Using SSH terminal (stdio) as a tunnel for mongo (-L option not allowed)

Question

Here's the thing:

• a mongodb client on the client machine
• a SSH terminal on a jump machine
• the mongodb server on the target machine

I would like the client to connect to the server through the jump machine, using SSH, but I CANNOT use regular tunneling

Here's what I've done:

• on the client:

socat TCP4-LISTEN:3340,fork,reuseaddr STDIO | ssh jump -W target:6003

• on the target:

socat TCP4-LISTEN:6003 TCP4:127.0.0.1:27017
mongo server is of course listening on 27017

Now on the client, I type this:

$ mongo 127.0.0.1:3340/admin
MongoDB shell version v3.6.8
connecting to: mongodb://127.0.0.1:3340/admin
show dbs
2021-03-02T03:02:39.162+0100 E QUERY    [thread1] Error: network error while attempting to run command 'isMaster' on host '127.0.0.1:3340'  :
connect@src/mongo/shell/mongo.js:257:13
@(connect):1:6
exception: connect failed

Here's what I see in the SSH session:

Authenticated to jump ([<jump_IP>]:22).
debug1: channel_connect_stdio_fwd <target_IP>:6003
debug1: channel 0: new [stdio-forward]
debug1: getpeername failed: Bad file descriptor
debug1: Requesting no-more-sessions@openssh.com
debug1: Entering interactive session.
debug1: pledge: network
debug1: client_input_global_request: rtype hostkeys-00@openssh.com want_reply 0
?AHsmastertopologyVersion-processId`"�蚇���countermaxBsonObjectSizemaxMessageSizeBytesl�maxWriteBatchSize��       localTime\[��wlogicalSessionTimeoutMinutesconnectionIdmminWireVersionmaxWireVersion       readOnlyok�?^Cdebug1: channel 0: free: direct-tcpip: listening port 0 for <target_IP> port 6003, connect from 127.0.0.1 port 65535 to UNKNOWN port 65536, nchannels 1
debug1: fd 0 clearing O_NONBLOCK


show dbs
�@H�okerrmsgDThe client metadata document may only be sent in the first isMastercode�codeNameClientMetadataCannotBeMutated


Killed by signal 2.

So I can somehow see some "text" being transfered, but obviously not working

It's not the simplest architecture. Any help is appreciated

PS: the reason I can't use the -L tunnel option, which would work perfectly, is that:

• the SSH terminal arrives in a container
• the SSH tunnel would arrive on the host machine

The reason? When I SSH with a certain user, the shell used is actually the inside of the docker container. This allows me to "lock" some users inside containers, without having to create SSH/passwd inside the containers. Also, only a certain container IP address is allowed for a certain mongo account, so the client mongo has to go through the container terminal.