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diff --git a/content/blog/201502-ssh-tunnel-and-postfix.rst b/content/blog/201502-ssh-tunnel-and-postfix.rst deleted file mode 100644 index b0d7422..0000000 --- a/content/blog/201502-ssh-tunnel-and-postfix.rst +++ /dev/null @@ -1,247 +0,0 @@ -SSH tunnel with single hop, using systemd-networkd and autossh -############################################################## - -:date: 2015-02-01 20:00 -:modified: 2015-02-01 20:00 -:tags: archlinux, autossh, ssh, tunnel, systemd, systemd.network, postfix, TUN -:category: admin -:slug: ssh-tunnel-with-single-hop-using-systemd-networkd-and-autossh -:summary: HOWTO on setting up a SSH tunnel with the help of a systemd-networkd between two machines, with no direct access to each other and modifying Postfix to use that tunnel. -:authors: David Runge - -| Recently I had the pleasure of setting up a :abbr:`SSH (Secure Shell)` 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 :abbr:`TUN (network TUNnel (virtual-network kernel devices))` devices (aka. "poor man's :abbr:`VPN (Virtual Private Network)`"). -| 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| 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): - - .. code:: apache - - 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): - - .. code:: 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: - - .. code:: 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**: - - .. code:: apache - - 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 - - .. code:: 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 feature similar to VPN, 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: - - .. code:: 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: - - .. code:: bash - - ssh -NCTv -w5:5 client2 - -Setting up the TUN devices --------------------------- - -| A short - - .. code:: 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| with the help of |systemd-networkd|. 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*): - - .. code:: 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*): - - .. code:: 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. - - .. code:: bash - - systemctl restart systemd-networkd - -| Now starting the tunnel again should give a fully working point-to-point :abbr:`TCP (Transmission Control Protocol)` 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| or |systemd-networkd|), 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*): - - .. code:: bash - - 10.0.10.2 client2.org client2 - -| On **client2** (*/etc/hosts*): - - .. code:: bash - - 10.0.10.1 client1.org client1 - -Postfix -_______ - -| If using |postfix| as :abbr:`MTA (Message Transfer Agent)`, 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*): - - .. code:: 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| . -| A simple |systemd_service| file can then be used to manage this behavior. -| On **client1** (*/etc/systemd/system/tunnel@.service*): - - .. code:: 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 - - .. code:: bash - - systemctl enable tunnel@client2 - -| Start the service with - - .. code:: bash - - systemctl start tunnel@client2 - - -.. |openssh| raw:: html - - <a href="http://openssh.com" target="_blank">OpenSSH</a> - -.. |systemd_network| raw:: html - - <a href="http://www.freedesktop.org/software/systemd/man/systemd.network.html" target="_blank">systemd network</a> - -.. |systemd-networkd| raw:: html - - <a href="http://www.freedesktop.org/software/systemd/man/systemd-networkd.service.html" target="_blank">systemd-networkd</a> - -.. |netfilter| raw:: html - - <a href="http://www.netfilter.org/" target="_blank">netfilter</a> - -.. |systemd_service| raw:: html - - <a href="http://www.freedesktop.org/software/systemd/man/systemd.service.html" target="_blank">systemd service</a> - -.. |autossh| raw:: html - - <a href="http://www.harding.motd.ca/autossh/" target="_blank">autossh</a> - -.. |postfix| raw:: html - - <a href="http://www.postfix.org/" target="_blank">postfix</a> |