On Juniper SRX devices operating in flow mode (stateful firewall), when a policy permits traffic with ‘session-init’, it creates a session entry in the state table. Return traffic for that established session is automatically permitted by the stateful inspection engine, without needing a separate policy. This is the core function of a stateful firewall for connection-oriented protocols like TCP.

The ‘show security match-policies’ command is the primary troubleshooting tool for policy lookup. It simulates a packet with the specified parameters (source IP, destination IP, port, etc.) and shows which security policy, if any, it matches and the resulting action (permit/deny). This directly tests the policy logic without relying on active traffic flows.

To log all sessions for a policy, you need to log both the initiation and the closure. `log session-init` creates a log entry when the session is established. `log session-close` creates an entry when the session terminates, which includes valuable information like duration and byte counts. Using only `count` enables counters but not logging, and using only `session-init` would miss the final session record.

The ‘show security flow session’ command displays the post-NAT IP addresses in the session table. If the source IP in the session entry differs from the initiating host’s original IP, it confirms that a source NAT rule (like source NAT with a pool or interface NAT) has been successfully applied to that traffic flow, translating the private source IP to a public one.

Firewall policies are processed in order. A rule with source ‘any’, destination ‘any’, and action ‘permit’ will match all traffic that reaches that point in the policy set (e.g., from-zone trust to-zone untrust). This acts as a catch-all, allowing all traffic and negating the security function of any more specific deny rules placed after it, as they would never be evaluated.

Destination NAT requires a destination NAT rule that matches the public IP/port (203.0.113.5:443) and translates it to the private IP (192.168.1.10). Crucially, the security policy must evaluate the traffic *after* NAT has occurred. Therefore, the policy’s destination address must be the *translated* private server address (192.168.1.10), not the original public IP.

Source NAT is used for outbound traffic originating from your network. Since the requirement is for traffic initiated from the trust zone to use a specific public IP address (not the interface IP), you must configure source NAT with a pool containing that dedicated public IP address. Interface NAT would use the IP of the egress interface, and static/destination NAT are for inbound traffic.

The ‘messages’ log is the default log where firewall deny events are sent when `set syslog file messages any any` and `set security policies default-policy deny-all` (or specific deny policies with `log`) are configured. `RT_FLOW_SESSION_DENY` is the specific syslog message tag for denied sessions. Filtering by this tag and the suspicious IP provides a direct view of denied connection attempts.

The core concept of a zone-based firewall is that security policies are defined based on the source and destination zones of traffic (e.g., from-zone trust to-zone untrust). Assigning an interface to a zone places it into a trust domain. All traffic entering or leaving that interface is associated with that zone, and policies are applied when traffic moves from one zone to another. This abstracts policy from specific interfaces or IPs.

On Juniper SRX devices operating in flow mode (stateful firewall), when a policy permits traffic with ‘session-init’, it creates a session entry in the state table. Return traffic for that established session is automatically permitted by the stateful inspection engine, without needing a separate policy. This is the core function of a stateful firewall for connection-oriented protocols like TCP.0