Limitations and caveats for stateful rules in AWS Network Firewall

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AWS Network Firewall stateful rules are Suricata compatible. Most Suricata rules work out of the box with Network Firewall. Your use of Suricata rules with Network Firewall has the restrictions and caveats listed in this section.

Suricata features that Network Firewall doesn't support

The following Suricata features are not supported by Network Firewall:

• Datasets. The keywords dataset and datarep aren't allowed.

• ENIP/CIP keywords.

• File extraction. File keywords aren't allowed.

• FTP-data protocol detection.

• IP reputation. The iprep keyword is not allowed.

• Lua scripting.

• Rules actions except for pass, drop, reject, and alert. Pass, drop, reject, and alert are supported. For additional information about stateful rule actions, see Stateful actions.

• SCTP protocol.

• Thresholding.

• IKEv2 protocol.

Suricata features that Network Firewall supports with caveats

The following Suricata features have caveats for use with Network Firewall:

• If you want a rule group to use settings for HOME_NET and EXTERNAL_NET that are different from those that are set for the firewall policy, you must explicitly set both of these variables.

  • In a firewall policy's variables, you can set a custom value for HOME_NET. The default HOME_NET setting is the CIDR of the inspection VPC. The policy's EXTERNAL_NET setting is always the negation of the policy's HOME_NET setting. For example, if the HOME_NET is 11.0.0.0, the EXTERNAL_NET is set to !11.0.0.0.

  • In a rule group's variables, you can set custom values for both HOME_NET and EXTERNAL_NET. If you explicitly set rule group variables, those are used. Otherwise, rule group variables inherit their settings from the corresponding policy variables.

    This means that, if you don't specify the rule group's EXTERNAL_NET, it inherits the setting from the policy's EXTERNAL_NET setting, regardless of the value of the rule group's HOME_NET setting.

    For example, say you set the rule group's HOME_NET to 10.0.0.0, and the firewall policy's HOME_NET to 11.0.0.0. If you don't set the rule group's EXTERNAL_NET, then Network Firewall sets it to !11.0.0.0, based on the policy's HOME_NET setting.

• The AWS Network Firewall stateful inspection engine supports inspecting inner packets for tunneling protocols such as Generic Routing Encapsulation (GRE). If you want to block the tunneled traffic, you can write rules against the tunnel layer itself or against the inner packet. Due to the service inspecting the different layers, you might see flows and alerts for the packets within the tunnel.

• To create a rule that requires a variable, you must specify the variable in the rule group. Without the required variables, the rule group isn't valid. For an example of a rule group that's configured with variables, see Stateful rules examples: rule variables.

• In payload keywords, the pcre keyword is only allowed with content, tls.sni, http.host, and dns.query keywords.

• The priority keyword is not supported for rule groups that evaluate rules using strict evaluation order.

• When you use a stateful rule with a layer 3 or 4 protocol such as IP or TCP, and you don't include any flow state context, for example "flow:not_established", then Suricata treats this rule as an IP-only rule. Suricata only evaluates IP-only rules for the first packet in each direction of the flow. For example, Suricata will process the following rule as an IP-only rule:

  pass tcp $HOME_NET any -> [10.0.0.0/8] $HTTPS_PORTS (sid: 44444; rev:2;)

  However, if the destination IP contains a !, then Suricata treats this as per the protocol specified in the rule. Suricata will process the following rule as a TCP rule.

  pass tcp $HOME_NET any -> [!10.0.0.0/16] $HTTPS_PORTS (sid: 44444; rev:2;)