[ssh] Host key sync - "global-requests-ok" extension

Hello, fellow kids!

... or whoever may still be subscribed here.

I also BCC'd a bunch of people to whom this may be relevant. Apologies 
if you're no longer involved in SSH.

Pursuant to previous discussion about *host key synchronization* in SSH, 
Bitvise SSH Server and SSH Client now implement the mechanism 
"[email protected] <mailto:[email protected]>" as defined by 
OpenSSH:

https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL?annotate=HEAD

In our 8.xx versions up to 8.19, the SSH Server will unconditionally 
send this global request after successful authentication to inform the 
client of its other host keys. This allows a client to auto-verify those 
host keys, allowing for host key rotation.

Within 6 weeks after our 8.xx release, we found that several significant 
client implementations will *disconnect on receiving a global request*. 
Affected client version strings that I know of so far include:

"Cisco" - a bunch of Cisco appliances, quite common

"AutoMate" - an automated file transfer client

"WeOnlyDo" - old versions of WeOnlyDo libraries, which unfortunately are 
indistinguishable from new versions (same version string)

We suspect some users may also be using ancient OpenSSH versions on 
their production servers that cannot be upgraded. This may include 
versions before 3.1, which first supported client-side receipt of global 
requests.

Furthermore, the following server (!) disconnects on global requests 
sent by the client in versions before 8.0.8.3:

"CerberusFTPServer"

With Bitvise software versions 8.21 (being released), we are adopting 
the following strategy:

- We are *whitelisting* a bunch of software - everything we could 
reasonably test to confirm it handles global requests - so that clients 
will receive host key sync and can implement it. (In 10 years or when 
people get around to it)

- We are *blacklisting* a few implementations - in particular the above 
- so that they will not receive global requests.

- I have defined a new SSH_MSG_EXT_INFO extension which SSH clients can 
use to indicate support for global requests. This is meant to override a 
server's blacklist/whitelist heuristics.

I have published an initial draft for the extension, 
"*global-requests-ok*", here:

https://tools.ietf.org/html/draft-ssh-global-requests-ok-00

The wire details are not flexible (we have implemented and released), 
but the text could be tweaked.

Clients which find themselves on a blacklist, but wish to correct their 
ways; and new implementations that want to use host key sync; are 
encouraged to include "global-requests-ok" in their EXT_INFO.

Side note: although OpenSSH first defined and implemented host key sync, 
it seems to be disabled by default. Perhaps this is due to problems with 
global requests in software like the above.

I suggest that, for clients that send "global-requests-ok", host key 
sync could be reliably enabled by default. Bitvise SSH Client would 
welcome it.

denis