A connect request is the most common operation in SOCKS clients. This functionality is provided by the following functions:

Connecting to Application Server

Once a socket is connected to the proxy server, a single connect request is made to establish a connection to the application server:

error_code ec;
endpoint bound_ep = connect(
    socket, target_ep, auth_options::none{}, ec);

When the request is successful, the SOCKS server starts relaying traffic on both directions. Thus, the client can now perform I/O operations on the same connection as if it were directly connected to the application server.

The function returns the endpoint with the address and port the SOCKS proxy assigned to connect to the target host. This is part of the connect reply message.

Because SOCKS servers might be multi-homed, this endpoint might be different from the address the client used to reach the SOCKS server.

Connecting to Application Server

An asynchronous request is a composed operation that follows the usual pattern of Asio async functions:

async_connect(
    socket, target_ep, auth_options::none{},
    [&](error_code ec, endpoint bound_ep)
{
    // Continuation
    if (!ec.failed())
        do_write(bound_ep);
});

The asynchronous functions support any Asio continuation token, as other Asio functionalities. After the connect operation, the client can perform I/O on the socket as if connected to the application server.

Authentication

All connect functions include a parameter for authentication.

SOCKS authentication happens through a composed operation that performs sub-negotiation of authentication parameters before the final connect request is sent.

An object of type auth_options can be used to specify the parameters of the authentication:

error_code ec;
endpoint bound_ep = connect(
    socket,
    target_ep,
    auth_options::userpass{
        "user_id", "password"},
    ec);

In SOCKS4 (connect_v4 / async_connect_v4) operations, authentication happens via an string_view describing the user identity. According to the protocol, this field is mandatory, although it can be empty.

error_code ec;
endpoint bound_ep = connect_v4(
    socket, target_ep, "user_id", ec);