Paranoid Penguin - Linux VPNs with OpenVPN, Part II
What about Static Keys?
Conspicuously absent from my OpenVPN examples are static keys (also known as pre-shared secret keys), which provide a method for authenticating OpenVPN tunnels that is, arguably, simpler to use than the RSA certificates described herein. Why?
An OpenVPN shared secret takes the form of a small file containing cryptographically generated random data that is highly, highly infeasible for an external attacker to guess via some sort of dictionary or brute-force attack. However, unlike WPA or IPsec shared secrets, an OpenVPN shared secret is used as a de facto session encryption key for every instance of every tunnel that uses it; it is not used to generate other, temporary, session keys that change over time.
For this reason, if attackers were to collect encrypted OpenVPN packets from, say, four different OpenVPN sessions between the same two endpoints and were to later somehow obtain a copy of that tunnel's shared secret file, they would be able to decrypt all packets from all four captured sessions.
In contrast, if you instead authenticate your OpenVPN tunnel with RSA certificates, OpenVPN uses the certificates dynamically to re-key the tunnel periodically—not just every time the tunnel is established, but even during the course of a single tunnel session. Furthermore, even if attackers somehow obtain both RSA certificates and keys used to key that tunnel, they can't easily decrypt any prior captured OpenVPN session (which would involve reconstructing the entire key negotiation process for every session key used in a given session), although they easily can initiate new sessions themselves.
So in summary, although it is a modest hassle to set up a CA and generate RSA certificates, in my opinion, using RSA certificates provides an increase in security that is much more significant than the simplicity of using shared secrets.
Let's get to work configuring the server. Here, I explain how to create a configuration file that puts OpenVPN into “server” mode, authenticates a single client by checking its RSA certificate for a valid CA signature, transparently generates dynamic session keys, establishes the tunnel, and then pushes settings back to the client that give the client a route back to the home network. And, let's even force the client to use the tunnel (and therefore the home network) as its default route back to the outside world, which is a potent protection against DNS spoofing and other attacks you otherwise might be vulnerable to when using an untrusted network.
Configuring OpenVPN consists of creating a configuration file for each OpenVPN listener you want to run and creating any additional files (certificates and so forth) referenced by that file. Prior to OpenVPN 2.0, you needed one listener per tunnel. If ten people needed to connect to your OpenVPN server concurrently, they'd each connect to a different UDP or TCP port on the server.
OpenVPN as of version 2.0, however, is multithreaded, and running in “server” mode, multiple clients can connect to the same TCP or UDP port using the same tunnel profile (that is, you can't have some users authenticate via TLS certificates and other users authenticate via shared secret on the same port). You still need to designate different ports for different tunnel configurations.
Even though the example scenario involves only one client, which would be amply served by a “peer-to-peer” OpenVPN tunnel, it really isn't any more complicated to use a “server mode” tunnel instead (that, again, you can use to serve multiple clients after changing only one line). As far as I can tell, using server mode for a single user doesn't seem to have any noticeable performance cost either. In my testing, even the relatively computationally intensive RSA public key routines involved in establishing my tunnels completed very rapidly.
Listing 1 shows a tunnel configuration file, server.ovpn, for our home network's OpenVPN server.
Listing 1. Server's server.ovpn File
port 1194 proto udp dev tun ca 2.0/keys/ca.crt cert 2.0/keys/server.crt key 2.0/keys/server.key # This file should be kept secret dh 2.0/keys/dh1024.pem tls-auth 2.0/keys/ta.key 0 server 10.31.33.0 255.255.255.0 ifconfig-pool-persist ipp.txt push "redirect-gateway def1 bypass-dhcp" keepalive 10 120 cipher BF-CBC # Blowfish (default) comp-lzo max-clients 2 user nobody group nogroup persist-key persist-tun status openvpn-status.log verb 3 mute 20
Let's walk through Listing 1's settings. port obviously designates this listener's port number. In this case, it's OpenVPN's default of 1194. proto specifies that this tunnel will use fast, connectionless UDP packets rather than slower but more reliable TCP packets (the other allowable value being tcp). Note that OpenVPN uses information in its UDP data payloads to maintain tunnel state. Even though UDP is by definition a “stateless” protocol, the OpenVPN process on either end of an OpenVPN UDP tunnel can detect dropped packets and request the other side to retransmit them.
dev sets the listener to use the Linux kernel's /dev/tun (tun) special device rather than /dev/tap (which would be specified by tap). Whereas the tap device is used to encapsulate entire Ethernet frames, the tun device encapsulates only IPv4 or IPv6 packets. In other words, the tap device tunnels all network traffic regardless of protocol (IPX/SPX, Appletalk, Netbios, IP). For this example, let's stick to the tun device; this will be an IP-only tunnel.
Next, there is the RSA certificate information: ca, cert and key, which specify the respective paths of a CA certificate, the server's certificate and the server's private key. The CA certificate is used to validate client certificates. If the certificate presented by a client contains a valid signature corresponding to the CA certificate, tunnel authentication succeeds. The server key is used during this authentication transaction and also, subsequently, during key negotiation transactions.
Note that certificate files are public information and as such don't need highly restrictive file permissions, but key files must be kept secret and should be root-readable only. Never transmit any key file over any untrusted channel! Note also that all paths in this configuration file are relative to the configuration file itself. If the file resides in /etc/openvpn, then the ca path 2.0/keys/ca.cert actually expands to /etc/openvpn/2.0/keys/ca.cert.
dh specifies a file containing seed data for the Diffie-Hellman session-key negotiation protocol. This data isn't particularly secret. tls-auth, however, specifies the path to a secret key file used by both server and client dæmons to add an extra level of validation to all tunnel packets (technically, “authentication”, as in “message authentication” rather than “user authentication”). Although not necessary for the tunnel to work, I like tls-auth because it helps prevent replay attacks.
Before I go any further explaining Listing 1, let's generate the files I just described. The first three, ca, cert and key, require a PKI, but like I mentioned, OpenVPN includes scripts to simplify PKI tasks. On my Ubuntu systems, these scripts are located in /usr/share/doc/openvpn/examples/easy-rsa/2.0. Step one in creating a PKI is to copy these files to /etc/openvpn, like so:
bash-$ cd /usr/share/doc/openvpn/examples/easy-rsa bash-$ su bash-# cp -r 2.0 /etc/openvpn
Notice that contrary to preferred Ubuntu/Debian practice, I “su-ed” to root. This is needed to create a PKI, a necessarily privileged set of tasks.
Step two is to customize the file vars, which specifies CA variables. First, change your working directory to the copy of easy-rsa you just created, and open the file vars in vi:
bash-# cd /etc/openvpn/2.0 bash-# vi vars
Here are the lines I changed in my own vars file:
export KEY_COUNTRY="US" export KEY_PROVINCE="MN" export KEY_CITY="Saint Paul" export KEY_ORG="Wiremonkeys" export KEY_EMAIL="email@example.com"
Next, initialize your new PKI environment:
bash-# source ./vars bash-# ./clean-all bash-# ./build-dh
And now, finally, you can create some certificates. First, of course, you need the CA certificate and key itself, which will be necessary to sign subsequent keys:
bash-# ./pkitool --initca
The output of that command consists of the files keys/ca.crt and keys/ca.key. By the way, if you want pkitool's output files to be written somewhere besides the local directory keys, you can specify a different directory in the file vars via the variable KEY_DIR.
Next, generate your OpenVPN server certificate:
bash-# ./pkitool --server server
This results in two files: keys/server.crt and keys/server.key. There's nothing magical about the last parameter in the above command, which is simply the name of the server certificate; to name it chuck (resulting in keys/chuck.crt and keys/chuck.key), you'd use ./pkitool --server chuck.
Last comes the client certificate. Unlike the server certificate, whose key may need to be used by some unattended dæmon process, we expect client certificates to be used by human beings. Therefore, let's create a client certificate with a password-protected (encrypted) key file, like so:
bash-# ./pkitool --pass minion
You'll be prompted twice for the key file's passphrase, which will be used to encrypt the file keys/minion.key (keys/minion.crt also will be created by not password-protected). If minion.key falls into the wrong hands, it won't be usable unless the thief also knows its password. However, this also means that every time you use this certificate, you'll be prompted for the key file's password, which I think is a reasonable expectation for VPN clients.
Now that you've got a working PKI set up, all you'll need to do to generate additional client certificates is repeat that last command, but with different certificate names, for example ./pkitool --pass minion102.
Warning: be careful about how you transmit client certificates and keys to end users! Unencrypted e-mail is a poor choice for this task. You should instead use scp, sftp or some other secure file-transfer protocol, or even transport them manually with a USB drive. Once the client certificate and key have been copied where they need to go (for example, /etc/openvpn/keys on the client system), make sure the key file is root-readable only! Erase any temporary copies of this file you may have made in the process of transporting it—for example, on a USB drive.
The OpenVPN server does not need local copies of client certificate or key files, though it may make sense to leave the “original” copies of these in the server's /etc/openvpn/2.0/keys directory (in my examples) in the event of users losing theirs due, for example, to a hard drive crash.
In the interest of full disclosure, I should note that contrary to my examples, it is a PKI best practice not to run a PKI (CA) on any system that actually uses the PKI's certificates. Technically, I should be telling you to use a dedicated, non-networked system for this purpose! Personally, I think if all you use this particular PKI for is OpenVPN RSA certificates, if your OpenVPN server is configured securely overall, and you keep all key files root-readable only, you probably don't need to go that far.
Okay, we've got a working PKI and some certificates. That may have been a lengthy explanation, but in my opinion, the process isn't too difficult or unwieldy. It probably will take you less time to do it than it just took you to read about it.
You've got two more files to generate before continuing working down server.ovpn. To generate your Diffie-Hellman seed file (still working as root within the directory /etc/openvpn/2.0), use this command:
bash-# openssl dhparam -out keys/dh1024.pem 1024
And, last of all the supplemental files, generate that TLS-authentication file, like so:
bash-# openvpn --genkey --secret 2.0/keys/ta.key
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