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Writting rules¶

Mole IDS rule system is built on top of yara. You can find information about writting basic yara rules

Syntax¶

With Mole IDS we will be able to match network packets payloads instead of file payloads, that means we will need to use Yara in a different way.

When you use Yara to find patterns on files you mainly define the strings section and the condition one. Well, in Mole IDS, you will need to use the meta section as well.

The meta section will help Mole IDS to determine when the rule must be executed and match the defined pattern.

Moreover, think for a moment that Mole IDS will execute Yara rules for a stream of data, which means it has a variable length or the order is not always the same.

Extra meta fields¶

As said before, there are some key entries in the meta section that defined how the rule have to be executed based on the traffic.

uuid¶

Mole IDS does not use it at the moment. It is there for future purposes.

type¶

Type of event. Possible values:

alert
log
block

Warning

At the moment Mole IDS only manages alerts so the unique type that is recognized by Mole IDS right now is álert´.

proto¶

proto stands for protocol and it defines either network or transport protocols. The Yara rules associated to that protocol will be elected among others to be executed when all conditions are met.

Possible values:

Network¶

IP

Transport¶

TCP
UDP
SCTP

proto accepts the negation operator (!). This operator can be used at the begining of the sting and its function is to negate the value. For example a rule defined like proto = "!tcp" will be executed on the following protocols IP, UDP, and SCTP.

Warning

Mole IDS only recognize the following protocols:

IP
TCP
UDP
SCTP

src¶

Origin address of the packet. It must be an ip address, list of ip address or range of ip addresses.

Example values:

192.168.0.1
192.168.0.1/32
192.168.0.0/24
192.168.0.1,192.168.0.2

src also accepts the negation operator (!). You can use it like this !192.168.0.1,192.168.0.2 and Mole IDS will match traffic comming from any address but not from those two.

sport¶

Origin port of the packet. It can be a port, a list of ports or a range of ports.

Example values:

1234
10:100
1,2,3

sport also supports the negation operator (!). An example of using the negation operation sport = "!80,443".

dst¶

Destination address of the packet. It must be an ip address, list of ip address or range of ip addresses.

dport¶

Destination port of the packet. It can be a port, a list of ports or a range of ports.

Variables¶

Variables are handy utility for writing more generic rules or rules that can be addapted to other environmentes esaly.

Variables are defined in the configuration file under the rules section. They are basically a set of key:value.

This variables can be only used in the meta section and Mole IDS has some variables already defined and they can not be overwrite, those are:

$tcp = tcp
$udp = udp
$sctp = sctp

Finally, there is a variable called any that can be used to define any soruce or destination address as wel as any source or destination port.

Examples¶

Following several rule examples.

Example 1¶

rule ExampleRule {
  meta:
    description = "Port range from 1 to 1024"
    type = "alert"
    proto = "tcp"
    src = "any"
    sport = "any"
    dst = "any"
    dport = "1:1024"
  strings:
    $dnp3_header = { 05 64 }
    $unsolicited_response = { 82 }
  condition:
    $dnp3_header at 0 and $unsolicited_response at 12 and #dnp3_header < 2
}