24.1. Firewall Mode Design

Note

In Suricata 8 the firewall mode is experimental and subject to change.

The firewall mode in Suricata allows the use of a ruleset that has different properties than the default "threat detection" rulesets:

1. default policy is drop, meaning a firewall ruleset needs to specify what is allowed

2. firewall rules are loaded from separate files

3. firewall rules use a new action accept

4. firewall rules are required to use explicit action scopes and rule hooks (see below)

5. evaluation order is as rules are in the file(s), per protocol state

24.1.1. Concepts

• table collection of rules with different properties: packet:filter, packet:td, app:<proto>:<state>, app:td. These are built-in. No custom tables can be created.

• state controls a specific protocol state at which a rule is evaluated. Examples are tcp.flow_start or tls.client_body_done.

The tables for the application layer are per app layer protocol and per protocol state. For example, app:http:request_line.

24.1.2. Actions and Action Scopes

Firewall rules require action scopes to be explicitly specified.

24.1.2.1. accept

accept is used to issue an accept verdict to the packet, flow or hook.

• packet accept this packet

• flow accept the rest of the packets in this flow

• hook accept rules for the current hook/state, evaluate the next tables

• tx accept rules for the current transaction, evaluate the next tables

The accept action is only available in firewall rules.

Note

some protocol implementations like dns use a transaction per direction. For those accept:tx will only accept packets that are part of that direction.

24.1.2.2. drop

drop is used to drop either the packet or the flow

• packet drop this packet directly, don't eval any further rules

• flow drop this packet as with packet and drop all future packets in this flow

Note

the action pass is not available in firewall rules due to ambiguity around the existing meaning for threat detection rules.

24.1.2.3. Explicit rule hook (states)

In the regular IDS/IPS rules the engine infers from the rule's matching logic where the rule should be "hooked" into the engine. While this works well for these types of rules, it does lead to many edge cases that are not acceptable in a firewall ruleset. For this reason in the firewall rules the hook needs to be explicitly set.

This is done in the protocol field of the rule. Where in threat detection a rule might look like:

alert http ... http.uri; ...

In the firewall case it will be:

accept:hook http1:request_line ... http.uri; ...

The application layer states / hooks are defined per protocol. Each of the hooks has its own default-drop policy, so a ruleset needs an accept rule for each of the states to allow the traffic to flow through.

24.1.2.3.1. general

Each protocol has at least the default states.

Request (to_server) side:

• request_started

• request_complete

Response (to_client) side:

• response_started

• response_complete

24.1.2.3.2. http

For the HTTP protocol there are a number of states to hook into. These apply to HTTP 0.9, 1.0 and 1.1. HTTP/2 uses its own state machine.

Available states:

Request (to_server) side:

• request_started

• request_line

• request_headers

• request_body

• request_trailer

• request_complete

Response (to_client) side:

• response_started

• response_line

• response_headers

• response_body

• response_trailer

• response_complete

24.1.2.3.3. tls

Available states:

Request (to_server) side:

• client_in_progress

• client_hello_done

• client_cert_done

• client_handshake_done

• client_finished

Response (to_client) side:

• server_in_progress

• server_hello

• server_cert_done

• server_hello_done

• server_handshake_done

• server_finished

24.1.2.3.4. ssh

Available states are listed in Hooks.

24.1.2.4. Firewall pipeline

The firewall pipeline works in the detection engine, and is invoked after packet decoding, flow update, stream tracking and reassembly and app-layer parsing are all done in the context of a single packet.

For each packet rules in the first firewall hook packet:filter are then evaluated. Assuming the verdict of this hook is accept:hook, the next hook is evaluated: packet:td (packet threat detection). In this hook the IDS/IPS rules are evaluated. Rule actions here are not immediate, as they can still be modified by alert postprocessing like rate_filter, thresholding, etc.

The default drop for the packet:filter table is drop:packet. Thus the drop is only applied to the current packet.

If the packet has been marked internally as a packet with an application layer update, then the next table is app:*:*.

In app:*:* the per application layer states are all evaluated at least once. At each of these states an accept:hook is required to progress to the next state. When all available states have been accepted, the pipeline moves to the final table app:td (application layer threat detection). A drop in the app_filter table is immediate, however and accept is conditional on the verdict of the app:td table.

The default drop in one of the app:*:* tables is a drop:flow. This means that the current packet as well as all future packets from that flow are dropped.

In app:td the IDS/IPS rules for the application layer are evaluated. drop actions in this table are queued in the alert queue.

When all tables have been evaluated, the alert finalize process orders threat detection alerts by action-order logic. It can then apply a drop or default to accept-ing.

24.1.2.5. Pass rules with Firewall mode

In IDS/IPS mode, a pass rule with app-layer matches will bypass the detection engine for the rest of the flow. In firewall mode, this bypass no longer happens in the same way, as pass rules do not affect firewall rules. So the detection engine is still invoked on packets of such a flow, but the packet_td and app_td tables are skipped.