Reverse SSH tunnel and autossh
Charly's Column – Reverse SSH Tunnel
This month, Charly Kühnast draws attention to a widely unknown weather phenomenon: The instability of rarely used tunnels leading to a Raspberry Pi. Read on for greater insights.
Recording environmental data is one of my hobbies; I already reported about my magnificent dust sensor [1]. I also own a small weather station: the popular WH1080, which is sold under names like Fine Offset, Nextrend, Froggit, or TFA Nexus. It includes various external sensors, as well as an indoor base station.
I continually extract the measurement data via the USB port of the base system – any Linux machine would be capable of it, but my old Raspberry Pi 2B is the perfect choice. I use RRDtool to write the data to a round-robin database and conjure up colorful graphs for a web server (Figure 1.)
I had to come up with something to publish the data on the web. My ISP uses Dual-Stack Lite [2] and doesn't even offer static IPs for an extra charge – and, I have an aversion to dynamic DNS services.
A reverse SSH tunnel should fix it. The Raspberry Pi opens a connection from the inside through the NAT to a server outside, which I rented for a little money. (Performance doesn't matter; it's all about having a static IP address.) This SSH connection creates a direct tunnel between port 80 of the web server and port 80 of the Raspberry Pi. This was fine as a proof of concept; later, I converted it to HTTPS with a certificate from Let's Encrypt.
The syntax for tunneling is simple. On the Raspberry Pi, you enter:
ssh -R <Webserver>:80:localhost:80<User>@<Webserver>
The tunnel is set up; I access the web server and see Connection refused. So what went wrong? After five minutes with a search engine that I really don't trust, I found this out: On the server, I have to add a GatewayPorts clientspecified
line to /etc/ssh/sshd_config
. Without this, the port is only bound to localhost.
Not Enough Action in the Tunnel
Unfortunately SSH disconnects after a period of inaction; because my weather server is not quite as popular as the one run by the Met Office, this happens quite soon. I can set the timeouts, but sooner or later the tunnel always breaks down, and it does not reopen. I succeeded in eliminating this annoyance with autossh
[3]. The tool monitors the connection and restarts crashed tunnels. The syntax for doing this is:
autossh -M 9999 -N -R <Webserver>:80:localhost:80 <User>@<Webserver>
The -M
switch is the monitoring connection. I chose the port number arbitrarily; it and the next one (i.e., 10000) both need to be free.
Caution: The machine (here, the Raspberry Pi) must not have access to the internal LAN at the same time, because if an attacker took over, they would be handed my entire home network on a silver platter. In my environment, the Raspberry Pi is connected to special DMZ ports on the firewall, which isolates it from the LAN. So let's see: 21°C – time to go out into the garden and see if the watering Raspberry Pi is doing its job properly.
Infos
- "Charly's Column: Particulate Matter Measurement with the Raspberry Pi" by Charly Kühnast, Linux Pro Magazine, issue 213, August 2018, p. 54, http://www.linuxpromagazine.com/Issues/2018/213/Breathe-deeply/(language)/eng-US
- Dual-Stack Lite: https://en.wikipedia.org/wiki/IPv6_transition_mechanism#Dual-Stack_Lite_(DS-Lite)
- autossh: https://linux.die.net/man/1/autossh
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Red Hat Adds New Deployment Option for Enterprise Linux Platforms
Red Hat has re-imagined enterprise Linux for an AI future with Image Mode.
-
OSJH and LPI Release 2024 Open Source Pros Job Survey Results
See what open source professionals look for in a new role.
-
Proton 9.0-1 Released to Improve Gaming with Steam
The latest release of Proton 9 adds several improvements and fixes an issue that has been problematic for Linux users.
-
So Long Neofetch and Thanks for the Info
Today is a day that every Linux user who enjoys bragging about their system(s) will mourn, as Neofetch has come to an end.
-
Ubuntu 24.04 Comes with a “Flaw"
If you're thinking you might want to upgrade from your current Ubuntu release to the latest, there's something you might want to consider before doing so.
-
Canonical Releases Ubuntu 24.04
After a brief pause because of the XZ vulnerability, Ubuntu 24.04 is now available for install.
-
Linux Servers Targeted by Akira Ransomware
A group of bad actors who have already extorted $42 million have their sights set on the Linux platform.
-
TUXEDO Computers Unveils Linux Laptop Featuring AMD Ryzen CPU
This latest release is the first laptop to include the new CPU from Ryzen and Linux preinstalled.
-
XZ Gets the All-Clear
The back door xz vulnerability has been officially reverted for Fedora 40 and versions 38 and 39 were never affected.
-
Canonical Collaborates with Qualcomm on New Venture
This new joint effort is geared toward bringing Ubuntu and Ubuntu Core to Qualcomm-powered devices.