Various improvements to the SNMP forwarding/proxying container setup

doc/hacking/snmp-tunnels.rst

@@ -26,7 +26,8 @@ Course of action - with docker
 
 2. Change the line ``command: 192.168.0.1 user@my-hop-host 10000`` to the ip
    address or name of the device you want to reach, the relevant hop host and
-   whatever port you want to tunnel through.
+   whatever port you want to tunnel through.  This port should be free to use
+   on the hop-host.
 
 3. Make sure that ssh key to the hop host is saved (you can test this by doing
    ``ssh user@my-hop-host``, it is saved if you're not prompted for a password).
@@ -54,7 +55,8 @@ When starting docker:
 - if the error message
   ``mydevice.mydomain_1  | 2023/02/21 13:36:11 socat[1744] E bind(5,
   {AF=2 0.0.0.0:10000}, 16): Address already in use``
-  appears: change the port in the docker file
+  appears: change the port in the docker file. Some other process on the hop-host is
+  using this port.
 
 When adding IP device in SeedDB:
 
@@ -111,14 +113,14 @@ Troubleshooting - without docker
 
       sudo netstat -aupn
 
-  (these flags are Linux specific, use 
-  
+  (these flags are Linux specific, use
+
   .. code-block:: sh
-   
+
       man netstat
 
   to figure out which flags might be helpful on other operating systems).
-  
+
   Then kill the process by running
 
   .. code-block:: sh

tools/forward/Dockerfile

@@ -1,10 +1,10 @@
 FROM debian:bullseye
 RUN apt-get update && apt-get install -y openssh-server socat sudo tini
-COPY snmp_forward.sh /
 RUN echo '%adm ALL=NOPASSWD: /usr/bin/socat' > /etc/sudoers.d/socat
 RUN chmod 0440 /etc/sudoers.d/socat
 ARG USER
 RUN useradd -g adm --no-create-home $USER
 USER $USER
 
+COPY snmp_forward.sh /
 ENTRYPOINT ["tini", "/snmp_forward.sh"]

tools/forward/snmp_forward.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # -----------------------------------------------------------------------
 # Shell script snmp_forward.sh
 # Create an SNMP tunnel to remote Agent through a hop host
@@ -11,17 +11,35 @@
 # -----------------------------------------------------------------------
 
 PROGNAME=$0
+PGID=$$
 
 snmp_agent=${1:-158.38.12.155}
 hop_host=${2:-teknobyen-vk.uninett.no}
 tunnel_port=${3:-10000}
 
 remote_tunnel ()
 {
-    ssh -f -L${tunnel_port}:127.0.0.1:${tunnel_port} $hop_host socat -T10 TCP4-LISTEN:${tunnel_port},fork UDP4:${snmp_agent}:161
+    echo "Setting up SSH tunnel to $snmp_agent via $hop_host ..."
+    if [[ "$snmp_agent" == *":"* ]]; then
+        echo "$snmp_agent looks like an IPv6 address, using an IPv6 tunnel"
+        remote_addr=UDP6:\\[${snmp_agent}\]:161
+    else
+        remote_addr=UDP4:${snmp_agent}:161
+    fi
+    ssh -tt -o ConnectTimeout=4 -L${tunnel_port}:127.0.0.1:${tunnel_port} $hop_host socat -T10 TCP4-LISTEN:${tunnel_port},fork ${remote_addr}
 }
 
-remote_tunnel
+local_tunnel ()
+{
+    echo "Setting up local socat tunnel to SSH tunnel..."
+    sudo socat UDP4-LISTEN:161,fork TCP4:localhost:${tunnel_port}
+}
+
+
+# Ensure everything in process group is stopped if either tunnel process dies
+trap "echo A tunnel subprocess died, stopping all forwarding; kill -HUP -$PGID" CHLD
+
+remote_tunnel &
+local_tunnel &
+wait  # Just wait for all background processes to die
 
-echo "local tunnel..."
-sudo socat UDP4-LISTEN:161,fork TCP4:localhost:${tunnel_port}