Set up a Hardened OpenVPN Server on Debian 9

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OpenVPN is a tool for creating network tunnels between groups of computers that are not on the same local network. This is useful to remotely access services on a network or computer without making those services publicly accessible. When integrated with OpenSSL, OpenVPN encrypts all VPN traffic providing a secure connection between machines.

Set up a Hardened OpenVPN Server on Debian

An OpenVPN connection consists of two flow channels between the server and clients: the Control Channel and the Data Channel. A client connects to the server by initiating a TLS session over the control channel, in which credentials are exchanged between server and clients to establish the data channel. The data channel is the encrypted pipeline in which all traffic between server and clients is then transmitted.

This guide is the first of a three-part series. Part one sets up a VPN server on Debian and prepares the access credentials for client devices. This VPN can be used to host internal services such as websites, game servers or file servers.

Part two shows you how to set up a routed VPN so all traffic from client devices is tunneled through your Linode to the internet. Part three takes you through setting up the client-side software for various operating systems, including mobile platforms.

Before You Begin

  1. Familiarize yourself with our Getting Started guide and set your Linode’s timezone.

  2. This guide will use sudo wherever possible. Complete the sections of our Securing Your Server guide to create a standard user account, harden SSH access and remove unnecessary network services. Do not complete the steps in the Creating a Firewall section. This guide has instructions specifically for firewall rules for an OpenVPN server.

  3. Update the system:

    sudo apt update && sudo apt upgrade
    

Managing the OpenVPN Environment

You can manage the OpenVPN environment in two ways:

  • The first method uses OpenVPN Community Edition, where server and client configurations are manually edited and client credentials are distributed to their respective devices using SCP or SFTP. Alternatively, you could copy credentials to a desktop computer, edit them and then transfer them by USB.

  • The second method uses OpenVPN Access Server, a server-side application which lets you configure OpenVPN with your web browser. Client machines access a private URL from which to download their credential packages.

For small applications, OpenVPN Access Server is the more streamlined and user-friendly solution. The free version allows up to two simultaneous users. Although each user can have as many client devices as they like, a user’s clients will all have the same keys and certificates; more can be added by buying licensing. For more advanced configurations than what the GUI offers, you would still need to edit the VPN’s configuration files.

If you are interested in running OpenVPN Access Server on your Linode, see our guide: Secure Communications with OpenVPN Access Server. The remainder of this guide will focus on manual configuration using OpenVPN Community Edition.

Networking Configuration

OpenVPN currently does not support a dual-stack configuration from a single instance where clients can connect to a server simultaneously using IPv4 and IPv6; one transport layer must be chosen or the other. Furthermore, only OpenVPN versions 2.3 and above support IPv6 over TUN devices. With versions earlier than 2.3, IPv6 can only be used with TAP devices.

This series assumes your VPN will operate over IPv4 only. If you instead wish to use IPv6, see OpenVPN’s documentation for more information.

IPv4 Firewall Rules

  1. Switch to the root user.

    su - root
    
  2. Flush any pre-existing rules and non-standard chains which may be in the system:

    iptables -F && iptables -X
    
  3. Install iptables-persistent so any iptables rules we make now will be restored on succeeding bootups. When asked if you want to save the current IPv4 and IPv6 rules, choose No for both protocols.

    apt install iptables-persistent
    
  4. Add IPv4 rules: iptables-persistent stores its rulesets in the files /etc/iptables/rules.v4 and /etc/iptables/rules.v6. Open the rules.v4 file and replace everything in it with the information below:

    /etc/iptables/rules.v4
     1 2 3 4 5 6 7 8 910111213141516171819202122232425262728293031323334353637383940414243444546
    *filter
    
    # Allow all loopback (lo) traffic and reject anything
    # to localhost that does not originate from lo.
    -A INPUT -i lo -j ACCEPT
    -A INPUT ! -i lo -s 127.0.0.0/8 -j REJECT
    -A OUTPUT -o lo -j ACCEPT
    
    # Allow ping and ICMP error returns.
    -A INPUT -p icmp -m state --state NEW --icmp-type 8 -j ACCEPT
    -A INPUT -p icmp -m state --state ESTABLISHED,RELATED -j ACCEPT
    -A OUTPUT -p icmp -j ACCEPT
    
    # Allow SSH.
    -A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 22 -j ACCEPT
    -A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 22 -j ACCEPT
    
    # Allow UDP traffic on port 1194.
    -A INPUT -i eth0 -p udp -m state --state NEW,ESTABLISHED --dport 1194 -j ACCEPT
    -A OUTPUT -o eth0 -p udp -m state --state ESTABLISHED --sport 1194 -j ACCEPT
    
    # Allow DNS resolution and limited HTTP/S on eth0.
    # Necessary for updating the server and timekeeping.
    -A INPUT -i eth0 -p udp -m state --state ESTABLISHED --sport 53 -j ACCEPT
    -A OUTPUT -o eth0 -p udp -m state --state NEW,ESTABLISHED --dport 53 -j ACCEPT
    -A INPUT -i eth0 -p tcp -m state --state ESTABLISHED --sport 80 -j ACCEPT
    -A INPUT -i eth0 -p tcp -m state --state ESTABLISHED --sport 443 -j ACCEPT
    -A OUTPUT -o eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 80 -j ACCEPT
    -A OUTPUT -o eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 443 -j ACCEPT
    
    # Allow traffic on the TUN interface so OpenVPN can communicate with eth0.
    -A INPUT -i tun0 -j ACCEPT
    -A OUTPUT -o tun0 -j ACCEPT
    
    # Log any packets which don't fit the rules above.
    # (optional but useful)
    -A INPUT -m limit --limit 3/min -j LOG --log-prefix "iptables_INPUT_denied: " --log-level 4
    -A FORWARD -m limit --limit 3/min -j LOG --log-prefix "iptables_FORWARD_denied: " --log-level 4
    -A OUTPUT -m limit --limit 3/min -j LOG --log-prefix "iptables_OUTPUT_denied: " --log-level 4
    
    # then reject them.
    -A INPUT -j REJECT
    -A FORWARD -j REJECT
    -A OUTPUT -j REJECT
    
    COMMIT
  5. You will disable IPv6 in the next section, so add an ip6tables ruleset to reject all IPv6 traffic:

    cat >> /etc/iptables/rules.v6 << END
    *filter
    
    -A INPUT -j REJECT
    -A FORWARD -j REJECT
    -A OUTPUT -j REJECT
    
    COMMIT
    END
    
  6. Activate the rulesets immediately and verify:

    iptables-restore < /etc/iptables/rules.v4
    ip6tables-restore < /etc/iptables/rules.v6
    

    You can see your loaded rules with sudo iptables -S. For more specialized firewall rules, see /usr/share/doc/openvpn/examples/sample-config-files/firewall.sh on your server.

  7. Load the rulesets into iptables-persistent. Answer Yes when asked if you want to save the current IPv4 and IPv6 rules.

    dpkg-reconfigure iptables-persistent
    

Disable IPv6

If you are exclusively using IPv4 on your VPN, IPv6 should be disabled unless you have a specific reason not to do so.

  1. Add the following kernel parameters for systemd-sysctl to set on boot:

    cat >> /etc/sysctl.d/99-sysctl.conf << END
    
    net.ipv6.conf.all.disable_ipv6 = 1
    net.ipv6.conf.default.disable_ipv6 = 1
    net.ipv6.conf.lo.disable_ipv6 = 1
    net.ipv6.conf.eth0.disable_ipv6 = 1
    END
    
  2. Activate them immediately:

    sysctl -p
    
  3. Comment out the line for IPv6 resolution over localhost in /etc/hosts:

    /etc/hosts
    1
    #::1     localhost ip6-localhost ip6-loopback

Install and Begin Configuring OpenVPN

  1. Install OpenVPN from the OpenVPN Project’s repository. Choose OpenVPN’s repo over Debian’s so that you’ll always have the most up to date build for Debian Stretch:

    wget -O - https://swupdate.openvpn.net/repos/repo-public.gpg|apt-key add -
    echo "deb http://build.openvpn.net/debian/openvpn/stable stretch main" > /etc/apt/sources.list.d/openvpn-aptrepo.list
    apt update && apt install openvpn
    
  2. By default, OpenVPN runs as root. The user nobody given in the server.conf template has fewer privileges than root. If nobody gets compromised, the intruder will have access to processes running as the nobody user. This can include Apache when using mod_php, various NFS mounts and some cron jobs. Forcing OpenVPN to run as its own exclusive user and group is a good way to isolate it from other processes, especially if you will also be hosting a web or file server on the same host as your VPN.

    Create a new user account and group for the OpenVPN daemon to run as after startup. The name ovpn is used as an example.

    adduser --system --shell /usr/sbin/nologin --no-create-home ovpn
    groupadd ovpn
    usermod -g ovpn ovpn
    
  3. Require a matching HMAC signature for all packets involved in the TLS handshake between the server and connecting clients. Packets without this signature are dropped. To generate the HMAC signature file:

    openvpn --genkey --secret /etc/openvpn/server/ta.key
    
  4. Generate Diffie-Hellman parameter. This is a set of randomly generated data used when establishing Perfect Forward Secrecy during creation of a client’s session key. The default size is 2048 bits, but OpenVPN’s documentation recommends to use a prime size equivalent to your RSA key size. Since you will be using 4096 bit RSA keys, create a 4096 bit DH prime. Depending on the size of your Linode, this could take approximately 10 minutes to complete.

    openssl genpkey -genparam -algorithm DH -out /etc/openvpn/server/dhp4096.pem -pkeyopt dh_paramgen_prime_len:4096
    
    Note
    According to OpenSSL’s man page, genpkey -genparam supersedes dhparam.
  5. Exit from the root shell and back to your standard user account.

VPN Certificate Authority

Client certificates and keys should be not be managed directly on your VPN server. They should be created locally on a computer and stored offline. For the best quality entropy, they should be created on a computer which has a powerful CPU. You should avoid doing this on a virutal machine.

You can generate certificates and keys two ways: by using EasyRSA scripts, or by creating your own public key infrastructure for your VPN, which includes customizations not included in the default OpenSSL configuration file.

The rest of this guide will use EasyRSA.

Configure EasyRSA

  1. If you’re using a local Unix-based operating system, install the package easy-rsa onto your local computer (CentOS will first need the EPEL repositories installed). For OS X or Windows, grab the .zip archive from the project’s Github repository.

  2. Make your a root directory for the Certificate Authority. The location of your CA is arbitrary, but for the purposes of this guide make a folder called ca in your user’s home directory as an example. The certificates are created from that directory.

    make-cadir ~/ca && cd ~/ca
    
  3. Create a symbolic link from openssl-1.1.0.cnf to openssl.cnf:

    ln -s openssl-1.0.0.cnf openssl.cnf
    
  4. The vars file created in /ca contains presets used by EasyRSA. Here you can specify a distinguished name for your certificate authority that will be passed to client certificates. Changing these fields is optional, and you can always input them manually during certificate creation, but setting them here creates less work during client certificate creation.

    ~/ca/vars
    123456789
    # These are the default values for fields
    # which will be placed in the certificate.
    # Don't leave any of these fields blank.
    export KEY_COUNTRY="US"
    export KEY_PROVINCE="CA"
    export KEY_CITY="SanFrancisco"
    export KEY_ORG="Fort-Funston"
    export KEY_EMAIL="me@myhost.mydomain"
    export KEY_OU="MyOrganizationalUnit"
  5. From ~/ca, source the vars script:

    source ./vars
    

    This will return:

    NOTE: If you run ./clean-all, I will be doing a rm -rf on /home/user/ca/keys
    
  6. Run the clean-all script to create the keys directory and its containing files:

    ./clean-all
    

Server Credentials

  1. A root certificate, sometimes called a Certificate Authority, is the certificate and key pair that will be used to generate key pairs for clients and intermediate authorities (on this VPN server there are none). At each prompt, add or edit the information to be used in your certificate, or leave them blank. Use your VPN server’s hostname or some other identifier as the Common Name and leave the challenge password blank.

    ./build-ca
    
  2. Then create the server’s private key, add or edit the information prompts as needed:

    ./build-key-server server
    

    When you’ve completed the question section for the private key, confirm the signing of the certificate and the certificate requests certified by answering yes to those two questions.

  3. Upload the server credentials to your Linode, using scp from your local computer:

    scp ./keys/{ca.crt,server.crt,server.key} root@<your_linode's_IP>:/etc/openvpn/server
    
  4. You’ll also want a copy of the HMAC key you created earlier to distribute to each client device:

    scp root@<your_linode's_IP>:/etc/openvpn/server/ta.key ./keys
    

Client Credentials

Each client device connecting to the VPN should have its own unique key and identifier (client1, client2, etc.). All other certificate information can remain the same and be shared across all client devices. If you need to add users at any time later, just repeat this step using a different client name.

cd ~/ca && source ./vars && ./build-key client1
Note
Anyone with access to client1.key will be able to access your VPN. To better protect against this scenario, you can issue ./build-key-pass client1 instead to build a client key which is encrypted with a passphrase.

OpenVPN Configuration Files

Server Configuration File

OpenVPN’s server-side configuration file is /etc/openvpn/server.conf. Use the contents below to create a new file at that location on your server:

/etc/openvpn/server.conf
 1 2 3 4 5 6 7 8 910111213141516171819202122232425262728293031323334353637383940414243444546
dev tun
persist-key
persist-tun
topology subnet
port 1194
proto udp
keepalive 10 120

# Location of certificate authority's cert.
ca /etc/openvpn/server/ca.crt

# Location of VPN server's TLS cert.
cert /etc/openvpn/server/server.crt

# Location of server's TLS key
key /etc/openvpn/server/server.key

# Location of DH parameter file.
dh /etc/openvpn/server/dhp4096.pem

# The VPN's address block starts here.
server 10.89.0.0 255.255.255.0

explicit-exit-notify 1

# Drop root privileges and switch to the `ovpn` user after startup.
user ovpn

# OpenVPN process is exclusive member of ovpn group.
group ovpn

# Cryptography options. We force these onto clients by
# setting them here and not in client.ovpn. See
# `openvpn --show-tls`, `openvpn --show-ciphers` and
#`openvpn --show-digests` for all supported options.
tls-crypt /etc/openvpn/server/ta.key
auth SHA512    # This needs to be in client.ovpn too though.
tls-version-min 1.2
tls-cipher TLS-DHE-RSA-WITH-AES-256-GCM-SHA384:TLS-DHE-RSA-WITH-AES-256-CBC-SHA256
ncp-ciphers AES-256-GCM:AES-256-CBC

# Logging options.
ifconfig-pool-persist ipp.txt
status openvpn-status.log
log /var/log/openvpn.log
verb 3
Note

You can extract a server template from OpenVPN’s sample configuration files using:

gunzip -c /usr/share/doc/openvpn/examples/sample-config-files/server.conf.gz > /etc/openvpn/server.conf

Client Configuration File

OpenVPN’s client-side configuration file is client.ovpn. When you import an OpenVPN profile, the location of the directory where the credentials are stored doesn’t matter, but this .ovpn file needs to be in the same directory as the client certificate and all other credentials. OpenVPN does not refer to any of these files after importing and they do not need to remain on the client system. Create this file on your Linode so that it can be distributed to your client devices:

client.ovpn
 1 2 3 4 5 6 7 8 91011121314151617181920212223242526272829
# No cryptography options are specified here because we want
# the VPN server to push those settings to clients rather than
# allow clients to dictate their crypto.

client
dev tun
persist-key
persist-tun
proto udp
nobind
user ovpn
group ovpn
remote-cert-tls server
auth SHA512
verb 3

# Remote server's IP address and port. IP is
# preferable over hostname so as not to rely
# on DNS lookups.
remote <your_linode's IP address> 1194

# To successfully import this profile, you
# want the client device's CA certificate copy,
# client certificate and key, and HMAC signature
# all in the same location as this .ovpn file.
ca ca.crt
cert client1.crt
key client1.key
tls-crypt ta.key
Note

You can use a client template from OpenVPN’s sample configuration files using the command below. Most clients require a .ovpn file format instead of .conf.

cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf /etc/openvpn/client/client.ovpn

Distribute Credentials

Each client device needs to contain the following files:

  • client1.key # Exclusive to this device.
  • client1.cert # Exclusive to this device.
  • CA.pem # Is shared among server and client devices.
  • ta.key # Is shared among server and client devices.
  • client.ovpn # Is shared among client devices.

Initial Startup and Log Monitoring

Start the OpenVPN daemon and enable it on reboot:

sudo systemctl enable openvpn.* && sudo systemctl start openvpn.*
Note
This will scan the /etc/openvpn directory on the server for files with a .conf extension. For every file that it finds, it will spawn a VPN daemon (server instance) so make sure you don’t have a client.conf or client.ovpn file in there.

The logs of both the OpenVPN client and servers will contain all the information you need to confirm connection specifications, view client address assignments, and debug connection issues. Even if the connection completes without problems, the logs may contain alerts and messages, to aid in improving your setup.

To check the status of the OpenVPN process:

sudo systemctl status openvpn*

That should return:

● openvpn.service - OpenVPN service
   Loaded: loaded (/lib/systemd/system/openvpn.service; enabled; vendor preset: enabled)
  Active: active (exited) since Tue 2017-08-15 17:05:53 UTC; 3min 13s ago
 Process: 3786 ExecStart=/bin/true (code=exited, status=0/SUCCESS)
Main PID: 3786 (code=exited, status=0/SUCCESS)
Tasks: 0 (limit: 4915)
CGroup: /system.slice/openvpn.service

Aug 15 17:05:53 debian systemd[1]: Starting OpenVPN service...
Aug 15 17:05:53 debian systemd[1]: Started OpenVPN service.

● openvpn@server.service - OpenVPN connection to server
   Loaded: loaded (/lib/systemd/system/openvpn@.service; disabled; vendor preset: enabled)
  Active: active (running) since Tue 2017-08-15 17:05:53 UTC; 3min 13s ago
 Docs: man:openvpn(8)
       https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage
       https://community.openvpn.net/openvpn/wiki/HOWTO
 Process: 3793 ExecStart=/usr/sbin/openvpn --daemon ovpn-server --status /run/openvpn/server.status 10 --cd /etc/openvpn --config /etc/openvpn/server.conf --writepid /run/openvpn/server.pid (code=exited, status=0/SU)
Main PID: 3795 (openvpn)
Tasks: 1 (limit: 4915)
CGroup: /system.slice/system-openvpn.slice/openvpn@server.service
       └─3795 /usr/sbin/openvpn --daemon ovpn-server --status /run/openvpn/server.status 10 --cd /etc/openvpn --config /etc/openvpn/server.conf --writepid /run/openvpn/server.pid

Aug 15 17:05:53 debian systemd[1]: Stopped OpenVPN connection to server.
Aug 15 17:05:53 debian systemd[1]: Starting OpenVPN connection to server...
Aug 15 17:05:53 debian systemd[1]: Started OpenVPN connection to server.

Use sudo journalctl -f | grep vpn to monitor the logs of your OpenVPN server in realtime; press Control+C to stop monitoring. The command sudo journalctl -xe | grep openvpn can also be useful for troubleshooting.

Next Steps

You should now have an operational OpenVPN server and a set of certificate/key pairs for your desired client devices. If you intend to use your OpenVPN server as an extension of your local network, or for hosting services you want to access from your LAN, you would need to configure the specific applications for your use.

If you want your VPN server to forward and receive traffic to/from the internet on behalf of VPN clients, see part two of this series: Tunnel Your Internet Trafic Through an OpenVPN Server. To set up the connecting client devices, see part three: Configuring OpenVPN Client Devices.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

See Also

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