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ssherlangportforwardingssh-tunnel

Run Erlang Observer with ssh port forwarding

I have a remote Erlang node and local developer's PC. I want to start local node debug@127.0.0.1, start observer, call c:nl/1 and other debug actions. I wrote this:

   #!/bin/sh
   export ERL_EPMD_PORT=43690
   PORT=`ssh -l user target -p 5022 "/usr/bin/epmd -names" | awk '$2 == "target-node" {print $5}'`
   pkill -f ssh.*-fxN.*target
   ssh -fxNL 43690:`hostname`:4369 target -p 5022 -l user
   ssh -fxNL $PORT:`hostname`:$PORT target -p 5022 -l user
   ssh -fxNR 9001:`hostname`:9001 target -p 5022 -l user
   erl -name dev@127.0.0.1 -setcookie ABCDEFFJKGSK \
   -kernel inet_dist_listen_min 9001 inet_dist_listen_max 9001 \
   -eval "net_adm:ping('nodename@target')."
   pkill -f ssh.*-fxN.*target

But when I run this script I get message like:

bind: Address already in use Bad local forwarding specification ':debian:' Erlang/OTP 17 [erts-6.1] [source] [64-bit] [smp:4:4] [async-threads:10] [kernel-poll:false]

Eshell V6.1 (abort with ^G) (dev@127.0.0.1)1>

How can I run observer on local machine and connect them to the remote node?

Solution

  • There are at least three things:

    • not closing TCP connections properly
    • node names
    • tunneling wrong

    If you close process, that holds TCP connection with kill it can go to TIME_WAIT state for a couple of minutes: read more

    In the ssh -L port:host:hostport, you have 2 components:

    • first one is port and this says, use this port on my local machine
    • host:hostport mean: "connect to this host and port from remote machine", you can imagine, that you are already on the remote machine and you want to connect to the third one

    You are using hostname inside, which is evaluated on your local machine and returns debian and this doesn't make sense, because probably debian doesn't mean anything on your remote machine. Try using:

    ssh -fxNR 9001:localhost:9001 target -p 5022 -l user

    This should work as expected without the error about specification.

    Erlang distributed uses node names to locate the machines. On your local node you are calling:

    net_adm:ping('nodename@target').

    which tries to contact port 4369 directly on target without port forwarding.

    Erlang distributed was designed to be used in local trusted networks. It is really hard to set it up with port forwarding. You can start the debug node on the remote machine and connect to your system - this should be easy. If the remote system has X server, you can even forward the observer window (ssh -Y).