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{{tag>vpn openvpn}}

====== Openvpn installation ======
Install
  apt install openvpn

Enable ip forwarding
  echo 1 > /proc/sys/net/ipv4/ip_forward


===== Simple setup with static key =====

<WRAP center round important 60%>
This simple setup with static key is really only usable with single client and server, unless you want to run separate openvpn instances on different server ports with different server configurations.
</WRAP>

  cd /etc/openvpn/
  openvpn --genkey --secret static.key

Set up the server.conf, here's a minimum konfiguration

  dev tun
  ifconfig 10.8.0.1 10.8.0.2
  secret /etc/openvpn/static.key
  cipher AES-256-CBC

And client.conf

  remote 1.2.3.4
  dev tun
  ifconfig 10.8.0.2 10.8.0.1
  secret static.key
  
  #route 4.5.6.7 255.255.255.255 vpn_gateway

  redirect-gateway def1
  
  cipher AES-256-CBC

Note the ''static key'' and ''redirect-gateway'' options. Redirect gateway will route all client's traffic to go through VPN. For visiting public sites you will need to setup SNAT/MASQUERADING on firewall. See below.
Uncomment ''route'' and comment out ''redirect-gateway'' if you only want to route traffic to specific IP e.g. 4.5.6.7

Start the openvpn on both server and client. Server:
  /etc/openvpn# openvpn server/server.conf

and client
  sudo openvpn client.ovpn

Both commands should show at the end something like:
  ...
  2024-04-09 10:25:08 Peer Connection Initiated with [AF_INET]1.2.3.4:port
  2024-04-09 10:25:08 Initialization Sequence Completed

To verify that the VPN is running, you should be able to ping 10.8.0.2 from the server and 10.8.0.1 from the client.

===== Setup for multiple clients - one server =====
This is the "proper" way to do it when there are multiple clients involved.

==== Certificate generation ====

Download the easy-rsa: 

  wget https://github.com/OpenVPN/easy-rsa/releases/download/v3.1.7/EasyRSA-3.1.7.tgz
  tar xf EasyRSA-3.1.7.tgz
  cd EasyRSA-3.1.7

Copy vars.example to vars and change variables accordingly. vars will be automatically sourced by easy-rsa script.
  cp vars.example vars

Also add the ''keyEncipherment'' option in //x509-types/client// file otherwise you will get an error like
''Client failed: unsupported certificate purpose'' eg.
<code>
etc/openvpn/EasyRSA-3.1.7# cat x509-types/client 
# X509 extensions for a client

basicConstraints = CA:FALSE
subjectKeyIdentifier = hash
authorityKeyIdentifier = keyid,issuer:always
extendedKeyUsage = clientAuth
keyUsage = digitalSignature, keyEncipherment
</code>

1. Initialize the PKI directories
  ./easyrsa init-pki

2. Create the CA. Used in client and server configuration:
  ./easyrsa build-ca

3. Generate Diffie-Hellman (DH) params.
  ./easyrsa gen-dh

4. Create the vpn server's sign request and then certificate. This should be used in server.conf ''cert'' along with ''key'' which will also be generated with these commands.  Here you need to provide the password of the CA.key in step 2:
  ./easyrsa gen-req server nopass
  ./easyrsa sign-req server server
  
5. Create client signing request and certificate. Change EntityName, ''nopass'' if you don't want private key to be encrypted
  ./easyrsa gen-req EntityName nopass
  ./easyrsa sign-req client EntityName


  
For each new client just repeat last step.

==== Creating configuration files for server and clients ====
=== Server ===

You can copy the example file and adapt the fields to your config
  cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf server.conf

Edit the ca, cert, key, and dh parameters to point to the files you generated and any other configuration you need. Also create the ta.key

  openvpn --genkey tls-auth ta.key

Also create the openvpn user
  adduser --no-create-home --disabled-login openvpn

and uncomment this in server.conf
  ...
  ;user openvpn
  ;group openvpn
  ...

=== Client ===
  cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf client.conf.example

  * Like the server configuration file, first edit the ca, cert, and key parameters to point to the files you generated.
  * Edit the ''remote'' directive to point to the hostname/IP address and port number of the OpenVPN server
  * Ensure that the client configuration file is consistent with the directives used in the server configuration. The major thing to check for is that the dev (tun or tap) and proto (udp or tcp) directives are consistent. Also make sure that comp-lzo and fragment, if used, are present in both client and server config files.

You can now try running the openvpn on client and server like mentioned in Simple setup above.

==== Redirecting traffic over VPN ====
=== Redirect all traffic ===
In server.conf add

  push "redirect-gateway def1 bypass-dhcp"


In client.conf add:
  redirect-gateway def1

=== Redirect only some IPs (aka Split tunnel) ===
In server.conf add:

  push "route 192.168.10.0 255.255.255.0"
  push "route 192.168.20.0 255.255.255.0"
  ...

Remove the redirect-gateway lines from conf.

<WRAP center round important 60%>
[[https://web.archive.org/web/20240521145319/https://i.sstatic.net/KNr6M.png|Ubuntu Network Manager]] doesn't seem to need above settings, so you can just uncheck the "Use this connection only for resources on its network" to redirect all traffic over VPN.
</WRAP>

===== Redirect DNS through VPN =====
First uninstall any other DNS server/resolver like ''systemd-resolved''. Now install dnsmasq

  apt install dnsmasq

Now make the ///etc/resolv.conf// look like this

  nameserver ::1
  nameserver 127.0.0.1
  options trust-ad

You might need to [[https://wiki.archlinux.org/title/Domain_name_resolution#Overwriting_of_/etc/resolv.conf|write protect]] this file because some other programs might overwrite this.

In ///etc/dnsmasq.conf// add one or more upstream servers that dnsmasq will use for name resolution i.e.

  listen-address=::1,127.0.0.1,10.8.0.1
  interface=tun0
  # Google's nameservers, for example
  server=8.8.8.8
  server=8.8.4.4

Adapt the listen-address IP to your VPN interface IP and interface as well and uncomment ''no-resolv''.

In ///etc/openvpn/server/server.conf// make sure you push DNS and redirect gateway:

  ...
  push "redirect-gateway def1 bypass-dhcp"
  push "dhcp-option DNS 10.13.13.1"
  ...
  
<WRAP center round info 60%>
If you run into problems with DNS not going through VPN in Windows installed as a VM in Virtualbox for example, you will need to disable automatic metric on VPN interface and set it to lower then the default Ethernet or wifi. To check open the powershell and type ''nslookup example.org'' and you should get a response from VPN IP e.g. 10.8.0.1.
{{ :wiki:screenshots:windows:windows_metric-2024-05-22_14-17.png?400 |}}
</WRAP>

==== Resolve specific domains names to hard-coded IP ====
Say you need to always resolve example.org to 1.2.3.4 address, you have two options.

1. Add the resolution to ///etc/hosts// i.e.

  ...
  1.2.3.4 www.example.org example.org

2. Remove the above from ///etc/hosts// and in ///etc/dnsmasq.conf// add following
  address=/example.org/1.2.3.4

Also [[https://wiki.archlinux.org/title/dnsmasq#Tips_and_tricks|there are other]] stuff you can do.

===== Firewall setup =====
==== Shorewall ====
Example configuration.

///etc/shorewall/interfaces//

<code>
###############################################################################
#ZONE   INTERFACE       BROADCAST       OPTIONS
net     eth0      detect          tcpflags,logmartians,nosmurfs,dhcp
vpn     tun0      detect
</code>

///etc/shorewall/zones//
<code>
###############################################################################
#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS

fw      firewall
net     ipv4
vpn     ipv4
</code>

///etc/shorewall/snat// - this will masq all traffic to appear to be coming from the IP of the server running Openvpn.
<code>
MASQUERADE      10.8.0.0/24     eth0
</code>

///etc/shorewall/policy//
<code>
###############################################################################
#SOURCE         DEST            POLICY          LOG LEVEL       LIMIT:BURST

$FW             net             ACCEPT

$FW             vpn             ACCEPT
vpn             $FW             ACCEPT

vpn             net             ACCEPT
net             all             DROP            info

# The FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info
</code>

///etc/shorewall/rules// - connect to openvpn service at this port
<code>
...
ACCEPT          net             $FW             udp     1194
...
</code>

==== iptables ====

  iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

===== Running openvpn server as a service =====

Debian 12 ships with systemd units for this. The way to run it is to first make sure the server configuration file is located in ///etc/openvpn/server// directory. The file must end with .conf. So if you have a configuration file called ''myvpnserver.conf'' you need to run it as

  systemctl enable --now openvpn-server@myvpnserver

Check logs with
  journalctl -xefu openvpn-server@server.service

====== Tested on ======
  * Debian 12 Bookworw
  * shorewall 5.2.8

====== See also ======
  * [[wiki:ovpn_file_example]]
  * [[wiki:vpn_troubleshooting|VPN troubleshooting]]
====== References ======
  * https://openvpn.net/community-resources/static-key-mini-howto/
  * https://wiki.archlinux.org/title/Shorewall#/etc/shorewall/masq
  * https://serverfault.com/questions/480069/how-to-force-all-traffic-through-vpn
  * https://openvpn.net/community-resources/how-to/#setting-up-your-own-certificate-authority-ca-and-generating-certificates-and-keys-for-an-openvpn-server-and-multiple-clients
  * https://www.openssl.org/docs/man1.1.1/man5/x509v3_config.html
  * https://shorewall.org/OPENVPN.html
  * https://openvpn.net/community-resources/reference-manual-for-openvpn-2-4/
  * [[https://superuser.com/a/966833|windows metric issue]]