Glossary

SOCKS5

A general-purpose proxy protocol that forwards TCP connections without inspecting them, optionally behind a username and password.

Also called: SOCKS5 proxy, RFC 1928.

SOCKS5, specified in RFC 1928, is a proxy protocol that operates below the application layer: the client asks the proxy to open a connection to a host and port, and from then on bytes are relayed without the proxy parsing or rewriting anything. That makes it protocol-agnostic in a way an HTTP proxy is not. A companion specification, RFC 1929, adds an optional username-and-password handshake, which is how most commercial SOCKS5 endpoints are sold.

That optional handshake is where things break in practice, and it is one of the clearest reasons to ship a browser rather than automate one. Chromium's own SOCKS5 client only speaks the no-authentication variant, so a proxy that requires a username and password cannot be used with it at all — which quietly rules out a large share of what providers sell. Argus handles it inside the browser process: a local relay performs the RFC 1929 handshake on the upstream leg while the profile is pointed at a loopback address, so no proxy authentication dialog ever appears. The proxies page has the detail, and it is listed in the capability reference as well.