SOCKSv6 and OpenSSH

Damien Miller djm at
Tue Sep 1 16:08:40 AEST 2020

On Mon, 31 Aug 2020, Vladimir Olteanu wrote:

> Hi there,
> I'm Vladimir Olteanu (or just Vlad, for short), and I'm working on the 
> SOCKSv6 protocol. It is being discussed at the IETF 
> ( The spec 
> has matured somewhat and is currently undergoing an adoption call at the 
> Intarea WG.
> I would like to get your opinion on this newer version of the protocol, 
> in particular on how it would work for OpenSSH and what features and 
> tweaks you'd like to see included.
> Version 6's core functionality is roughly equivalent to v4. It is 
> extensible and has several optional features, like:

OpenSSH currently implements a small subset of SOCKS4, 4A and 5 -
basically just unauthenticated TCP CONNECT and I don't recall any
requests for more protocol features to be implemented.

Looking at the draft, implementing similar basic support for SOCKS6
appears easy. The only parsing needed would be the initial request
message and the only reply would be a synchronous auth success + success

Note that for SOCKS4/SOCKS5 we cheat by not delaying sending the
success message until the onward forwarded connection has successfully
connected. As such we can't propogate failure reasons back to the
client. Adding this would require extending the channels state machine a

Anyway, I don't see any showstoppers preventing feature/bug parity in
OpenSSH for SOCKS6 compared to 4/5.


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