Easy-rsa
The first step when setting up OpenVPN is to create a Public Key Infrastructure (PKI). In summary, this consists of:
- a public master Certificate Authority (CA) certificate and a private key;
- a separate public certificate and private key pair for each server;
- a separate public certificate and private key pair for each client.
One can think of the key-based authentication in terms similar to that of how SSH keys work with the added layer of a signing authority (the CA). OpenVPN relies on a bidirectional authentication strategy, so the client must authenticate the server's certificate and in parallel, the server must authenticate the client's certificate. This is accomplished by the 3rd party's signature (the CA) on both the client and server certificates. Once this is established, further checks are performed before the authentication is complete.
Contents
1 Certificate Authority (CA)
For security purposes, it is recommended that the CA machine be separate from the machine running OpenVPN.
On the CA machine, install easy-rsa, initialize a new PKI and generate a CA keypair that will be used to sign certificates:
cd /etc/easy-rsa easyrsa init-pki easyrsa build-ca
2 OpenVPN server files
A functional OpenVPN server requires the following (in alphabetical order):
- The CA's public certificate
- The Diffie-Hellman (DH) parameters file (needed for TLS mode which is recommended).
- The server key pair (a public certificate and a private key).
- The Hash-based Message Authentication Code (HMAC) key.
Upon completing the steps outlined in this article, users will have generated the following files on the server:
- /etc/openvpn/ca.crt
- /etc/openvpn/dh.pem
- /etc/openvpn/servername.crt and /etc/openvpn/servername.key
- /etc/openvpn/ta.key
2.1 CA public certificate
The CA public certificate /etc/easy-rsa/pki/ca.crt generated in the previous step needs to be copied over to the machine that will be running OpenVPN.
On the CA machine:
cp /etc/easy-rsa/pki/ca.crt /tmp chown foo /tmp/ca.crt
$ scp /tmp/ca.crt foo@hostname-of-openvpn-server:/tmp
On the OpenVPN server machine:
mv /tmp/ca.crt /etc/openvpn chown root:root /etc/openvpn/ca.crt
2.2 Server certificate and private key
On the OpenVPN server machine, install easy-rsa and generate a key pair for the server:
cd /etc/easy-rsa easyrsa init-pki easyrsa gen-req servername nopass cp /etc/easy-rsa/pki/private/servername.key /etc/openvpn
This will create two files: /etc/easy-rsa/pki/reqs/servername.req /etc/easy-rsa/pki/private/servername.key
2.3 Diffie-Hellman (DH) parameters file
On the OpenVPN server machine, create the initial dh.pem file:
openssl dhparam -out /etc/openvpn/dh.pem 2048
2.4 Hash-based Message Authentication Code (HMAC) key
On the OpenVPN server machine, create the HMAC key:
openvpn --genkey --secret /etc/openvpn/ta.key
This will be used to add an additional HMAC signature to all SSL/TLS handshake packets. In addition any UDP packet not having the correct HMAC signature will be immediately dropped, protecting against:
- Portscanning.
- DOS attacks on the OpenVPN UDP port.
- SSL/TLS handshake initiations from unauthorized machines.
- Any eventual buffer overflow vulnerabilities in the SSL/TLS implementation.
3 OpenVPN client files
3.1 Client certificate and private key
Any machine can generate client files provided that easy-rsa is installed.
If the pki is not initialized, do so via:
cd /etc/easy-rsa easyrsa init-pki
Generate the client key and certificate:
cd /etc/easy-rsa easyrsa gen-req client1 nopass
This will create two files: /etc/easy-rsa/pki/reqs/client1.req /etc/easy-rsa/pki/private/client1.key
The gen-req set can be repeated as many times as needed for additional clients.
4 Sign the certificates and pass them back to the server and clients
4.1 Obtain and sign the certificates on the CA
The server and client(s) certificates need to be signed by the CA then transferred back to the OpenVPN server/client(s).
On the OpenVPN server (or the box used to generate the certificate/key pairs):
cp /etc/easy-rsa/pki/reqs/*.req /tmp chown foo /tmp/*.req
Securely transfer the files to the CA machine for signing:
$ scp /tmp/*.req foo@hostname-of-CA:/tmp
On the CA machine, import and sign the certificate requests:
cd /etc/easy-rsa easyrsa import-req /tmp/servername.req servername easyrsa import-req /tmp/client1.req client1 easyrsa sign-req server servername easyrsa sign-req client client1
This will create the following signed certificates which can be transferred back to their respective machines: /etc/easy-rsa/pki/issued/servername.crt /etc/easy-rsa/pki/issued/client1.crt
The leftover .req files can be safely deleted:
rm -f /tmp/*.req
4.2 Pass the signed certificates back to the server and client(s)
On the CA machine, copy the signed certificates and transfer them to the server/client(s):
cp /etc/easy-rsa/pki/issued/*.crt /tmp chown foo /tmp/*.crt $ scp /tmp/*.crt foo@hostname-of-openvpn_server:/tmp
On the OpenVPN server, move the certificates in place and reassign ownership:
mv /tmp/servername.crt /etc/openvpn chown root:root /etc/openvpn/servername.crt
The signed client certificate can be stored anywhere since it will be used in the subsequent step of preparing the ovpn client profile file.
mkdir /etc/easy-rsa/pki/signed mv /tmp/client1.crt /etc/easy-rsa/pki/signed
5 Revoking certificates and alerting the OpenVPN server
5.1 Revoke a certificate
Over time, it may become necessary to revoke a certificate thus denying access to the affected user(s). This example revokes the "client1" certificate.
On the CA machine:
cd /etc/easy-rsa easyrsa revoke client1 easyrsa gen-crl
This will produce the CRL file /etc/easy-rsa/pki/crl.pem that needs to be transferred to the OpenVPN server and made active there.
5.2 Alert the OpenVPN server
On the CA machine:
cp /etc/easy-rsa/pki/crl.pem /tmp chown foo /tmp/crl.pem
On the OpenVPN machine, copy crl.pem and inform the server to read it:
mv /tmp/crl.pem /etc/openvpn chown root:root /etc/openvpn/crl.pem
Edit /etc/openvpn/server.conf uncommenting the crl-verify directive, then restart openvpn@server.service to re-read it:
/etc/openvpn/server.conf
. crl-verify /etc/openvpn/crl.pem .