Trojan Protocol

The Trojan protocol operates over TLS, making proxy traffic indistinguishable from normal HTTPS connections. This page documents the wire format used by trojan-rs.

Protocol Stack

Application Data
Trojan Protocol
TLS 1.2/1.3
TCP

The client first completes a standard TLS handshake. On success, it sends the Trojan header followed by application data. On failure, the connection is dropped (identical to a failed HTTPS connection).

TCP Header Format

The Trojan header consists of a password hash, a request, and CRLF delimiters:

+-----------------------+---------+----------------+---------+----------+
| hex(SHA224(password)) |  CRLF   | Trojan Request |  CRLF   | Payload  |
+-----------------------+---------+----------------+---------+----------+
|          56           | X'0D0A' |    Variable    | X'0D0A' | Variable |

• Password hash — 56 bytes of lowercase hexadecimal ASCII, computed as hex(SHA224(password))
• CRLF — \r\n (0x0D 0x0A) delimiter
• Payload — Optional first-packet payload, bundled with the header to reduce latency

Trojan Request

The request follows the SOCKS5 address format:

+-----+------+----------+----------+
| CMD | ATYP | DST.ADDR | DST.PORT |
+-----+------+----------+----------+
|  1  |  1   | Variable |    2     |

CMD (Command):

• 0x01 — CONNECT (TCP proxy)
• 0x03 — UDP ASSOCIATE

ATYP (Address Type):

• 0x01 — IPv4 (4 bytes)
• 0x03 — Domain name (1 byte length + domain bytes)
• 0x04 — IPv6 (16 bytes)

DST.PORT — Destination port in network byte order (big-endian, 2 bytes)

UDP Packet Format

When CMD is 0x03 (UDP ASSOCIATE), subsequent data follows this per-packet format:

+------+----------+----------+--------+---------+----------+
| ATYP | DST.ADDR | DST.PORT | Length |  CRLF   | Payload  |
+------+----------+----------+--------+---------+----------+
|  1   | Variable |    2     |   2    | X'0D0A' | Variable |

• Length — Payload length in bytes (2 bytes, big-endian)
• CRLF — \r\n delimiter
• UDP packets do not include authentication. Authentication occurs once during the TCP header phase.

Authentication

Passwords are configured in plaintext and hashed at runtime:

1. Server stores the SHA-224 hash of each valid password
2. Client computes hex(SHA224(password)) — 56 lowercase hex characters
3. Client sends the hash as the first 56 bytes of the Trojan header
4. Server checks the hash against its valid set

SHA-224 produces a 28-byte digest, which is 56 hex characters. trojan-rs enforces lowercase hex and validates the 56-byte length.

Fallback Behavior

When the server receives a connection that does not match the Trojan protocol:

1. TLS handshake succeeds, invalid header — The server buffers the received bytes and forwards the entire connection (including buffered bytes) to the fallback address
2. Password mismatch — Same as invalid header: forward to fallback

This makes the server behave identically to a normal HTTPS server for any traffic that isn't a valid Trojan connection. The fallback is typically a web server serving legitimate content.

trojan-rs implements this with a PrefixedStream that replays buffered bytes before reading new data from the socket.

First-Packet Payload

The protocol allows (and encourages) sending application data in the same TLS record as the header. This:

• Reduces the number of round trips
• Makes the traffic pattern less distinguishable from normal HTTPS

trojan-rs bundles the first data packet with the Trojan header when data is available during the handshake.

Compatibility

trojan-rs is wire-compatible with:

• The original Trojan C++ implementation
• trojan-go

The protocol format is identical. Any Trojan-compatible client can connect to a trojan-rs server and vice versa.