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--- router:ssh_tunnel [2024/10/25 08:36] – willy
+++ router:ssh_tunnel [Unknown date] (current) – removed - external edit (Unknown date) 127.0.0.1
@@ Line 1: / Line 1: @@
-====== Remote Access via SSH tunneling ======
-This is common between simple and advanced approaches, if you choose SSH tunneling for remote access.
-I choose the external server + SSH tunneling, because i want to be able to access my services even if my devices are broken / unavailable at times. Is it secure? Yes, security is provided by the reverse-proxy + SSO, and privacy is guaranteed by using HTTPS on all services + encrypted SSH tunnels.
-=== Architecture ===
-So, here are the assumption:
-  * You choose the "external server" + "SSH tunnel" solution
-  * You have two external servers: **external** and  **failback**.
-  * You have two ISPs at home (optional)
-I will try with some ASCII art to illustrate:
-<code>
-        ┌───────────┐
-        │Home       │
-        │ ▲ Server ▲│
-        └─┼────────┼┘
-          │        │
-          │        │
-          │        │
-   ┌──────┼─┐    ┌─┼──────┐
-   │ISP 1 │ │    │ │ISP 2 │
-   └──────┼─┘    └─┼──────┘
-          │        │
-          │        │
-          │        │
-┌─────────┴┐      ┌┴─────────┐
-│External  │      │External  │
-│ Server 1 │      │ Server 2 │
-│          │      │          │
-│┌────────┐│      │┌────────┐│
-││ SSH n.1││      ││ SSH n.2││
-│└────────┘│      │└────────┘│
-└──────────┘      └──────────┘
-</code>
-The **home-server** will port-forward to **external.mydomain.com** via **ISP 1** and, at the same time, to **failsafe.mydomain.com** via **ISP 2**.
-If you have only one ISP or one external server, just ignore the other. Of course you can use any mix of two ISPs and one external server, or one ISP and two external servers as you like, each step is a bit less redundant than the other of course.
-=== Prerequisites  ===
-I have already stated above, but i will again state it here since it's very important. For SSH tunnel approach to be safe, you **must**:
-  * Use HTTPS on __all__ your service. And i mean all.
-  * Use a reverse-proxy in front of __all__ your services.
-  * Use strong authentication (proxy-auth, or Authelia or a similar services)
-All these three points are covered by using NGINX reverse proxy + SSO as i describe in these pages.
-One last time: HTTPS is __mandatory__.
-=== The Basic Idea  ===
-You will create a specific user, called //tunnel// both on the local and the remote servers and allow the local user //tunnel// to login via private/public key exchange to the remote servers. This is always possible because the remote server is, by definition of internet, accessible from the home server.
-The user //tunnel// will then activate a number of port-forwarding from the remote server to the local server, exposing directly only your home most-secured only services:
-  * Home SSH port: so you can access your home server from outside. This will be mapped to something different than 22 both for security reason and not to clash with the external server port 22.
-  * Home Reverse Proxy ports (80 & 443): so you can access all your secured services from outside.
-Please note that you **must** expose port 80 of your reverse-proxy if you plan to use Let's Encrypt Certbot tool, or it will not work.
-What you will need:
-  * Create a //tunnel// user on home server
-  * Create a //tunnel// user on remote server(s)
-  * Share home //tunnel// user **public** key to the remote server's //tunnel// user authorized keys
-  * Login from home to remote as user //tunnel// with appropriate options to create all the needed tunnels.
-Everything will be properly encrypted and safe, and automated.
-=== Home Server Setup ===
-Create a new user called **tunnel**, but make sure it's home folder is on **root partition** because if, by any chance, your data partitions will not mount, you would lose the tunnels as well.
-<code bash>
-useradd -m tunnel
-</code>
-Create the SSH identity for the user tunnel:
-<code bash>
-su - tunnel
-ssh-keygen
-</code>
-I suggest you do not setup a password for this key. It is slightly less secure, but you can always revoke the key if it gets leaked. If you setup a password, then you will need to provide it in the connection script, which kind of make it a mooth point anyway.
-Your SSH public key will be stored under **~/.ssh/id_rsa.pub** (this might change in newer SSH versions where RSA is phased out for better keys, YMMV).
-=== Remote Server Setup ===
-Login to your remote server, create the user tunnel:
-<code bash>
-useradd -m tunnel
-</code>
-Copy / paste the home server user //tunnel// public key into the **~/.ssh/authorized_hosts** file of the remote server tunnel user, as a new line. You should create the folder and the file if missing.
-**BE CAREFUL:** you **must** set permissions properly for the **.ssh** folder and the **authorized_keys** file! They **must not** be accessible or readable by anybody but the user.
-You might want to edit SSH daemon configuration and set in your server **/etc/ssh/sshd_config**:
-<code>
-ClientAliveInterval 10
-ClientAliveCountMax 2
-</code>
-To ensure that on the **remote server** the user tunnel is allowed to create port-forwarding, add the following to your **/etc/ssh/sshd.conf** on the remote server:
-<code>
-Match User tunnel
-        GatewayPorts clientspecified
-</code>
-and restart the SSHD service.
-so that clients will be dropped and port freed automatically if they stop responding within 10 seconds for maximum two consecutive times.
-Since you will need to redirect low-ports (<1024), and this is not possible with default SSH by design, you will need also the [[http://www.dest-unreach.org/socat/|socat]] tool that allows you to create local redirections between the ports as needed. Emerge it:
-<code bash>
-emerge socat
-</code>
-__Note__: for security reasons, you might prefer to move the external server SSH port from the default 22 to some other value, like 222, in case you do so, remember to add //-p 222// to your ssh and autossh command below.
-=== SSH tunnel creation ===
-Which kind of port-forwarding do you need? And how many?
-SSH provides two kinds of port forwarding: **remote-to-local** and **local-to-remote**.
-**Remote to local**: the specified **port** on **remote server** will be opened by SSH and any traffic trough that port will be redirected, encrypted and transparently, to a specific **port** (and possibly ip address) on the **home server**. This is useful to access a home service from outside.
-**Local to remote**: the specified **port** on **home server** will be opened by SSH and any traffic will be redirected, encrypted and transparently, to a **port** of the remote server (and possibly also a specific IP address). This can be used to remap en external service on the local server, without giving an user direct access to the remote server.
-You will mostly need the first one, of course.
-The syntax is: **-R<remote_ip>:<remote_port>:<local_ip>:<local_port>**
-  * -R : specify it's a **remote to local** forwarding
-  * remote_ip: can always be 127.0.0.1, to use remote **public** IPs or **0.0.0.0** you need special configuration on remote server.
-  * remote_port: which port to listen to on remote server. Must be > 1024 (unprivileged)
-  * local_ip: destination IP for connection incoming from remote. Usually **127.0.0.1** but can be whatever you want
-  * local_port: destination port for connections. It's your exported service port.
-A good idea is to forward home server SSH port to some custom port on the remote server, so that you can access your home via SSH. From your home server, as tunnel user run:
-<code bash>
-ssh tunnel@external-server1 -R0.0.0.0:2022:127.0.0.1:22
-</code>
-will allow you to SSH on port 2022 of the remote server, and actually login to your home server SSH. Try that now, from the home server, as any user from a new terminal, run:
-<code bash>
-ssh myuser@external-server1 -p 2022
-</code>
-and you will login as user __myuser__... on your home server!
-__Note__: remember to login once manually, as user tunnel, to ensure remote host keys are accepted of your tunnels will fail.
-__Note__: running SSH with port forwarding like i showed you above is ok, but will not reconnect automatically in case the tunnels fails nor will survive a reboot. A better approach is discussed down below, for now, you can test your connections with SSH.
-=== Forwarding the home services ===
-So, assuming the prerequisites are satisfied, you have all your home services behind a reverse-proxy protected with HTTPS and Let's Encrypt real certificates, all you need to do it actually forward the HTTP(80) and HTTPS(443) services from your home to your remote server. Yes, also port 80 must be forwarded for Let's Encrypt automatic certificate renewal to work.
-There is a catch that you need to be aware of: SSH is **not** capable (by design, and because of security) to forward ports below 1024. Unfortunately ports 80 (HTTP) and 443 (HTTPS) are both below 1024. The solution to this issue is to let SSH forward those ports only on the external server 127.0.0.1 (non-public) network on higher ports (8080 instead of 80 and 8443 instead of 443) and then add a second internal forwarding on the remote server //from port// **127.0.0.1:8080** //to// **0.0.0.0:80** and //from port// **127.0.0.1:8443** //to// **0.0.0.0:443**. (Check note below, you might need 8443 local as well if using my reverse proxy setup)
-Due to this limitation, the command, to be run from the home server looks like:
-<code bash>
-ssh tunnel@external-server1 -R0.0.0.0:2022:127.0.0.1:22 -R127.0.0.1:8080:127.0.0.1:80 -R127.0.0.1:8443:127.0.0.1:8443
-</code>
-The SSH rule (port 2022/22) it has been already discussed above.
-__Note__: my reverse proxy [[selfhost:nginx|guide]] will use port 8443 for external connections to differentiate from internal connections. This is why the above rule forwards to home server port 8443 instead of port 443.
-At this point you need to add the two internal redirections from 80 -> 127.0.0.18080 and 443 -> 127.0.0.1:8443 on the remote server using __socat__. Create the redirect start file under **/etc/local.d/01-redirect.start**:
-<file - 01-redirect.start>
-#!/bin/bash
-(socat TCP-LISTEN:443,fork,reuseaddr TCP:127.0.0.1:8443)&
-(socat TCP-LISTEN:80,fork,reuseaddr TCP:127.0.0.1:8080)&
-</file>
-make it executable and manually start it, as root, for this first time (it will autostart next reboot):
-<code bash>
-chmod +x /etc/local.d/01-redirect.start
-/etc/local.d/01-redirect.start
-</code>
-At this point, you should be able to reach your home services from your external server.
-=== Autoreconnect & Autostart the home server side ===
-There is a simple and pretty solid tool, [[https://github.com/Autossh/autossh|AutoSSH]], that will help you pack it all up and make is more resilient with autoreconnection capabilties. Autossh will start SSH for you and restart it when it fails. but there are some caveats as autossh will exit and might end up quitting in some special cases, like if the remote server is not reachable at boot or the network connection gives up for a prolonged period of time. To ensure this does not happen, you need to set **AUTOSSH_GATETIME=0**.
-First of all, install //autossh//:
-<code bash>
-emerge autossh
-</code>
-Now create the following start-up script in **/home/tunnel/tunnels.sh**:
-<file - tunnels.sh>
-#!/bin/bash
-export AUTOSSH_GATETIME=0
-function run_tunnel()
-{
-        while true
-        do
-                autossh -M 0 \
-                        -nNT -q \
-                        -o ServerAliveInterval=30 \
-                        -o ServerAliveCountMax=2 \
-                        -o ExitOnForwardFailure=yes \
-                        -R0.0.0.0:5022:127.0.0.1:22 \
-                        -R127.0.0.1:8080:127.0.0.1:80 \
-                        -R127.0.0.1:8443:127.0.0.1:8443 \
-                        -p $2
-                        $1
-                sleep 30
-        done
-}
-run_tunnel <<ip address of external-server1>> <<SSH port of external-server1>>&
-run_tunnel <<ip address of external-server2>> <<SSH port of external-server1>>&
-wait
-</file>
-For additional resillience i have also wrapped those tunnels with a while loop, and notice that due to this reason i am not using the //-f// option for SSH or AutoSSH. You might never be carefull enough when it comes to losing remote access to your server, and maybe you are traveling abroad.
-Also, for additional resillience, you want to use the real **ip** address instead of //external-server1&2// so that is DNS fails but network is working, the tunnels will connect anyway.
-To start it on boot, drop this init script to **/etc/init.d/tunnel**:
-<file - tunnel>
-#!/sbin/openrc-run
-# Copyright 2024 Willy Garidol
-# Distributed under the terms of the GNU General Public License v3
-depend() {
-        need localmount net
-}
-description="SSH tunnel manager"
-pidfile="/run/tunnel.pid"
-command_background=true
-command="/home/tunnel/tunnel.sh"
-command_args=""
-command_user="tunnel:tunnel"
-start_stop_daemon_args="--stdout /var/log/tunnel/tunnel.log --stderr /var/log/tunnel/tunnel.log"
-start_pre() {
-        test -e "/var/log/tunnel" || {
-                mkdir "/var/log/tunnel"
-        } && chown -R tunnel:tunnel "/var/log/tunnel"
-}
-</file>
-and make it executable, set it on boot and start it now:
-<code bash>
-chmod +x /etc/init.d/tunnel
-rc-update add tunnel default
-/etc/init.d/tunnel start
-</code>
-You can improve and generalize these script as much as you like. My suggestion is: keep it simple, this is __core__ instrastructure and any complexity might introduce errors, bugs where they are not needed.

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