- analysis of ssh connections for callback
    SSH_CLIENT='::1 38502 22'
    SSH_CONNECTION='::1 38502 ::1 22'

    -> callback possible to source host



[ target host ]  <--------------|
    |                           |
    |                           |
    |                           |
    | trigger                   |   configuration
    |                           |
    v                           |
[ configuration host ]      ----|


- dynamic port allocation for tunneling

    [1:37] bento:~% ssh -R  0:localhost:22 localhost
    Warning: Permanently added 'localhost' (ECDSA) to the list of known hosts.
    Allocated port 53161 for remote forward to localhost:22

    SSH_AUTH_SOCK=/tmp/ssh-zDCWbUVcUK/agent.30749
    SSH_CLIENT='::1 38587 22'
    SSH_CONNECTION='::1 38587 ::1 22'
    SSH_TTY=/dev/pts/21


- ssh_config:
    DynamicForward
    LocalForward
    RemoteForward

- testing

[1:52] bento:cdist% netstat -anp | grep 56844
(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
tcp        0      0 127.0.0.1:56844         0.0.0.0:*               LISTEN      -                   
tcp6       0      0 ::1:56844               :::*                    LISTEN      -                   
[1:53] bento:cdist% 


[1:48] bento:~% ssh -R  0:localhost:22  localhost       
Allocated port 56844 for remote forward to localhost:22
...

- chatting

01:42 -!- Irssi: Join to #openssh was synced in 0 secs
01:42 < telmich> good evening
01:43 < telmich> I am trying to make use of remote port forwarding using dynamic port 
                 allocation (port=0) -- I am wondering if there is an easy way to 
                 access the port number on the remote side easily?
01:44 < telmich> background for this question is: I'd like to allow various clients to 
                 login to a configuration server, which then configures the clients by 
                 using the tunnel the client provides for the server to ssh back into
02:07 < BasketCase> telmich: afaik you need to use a tool like ss/netstat/lsof to see what port it has open