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Routing parsers (WASM)

Prism extracts the routing hostname from the first bytes of each TCP connection using routing parsers.

A routing parser is a .wat file (WebAssembly text format). Prism loads these modules from a directory on disk and executes them to extract the hostname.

Built-in parsers​

Prism ships two built-in parsers:

  • minecraft_handshake
  • tls_sni

At startup, Prism materializes (writes) the built-in .wat modules into the routing parser directory if they are missing.

Where parsers live​

The routing parser directory is configured via:

  • CLI flag: --routing-parser-dir /path/to/parsers
  • Environment variable: PRISM_ROUTING_PARSER_DIR=/path/to/parsers
  • Default: <config_dir>/parsers (Linux default: /etc/prism/parsers)

Each parser name maps to a file:

  • parsers = ["minecraft_handshake"] → <routing_parser_dir>/minecraft_handshake.wat

Relative paths​

If --routing-parser-dir is relative (like parsers), Prism resolves it relative to the config directory.

Security & format notes​

  • Prism intentionally does not load raw .wasm binaries for routing parsers.
  • Routing parsers execute on untrusted client input (the first bytes of a TCP stream). Keep them small and defensive.
  • If you add custom parsers, prefer reviewing them like you would any code that processes network bytes.

Troubleshooting​

My parser isn’t being found​

  • Confirm the parser name in routes[*].parsers matches the filename without .wat.
  • Confirm the file exists in the resolved routing parser directory.
  • Check Prism logs at logging.level = "debug".