Warning, this article has been created more than 6 month ago. The information it contains may not be up to date. A highly secure OpenVPN 2.4 configuration in 2018 Written by Mirabellette / 04 november 2018 / 17 comments Hello everyone, Introduction Today I would like to talk about OpenVPN. For those who did not know, OpenVPN is a free and open-source software application that implements virtual private network (VPN) techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities * from Wikipedia. It is developed by OpenVPN Incorporation and they offered a service of VPN with the product privatetunnel. Nowadays, there a plenty of OpenVPN tutorial which describe how to install and configure it. Contrary to them, I will try to choose the most secure configuration possible in 2018 of OpenVPN 2.4.0 with Debian 9.5. I will try to describe as much as possible each step of the tutorial in order to be clearly understood. Preliminaries First of all, we have to install the OpenVPN package and some extra tools with the user root apt update apt upgrade apt install -y iptables-persistent openvpn vim sudo Generate certificates Downloads and configuration of EasyRSA To generate certificates, we will use EasyRSA. EasyRSA is command line interface utility to build and manage keys and certificates. You can download the latest version for your desktop here EasyRSA is one the easiest tool to use to generate Certificate for OpenVPN. This article was focused on OpenVPN configuration and not the certificate part. That means I used the recommend certificate generation tool by OpenVPN, EASYRSA. Unfortunately, it appears it does not allow to use the more secure cryptographic digest available today to generate a certificate. If you really want to use them, you need to use OpenSSL directly. An amazing tutorial is available here and could help you. mkdir /tmp/openvpn cd /tmp/openvpn/ wget https://github.com/OpenVPN/easy-rsa/releases/download/v3.0.5/EasyRSA-nix-3.0.5.tgz tar xf EasyRSA-nix-3.0.5.tgz cd EasyRSA-3.0.5 cp vars.example vars vim vars You must now modify the vars file in order to enable elliptic curve mode and improves the hash algorithm uses. Regarding the elliptic curve choosen, it appears that curves and cryptographic tools provided by the National Institute of Standards and Technology (NIST) were deliberately weaken. That's why, I recommend the curve Curve25519 because it is similarly strong than secp256, faster, safer and was not create by the NIST. They are curve which are more secure but they are also dramatically slow. # Enable elliptic crypto mode set_var EASYRSA_ALGO ec # Define the named curve - choose what you like and what is supported - openvpn --show-curves # Impossible to choose Curve25519 with EasyRSA set_var EASYRSA_CURVE secp521r1 # In how many days should the root CA key expire? set_var EASYRSA_CA_EXPIRE 3650 # In how many days should certificates expire? set_var EASYRSA_CERT_EXPIRE 3650 # In how many days should the control happen? set_var EASYRSA_CRL_DAYS 3650 # Define the Cryptographic digest use, unfortunately, only the md5 and sha family is currently available with EasyRSA set_var EASYRSA_DIGEST "sha512" Generate certificates Please write carefully about the Common name you choose for each certificate, especially for the server certificate. This one will be used by the client to verify the server certificate in order to avoid Man in the middle attack. # generate a directory to store all files ./easyrsa init-pki # generate an autority certificate ./easyrsa build-ca nopass # create server key (server.key) and certificate signing request (server.req) ./easyrsa gen-req server nopass # sign server certificate signing request by autority certificate (server.ca) ./easyrsa sign-req server server nopass # create client key (client.key) and certificate signing request (client.req) ./easyrsa gen-req client nopass # sign client certificate signing request by autority certificate (client.ca) ./easyrsa sign-req client client nopass Directory structure # we will now create a more easy to read directory tree mkdir /tmp/openvpn/server/ mkdir /tmp/openvpn/server/certificates cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/server/certificates/ca.crt cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/server.crt /tmp/openvpn/server/certificates/server.crt cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/server.key /tmp/openvpn/server/certificates/server.key mkdir /tmp/openvpn/client/ mkdir /tmp/openvpn/client/certificates cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/client/certificates/ca.crt cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/client.crt /tmp/openvpn/client/certificates/client.crt cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/client.key /tmp/openvpn/client/certificates/client.key Network configuration Let's consider that the ssh server listens the 22 port and 443 for the VPN server. Firewall rules We have to define firewall rules in the file /etc/iptables/rules.v4: *nat :PREROUTING ACCEPT [0:0] :INPUT ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :POSTROUTING ACCEPT [0:0] # Forward the VPN traffic to eth0 -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE COMMIT *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 TCP traffic. -A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 443 -j ACCEPT -A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 443 -j ACCEPT # Allow DNS resolution and limited HTTP/S on eth0. # Necessary for updating the server and keeping time. -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 # Allow traffic on the TUN interface. -A INPUT -i tun0 -j ACCEPT -A OUTPUT -o tun0 -j ACCEPT # then reject them. -A INPUT -j REJECT -A OUTPUT -j REJECT COMMIT Be sure to adapt these rules to your needs before applying them. sudo iptables-restore < /etc/iptables/rules.v4 We can check if the rules are correctly implied: sudo iptables -L Enable ipv4 forwarding and disable ipv6 In the file /etc/sysctl.d/99-sysctl.conf, add the following lines to enable ipv4 forwarding and disable ipv6: net.ipv4.ip_forward = 1 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 and then apply the new configuration sudo sysctl -p Remove ipv6 lines in /etc/hosts: #::1 localhost ip6-localhost ip6-loopback Reject ipv6 traffic by editing the file /etc/iptables/rules.v6, it must contains: *filter -A INPUT -j REJECT -A FORWARD -j REJECT -A OUTPUT -j REJECT COMMIT and apply: sudo ip6tables-restore < /etc/iptables/rules.v6 Examples of very secure configuration In order to continue, we will use the highly secure configuration below. Be careful, you need to follow the tutorial until the end to make it workable. Example of server configuration Please copy the following code in /tmp/openvpn/server/server.conf. It is the OpenVPN server configuration. Don't forget to replace local IP_OF_YOUR_OPENVPN_SERVER by your server ip. #/tmp/openvpn/server/server.conf local IP_OF_YOUR_OPENVPN_SERVER dev tun topology subnet proto tcp port 443 server 10.8.0.0 255.255.255.0 tls-server ca /etc/openvpn/server/certificates/ca.crt # crl-verify /etc/openvpn/server/certificates/crl.pem cert /etc/openvpn/server/certificates/server.crt key /etc/openvpn/server/certificates/server.key tls-crypt /etc/openvpn/server/certificates/tls_crypt.key dh none ecdh-curve ED25519 tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384 cipher AES-256-GCM ncp-ciphers AES-256-GCM tls-version-min 1.2 persist-tun compress persist-key keepalive 10 120 user ovpn group ovpn status /var/log/openvpn-status.log log /var/log/openvpn.log push "redirect-gateway" push "dhcp-option DNS 10.8.0.1" push "dhcp-option WINS 10.8.0.1" push "route-ipv6 2000::/3" Example of client configuration Please copy the following code in /tmp/openvpn/client/client.conf. It is the OpenVPN client configuration. Don't forget to replace remote IP_OF_YOUR_OPENVPN_SERVER by your server ip and COMMON_NAME_OF_THE_SERVER_CERTIFICATE by the Common name you gave to the server certificate .cf Generating certificates. # /tmp/openvpn/client client dev tun remote IP_OF_YOUR_OPENVPN_SERVER 443 proto tcp resolv-retry infinite compress nobind verify-x509-name "COMMON_NAME_OF_THE_SERVER_CERTIFICATE" name remote-cert-tls server tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384 cipher AES-256-GCM tls-version-min 1.2 auth-nocache persist-key persist-tun status /var/log/openvpn-status.log log /var/log/openvpn.log verb 3 ca /etc/openvpn/client/certificates/ca.crt cert /etc/openvpn/client/certificates/client.crt key /etc/openvpn/client/certificates/client.key tls-crypt /etc/openvpn/client/certificates/tls_crypt.key Harden OpenVPN configuration This part will describe most of the security parameters chosen in the final configuration. TCP/IP protocol and port listening In order to avoid most of firewall limit, it is highly recommended to switch from udp protocol to tcp. OpenVPN configured with TCP protocol is a little bit slower but it has more chances to be accessible from a network you do not control. You also have to configure the OpenVPN server to listen the port 443. Port 443 is the usual port used by a web server configuration and it is most of the time open in firewall output. Moreover, it will also make your OpenVPN configuration harder to detect because it is not the standard OpenVPN port. The standard port is 1194. proto tcp4 port 443 Diffie-Hellman Diffie–Hellman key exchange is a method of securely exchanging cryptographic keys over a public channel. In OpenVPN, this protocol is used in the first steps of TLS establishment connection. With a key of 1024 bits and lower size, it is vulnerable to Logjam exploits. The authors of the vulnerability recommend using primes of 2048 bits or more as a defense or switching to elliptic-curve Diffie–Hellman. In this tutorial, we use elliptic-curve Diffie–Hellman HMAC signature during TLS handshake Since openVPN 2.4, it is recommended to replace tls-auth by tls-crypt. Mainly because tls-crypt will also encrypt the TLS control channel. That means, to add the following line in server.conf: tls-crypt /etc/openvpn/certificates/ta.key 0 and generate the HMAC key file in the openvpn server directory and client directory: openvpn --genkey --secret /tmp/openvpn/server/certificates/tls_crypt.key cp /tmp/openvpn/server/certificates/tls_crypt.key /tmp/openvpn/client/certificates/tls_crypt.key Persistent tun/tap device and key While your connection might be interrupted and OpenVPN is trying to reconnect, you may be using the default network routes again, bypassing the tunnel. For accessing private networks this might not be a big issue as the network addresses may not be reachable from outside the tunnel, but it may expose information you'd rather keep private like an HTTP request containing cookies. persist-key is not a security option but a problem solving in case OpenVPN restart and the OpenVPN user is not able to read the key file anymore. This parameter avoid this situation. persist-tun persist-key Limited user Probably one of the most important configuration setting. In order to limit the impact of an OpenVPN vulnerability, it is highly recommended to run it with a user with limited rights. To do that, we have to create a user for our OpenVPN applications. This user has limited privileges: adduser --system --shell /usr/sbin/nologin --no-create-home ovpn groupadd ovpn usermod -a -G ovpn ovpn and specify the user in the OpenVPN configuration: user ovpn group ovpn Ciphers and digests In the file /etc/openvpn/server.conf, we have to specify the ciphers and digest we want to use. It looks recommended to use GCM instead of CBC. More information about why here and here tls-version-min 1.2 ncp-ciphers AES-256-GCM:AES-256-CBC tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384 cipher AES-256-GCM ecdh-curve ED25519 dh none We will force to use SHA512 to manage the authentication mechanism. unfortately the best digest available is still sha family. Enabling the auth-nocache parameter will prevent to cache passwords in memory. auth-nocache auth SHA512 Compression algorithms In the OpenVPN version 2.3, it was the LZ0 compression algorithm which was by default. Since OpenVPN version 2.4, the compression algorithm LZ4 is available. Contrary to what the documentation says, it is not the best one available. There is also the version 2, lz4-v2 of it which is available but not documented yet. If security if your only criteria, you should disable this feature. Indeed, a family of vulnerability like Beast, Crime and Voracle exist . Those vulnerabilities could allow an attacker to gain information about an encrypted communication in very specific circumstances. To explain differently, if the attacker knows exactly what he is supposed to receive and capture enough packet with little predictable changes, this could help him to reduce the time required to break the encryption. For me, in the real world, it is really complicated to use this kind of vulnerability. The OpenVPN company recommends to disable compression algorithm and already did it on their products. In the configuration files provide here, it is, of course, disable because we are only focused on security. Feel free to make some performance test with speedtest if you hesitate. Pushing configuration to the client OpenVPN is allowed to push some network rules to the server. The most important is push "redirect-gateway" which forces to route all the client traffic throw the vpn. This parameter replaces the gateway route in the client configuration. A second very useful possibility is to push a route for ipv6 which does not work. This will avoid in case your client is configured to work with ipv6 to make it unusable. push "route-ipv6 2000::/3" Revoke and unrevoke a client Revoke a client Sometime, you want to revoke an access to the vpn to a client. OpenVPN uses Certification Revocation List (CRL) to determine if a certificate is revoked or not. You need to execute the following command: ./easyrsa revoke client ./easyrsa gen-crl Those commands also modify the file index.txt in the directory of /tmp/openvpn/EasyRSA-3.0.5/pki. For our setup and since we had revoked client, this file should be like that. V 893632988188Z CWGT67WFA9QLQ9YZ65VGB8FLJKUKV3JL unknown /CN=server R 943755258824Z Y9BFEB757EFV47Y9ENNBUGJLKGP9KQCD unknown /CN=client As you can see,the first column is V or R. V means valide and R means Revoke. We also have a new file crl.pem in /tmp/openvpn/EasyRSA-3.0.5/pki. It is this file which contains the list of revoked certificate. We have to copy it in a place accessible for the OpenVPN application and modify server.confwith a command. cp /tmp/openvpn/EasyRSA-3.0.5/pki/crl.pem /etc/openvpn/server/certificates/crl.pem chown ovpn:ovpn /etc/openvpn/server/certificates/crl.pem # in server.conf crl-verify /etc/openvpn/server/certificates/crl.pem systemctl restart openvpn Unrevoke a client The certificate revocation list contains is a unique value even if you revoke multiple client certificate. If you want to unrevoke a client, you need to generate a new certificate revocation list. To do that, you need to modify /tmp/OpenVPN/EasyRSA-3.0.5/index.txt and replace the R for the certificate by V then generate a new certificate revocation list and finally replace the crl.pem loaded by OpenVPN. Tuning and performance improvement OpenVPN could be tuned in a lot of way to improve performance. I will not talk a lot about that because it requires an article by his own. You could change for example, the encryption algorithm, the TCP/IP protocol or modify those parameters to improve it. mssfix fragment tun-mtu compression algorithm Feel free to read this page to know more about tuning OpenVPN. Deployement and cleaning Congratulation! The most complicated part is over. We just have now to deploy each directory in the proper place. Directory structure You should have this directory structure in /tmp/openvpn/server/. The server will need the following files to run properly: server.conf certificates/ca.crt certificates/crl.pem certificates/server.key certificates/server.crt certificates/tls_crypt.key You should now have this directory structure in /tmp/openvpn/client/. The client will need the following files to run properly: client.conf certificates/client.crt certificates/client.key tls_crypt.key A reader made a very good comment about the client configuration file. You can summarize in client.conf all the cryptography data required to establish the connection to the OpenVPN server. It makes it easier to transport and manage. To do that, you should remove the line about ca, cert, key and tls-crypt and replace them with the following lines. <ca> --STRIPPED INLINE CA CERT-- </ca> <cert> --STRIPPED INLINE CERT-- </cert> <key> --STRIPPED INLINE KEY-- </key> <tls-crypt> --STRIPPED INLINE KEY-- </tls-crypt> Last configuration and deployement Before moving directories, we will make them available only for the root user in reading mode. chmod 400 -R /tmp/openvpn/ We will also move the all public and private key to a safe place and move the server directory to the right place: cp -r /tmp/openvpn/server /etc/openvpn/ cp -r /tmp/openvpn/ /etc/ssl/openvpn-pki ln -s /etc/openvpn/server/server.conf /etc/openvpn/server.conf In the client which should be another computer. You just have to copy and paste the directory /tmp/openvpn/client to /etc/openvpn/ and create a symbolic link from /etc/openvpn/client/client.conf to /etc/openvpn/client.conf Running To run OpenVPN with Systemd, you need to modify /etc/default/openvpn by uncommenting AUTOSTART="all" and replacing "all" by the name of configuration file. For example with in the server side, you should replace "all" by "server". systemctl daemon-reload You are now able to run it with systemctl and the following command: systemctl start openvpn You need to do the same thing for the client. Cleaning Be careful, do not forget to remove the directory /tmp/openvpn. It contains all your certificates. Conclusion Social media Thank you for your reading, I hope this article was helpful for you. Don't hesitate to comment and if you think I made a mistake, it will be a pleasure to discuss it. If you find this article interesting, feel free to subscribe to the RSS flux of the blog and to follow me on Mastodon. Sources This article would not have been possible if other people does not share information about it. Their work help me a lot, thank you! Hardening page from openVPN official website Hardening openVPN from linode Hardening openVPN from ester Link to a tutorial of openVPN Link to the best configuration of openvpn with Eliptic Curve Difference between GCM and CBC mode Why persistent tun/tap is usefull Official man page about OpenVPN 2.4 Feedbacks from readers after the article was published Updates: 31/07/2019 : Replace TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384 by TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384. As the root certificate was built with elliptic curve secp521, it is not possible to use it to sign a certificate with the RSA algorithme, only ECDSA.