In Juniper SRX flow-based packet processing, the flow module is responsible for security functions such as screening, session management, NAT, and policy enforcement. The processing order is critical:

Screens are applied before any session lookup. This ensures that packets are inspected for anomalies, floods, or protocol violations before consuming resources for session management. Examples of these screens include TCP SYN flood protection, ICMP flood protection, and port scanning protection.

After screening, the session lookup occurs. At this point, the firewall checks whether the packet belongs to an existing session in the session table. If a matching session is found, the packet bypasses policy evaluation and is forwarded according to the session state.

If no existing session is found, the packet continues through route lookup, NAT processing, and security policy evaluation before a new session is created.

Thus, screening occurs before the session lookup , protecting the system early in the flow process. This design ensures efficiency by dropping malicious or malformed traffic before allocating session resources.

The SRX Series devices support third-party antivirus scanning engines such as Avira . To use the Avira antivirus engine, administrators must explicitly enable the engine and ensure that the required components are properly loaded.

Enable in configuration mode:

The Avira antivirus engine must be enabled under UTM configuration mode. This step ensures the SRX device uses the Avira scanning engine for antivirus inspection.

Example:

set security utm feature-profile anti-virus avira-engine enable

Reboot the SRX device:

A system reboot is required after enabling the Avira engine to load the Avira antivirus components into memory.

Without a reboot, the Avira engine will not become active.

Why not the others?

Restarting the mgd process (Option A) only reloads the management daemon and does not load antivirus engines.

Enabling in operational mode (Option B) is not supported; the configuration must be applied in configuration mode.

Therefore, the correct actions to use Avira Antivirus are: Enable the Avira engine in configuration mode (Option D) and reboot the SRX device (Option C).

In Junos OS, security zones are the foundation of SRX firewall policy enforcement. Logical interfaces must be assigned to zones. This enables:

Separation of traffic by zone boundaries.

Enforcement of security policies for traffic traversing between zones.

Control of traffic across VLANs, subnets, or functional areas (e.g., trust, untrust, DMZ).

Other options:

Zone assignment is not used to simplify interface configuration (A).

Routing protocols and updates (B) are handled by routing instances, not zones.

SNMP monitoring (D) is enabled under system or services configuration, not zones.