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-Title: SSH tunnel with single hop, using systemd network and autossh
-Date: 2015-02-01 20:00
-Modified: 2015-02-01 20:00
-Tags: archlinux, autossh, ssh, tunnel, systemd, systemd.network, postfix, TUN
-Slug: ssh-tunnel-with-single-hop-using-systemd-network-and-autossh
-Authors: David Runge
-Summary: Howto on setting up a SSH tunnel with the help of a systemd .network file between two machines, with no direct access to each other and modifying Postfix to use that tunnel.
-
-Recently I had the pleasure of setting up a SSH tunnel between two virtual machines that share no route and are located in two different subnets.  
-They can however reach each other via SSH, hopping their host.
-Let's assume the following setup:  
-
-* **client1** (Arch Linux) has *10.0.5.2/24*
-* **client2** (Arch Linux) has *10.0.6.2/24*
-* **host** (Debian) is *10.0.5.1/24* to **client1** and *10.0.6.1/24* to **client2**
-
-As I needed the two clients to be able to send mail to each other and reach each others' services, I did some digging and opted for a SSH connection using TUN devices (aka. *"poor man's VPN"*).
-The following is needed to set this up:  
-
-* root access on both virtual machines (**client1** & **client2**)
-* a user account on the **host** system
-* SSH ([OpenSSH](http://www.openssh.com/) assumed) installed on all three machines
-
-# Connect the clients
-
-## Change sshd_config
-The following two settings have to be made in each clients */etc/ssh/sshd_config* (to allow root login and the creation of TUN devices):  
-
-    #!aconf
-    PermitRootLogin yes  
-    PermitTunnel yes  
-
-I hope it is needless to say, that permitting root access via SSH has its caveats. You should make sure to set a very secure password, or only allow SSH keys for login.
-
-## Generate and exchange keys
-Generate SSH keys on **client1** (you can of course use other key types, if your OpenSSH installation allows and supports it):  
-
-    :::bash
-    ssh-keygen -t rsa -b 4096 -C "$(whoami)@$(hostname)-$(date -I)"  
-
-Here you can choose between setting a password for the key (to unlock the key with *ssh-add* yourself) or not setting one (to be able to use the key on system boot with an automated service).  
-Add them to your user at **host** like this:  
-
-    :::bash
-    ssh-copy-id -i .ssh/id_rsa user@host  
-
-Also add it to */root/.ssh/authorized_keys* on **client2**.
-
-## Use ProxyCommand to connect
-To make a first connection between the clients, one can use the following settings in */root/.ssh/config* of **client1** to hop **host** and connect to **client2**:  
-
-    #!aconf
-    Host client2
-      ProxyCommand ssh user@10.0.5.1 -W 10.0.6.2:%p
-      ForwardAgent yes
-      User root
-      ServerAliveInterval 120
-      Compression yes
-      ControlMaster auto
-      ControlPath ~/.ssh/socket-%r@%h:%p
-
-The *ForwardAgent yes* setting here is especially interesting, as it forwards the SSH key of **client1** to **client2**.  
-On **client1** a simple  
-
-    :::bash
-    ssh client2 -v
-should now directly connect to **client2** by hopping **host**.
-
-# Tunneling
-## Start the tunnel
-Now to the fun part: Creating the tunnel.  
-OpenSSH supports a VPN-like feature, that creates a TUN device on both ends of the connection. As the "direct" (hopping **host**) connection between **client1** and **client2** has been setup already, let's try the tunnel:  
-
-    :::bash
-    ssh -w5:5 client2 -v
-
-The *-w* switch will create a TUN device (*tun5* to be exact) on each client.  
-Now, to start the tunnel without executing a remote command (*-N*), compression of the data (*-C*) and disabling pseudo-tty allocation (*-T*), one can use the following:  
-
-    :::bash
-    ssh -NCTv -w5:5 client2
-
-## Setting up the TUN devices
-A short  
-
-    :::bash
-    ip a s
-
-on **client1** and **client2** shows, that the *tun5* devices have been created on both clients. However they don't feature a link yet.  
-This can be achieved by setting up a [systemd.network](http://www.freedesktop.org/software/systemd/man/systemd.network.html) with the help of [systemd-networkd](http://www.freedesktop.org/software/systemd/man/systemd-networkd.service.html). By placing a *.network* file in */etc/systemd/network/*, the TUN device will be configured as soon as it shows up.  
-Here I chose the *10.0.10.0/24* subnet, but you could use any other private subnet (that's still available in your setup).  
-On **client1** (*/etc/systemd/network/client1-tun.network*):  
-
-    #!ini
-    [Match]
-    Name=tun5
-    Host=client1
-    
-    [Network]
-    Address=10.0.10.1/24
-    
-    [Address]
-    Address=10.0.10.1/24
-    Peer=10.0.10.2/24
-
-On **client2** (*/etc/systemd/network/client2-tun.network*):  
-
-    #!ini
-    [Match]
-    Name=tun5
-    Host=client2
-    
-    [Network]
-    Address=10.0.10.2/24
-    
-    [Address]
-    Address=10.0.10.2/24
-    Peer=10.0.10.1/24
-
-After adding the files a restart of the **systemd-networkd** service on both machines is necessary.  
-
-    :::bash
-    systemctl restart systemd-networkd
-
-Now starting the tunnel again should give a fully working point-to-point TCP connection between the two (virtual) machines using the TUN devices.  
-If you need a more complex setup (i.e. to access the other clients' subnet), you will have to apply some routes (either using [Netfilter](http://www.netfilter.org/) or [systemd.network](http://www.freedesktop.org/software/systemd/man/systemd.network.html)), depending on your individual setup.
-
-# Hosts
-To make both hosts know about each other by hostname (and domain, if any), too, those can be added to the clients' */etc/hosts* files.  
-On **client1** (*/etc/hosts*):  
-
-    10.0.10.2 client2.org client2
-On **client2** (*/etc/hosts*):  
-
-    10.0.10.1 client1.org client1
-
-# Postfix
-If using [postfix](http://www.postfix.org/) as MTA, the service has to be configured to use */etc/hosts* before resolving to your networks DNS resolving.  
-On **client1** and **client2** (*/etc/postfix/main.cf*):  
-
-    #!ini
-    lmtp_host_lookup = native
-    smtp_host_lookup = native
-    ignore_mx_lookup_error = yes
-
-# Autossh and system boot
-Wrapping it all up, it's usually intended to have a tunnel service be started on system boot. SSH tunnels are supposedly known for their poor connectivity. One way to get around this issue is to manage them with [autossh](http://www.harding.motd.ca/autossh/).
-A simple [systemd service](http://www.freedesktop.org/software/systemd/man/systemd.service.html) file can then be used to manage this behavior.  
-On **client1** (*/etc/systemd/system/tunnel@.service*):  
-
-    #!ini
-    [Unit]
-    Description=AutoSSH tunnel to a host
-    After=network.target
-    
-    [Service]
-    Environment="AUTOSSH_GATETIME=0"
-    ExecStart=/usr/bin/autossh -M 0 -NCTv -o ServerAliveInterval=45 -o ServerAliveCountMax=2 -o TCPKeepAlive=yes -w 5:5 %I
-    
-    [Install]
-    WantedBy=multi-user.target
-
-Enable the service with  
-
-    :::bash
-    systemctl enable tunnel@client2
-
-Start the service with  
-
-    :::bash
-    systemctl start tunnel@client2
-
-
-*[SSH]: Secure Shell
-*[MTA]: Message Transfer Agent
-*[TCP]: Transmission Control Protocol
-*[TUN]: network TUNnel (virtual-network kernel devices)
-*[VPN]: Virtual Private Network