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diff --git a/content/server/ssh-tunnel-and-postfix.md b/content/server/ssh-tunnel-and-postfix.md new file mode 100644 index 0000000..30e8ab6 --- /dev/null +++ b/content/server/ssh-tunnel-and-postfix.md @@ -0,0 +1,178 @@ +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 |