This article describes a basic installation and configuration of OpenVPN, suitable for private and small business use. For more detailed information, please see the OpenVPN 2.4 man page and the OpenVPN documentation. OpenVPN is a robust and highly flexible VPN daemon. It supports SSL/TLS security, Ethernet bridging, TCP or UDPtunnel transport through proxies or NAT. Additionally it has support for dynamic IP addresses and DHCP, scalability to hundreds or thousands of users, and portability to most major OS platforms.

OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it. It supports conventional encryption using a pre-shared secret key (Static Key mode) or public key security (SSL/TLS mode) using client & server certificates. Additionally it supports unencrypted TCP/UDP tunnels.

OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on most platforms. Overall, it aims to offer many of the key features of IPSec but with a relatively lightweight footprint. OpenVPN was written by James Yonan and is published under the GNU General Public License (GPL).

The Windows service did not properly quote the executable filename passed to CreateService. A local attacker with write access to the root directory C: could create an executable that would be run with the same privilege level as the OpenVPN Windows service.

• 5A basic Layer-3 IP routing configuration
  • 5.2The server configuration file
  • 5.3The client config profile
  • 5.7IPv6
• 6Starting OpenVPN
• 7Routing client traffic through the server
  • 7.1Firewall configuration
  • 7.2Prevent leaks if VPN goes down
• 8Layer-3 IPv4 routing
  • 8.1Prerequisites for routing a LAN
• 9DNS
• 11Config generators
• 12Troubleshooting

Installation

Install the openvpn package, which provides both server and client mode.

Available frontends:

• NetworkManager OpenVPN — NetworkManager VPN plugin for OpenVPN.

https://wiki.gnome.org/Projects/NetworkManager/VPN networkmanager-openvpn

• QOpenVPN — Simple OpenVPN GUI written in PyQt for systemd based distributions.

https://github.com/xmikos/qopenvpn qopenvpn

Kernel configuration

OpenVPN requires TUN/TAP support, which is already configured in the default kernel. Users of custom kernel should make sure to enable the tun module:

Read Kernel modules for more information.

Connect to a VPN provided by a third party

To connect to a VPN service provided by a third party, most of the following can most likely be ignored, especially regarding server setup. Begin with #The client config profile and skip ahead to #Starting OpenVPN after that. One should use the provider certificates and instructions, see Category:VPN providers for examples that can be adapted to other providers. OpenVPN client in Linux Containers also has general applicable instructions, while it goes a step further by isolating an OpenVPN client process into a container.

Create a Public Key Infrastructure (PKI) from scratch

When setting up an OpenVPN server, users need to create a Public Key Infrastructure (PKI) which is detailed in the Easy-RSA article. Once the needed certificates, private keys, and associated files are created via following the steps in the separate article, one should have 5 files in /etc/openvpn/server at this point:

Alternatively, as of OpenVPN 2.4, one can use Easy-RSA to generate certificates and keys using elliptic curves. See the OpenVPN documentation for details.

A basic Layer-3 IP routing configuration

OpenVPN is an extremely versatile piece of software and many configurations are possible, in fact machines can be both servers and clients.

With the release of v2.4, server configurations are stored in /etc/openvpn/server and client configurations are stored in /etc/openvpn/client and each mode has its own respective systemd unit, namely, openvpn-client@.service and openvpn-server@.service.

Example configuration

The OpenVPN package comes with a collection of example configuration files for different purposes. The sample server and client configuration files make an ideal starting point for a basic OpenVPN setup with the following features:

• Uses Public Key Infrastructure (PKI) for authentication.
• Creates a VPN using a virtual TUN network interface (OSI Layer-3 IP routing).
• Listens for client connections on UDP port 1194 (OpenVPN's official IANA port number[1]).
• Distributes virtual addresses to connecting clients from the 10.8.0.0/24 subnet.

For more advanced configurations, please see the openvpn(8) man page and the OpenVPN documentation.

The server configuration file

Copy the example server configuration file /usr/share/openvpn/examples/server.conf to /etc/openvpn/server/server.conf.

Edit the file making a minimum of the following changes:

If TLS with elliptic curves is used, specify dh none and ecdh-curve secp521r1. DH parameters file is not used when using elliptic curves. Starting from OpenVPN 2.4.8, it is required to specify the type of elliptic curves in server configuration. Otherwise the server would fail to recognize the curve type and possibly use an incompatible one, resulting in authentication errors.

Hardening the server

If security is a priority, additional configuration is recommended including: limiting the server to use a strong cipher/auth method and (optionally) limiting the set of enabled TLS ciphers to the newer ciphers. Starting from OpenVPN 2.4, the server and the client will automatically negotiate AES-256-GCM in TLS mode.

Add the following to /etc/openvpn/server/server.conf:

• The .ovpn client profile must contain a matching cipher and auth line to work properly (at least with the iOS and Android client).
• Using tls-cipher incorrectly may cause difficulty with debugging connections and may not be necessary. See OpenVPN’s community wiki for more information.

Enabling compression

Enabling compression is not recommended by upstream; doing so opens to the server the so-called VORACLE attack vector. See this article.

Deviating from the standard port and/or protocol

It is generally recommended to use OpenVPN over UDP, because TCP over TCP is a bad idea[2].

Some networks may disallow OpenVPN connections on the default port and/or protocol. One strategy to circumvent this is to mimic HTTPS traffic which is very likely unobstructed.

To do so, configure /etc/openvpn/server/server.conf as such:

Running multiple instances of OpenVPN on different ports on the physical machine

One can have multiple, concurrent instances of OpenVPN running on the same box. Each server needs to be defined in /etc/openvpn/server/ as a separate .conf file. At a minimum, the parallel servers need to be running on different ports. A simple setup directs traffic connecting in to a separate IP pool. More advanced setups are beyond the scope of this guide.

Consider this example, running 2 concurrent servers, one port 443/udp and another on port 80/tcp.

First modify /etc/openvpn/server/server.conf created as so:

Now copy it and modify the copy to run on 80/tcp:

Be sure to setup the corresponding entries in the firewall, see the relevant sections in #Firewall configuration.

The client config profile

Copy the example client configuration file /usr/share/openvpn/examples/client.conf to /etc/openvpn/client/.

Edit the following:

• The remote directive to reflect either the server's Fully Qualified Domain Name, hostname (as known to the client), or its IP address.
• Uncomment the user and group directives to drop privileges.
• The ca, cert, and key parameters to reflect the path and names of the keys and certificates.
• Enable the TLS HMAC handshake protection (--tls-crypt or --tls-auth).

Run as unprivileged user

Using the options user nobody and group nobody in the configuration file makes OpenVPN drop its root privileges after establishing the connection. The downside is that upon VPN disconnect the daemon is unable to delete its set network routes again. If one wants to limit transmitting traffic without the VPN connection, then lingering routes may be considered beneficial. It can also happen, however, that the OpenVPN server pushes updates to routes at runtime of the tunnel. A client with dropped privileges will be unable to perform the update and exit with an error.

As it could seem to require manual action to manage the routes, the options user nobody and group nobody might seem undesirable. Depending on setup, however, there are different ways to handle these situations:

• For errors of the unit, a simple way is to edit it and add a Restart=on-failure to the [Service] section. Though, this alone will not delete any obsoleted routes, so it may happen that the restarted tunnel is not routed properly.
• The package contains the /usr/lib/openvpn/plugins/openvpn-plugin-down-root.so, which can be used to let openvpn fork a process with root privileges with the only task to execute a custom script when receiving a down signal from the main process, which is handling the tunnel with dropped privileges (see also its README).

The OpenVPN HowTo's linked below go further by creating a dedicated non-privileged user/group, instead of the already existing nobody. The advantage is that this avoids potential risks when sharing a user among daemons:

• The OpenVPN HowTo explains another way how to create an unprivileged user mode and wrapper script to have the routes restored automatically.
• It is possible to let OpenVPN start as a non-privileged user in the first place, without ever running as root, see this OpenVPN wiki (howto). The howto assumes the presence of System V init, rather than Systemd and does not cover the handling of --up/--down scripts - those should be handled the same way as the ip command, with additional attention to access rights.

Converting certificates to encrypted .p12 format

Some software will only read VPN certificates that are stored in a password-encrypted .p12 file. These can be generated with the following command:

Testing the OpenVPN configuration

Run # openvpn /etc/openvpn/server/server.conf on the server, and # openvpn /etc/openvpn/client/client.conf on the client. Example output should be similar to the following:

Find the IP address assigned to the tunX interface on the server, and ping it from the client.

Find the IP address assigned to the tunX interface on the client, and ping it from the server.

Configure the MTU with Fragment and MSS

If experiencing issues when using (remote) services over OpenVPN (e.g. web browsing, DNS, NFS), it may be needed to set a MTU value manually.

The following message may indicate the MTU value should be adjusted:

In order to get the maximum segment size (MSS), the client needs to discover the smallest MTU along the path to the server. In order to do this ping the server and disable fragmentation, then specify the maximum packet size [3]:

Decrease the 1500 value by 10 each time, until the ping succeeds.

Update the client configuration to use the succeeded MTU value, e.g.:

OpenVPN may be instructed to test the MTU every time on client connect. Be patient, since the client may not inform about the test being run and the connection may appear as nonfunctional until finished. The following will add about 3 minutes to OpenVPN start time. It is advisable to configure the fragment size unless a client will be connecting over many different networks and the bottle neck is not on the server side:

IPv6

Connect to the server via IPv6

Starting from OpenVPN 2.4, OpenVPN will use AF_INET defined by the OS when just using proto udp or proto tcp, which in most cases will be IPv4 only. To use both IPv4 and IPv6, use proto udp6 or proto tcp6. To enforce only IPv4-only, you need to use proto udp4 or proto tcp4. On older OpenVPN versions, one server instance can only support either IPv4 or IPv6.

Provide IPv6 inside the tunnel

In order to provide IPv6 inside the tunnel, have an IPv6 prefix routed to the OpenVPN server. Either set up a static route on the gateway (if a static block is assigned), or use a DHCPv6 client to get a prefix with DHCPv6 Prefix delegation (see IPv6 Prefix delegation for details). Also consider using a unique local address from the address block fc00::/7. Both methods have advantages and disadvantages:

• Many ISPs only provide dynamically changing IPv6 prefixes. OpenVPN does not support prefix changes, so change the server.conf every time the prefix is changed (Maybe can be automated with a script).
• ULA addresses are not routed to the Internet, and setting up NAT is not as straightforward as with IPv4. This means one cannot route the entire traffic over the tunnel. Those wanting to connect two sites via IPv6, without the need to connect to the Internet over the tunnel, may want to use the ULA addresses for ease.

Alternatively, if you have no access to these mentioned methods, an NDP proxy should work. See this StackExchange post.

After having received a prefix (a /64 is recommended), append the following to the server.conf:

This is the IPv6 equivalent to the default 10.8.0.0/24 network of OpenVPN and needs to be taken from the DHCPv6 client. Or use for example fd00:1234::/64.

Those wanting to push a route to a home network (192.168.1.0/24 equivalent), need to also append:

OpenVPN does not yet include DHCPv6, so there is no method to e.g. push DNS server over IPv6. This needs to be done with IPv4. The OpenVPN Wiki provides some other configuration options.

Starting OpenVPN

Manual startup

To troubleshoot a VPN connection, start the client's daemon manually with openvpn /etc/openvpn/client/client.conf as root. The server can be started the same way using its own configuration file (e.g., openvpn /etc/openvpn/server/server.conf).

systemd service configuration

To start the OpenVPN server automatically at system boot, enableopenvpn-server@<configuration>.service on the applicable machine. For a client, enableopenvpn-client@<configuration>.service instead. (Leave .conf out of the <configuration> string.)

For example, if the client configuration file is /etc/openvpn/client/client.conf, the service name is openvpn-client@client.service. Or, if the server configuration file is /etc/openvpn/server/server.conf, the service name is openvpn-server@server.service.

Letting NetworkManager start a connection

One might not always need to run a VPN tunnel and/or only want to establish it for a specific NetworkManager connection. This can be done by adding a script to /etc/NetworkManager/dispatcher.d/. In the following example 'Provider' is the name of the NetworkManager connection:

See NetworkManager#Network services with NetworkManager dispatcher for more details.

Gnome configuration

To connect to an OpenVPN server through Gnome's built-in network configuration do the following. First, install networkmanager-openvpn. Then go to the Settings menu and choose Network. Click the plus sign to add a new connection and choose VPN. From there, choose OpenVPN and manually enter the settings. One can optionally import #The client config profile. Yet, be aware NetworkManager does not show error messages for options it does not import. To connect to the VPN simply turn the connection on and check the options are applied (e.g. via journalctl -b -u NetworkManager).

Routing client traffic through the server

Without further configuration only traffic directly to and from the OpenVPN server's IP passes through the VPN. To have other traffic, like web traffic pass through the VPN, correspondent routes must be added. You can either add routes in the client's configuration or configure the server to push these routes to the client.

To redirect traffic to and from a subnet of the server, add push 'route <address pool> <subnet>' right before the remote <address> <port> udp/tcp, like:

To redirect all traffic including Internet traffic to the server, add the following in the client's configuration:

If you are running an IPv4-only server, drop the ipv6 option. If you are going IPv6-only, use redirect-gateway ipv6 !ipv4.

To make the server push routes, appendpush 'redirect-gateway def1 bypass-dhcp ipv6' to the configuration file (i.e. /etc/openvpn/server/server.conf) [4] of the server. Note this is not a requirement and may even give performance issue:

If you are running an IPv4-only server, drop the ipv6 option. If you are going IPv6-only, use push 'redirect-gateway ipv6 !ipv4'

Use the push 'route <address pool> <subnet>' option to allow clients reaching other subnets/devices behind the server:

Optionally, push local DNS settings to clients (e.g. the DNS-server of the router and domain prefix .internal):

After setting up the configuration file, enable packet forwarding on the server. Additionally, the server's firewall needs to be adjusted to allow VPN traffic, which is described below for both ufw and iptables.

Firewall configuration

firewalld

If you use the default port 1194, enable the openvpn service. Otherwise, create a new service with a different port.

Now add masquerade to the zone:

Make these changes permanent:

ufw

In order to allow ufw forwarding (VPN) traffic append the following to /etc/default/ufw:

Change /etc/ufw/before.rules, and append the following code after the header and before the '*filter' line:

• Change the IP/subnet mask to match the server set in the OpenVPN server configuration.
• Change the network interface to the connection used by OpenVPN server.

Make sure to open the chosen OpenVPN port (default 1194/udp):

To apply the changes. reload/restart ufw:

iptables

In order to allow VPN traffic through an iptables firewall, first create an iptables rule for NAT forwarding [5] on the server. An example (assuming the interface to forward to is named eth0):

If running multiple servers on different IP pools, add a corresponding line for each one, for example:

If the server cannot be pinged through the VPN, one may need to add explicit rules to open up TUN/TAP interfaces to all traffic. If that is the case, do the following [6]:

Additionally be sure to accept connections from the OpenVPN port (default 1194) and through the physical interface.

When satisfied, make the changes permanent as shown in iptables#Configuration and usage.

Those with multiple tun or tap interfaces, or more than one VPN configuration can 'pin' the name of the interface by specifying it in the OpenVPN config file, e.g. tun22 instead of tun. This is advantageous if different firewall rules for different interfaces or OpenVPN configurations are wanted.

Prevent leaks if VPN goes down

This prevents all traffic through the default interface (enp3s0 for example) and only allows traffic through tun0.If the OpenVPN connection drops, the system will lose its internet access thereby preventing connections through the default network interface.

One may want to set up a script to restart OpenVPN if it goes down.

ufw

Alternatively, one can allow DNS leaks. Be sure to trust your DNS server!

vpnfailsafe

Alternatively, the vpnfailsafe (vpnfailsafe-git^AUR) script can be used by the client to prevent DNS leaks and ensure that all traffic to the internet goes over the VPN. If the VPN tunnel goes down, internet access will be cut off, except for connections to the VPN server(s). The script contains the functionality of update-resolv-conf, so the two do not need to be combined.

Layer-3 IPv4 routing

This section describes how to connect client/server LANs to each other using Layer-3 IPv4 routing.

Prerequisites for routing a LAN

For a host to be able to forward IPv4 packets between the LAN and VPN, it must be able to forward the packets between its NIC and its tun/tap device. See Internet sharing#Enable packet forwarding for configuration details.

Routing tables

The factual accuracy of this article or section is disputed.

By default, all IP packets on a LAN addressed to a different subnet get sent to the default gateway. If the LAN/VPN gateway is also the default gateway, there is no problem and the packets get properly forwarded. If not, the gateway has no way of knowing where to send the packets. There are a couple of solutions to this problem.

• Add a static route to the default gateway routing the VPN subnet to the LAN/VPN gateway's IP address.
• Add a static route on each host on the LAN that needs to send IP packets back to the VPN.
• Use iptables' NAT feature on the LAN/VPN gateway to masquerade the incoming VPN IP packets.

Connect the server LAN to a client

The server is on a LAN using the 10.66.0.0/24 subnet. To inform the client about the available subnet, add a push directive to the server configuration file:

Connect the client LAN to a server

Prerequisites:

• Any subnets used on the client side, must be unique and not in use on the server or by any other client. In this example we will use 192.168.4.0/24 for the clients LAN.
• Each client's certificate has a unique Common Name, in this case bugs.
• The server may not use the duplicate-cn directive in its config file.
• The CCD folder must be accessible via user and group defined in the server config file (typically nobody:nobody)

Create a client configuration directory on the server. It will be searched for a file named the same as the client's common name, and the directives will be applied to the client when it connects.

Create a file in the client configuration directory called bugs, containing the iroute 192.168.4.0 255.255.255.0 directive. It tells the server what subnet should be routed to the client:

Add the client-config-dir and the route 192.168.4.0 255.255.255.0 directive to the server configuration file. It tells the server what subnet should be routed from the tun device to the server LAN:

Connect both the client and server LANs

Combine the two previous sections:

Connect clients and client LANs

By default clients will not see each other. To allow IP packets to flow between clients and/or client LANs, add a client-to-client directive to the server configuration file:

In order for another client or client LAN to see a specific client LAN, add a push directive for each client subnet to the server configuration file (this will make the server announce the available subnet(s) to other clients):

DNS

For Linux, the OpenVPN client can receive DNS host information from the server, but the client expects an external command to act on this information. No such commands are configured by default. They must be specified with the up and down config options. There are a few alternatives for what scripts to use, but none are officially recognised by OpenVPN, so in order for any of them to work, script-security must be set to 2. The down-root plugin can be used instead of the down option if running as an unprivileged user.

The pull-resolv-conf custom scripts

These scripts are maintained by OpenVPN. They are client.up and client.down, and they are packaged in /usr/share/openvpn/contrib/pull-resolv-conf/. The following is an excerpt of a resulting client configuration using the scripts in conjunction with the down-root plugin:

These scripts use the resolvconf command if present. Systemd-resolvconf and Openresolv both implement this command. See their wiki pages for more information on getting a working resolvconf implementation.

If no implementation of resolvconf is present, client.up preserves the existing resolv.conf at /etc/resolv.conf.ovpnsave and writes a new one. This new one will not have any of the original DNS servers.

If you need to edit these scripts, copy them somewhere else and edit them there, so that your changes don't get overwritten by the next openvpn package upgrade. /etc/openvpn/client is a pretty good place.

The update-resolv-conf custom script

The openvpn-update-resolv-conf script is available as an alternative to packaged scripts. It needs to be saved for example at /etc/openvpn/update-resolv-conf and made executable.

If you prefer a package, there is openvpn-update-resolv-conf-git^AUR that does above for you. You still need to do the following.

Once the script is installed add lines like the following into the OpenVPN client configuration file:

Now, when launching the OpenVPN connection, resolv.conf should be updated accordingly, and also should get returned to normal when the connection is closed.

The update-systemd-resolved custom script

The update-systemd-resolved script links OpenVPN with systemd-resolved via DBus to update the DNS records.

Copy the script into /etc/openvpn/scripts and mark as executable (or installopenvpn-update-systemd-resolved^AUR) and append the following lines into the OpenVPN client configuration file:

In order to send all DNS traffic through the VPN tunnel and prevent DNS leaks, also add the following line (see [7]):

Make sure that the systemd-resolved service is configured and running.

Override DNS servers using NetworkManager

By default networkmanager-openvpn plugin appends DNS servers provided by OpenVPN to /etc/resolv.conf. This may result in DNS instability (leakage).

The NetworkManager GUI does not provide any way to change this behavior, but it is possible to completely override DNS using connection configuration file.

If the override is applied, queries for non-public DNS records are sent to the external resolver, see [8].

To use DNS settings provided by the VPN connection add dns-priority=-1 (ipv4 section) to the file located at /etc/NetworkManager/system-connections/your_vpn_name, where your_vpn_name is the name of your VPN connection.

Making changes to the VPN connection with the NetworkManager GUI will remove the setting above.

To verify that the correct DNS server(s) are configured, see resolvectl status if systemd-resolved is in use, for other resolvers see Domain name resolution.

Layer-2 Ethernet bridging

This article or section needs expansion.

For now see: OpenVPN Bridge

Config generators

ovpngen

The ovpngen^AUR package provides a simple shell script that creates OpenVPN compatible tunnel profiles in the unified file format suitable for the OpenVPN Connect app for Android and iOS.

Simply invoke the script with 5 tokens:

1. Server Fully Qualified Domain Name of the OpenVPN server (or IP address).
2. Full path to the CA cert.
3. Full path to the client cert.
4. Full path to the client private key.
5. Full path to the server TLS shared secret key.
6. Optionally a port number.
7. Optionally a protocol (udp or tcp).

Example:

If the server is configured to use tls-crypt, as is suggested in #The server configuration file, manually edit the resulting foo.ovpn replacing <tls-auth> and </tls-auth> with <tls-crypt> and </tls-crypt>.

The resulting foo.ovpn can be edited if desired as the script does insert some commented lines. foo.ovpn will not automatically route all traffic through the VPN, so you may want to follow #Routing client traffic through the server to enable redirection.

E Designer Configuration tool for the E1000 series of operator panels. E Designer makes it easy to create applications that are logic and user-friendly for the operator, and make flexible and efficient interfaces between systems, machines and operators. Mitsubishielectric E-designer version 7.52: Security vulnerabilities, exploits, vulnerability statistics, CVSS scores and references.

The client expects this file to be located in /etc/openvpn/client/foo.conf. Note the change in file extension from 'ovpn' to 'conf' in this case.

openvpn-unroot

The steps necessary for OpenVPN to #Run as unprivileged user, can be performed automatically using openvpn-unroot (openvpn-unroot-git^AUR).

It automates the actions required for the OpenVPN howto by adapting it to systemd, and also working around the bug for persistent tun devices mentioned in the note.

Troubleshooting

Client daemon not reconnecting after suspend

openvpn-reconnect^AUR, available on the AUR, solves this problem by sending a SIGHUP to openvpn after waking up from suspend.

Alternatively, restart OpenVPN after suspend by creating the following systemd service:

Enable this service for it to take effect.

Connection drops out after some time of inactivity

If the VPN-Connection drops some seconds after it stopped transmitting data and, even though it states it is connected, no data can be transmitted through the tunnel, try adding a keepalivedirective to the server's configuration:

In this case the server will send ping-like messages to all of its clients every 10 seconds, thus keeping the tunnel up.If the server does not receive a response within 120 seconds from a specific client, it will assume this client is down.

A small ping-interval can increase the stability of the tunnel, but will also cause slightly higher traffic. Depending on the connection, also try lower intervals than 10 seconds.

PID files not present

The default systemd service file for openvpn-client does not have the --writepid flag enabled, despite creating /var/run/openvpn-client. If this breaks a config (such as an i3bar VPN indicator), simply change openvpn-client@.service using a drop-in snippet:

Route configuration fails with systemd-networkd

When using systemd-networkd to manage network connections and attempting to tunnel all outgoing traffic through the VPN, OpenVPN may fail to add routes. This is a result of systemd-networkd attempting to manage the tun interface before OpenVPN finishes configuring the routes. When this happens, the following message will appear in the OpenVPN log.

From systemd-233, systemd-networkd can be configured to ignore the tun connections and allow OpenVPN to manage them. To do this, create the following file:

Restartsystemd-networkd.service to apply the changes. To verify that the changes took effect, start the previously problematic OpenVPN connection and run networkctl. The output should have a line similar to the following:

tls-crypt unwrap error: packet too short

This error shows up in the server log when a client that does not support tls-crypt, or a client that is misconfigured to use tls-auth while the server is configured to use tls-crypt, attempts to connect.

To support clients that do not support tls-crypt, replace tls-crypt ta.key with tls-auth ta.key 0 (the default) in server.conf. Also replace tls-crypt ta.key with tls-auth ta.key 1 (the default) in client.conf.

See also

• Securing Network Communication with Stunnel, OpenSSH, and OpenVPN (PDF)

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