connections from anywhere (the normal ports for websites) -A INPUT -p tcp --dport 80 -j ACCEPT -A INPUT -p tcp --dport 443 -j ACCEPT # Allows SSH connections # THE -dport NUMBER IS THE SAME ONE YOU SET UP IN THE SSHD_CONFIG FILE -A INPUT -p tcp -m state --state NEW --dport 30000 -j ACCEPT # Now you should read up on iptables rules and consider whether ssh access # for everyone is really desired. Most likely you will only allow access from certain IPs. # Allow ping -A INPUT -p icmp -m icmp --icmp-type 8 -j ACCEPT # log iptables denied calls (access via 'dmesg' command) -A INPUT -m limit --limit 5/min -j LOG --log-prefix "iptables denied: " --log-level 7 # Reject all other inbound - default deny unless explicitly allowed policy: -A INPUT -j REJECT -A FORWARD -j REJECT COMMIT
What are the pros and cons of both ways of setting up iptables? Background info is much appreciated because I'm quite new to the whole thing. For example, I don't get who is reading the file from the latter howto, and how it is processed. My feeling tells me the second howto suggests a better solution, but why exactly?