4.1. Rules Format

Signatures play a very important role in Suricata. In most occasions people are using existing rulesets.

The official way to install rulesets is described in Rule Management with Suricata-Update.

This Suricata Rules document explains all about signatures; how to read, adjust and create them.

A rule/signature consists of the following:

  • The action, that determines what happens when the signature matches
  • The header, defining the protocol, IP addresses, ports and direction of the rule.
  • The rule options, defining the specifics of the rule.

An example of a rule is as follows:

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

In this example, red is the action, green is the header and blue are the options.

We will be using the above signature as an example throughout this section, highlighting the different parts of the signature. It is a signature taken from the database of Emerging Threats, an open database featuring lots of rules that you can freely download and use in your Suricata instance.

4.1.1. Action

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

For more information see Action-order.

4.1.2. Protocol

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

This keyword in a signature tells Suricata which protocol it concerns. You can choose between four basic protocols:

  • tcp (for tcp-traffic)
  • udp
  • icmp
  • ip (ip stands for ‘all’ or ‘any’)

There are also a few so-called application layer protocols, or layer 7 protocols you can pick from. These are:

  • http
  • ftp
  • tls (this includes ssl)
  • smb
  • dns
  • dcerpc
  • ssh
  • smtp
  • imap
  • msn
  • modbus (disabled by default)
  • dnp3 (disabled by default)
  • enip (disabled by default)
  • nfs (depends on rust availability)
  • ikev2 (depends on rust availability)
  • ntp (depends on rust availability)

The availability of these protocols depends on whether the protocol is enabled in the configuration file suricata.yaml.

If you have a signature with for instance a http protocol, Suricata makes sure the signature can only match if it concerns http-traffic.

4.1.3. Source and destination

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

The first emphasized part is the source, the second is the destination (note the direction of the directional arrow).

With source and destination, you specify the source of the traffic and the destination of the traffic, respectively. You can assign IP addresses, (both IPv4 and IPv6 are supported) and IP ranges. These can be combined with operators:

Operator Description
../.. IP ranges (CIDR notation)
! exception/negation
[.., ..] grouping

Normally, you would also make use of variables, such as $HOME_NET and $EXTERNAL_NET. The configuration file specifies the IP addresses these concern, and these settings will be used in place of the variables in you rules. See Rule-vars for more information.

For example:

Example Meaning
! 1.1.1.1 Every IP address but 1.1.1.1
![1.1.1.1, 1.1.1.2] Every IP address but 1.1.1.1 and 1.1.1.2
$HOME_NET Your setting of HOME_NET in yaml
[$EXTERNAL_NET, !$HOME_NET] EXTERNAL_NET and not HOME_NET
[10.0.0.0/24, !10.0.0.5] 10.0.0.0/24 except for 10.0.0.5
[…, [….]]  
[…, ![…..]]  

Warning

If you set your configuration to something like this:

HOME_NET: any
EXTERNAL_NET: ! $HOME_NET

You can not write a signature using $EXTERNAL_NET because it stands for ‘not any’. This is an invalid setting.

4.1.4. Ports (source and destination)

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

The first emphasized part is the source, the second is the destination (note the direction of the directional arrow).

Traffic comes in and goes out through ports. Different ports have different port numbers. For example, the default port for HTTP is 80 while 443 is typically the port for HTTPS. Note, however, that the port does not dictate which protocol is used in the communication. Rather, it determines which application is receiving the data.

The ports mentioned above are typically the destination ports. Source ports, i.e. the application that sent the packet, typically get assigned a random port by the operating system. When writing a rule for your own HTTP service, you would typically write any -> 80, since that would mean any packet from any source port to your HTTP application (running on port 80) is matched.

In setting ports you can make use of special operators as well, like described above. Signs like:

Operator Description
: port ranges
! exception/negation
[.., ..] grouping

For example:

Example Meaning
[80, 81, 82] port 80, 81 and 82
[80: 82] Range from 80 till 82
[1024: ] From 1024 till the highest port-number
!80 Every port but 80
[80:100,!99] Range from 80 till 100 but 99 excluded
[1:80,![2,4]] Range from 1-80, except ports 2 and 4
[.., [..,..]]  

4.1.5. Direction

drop tcp $HOME_NET any -> $EXTERNAL_NET any (msg:”ET TROJAN Likely Bot Nick in IRC (USA +..)”; flow:established,to_server; flowbits:isset,is_proto_irc; content:”NICK “; pcre:”/NICK .*USA.*[0-9]{3,}/i”; reference:url,doc.emergingthreats.net/2008124; classtype:trojan-activity; sid:2008124; rev:2;)

The direction tells in which way the signature has to match. Nearly every signature has an arrow to the right (->). This means that only packets with the same direction can match. However, it is also possible to have a rule match both ways (<>):

source -> destination
source <> destination  (both directions)

Warning

There is no ‘reverse’ style direction, i.e. there is no <-.

The following example illustrates this. Say, there is a client with IP address 1.2.3.4 and port 1024, and a server with IP address 5.6.7.8, listening on port 80 (typically HTTP). The client sends a message to the server, and the server replies with its answer.

../_images/TCP-session.png

Now, let’s say we have a rule with the following header:

alert tcp 1.2.3.4 1024 -> 5.6.7.8 80

Only the first packet will be matched by this rule, as the direction specifies that we do not match on the response packet.

4.1.6. Rule options

The rest of the rule consists of options. These are enclosed by parenthesis and separated by semicolons. Some options have settings (such as msg), which are specified by the keyword of the option, followed by a colon, followed by the settings. Others have no settings, and are simply the keyword (such as nocase):

<keyword>: <settings>;
<keyword>;

Rule options have a specific ordering and changing their order would change the meaning of the rule.

Note

The characters ; and " have special meaning in the Suricata rule language and must be escaped when used in a rule option value. For example:

msg:"Message with semicolon\;";

As a consequence, you must also escape the backslash, as it functions as an escape character.

The rest of this chapter in the documentation documents the use of the various keywords.

Some generic details about keywords follow.

4.1.6.1. Modifier Keywords

Some keywords function act as modifiers. There are two types of modifiers.

  • The older style ‘content modifiers’ look back in the rule, e.g.:

    alert http any any -> any any (content:"index.php"; http_uri; sid:1;)
    

    In the above example the pattern ‘index.php’ is modified to inspect the HTTP uri buffer.

  • The more recent type is called the ‘sticky buffer’. It places the buffer name first and all keywords following it apply to that buffer, for instance:

    alert http any any -> any any (http_response_line; content:"403 Forbidden"; sid:1;)
    

    In the above example the pattern ‘403 Forbidden’ is inspected against the HTTP response line because it follows the http_response_line keyword.

4.1.6.2. Normalized Buffers

A packet consists of raw data. HTTP and reassembly make a copy of those kinds of packets data. They erase anomalous content, combine packets etcetera. What remains is a called the ‘normalized buffer’:

../_images/normalization1.png

Because the data is being normalized, it is not what it used to be; it is an interpretation. Normalized buffers are: all HTTP-keywords, reassembled streams, TLS-, SSL-, SSH-, FTP- and dcerpc-buffers.

Note that there are some exceptions, e.g. the http_raw_uri keyword. See http_uri and http_raw_uri for more information.