Changing ASA access lists on the fly
Solution 1
Adding line x
to a line after the ACL name will insert it at that point in the list.
So, if you have:
access-list outside_in extended udp deny any any
access-list outside_in extended icmp deny any any
and you run:
access-list outside_in line 2 extended tcp deny any any
your config will end up as:
access-list outside_in extended udp deny any any
access-list outside_in extended tcp deny any any
access-list outside_in extended icmp deny any any
IP is inclusive of udp, tcp, and icmp; blocking IP will block all of these. So, in your config above, only the top rule will get hit.
Solution 2
By default Cisco ASA denies everything it is not explicitly allowed. So in your case your could perfectly do a:
access-list OUTSIDE_IN permit tcp any any eq 80
access-group OUTSIDE_IN interface DMZ
and by default everything else is going to be denied. There is an implicit deny ip any any at the end of your access-list.
You would only need and explicit deny ip any any if you want to know the number of packets hitting the access list.
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Nate
Updated on September 17, 2022Comments
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Nate over 1 year
I'm shortly going to be in a situation where I'll need to be updating a firewall on the fly. How does one update cisco ASA access lists on the fly? For example, if I start with:
access-list outside_in extended ip deny any any access-list outside_in extended tcp deny any any access-list outside_in extended udp deny any any access-list outside_in extended icmp deny any any
(A little harsh, I know, but bear with me. Out of curiosity, is there an easier way to deny everything?)
and then
access-group outside_in in interface DMZ
then how do I later update the access-list to open, say, port 80? Short of re-writing the entire access list. I can't just add a rule, because packets will be denied by the previous rules. So, I guess what I'm asking is, how do I add a rule to the beginning of an access list?
Thanks!
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ravi yarlagadda about 13 yearsCorrect me if I'm wrong, but I believe the default is to permit traffic to less secure networks (lower security level on the interface)?
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jliendo about 13 years@Shane. You are 100% correct. Traffic from a higher security interface to a lower security interface is allowed by default. From lower to higher it is not. It may very well be that my interpretation of the question was wrong. The way I interpreted it was that @Nate was asking how to block traffic from outside to DMZ.
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ravi yarlagadda about 13 yearsFair enough - your guidance is dead on in the context of the question, I just wanted to make sure he knew there's an asterisk on that
deny ip any any
. +1 :) -
Nate about 13 yearsThanks! And to remove a rule, is it sufficient to say "no access-list outside_in extended permit tcp any any eq 80" (for example), and have that rule be removed from wherever it shows up in the ACL?
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ravi yarlagadda about 13 years@Nate Yup, that will do it.