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IPsec Tunneling Between FreeBSD Hosts

by Mike DeGraw-Bertsch
12/10/2001

IPsec is short for IP security, which is a manner of encrypting and authenticating Internet traffic all the way down to the IP packet level. That means that protocols such as SMTP, which are inherently insecure, can be reliably secured. Inherent to IPv6, IPSec is somewhat bolted onto the side of IPv4. But it works, and it works very well.

There are two key components to IPsec: Authentication Header (AH) and Encapsulating Security Protocol (ESP). AH provides authentication, proving the packet sender really is the sender, and the data really is what they sent. ESP encrypts the payload of the packets, and can also provide authentication services (for more info on authentication via ESP, visit the IPsec working group.)

Both ESP and AH can be used in both transport mode and tunnel mode. Transport mode provides security between two endpoints, while tunnel mode provides VPN-like security (VPNs -- virtual private networks --allow off-site users secure access to other networks). In transport mode, packets' payloads (and, in the case of AH, immutable parts of the header, like source address) are signed and/or secured. The packet is then sent to its destination, where it is validated, decrypted, and then processed as a normal packet. Transport mode is most often used between two hosts. As the origination IP matters in transport mode, the packet cannot go through NAT'd networks.

In tunnel mode, AH or ESP (and sometimes both) secures the entire packet, and treats the result as the payload of a new packet. That packet is sent to the other side of the tunnel, where the payload is validated and decrypted. The original packet is then processed as a normal IP packet, and, if appropriate, is sent to its ultimate destination. Tunnel mode is obviously most useful between two routers or a host and a router. Since the original packet is handled in its entirety, tunnel mode ESP can be passed through NAT'd networks.

Tunnel mode ESP is an excellent way to bring security to insecure portions of a network. For example, I'm writing this article on my laptop in my living room, thanks to my wireless network. Since WEP is mostly worthless, all traffic between my laptop and the Internet is first encrypted and tunneled to my access point (a FreeBSD box). That way, no one can peek at my traffic as it travels through the air. This also insures that no one else can use my wireless connection -- but that's for another time.

Setting up IPSec

IKE (Not the Former US President)

First, you must configure the two hosts to use the Internet Key Exchange. IKE is a protocol that allows IPsec to exchange its bulk encryption keys securely and automagically. In FreeBSD (and NetBSD), IKE is handled by the racoon daemon. Racoon is standard in FreeBSD 4.0 and above. To ensure compatibility, however, it's a good idea to make sure both hosts are running the same version -- install racoon from /usr/ports/security/racoon.

Racoon's configuration is, by default, in /usr/local/etc/racoon/, and consists of racoon.conf and pke.txt (though you'll need to copy them from their .dist brethren on a fresh install.) There is little that needs to be done with the .conf file, though you might want to fiddle with the key lifetimes (found on the lifetime time xxx <min|sec|hour> lines). The shorter the key lifetime, the better the connection security. However, you may find excessively short lifetimes consume many CPU cycles and disrupt connections as keys are renegotiated.

The most important file for this exercise is pke.txt. This file should be chmod 600 and owned by root, otherwise racoon will refuse to read it. It contains pre-shared keys, a little bit of shared secret information that lets racoon setup its initial connection.

Practical example time. We'll set up a secure tunnel between mason (10.0.0.2), the tunnel server, and gluon (10.0.0.77), the tunnel client. Their preshared key is macplusbsdcool. On mason, pke.txt should read:

10.0.0.77     macplusbsdcool

Similarly, on gluon:

10.0.0.2     macplusbsdcool

Now start the daemon on both hosts:

/usr/local/sbin/racoon -f \
/usr/local/etc/racoon/racoon.conf -l /var/log/racoon.log

Policy

Both hosts are now ready to exchange keys... But they don't know they need to yet. The last remaining step is configuring each host's IPsec policy -- the rule that tells the host how and when to apply IPsec -- via the setkey command. setkey takes input either from a file or from the command line. Since we'll always want this tunnel, we'll put the policies into a file, and read them at boot via the command setkey -f /etc/ipsec.rules. (Launching racoon and setkey at boot is an exercise left to the reader, but think /usr/local/etc/rc.d/)

For tunneling, we'll want all the client packets to be secured and tunneled via mason. On the server, all packets to gluon must be secured. Thus, the configuration on gluon should read:

spdadd 10.0.0.77 0.0.0.0/0 any -P out ipsec 
esp/tunnel/10.0.0.77-10.0.0.2/require;
spdadd 0.0.0.0/0 10.0.0.77 any -P in ipsec 
esp/tunnel/10.0.0.2-10.0.0.77/require;

And on mason:

spdadd 0.0.0.0/0 10.0.0.77 any -P out ipsec 
esp/tunnel/10.0.0.2-10.0.0.77/require;
spdadd 10.0.0.77 0.0.0.0/0 any -P in ipsec 
esp/tunnel/10.0.0.77-10.0.0.2/require;

The lines are pretty self-explanatory. spdadd says that you're adding a new IPsec policy (as opposed to an encryption key, for manual IPsec setup -- don't bother going there unless you must). any says the policy applies to any protocol. out and in are the direction the packet is travelling. Finally, esp/.../require; specifies that the kernel should use IPsec in tunnel mode, and tunnel the packets from the first IP to the second IP.

To make all this apply to IPv6, simply use IPv6 addresses.

Apply the policy on both machines by running the above setkey command.

Testing It Out

With racoon running on both hosts, and the policies in place, ping one machine from the other. If everything is set up correctly, there should be a slight pause, then you'll start to see succesful pings.

To make sure your connection is using IPsec, run tcpdump on the client. Now pop around to a few sites outside of just the tunnel server -- to make sure all connections use the tunnel. You should see output like:

13:58:39.045831 gluon > mason: ESP(spi=0x099e806d,seq=0x723)
13:58:39.046566 mason > gluon: ESP(spi=0x0d8d15b6,seq=0x6b2)
13:58:39.187444 gluon > mason: ESP(spi=0x099e806d,seq=0x724) (frag 64877:1480@0+)

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If you see any non-ESP traffic, something is awry. Check to make sure your destination address is 0.0.0.0/0 on the client. If you don't see any ESP traffic, there's obviously a problem. Make sure your policies correspond on both the client and the server. Check pke.txt on both machines, making sure that the keys are identical and the IP addresses are correct. If everything checks out, look through /var/log/racoon.log to see if there are any problems.

One other word of warning -- if you reboot one of the hosts, and suddenly have connectivity problems, flush the keys on both machines by running setkey -F. It's possible for the keys to get out of sync.

Wrapping Up

You should now have a working IPsec tunnel between your client and server. Go outside with your wireless connection, and enjoy the knowledge that no one can snoop your data!

Mike DeGraw-Bertsch is a security and Unix system administration consultant in the Boston, Mass. area. When he's not at a job, writing, hacking with Perl, or playing with his wireless network, he can usually be found playing goal in ice hockey.


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