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Algorithm  

Firepower Variable Sets

Below is an example of a SNORT rule. This particular rule looks for a Trojan called Cryptowall.

Notice that the first line has the values $HOME_NET and $EXTERNAL_NET? These are variables. SNORT uses these variables to represent the protected and the unprotected networks.


FMC SNORTRule
 

Variables are useful, as values can be set or changed in a global area. This means that there is no need to edit individual SNORT rules. In Firepower, a global list of variables is a Variable Set.

FMC comes with a default variable set. It's HIGHLY RECOMMENDED to change these values to represent your network. You should create your own set and change that, rather than modifying the default set.


 

 


Customising Variable Sets

Variables Cheat Sheet

There are many variables in FMC. Below is a table of the common variables that you should consider changing. If nothing else, change the $HOME_NET variable.

You can use objects and object groups to make future changes simple.


 

Variable Usage Default Value Notes
$HOME_NET Specifies the protected network any Add trusted networks in here
$EXTERNAL_NET Specifies the outside network any Add $HOME_NET as an exclusion here
$SIP_SERVERS Define SIP servers HOME_NET A list of IP's
$SMTP_SERVERS Define SMTP servers HOME_NET A list of IP's
$SNMP_SERVERS Define SNMP servers HOME_NET A list of IP's
$SQL_SERVERS Define SQL servers HOME_NET A list of IP's
$SSH_SERVERS Define SSH servers HOME_NET A list of IP's
$TELNET_SERVERS Define telnet servers HOME_NET A list of IP's
$FTP_PORTS Ports that FTP runs on 21, 2100, 3535 Only change if non-default ports are needed
$HTTP_PORTS Ports that HTTP runs on Many ports Only change if non-default ports are needed
$ORACLE_PORTS Ports that Oracle databases run on any Only change if non-default ports are needed
$SSH_PORTS Ports that are used for SSH 22 Only change if non-default ports are needed

 

 

 

Creating a Variable Set

Variable sets are objects, which are in Objects -> Object Management -> Variable Set.


FMC VarSet1

 

 

The Default-Set object is available out of the box. Create a new variable set by clicking the Add Variable Set button.

The new set will use the values in the default set as a starting point. This is why it's a good idea to use custom sets, and leave the default as it is.


FMC VarSet2
 

 

Here you can see a list of every variable. Variables are system-wide. When you create a new variable, it is available in every variable set. The value that the variable contains can be different in each set.

If you look at the HOME_NET variable, you can see that it is set to any. This is why you should edit the default variable set. The default variable set cannot distinguish between trusted and untrusted networks.

Each variable is customisable. Variables can be static values, or reusable objects.


FMC VarSet3
 

 

Applying a Variable Set

 

Pair variable sets with an Intrusion Policy in an Access Control Policy. This can be per rule or a default action.


FMC VarSet4
 

 



Twitter: @NetwrkDirection
 

 


Suggested Articles

 


 

 

References

Cisco - Reusable Objects

 


Last update 2017-10-08 15:15
 


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