11. Rule syntax

Sagan rule syntax is very similar to that of Suricata <https://suricata-ids.org>_ or Snort . This s was intentionally done to maintain compatibility with rule management software like oinkmaster and pulledpork and allows Sagan to correlate log events with your Snort/Suricata IDS/IPS system.

This also means that if you are already familiar with signature writing in Suricata and Snort, you already understand the Sagan syntax!

To understand the basic Sagan rule syntax, we will be using the following simple rule. This section of the Sagan user guide only covers up to the first rule option. That is, this section will cover up to the msg portion of this rule only. The rest of the rule is considered rule options.

Basic Sagan rule:

alert any $EXTERNAL_NET any -> $HOME_NET any (msg: "[SYSLOG] System out of disk space"; pcre: "/file system full|No space left on device/i"; classtype: hardware-event; threshold: type limit, track by_src, count 1, seconds 300; reference: url,wiki.quadrantsec.com/bin/view/Main/5000116; sid:5000116; rev:2;)
alert

This informs Sagan how to flag the event. Valid options are alert or drop.

any

Valid options for this field are any, tcp, udp or icmp. In most cases, you will likely want to specify any. The protocal is determined by the parse_proto or parse_program_proto rule options.

$EXTERNAL_NET

This informs Sagan where the source IP address or addresses must be coming from in order to trigger. By default the variable $EXTERAL_NET is used. This is set in the sagan.yaml configurations file and defaults to any. most cases, “any” (any source) is what you want. In other cases, you might want the signature to trigger when it is from a particular host. For example:

192.168.1.1

Makes Sagan only trigger if the source of the event is from the address 192.168.1.1 (/32 is automatically assumed). You can also apply multple networks. For example:

[192.168.1.0/24, 10.0.0.0/24]

Is valid and will only trigger if the network address is within 192.168.1.0/24 or 10.0.0.0/24. You can also apply not logic to the addresses. For example.

!192.168.1.1/32

This will only trigger when the IP address is not 192.168.1.1.

This feild is populated by whatever the source IP address within the log might be. For example, if the signature lacks parse_src_ip or normalize (see rule options), then the syslog source is addopted. If parse_src_ip or normalize rule option is use, then data (if any) that is extracted from the log is used.

any

The next any is the source port. If the normalize or default_src_port rule option is used, it will be applied here. This can be useful in filtering out certain subnets or syslog clients.

->

This would be the direction. From the $EXTERNAL_NET -> $HOME_NETWORK.

$HOME_NETWORK

This works similar to how $EXTERNAL_NET functions. Rather than being the source of the traffic, this is the destination of the traffic. Like $EXTERNAL_NET, this is set in the sagan.yaml configuration file and defaults to any. Also like the $EXTERNAL_NET, network CIDR notation can be used ( ie - 192.168.1.0). Data from this is populated by the parse_dst_ip and normalize rule options.

any

The final rule option is the destination port. If the normalize or default_dst_port rule option is used, it will be applied here. This can be useful in filtering out events from certain subnet.