ACCEPT/DROP

Hi,

I was looking into the iptables implementation and was intrigued about how iptables would handle a situation in which we have identical rules except for their targets which are contradicting, say ACCEPT and DROP. By looking at ipt_do_table() function it seems that the first non-IPT_RETURN verdict from any standard target will end the traversal of a chain of a table, which seems to be a bit odd. First, such conflicting rules must not be allowed. Even beyond, this fails in the situation where you have a dropping rule added after an accepting rule.

For example, a packet from m.n.o.p to a.b.c.d would be accepted at a.b.c.d because of the first rule, although it had to be dropped according to the second rule. And this would result because of the order in which the rules are added to the table.

iptables -A INPUT -d a.b.c.d -j ACCEPT
iptables -A INPUT -s m.n.o.p -j DROP

Is my understanding correct? If so, I am curious to know how nf-hipac behaves in such situations?

waiting for reply,

Venkatesh Prasad Ranganath,
Dept. Computing and Information Science,
Kansas State University, US.
web: http://www.cis.ksu.edu/~rvprasad

Hi Venkatesh

You misunderstand the definition of conflicting rules in the context of the packet classification problem. A conflict occurs if there are two or more matching rules with the same minimal cost. Since the cost of a rule is equivalent to its position in the chain[1] and the position is unique there are no conflicts by definition.
This holds for both iptables and nf-hipac.

[1] at least if you're not using user-defined chains; but even if user- defined chains come into play the costs are in fact unique

This behaviour is intended. The packet classification problem is about finding the matching rule with minimal cost.

Thomas