Stunnel/Troubleshooting

Having issues getting stunnel client and server to connect.

Have been following this Digital Ocean guide: https://www.digitalocean.com/community/tutorials/how-to-set-up-an-ssl-tunnel-using-stunnel-on-ubuntu

Problem Description

I am trying to create an stunnel connection from client to server. The stunnel client will route traffic from local port 2222 to local port 8000. This traffic will then be encrypted by Stunnel and sent out over the network, to the stunnel server on the remote machine, also listening for traffic and connections on port 8000. Once the traffic reaches the stunnel server, it is decrypted and forwarded to the server's local port 22, the SSH service.

This allows the execution of an SSH command to localhost that ultimately connects to the remote server:

[local] $ ssh -p 2222 zappa@localhost

...login message...

[remote] $ 

This is useful because:

• You can wrap arbitrary traffic from any local port, and send it encrypted with SSL over any other port.
• You can bypass any firewall that allows HTTPS traffic only by disguising your traffic using Stunnel.

Server

Server configuration

Here is the server stunnel.conf (Ubuntu):

output  = /var/log/stunnel4/stunnel.log
cert    = /etc/stunnel/stunnel.fullchain.pem
key     = /etc/stunnel/stunnel.key.pem
client  = no
debug   = 7
[ssh]
accept = 8000
connect = 127.0.0.1:22

The debug level of 7 is maximum and gives a more detailed description of what's happening in the log file.

Server behavior

On the server, starting the stunnel client using the following steps:

Check the stunnel conf:

$ cat /etc/stunnel/stunnel.conf

Open port 8000 if needed:

$ iptables -A INPUT -p tcp --dport 8000 -j ACCEPT

Stop previous stunnel instances and start a new one:

$ killall stunnel && stunnel

Look at the last few lines of the log to verify it is running correctly and bound to port 8000:

$ tail /var/log/stunnel4/stunnel.log
...
2017.03.29 04:59:15 LOG5[ui]: Configuration successful
2017.03.29 04:59:15 LOG7[ui]: Listening file descriptor created (FD=7)
2017.03.29 04:59:15 LOG7[ui]: Service [ssh] (FD=7) bound to 0.0.0.0:8000

Verification that server is ok

I can also see the open ports on the server using the netstat utility:

$ netstat -tulpn

(Not all processes could be identified, non-owned process info
 will not be shown, you would have to be root to see it all.)
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      -
tcp        0      0 0.0.0.0:8000            0.0.0.0:*               LISTEN      -
tcp6       0      0 :::80                   :::*                    LISTEN      -
tcp6       0      0 :::22                   :::*                    LISTEN      -

(The SSH service is available on this machine on port 22, so we could just connect to the machine that way, but consider a scenario in which port 22 is blocked on a local network and port 8000 is not.)

Finally, an nmap scan of localhost and the server's IP also shows ports 22, 80, and 8000 open and listening:

$ nmap localhost; echo; nmap 196.116.112.336

Starting Nmap 7.01 ( https://nmap.org ) at 2017-03-29 05:12 UTC
Nmap scan report for localhost (127.0.0.1)
Host is up (0.00016s latency).
Other addresses for localhost (not scanned): ::1
Not shown: 997 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
8000/tcp open  http-alt

Nmap done: 1 IP address (1 host up) scanned in 0.08 seconds


Starting Nmap 7.01 ( https://nmap.org ) at 2017-03-29 05:12 UTC
Nmap scan report for 196.116.112.336
Host is up (0.00013s latency).
Not shown: 997 closed ports
PORT     STATE SERVICE
22/tcp   open  ssh
80/tcp   open  http
8000/tcp open  http-alt

Nmap done: 1 IP address (1 host up) scanned in 0.08 seconds

Client

Client configuration

Here is the client stunnel.conf (Mac):

output  = /var/log/stunnel4/stunnel.log
cert    = /usr/local/etc/stunnel/stunnel.fullchain.pem
key     = /usr/local/etc/stunnel/stunnel.key.pem
client  = yes
debug   = 7
[ssh]
accept  = 127.0.0.1:22
connect = 92.126.102.36:8000

Client behavior

When the stunnel command is run on the client,