The Packet Tracer tool allows you to test policy configuration by modeling a packet with source and destination addresses,
and protocol characteristics. The trace does a policy lookup to validate if the packet will be permitted or denied access
based on the configured access rules, NAT, routing, access policies and rate-limiting policies. The packet flow is simulated
based on interfaces, source address, destination address, ports, and protocols. This method of testing the packets allows
you to verify the effectiveness of your policies and test whether the types of traffic you want to allow or deny are handled
as required.
Besides verifying your configuration, you can use the tracer to debug unexpected behavior, such as packets being denied access
when they should be allowed. To simulate a packet fully, the packet tracer traces the data path—slow-path and fast-path modules.
Initially, processing was transacted on per-session and per-packet basis. The Packet Tracer tool and Capture with Trace feature
log the tracing data on per packet basis when the firewall processes packets per session or per packet.
PCAP File
You can initiate a packet tracer using a PCAP file, and that has a complete flow. Currently, only a PCAP with a single TCP/UDP-based
flow and a maximum of 100 packets is supported. The packet tracer tool reads the PCAP file, and initializes the state for
client and server replay entities. The tool starts replaying the packets in a synchronized manner by collecting and storing
the trace output of each packet within the PCAP for subsequent processing and display.
PCAP Replay
Packet replay is executed by the sequence of the packet in the PCAP file, and interferences, if any, to the replay activity
terminates it and concludes the replay. The trace output is generated for all the packets in the PCAP on the specified ingress
interface and egress interface, thereby providing a complete context for flow evaluation.
PCAP replay is not supported for some features that dynamically modify the packet during replay, such as IPsec, VPN, , HTTPs
decryption, and so on.
For a NAT-configured threat
defense device, the PCAP packet reflects the translated addresses while replaying, so that the addresses are processed without being
dropped. However, the PCAP replay does not support IPv4 to IPv6, or IPv6 to IPv4 NAT types.
For the packet tracer to capture the identity and TLS decryption related trace information, you must ensure that Snort 3 is
configured in the device as the detection engine.
For a more realistic packet replay simulation, this tool can imitate the actual timing of the packets. It replays the packet
as per the timestamps recorded in the PCAP file. To enable the timestamp option, use the honor-timestamp keyword in the packet-tracer command.
Note
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The accuracy to honor the timestamps in the PCAP is limited when the threat
defense device's processing time of the replayed packet is higher than the delay between the packets.
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You can store the threat
defense device-generated packet trace data as part of the PCAP file using the export-pcapng keyword in the show packet tracer command. You can view the resultant pcapng file using other external packet viewer tools, such as Wireshark.