iptables/firewall brainstorming

[Roberto Ragusa]

Thomas Woerner wrote:
> Please think of a scenario like this: Service A is adding
> firewall rules for opening port 20 and 21 (ftp-data and ftp) for
> everyone and service B is opening port 20 and 21 only for a specific
> network segment. What do you want to have here? If you apply A's rules
> first then 20 and 21 is open for everyone and the rules from B are not
> used at all. But if you apply B's rules first, ... What is the right
> ordering here? Should A or B win?

The conflict is there only if you are thinking something like this:

//A
if(port==(20-21)) PERMIT else DENY;
//B
if(port==(20-21)) {if(net==trusted) PERMIT; else DENY;}

where the behavior changes if you change the order of A and B.

But I have always seen firewalls rules written as "holes", with a default
deny rule at the end:

//A
if(port==(20-21)) PERMIT;
//B
if(port==(20-21) && net==trusted) PERMIT;
//default
DENY;

In this case there is no dependence on the ordering.

Am I missing something?

-- 
   Roberto Ragusa    mail at robertoragusa.it