Linux Security Cookbook

2.22.1 Problem

You want to construct complex firewall behaviors, but you are getting lost in the complexity.

2.22.2 Solution

Be modular: isolate behaviors into their own chains. Then connect the chains in the desired manner.

For iptables:

# iptables -N CHAIN1 # iptables -N CHAIN2 # iptables -N CHAIN3 # iptables -N CHAIN4 # iptables -N CHAIN5

Add your rules to each chain. Then connect the chains; for example:

# iptables -A INPUT ...specification... -j CHAIN1 # iptables -A CHAIN1 ...specification... -j CHAIN2 # iptables -A CHAIN2 ...specification... -j CHAIN3 # iptables -A INPUT ...specification... -j CHAIN4 # iptables -A INPUT ...specification... -j CHAIN5

to create a rule structure as in Figure 2-1.

For ipchains:

# ipchains -N chain1 # ipchains -N chain2 # ipchains -N chain3 # ipchains -N chain4 # ipchains -N chain5

Add your rules to each chain. Then connect the chains, for example:

# ipchains -A input ...specification... -j chain1 # ipchains -A chain1 ...specification... -j chain2 # ipchains -A chain2 ...specification... -j chain3 # ipchains -A input ...specification... -j chain4 # ipchains -A input ...specification... -j chain5

to create the same rule structure as in Figure 2-1.

2.22.3 Discussion

Connecting chains is like modular programming with subroutines. The rule:

# iptables -A CHAIN1 ...specification... -j CHAIN2

creates a jump point to CHAIN2 from this rule in CHAIN1, if the rule is satisfied. Once CHAIN2 has been traversed, control returns to the next rule in CHAIN1, similar to returning from a subroutine.

2.22.4 See Also

iptables(8), ipchains(8).