By default, Juniper SRX Series devices operate in flow mode. This is the standard mode for a stateful firewall, where the device inspects traffic based on connection states or "flows." The first packet of a new session is checked against the security policy, and if permitted, a session is created in the session table. Subsequent packets belonging to that session are processed quickly based on the existing state.

Additionally, SRX Series devices, running the Junos OS, fully support stateless firewall filters. These filters evaluate each packet individually against a set of rules without considering connection state. They are commonly used to protect the Routing Engine (RE) or to filter traffic at the interface level before it undergoes flow-based processing.

C: Packet mode is a non-default operational mode that disables stateful firewalling, causing the SRX to function more like a traditional stateless router.

D: SRX devices inherently support stateless firewall filters as a fundamental feature of the Junos operating system.

1. Juniper Networks TechLibrary

"Flow-Based and Packet-Based Processing." This document explicitly states

"By default

SRX Series devices operate in flow mode." It details how flow mode enables stateful security features

which is the primary function of an SRX device.

URL: https://www.juniper.net/documentation/us/en/software/junos/flow-packet-processing/topics/concept/flow-based-packet-based-processing-understanding.html

2. Juniper Networks TechLibrary

"Firewall Filters Overview." This guide describes the functionality of stateless firewall filters on Junos devices

including the SRX Series. It confirms that these filters are available to "filter data packets that are transiting the device" and to "protect the Routing Engine from malicious traffic."

URL: https://www.juniper.net/documentation/us/en/software/junos/firewall-filters/topics/concept/firewall-filter-overview.html