CCIE Security – Cisco ASA Modular Policy Framework Example

This is first in a serie of posts in english dealing with technical configurations and solutions. The reason for this is me studying for CCIE Security certification and along the road I will probably find interresting stuff to share with others in the same situation as I am. (I´ve already found that more good configuration examples can be found at blogs than in technical references as CCO. Another purpose for me posting about stuff like this is that I learn alot when trying to explain and write about it. )

I have been bangning my head a couple of ours now trying to understand the modularity of Cisco ASA MPF. There are Class-maps, policy-maps and service-policies. There are a lot of different types of each of them and different combinations are invalid. Deeper information can be found att CCO.

MPF is built with 3 different types of modules:

1. class-maps. Defining what to look for
2. policy-maps. What shall we do with it?
3. service-policy. where do we do it?

I made up an hypothetical example. Lets say that I want to prevent my users from downloading mp3-files with ftp. This is a good example of when MPF comes handy (yes I know that some users know how to rename files, do encrypted file-transfers or use other protocols than ftp.)

Ok. On my LAN I have this PC with IP 192.168.1.215 which should not be able to GET files whose name contains “.mp3”. Where do I start?

First I create a regexp defining the string to search for:

regex mp3 ".*.mp3.*"

Next I create a class-map type regex that uses the regex defined:


class-map type regex match-any class-map-TEST-mp3-regex
match regex mp3


Next thing to do is to create a class-map type inspect which is going to look into the ftp application-data sent and have a look for filenames with the regexp-string matched:

class-map type inspect ftp match-all class-map-TEST-ftpfilter
match filename regex class class-map-TEST-mp3-regex


After that I create a policy-map that defines the action (reset tcp connection and log the event) to be taken when the class-map above is triggered:

policy-map type inspect ftp policy-map-TEST-dropftp
parameters
class class-map-TEST-ftpfilter
reset log


Now I need to define which traffic to look into. Remember the host

-IP above? I create an acl:

access-list acl_TEST_mytraffic extended permit tcp host 192.168.1.215 any eq ftp


Before putting everything together you need to decide where to apply this. It can be done either globally or inbound to a specific interface. The most logical thing to do in this case is to apply it on my inside-interface where my ftp-clients are. If you have already a policy-map bound to that interface you need to reuse that policy-map in the next step. Otherwise, just create a new policy-map.

asa# sh run service-policy
service-policy policy-inside interface inside
asa#


So I reuse this policy and add a new class:
policy-map policy-inside
class class-map-TEST-mytraffic
inspect ftp strict policy-map-TEST-dropftp

Let´s see if it works…
ftp> (connected to FTP-server from my host)

ftp> ls

200 PORT command successful

150 Connecting to port 40538

Connection closed by remote host.

Finally, a bleeding rocket-science flow-chart with home-made arrows and sugar on top…