================== Command Line Flags ================== The following command line flags can be used when executing Merlin agent: .. code-block:: html Merlin Agent -debug Enable debug output -host string HTTP Host header -ja3 string JA3 signature string (not the MD5 hash). Overrides -proto flag -proto string Protocol for the agent to connect with [https (HTTP/1.1), http (HTTP/1.1 Clear-Text), h2 (HTTP/2), h2c (HTTP/2 Clear-Text), http3 (QUIC or HTTP/3.0)] (default "h2") -proxy string Hardcoded proxy to use for http/1.1 traffic only that will override host configuration -psk string Pre-Shared Key used to encrypt initial communications (default "merlin") -sleep duration Time for agent to sleep (default 30s) -url string Full URL for agent to connect to (default "https://127.0.0.1:443") -v Enable verbose output -version Print the agent version and exit Debug ^^^^^ By default, the Merlin Agent will not write anything to STDOUT while it is running. The ``-debug`` flag enables debug output and facilitates troubleshooting to identify the source of a problem. Host ^^^^ The ``-host`` flag is used to specify the HTTP *Host:* header when communicating with the server. This feature is predominately used for `Domain Fronting `_. JA3 ^^^ `JA3 is a method for fingerprinting TLS clients on the wire `_. Every TLS client has a unique signature depending on its configuration of the following TLS options: ``SSLVersion,Ciphers,Extensions,EllipticCurves,EllipticCurvePointFormats``. The ``-ja3`` flag allows the agent to create a TLS client based on the provided JA3 hash signature. This is useful to evade detections based on a JA3 hash for a known tool (i.e. Merlin). `This `_ article documents a JA3 fingerprint for Merlin. Known JA3 signatures can be downloaded from https://ja3er.com/ **NOTE:** Make sure the input JA3 hash will enable communications with the Server. For example, if you leverage a JA3 hash that only supports SSLv2 and the server does not support that protocol, then they will not be able to communicate. The ``-ja3`` flag will override the the ``-proto`` flag and will cause the agent to use the protocol provided in the JA3 hash. Proto ^^^^^ The ``-proto`` flag specifies what protocol the Merlin Agent will use to communicate with the server The ``http`` protocol communicates using the clear-text HTTP/1.1 protocol. This can be useful when leveraging Domain Fronting on a CDN that does not allow both fronting and TLS encrypted traffic. The ``https`` protocol communicates using SSL/TLS encrypted HTTP/1.1 protocol. The ``h2c`` protocol communicates using the clear-text HTTP/2 protocol. This clear-text version is not used by web browsers like Chrome and may stand out during traffic analysis. However, it also has the potential to evade detections if allowed out of the network and no network defenses are able to parse the traffic. The ``h2`` protocol communicates using the TLS encrypted HTTP/2 protocol. This will start the connection with prior knowledge and will not negotiate from HTTP/1.1 to HTTP/2. Some web proxies will not allow HTTP/2 communications. In this case you should use ``https``. Alternatively, the HTTP/2 protocol *might* bypass network defenses or detections. The ``http3`` protocol communicates using HTTP/2 transported over `QUIC `_ known as `HTTP/3 `_. It is important to note that QUIC is a UDP protocol and may not be allowed of the network depending on egress filtering. QUIC uses TLS transport encryption. Proxy ^^^^^ The ``-proxy`` flag is used to force HTTP/1.1 communications to go through a known proxy. At this time the Merlin Agent **WILL NOT** automatically detect if a host is configured to use a proxy. The HTTP/2 protocol does not support using a proxy. If a proxy is required to egress a network, use the ``http`` or ``https`` protocols. PSK ^^^ The ``-psk`` flag is used to specify the Pre-Shared Key (PSK) that the Merlin Agent uses to initiate communication with the Merlin Server. The first message is encrypted with the PSK and subsequent messages establish a new session based encryption key using the `OPAQUE protocol `_ from `this `__ IETF draft. Additional information about OPAQUE can be found here: `Merlin Goes OPAQUE for Key Exchange `_. Sleep ^^^^^ The ``-sleep`` flag is used to specify how long the agent will sleep between checkin attempts. **NOTE:** You must include the unit of measurement after the number. For example, ``30s`` is for thirty seconds and ``1m`` is for one minute. URL ^^^ The ``-url`` flag is used to specify the Uniformed Resource Locator (URL) that the agent will attempt to communicate with. Include the protocol (i.e. ``https``), the host (i.e. ``127.0.0.1``), the page (i.e ``/`` or ``/news.php``), and optionally port (i.e. ``:443``). This will result in ``https://127.0.0.1:443/``. **NOTE:** By default the Merlin agent will communicate on the loopback adapter. Verbose ^^^^^^^ The ``-v`` flag enables verbose output. By default a running Merlin Agent will not write any information to STDOUT. This can be used to see what the agent is doing along with what commands it is receiving. Version ^^^^^^^ The ``-version`` flag will print the Agent version to the screen and then exit.