The Wiert Corner – irregular stream of stuff

You know the drill: site that limits incoming traffic and has painful VPN. Luckily this time outgoing ssh traffic on port 22 was allowed (because they do SFTP which is SSH File Transfer).

Since I’ve outside Linux boxes and could run a Linux VM there (all Tumbleweed based), this allowed me to do a reverse SSH tunnel. Those are always a bit confusing, but this set of drawings really helps: What’s ssh port forwarding and what’s the difference between ssh local and remote port forwarding – Unix & Linux Stack Exchange [WayBack].

Which brings me to a statement like this:

ssh -o "ExitOnForwardFailure yes" -R :3389:192.168.199.114:3389 -p 33322 93.184.216.34

That didn’t work: a remote machine could not RDP to port 3389, but a local telnet localhost 3389 would. The reason is that by default sshd binds a remote port to the local address only and not the wildcard addres.

So you have to open up the remote config a bit: at least /etc/sshd_config and most likely also your firewall.

lekensteyn explains the sshd side at tunnel – Reverse port tunnelling – Ask Ubuntu [WayBack]; the below settings allows the ssh client to specify the server binding (local or wildcard).

If you are more paranoid, you can even tighten this up further as explained in security – How to create a restricted SSH user for port forwarding? – Ask Ubuntu [WayBack].

The client-side you can even go beyond ExitOnForwardFailure (which terminates the ssh connection if the port cannot be forwarded, i.e. you logged in twice with the same port forwarding, see Can I make SSH fail when a port forwarding fails? – Super User [WayBack]) which you might to complement by following the tips on ClientAliveInterval and ServerAliveInterval in linux – How to release ports on the SSH server when a reverse ssh tunnel disconnects abruptly/uncleanly – Super User [WayBack] and SSH remote port forwarding failed – Server Fault [WayBack].

The reason? If you don’t then you can get the below error after ssh authentication and need to manually kill the blocking ssh session:

Error: remote port forwarding failed for listen port 3389

I usually use this pstree like construct to check which ssh sessions were active:

ps axf -ejH | less

Finally you can even shell – List open SSH tunnels – Super User [WayBack] and use the -n -N -T and -f options to avoid a remote shell and other things (see networking – How does reverse SSH tunneling work? – Unix & Linux Stack Exchange [WayBack]).

Oh and there is Be Genius – Talk: SSH Can Do That? [WayBack]

Anyway, here is the diff in firewall and and sshd config:

commit c8a2c7ca64a6d1cfa0027e19450efb1a0621c58d
Author: Jeroen Wiert Pluimers <jeroen.bitbucket.org@pluimers.com>
Date:   Tue Oct 4 17:02:31 2016 +0200

    open 3389 in firewall and allow sshd GatewayPorts clientspecified so a remote ssh RDP tunnel can be established

diff --git a/ssh/sshd_config b/ssh/sshd_config
index b76bbe2..f6e8e49 100644
--- a/ssh/sshd_config
+++ b/ssh/sshd_config
@@ -113,7 +113,8 @@ UsePAM yes
 
 #AllowAgentForwarding yes
 #AllowTcpForwarding yes
-GatewayPorts no
+#GatewayPorts no
+GatewayPorts clientspecified
 X11Forwarding no
 #X11DisplayOffset 10
 #X11UseLocalhost yes
diff --git a/sysconfig/SuSEfirewall2 b/sysconfig/SuSEfirewall2
index 0596477..930883e 100644
--- a/sysconfig/SuSEfirewall2
+++ b/sysconfig/SuSEfirewall2
@@ -250,7 +250,7 @@ FW_PROTECT_FROM_INT="no"
 #
 # Note: this setting has precedence over FW_SERVICES_ACCEPT_*
 #
-FW_SERVICES_EXT_TCP=""
+FW_SERVICES_EXT_TCP="3389"
 
 ## Type:   string
 #