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More ASA Objects and Object-groups

A few years ago, I developed a Perl-based application that take a template file and pukes out standardized access rules for new hosts as they’re added to the network.  This works great for making sure that each host is able to be managed properly.  This solution, however, is not very flexible.  If I need to remove a host’s access, I may have to take out 20 rules individually.  That’s not really cool, so, at the suggestion of a coworker, I’m working on a solution that uses objects, object-groups, and nested object-groups.  This should minimize the configured rules and allow new host rules to be added and removed by simply adding hosts to object-groups.

Example time.  Let’s say you have a bunch of RFC1918 addresses behind your firewall that all need HTTP access to one network on the InterTubes.  First thing to do is to create the objects that will be involved; in this case, that’s all the networks and/or ranges.  To be more specific, 192.0.2.0/24 is the public IP to which the hosts need access.   The internal hosts are 192.168.0.0/24 and the IP range 10.0.0.1-25.  Yes, I know the names are terrible.

object network NET1
 subnet 192.0.2.0 255.255.255.0
object network NET2
 subnet 192.168.0.0 255.255.255.0
object network NET3
 range 10.0.0.1 10.0.0.25

Now, we can use some Snort-like configuration to create object-groups that include the objects we just created.  In this case, we’re creating an InterWebs-based object-group and another for local addresses.

object-group network REMOTE-NETS
 network-object object NET1
object-group network LOCAL-NETS
 network-object object NET2
 network-object object NET3

Now we can use these object-groups to create ACLs.   You’ve done this before, right?

access-list TEST-ACL extended permit tcp object-
   group LOCAL-NETS object-group REMOTE-NETS eq www

To be sure it worked as expected, let’s take a look at the ACLs.  The format sucks because the lines are so long; sorry about that.

firewall# show access-list TEST-ACL
access-list TEST-ACL; 7 elements; name hash: 0x5329ed72
access-list TEST-ACL line 1 extended permit tcp object-group LOCAL-NETS object-group REMOTE-NETS eq www 0x1abfa4a0
  access-list TEST-ACL line 1 extended permit tcp 192.168.0.0 255.255.255.0 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0x50797e0c
  access-list TEST-ACL line 1 extended permit tcp host 10.0.0.1 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0xa2159c9d
  access-list TEST-ACL line 1 extended permit tcp 10.0.0.2 255.255.255.254 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0x93f1c362
  access-list TEST-ACL line 1 extended permit tcp 10.0.0.4 255.255.255.252 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0x512fc827
  access-list TEST-ACL line 1 extended permit tcp 10.0.0.8 255.255.255.248 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0x7b11e96f
  access-list TEST-ACL line 1 extended permit tcp 10.0.0.16 255.255.255.248 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0xc302aa0e
  access-list TEST-ACL line 1 extended permit tcp 10.0.0.24 255.255.255.254 192.0.2.0 255.255.255.0 eq www (hitcnt=0) 0x2ea75962

Cool.  Everything looks great, and everyone should have the access they need.  If a new host with the IP of 172.16.0.28 comes online inside the network, you add a new nested object-group that includes that host.  Access is automagically updated, so there’s no need for more ACL lines.  Another method is to add the new host directly to the LOCAL-NETS object-group, but that’s going to limit the ways to address that box and related hosts in an ACL.  I suggest you just add the new object to the object-group.

As a bonus, you can also nest object-groups into each other.  For example, we can create an object-group that includes our the LOCAL-NETS and REMOTE-NETS object-groups.

object-group network ALL-NETS
 group-object LOCAL-NETS
 group-object REMOTE-NETS

I don’t know where you’d ever use that specific object-group, but you could use this technique in other ways.  I’m looking to create object-groups for each interface of the firewall and creating a super-object (my term) to allow the standard access stuff.  You could do the same for office networks; each office has it’s own object-group for access that is also nested in an object that provides basic access to the TubeWebs or something.  Use your imagination.  🙂

Send any questions my way.

This article is based on an ASA 5505 running 8.3.1.  Most of the config above should be portable to any 8.x except for declaring the objects. In other versions of 8.x, you may have to add host directly to the object-group.  Running on 7.x and below may be a different story.

Other reading:  https://aconaway.com/2009/10/01/object-groups-in-the-asafwsmpix/

Director’s Commentary:
[audio:/content/images/wordpress/2010/04/More-ASA-Objects-and-Object-groups.mp3|titles=More ASA Objects and Object-groups]

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