ssh-tunnel-and-postfix.md: Adding article about SSH tunneling, systemd.network, autossh and Postfix.

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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