1. Introduction

The EchoLink
[EchoLink® is a registered trademark of Synergenics, LLC]
Proxy provides a way to move the TCP port 5200 and UDP port 5198/5199 traffic used by EchoLink to a remote server, and tunnel it from the EchoLink client to the proxy over a single TCP connection. This is useful in situations where port forwarding of the two EchoLink UDP ports cannot be set up like when on a hotel LAN or something like that.

For more information about EchoLink proxy, have a look at the description at http://www.echolink.org/proxy.htm.

2. Protocol Description

In the following sections we will describe how the EchoLink proxy protocol is used. All messages are described in detail in [proxy_message_specification].

2.1. Connect and Authenticate

This chapter explains how to connect to the proxy and authenticaticate. After the authentication has secceeded, messages are exchanged in the format described in [proxy_message_specification].

  1. The proxy server listens for a TCP connection from a proxy client. The default port is 8100 but this may be changed in the configuration.

  2. The client establishes a TCP connection.

  3. The server accepts the connection and sends a credentials challenge, which is an 8-byte random "nonce". Each byte is a printable character.

  4. The client reads this 8-byte nonce, then calculates a 16-byte digest, which is the MD5 hash of: the uppercased password followed by the nonce.

  5. The client responds by sending the newline-terminated callsign, followed by the 16-byte digest. This is the raw bytes of the digest, not encoded.

  6. The server confirms the digest.

    • If the digest is correct and the client is authorized, the server proceeds with setting up the UDP ports and proxying TCP and UDP to and from the client, as described below.

    • If the digest is incorrect, the server responds with a SYSTEM message with a one-byte data block, the data byte being 1 (BAD_PASSWORD). If the digest is correct but the server rejects the callsign due to its ACL, a SYSTEM message with a data byte 2 is sent (ACCESS_DENIED).

2.2. TCP Traffic

When the client wishes to establish a remote TCP connection, it constructs and sends a TCP_OPEN message, containing the IP address of the remote server. When the connection is established or rejected, the proxy server will respond with a TCP_STATUS message. All four data bytes will be set to zero if the connection succeeded. When any of the data bytes are non-zero, that indicate a connection failure.

If and when the connection is established, the server expects to receive TCP_DATA messages from the client, and constructs TCP_DATA messages to send to the client for each block of TCP data received from the remote server. When the proxy receives a TCP_CLOSE message from the client, it closes the remote TCP connection. If the remote server close the connection, the proxy server sends a TCP_CLOSE message to the client.

2.3. UDP Traffic

The proxy listens on UDP ports 5198 and 5199. Whenever a packet of UDP data is received on either port from a remote host, the server wraps it in a UDP_DATA or UDP_CONTROL message as appropriate and relays it to the client, with the remote host’s IP address in the address field. Conversely, when the client wishes to send a block of UDP data to the remote host, it wraps it in either a UDP_DATA or UDP_CONTROL message and sends it to the client, again with the remote host’s IP address in the address field.

2.4. Shutdown

At any time, the client can disconnect from the proxy by closing the TCP connection. At this point the server is again made available for use by closing its socket to the client and setting itself up to accept a new one.

3. Proxy Message Specification

After a proxy session has been set up and authenticated, each block of data that moves between client and server is encapsulated as a ProxyMessage. Each ProxyMessage has the format described below.

Table 1. Proxy Message Block Format
Field Size [octets] Description

type

1

The message type (see below)

address

4

Remote peer’s IP address in network byte order

size

4

Size of the data payload portion, little-endian

data

0-

Sequence of bytes (payload). May be zero octets.

The type field indicates what type of message it is. The available message types are listed below. The "sender" column indicate which of the sides, client (c), server (s) or both (c/s) that can send the message.

Table 2. Protocol Message Specifications
Name Type code Sender Data size Description

TCP_OPEN

1

c

0

Open a TCP connection to the IP address specified in the address field of the header.

TCP_DATA

2

c/s

1-

Send TCP data to a previously opened connection. The data field have variable size and contains the TCP data to send.

TCP_CLOSE

3

c/s

0

Close a previously opened TCP connection. The data field should have length 0. When the server receives the TCP_CLOSE message, it will reply back with a TCP_CLOSE message when the connection have been closed. The server may also send a TCP_CLOSE if the remote server close the connection.

TCP_STATUS

4

s

4

Status report for the TCP connection. The server will send a TCP_STATUS message after the connection have been established. If the connection succeeds, all four data bytes will be set to 0.

UDP_DATA

5

c/s

1-

Send a UDP data message. UDP data messages are used for example to carry EchoLink audio.

UDP_CONTROL

6

c/s

1-

Send a UDP control message. UDP control messages are for example used to control EchoLink connection establishment and maintenance.

SYSTEM

7

s

1

System event information message. This message is used to indicate authentication or authorization failure. There are two valid values for the single data byte.

  • 1: BAD_PASSWORD - The client provided the wrong authentication password.

  • 2: ACCESS_DENIED - The client is not authorized to access this proxy.