IPSEC Tunnels Tom Eastep 2005-05-29 2001-2005 Thomas M. Eastep Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled GNU Free Documentation License. This documentation is incomplete regarding using IPSEC and the 2.6 Kernel. Netfilter currently lacks full support for the 2.6 kernel's implementation of IPSEC. Until that implementation is complete, only a simple network-network tunnel is described for 2.6. UPDATE: Some distributions such as SuSE are now shipping Kernels and iptables with the IPSEC-Netfilter patches and policy match support. Check this article for information concerning this support and Shorewall.
Preliminary Reading I recommend reading the VPN Basics article if you plan to implement any type of VPN.
Configuring FreeS/Wan and Derivatives Such as OpenS/Wan There is an excellent guide to configuring IPSEC tunnels at http://jixen.tripod.com/. I highly recommend that you consult that site for information about configuring FreeS/Wan. IPSEC and Proxy ARP do not work unless you are running Shorewall 2.0.1 Beta 3 or later or unless you have installed the fix to Shorewall 2.0.0 available from the Errata Page. The documentation below assumes that you have disabled opportunistic encryption feature in FreeS/Wan 2.0 using the following additional entries in ipsec.conf: conn block auto=ignore conn private auto=ignore conn private-or-clear auto=ignore conn clear-or-private auto=ignore conn clear auto=ignore conn packetdefault auto=ignore For further information see http://www.freeswan.org/freeswan_trees/freeswan-2.03/doc/policygroups.html.
IPSec Gateway on the Firewall System Suppose that we have the following sutuation: We want systems in the 192.168.1.0/24 sub-network to be able to communicate with systems in the 10.0.0.0/8 network. We assume that on both systems A and B, eth0 is the internet interface. To make this work, we need to do two things: Open the firewall so that the IPSEC tunnel can be established (allow the ESP and AH protocols and UDP Port 500). Allow traffic through the tunnel. Opening the firewall for the IPSEC tunnel is accomplished by adding an entry to the /etc/shorewall/tunnels file. In /etc/shorewall/tunnels on system A, we need the following /etc/shorewall/tunnels - System A TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 134.28.54.2
In /etc/shorewall/tunnels on system B, we would have: /etc/shorewall/tunnels - System B TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 206.161.148.9
If either of the endpoints is behind a NAT gateway then the tunnels file entry on the other endpoint should specify a tunnel type of ipsecnat rather than ipsec and the GATEWAY address should specify the external address of the NAT gateway. You need to define a zone for the remote subnet or include it in your local zone. In this example, we'll assume that you have created a zone called vpn to represent the remote subnet. Note that you should define the vpn zone before the net zone. /etc/shorewall/zones - Systems A and B ZONE DISPLAY COMMENTS vpn VPN Remote Subnet net Internet The big bad internet
If you are running kernel 2.4:
At both systems, ipsec0 would be included in /etc/shorewall/interfaces as a vpn interface: /etc/shorewall/interfaces - Systems A and B ZONE INTERFACE BROADCAST OPTIONS vpn ipsec0
If you are running kernel 2.6:
It is essential that the vpn zone be declared before the net zone in /etc/shorewall/zones. Remember the assumption that both systems A and B have eth0 as their internet interface. You must define the vpn zone using the /etc/shorewall/hosts file. /etc/shorewall/hosts - System A ZONE HOSTS OPTIONS vpn eth0:10.0.0.0/8
/etc/shorewall/hosts - System B ZONE HOSTS OPTIONS vpn eth0:192.168.1.0/24
In addition, if you are using Masquerading or SNAT on your firewalls, you need to elmiinate the remote network from Masquerade/SNAT. These entries replace your current masquerade/SNAT entries for the local networks. /etc/shorewall/masq - System A INTERFACE SUBNET ADDRESS eth0:!10.0.0.0/8 192.168.1.0/24 ...
/etc/shorewall/masq System B INTERFACE SUBNET ADDRESS eth0:!192.168.1.0/24 10.0.0.0/8 ...
You will need to allow traffic between the vpn zone and the loc zone -- if you simply want to admit all traffic in both directions, you can use the policy file: /etc/shorewall/policy - Systems A and B SOURCE DEST POLICY LOG LEVEL loc vpn ACCEPT vpn loc ACCEPT
Once you have these entries in place, restart Shorewall (type shorewall restart); you are now ready to configure the tunnel in FreeS/WAN.
VPN Hub using Kernel 2.4 Shorewall can be used in a VPN Hub environment where multiple remote networks are connected to a gateway running Shorewall. This environment is shown in this diatram. We want systems in the 192.168.1.0/24 sub-network to be able to communicate with systems in the 10.0.0.0/16 and 10.1.0.0/16 networks and we want the 10.0.0.0/16 and 10.1.0.0/16 networks to be able to communicate. To make this work, we need to do several things: Open the firewall so that two IPSEC tunnels can be established (allow the ESP and AH protocols and UDP Port 500). Allow traffic through the tunnels two/from the local zone (192.168.1.0/24). Deny traffic through the tunnels between the two remote networks. Opening the firewall for the IPSEC tunnels is accomplished by adding two entries to the /etc/shorewall/tunnels file. In /etc/shorewall/tunnels on system A, we need the following /etc/shorewall/tunnels system A TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 134.28.54.2 ipsec net 130.152.100.14
In /etc/shorewall/tunnels on systems B and C, we would have: /etc/shorewall/tunnels system B & C TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 206.161.148.9
If either of the endpoints is behind a NAT gateway then the tunnels file entry on the other endpoint should specify a tunnel type of ipsecnat rather than ipsec and the GATEWAY address should specify the external address of the NAT gateway. On each system, we will create a zone to represent the remote networks. On System A: /etc/shorewall/zones system A ZONE DISPLAY COMMENTS vpn1 VPN1 Remote Subnet on system B vpn2 VPN2 Remote Subnet on system C
On systems B and C: /etc/shorewall/zones system B & C ZONE DISPLAY COMMENTS vpn VPN Remote Subnet on system A
At system A, ipsec0 represents two zones so we have the following in /etc/shorewall/interfaces: /etc/shorewall/interfaces system A ZONE INTERFACE BROADCAST OPTIONS - ipsec0
The /etc/shorewall/hosts file on system A defines the two VPN zones: /etc/shorewall/hosts system A ZONE HOSTS OPTIONS vpn1 ipsec0:10.0.0.0/16 vpn2 ipsec0:10.1.0.0/16
At systems B and C, ipsec0 represents a single zone so we have the following in /etc/shorewall/interfaces: /etc/shorewall/interfaces system B & C ZONE INTERFACE BROADCAST OPTIONS vpn ipsec0
On systems A, you will need to allow traffic between the vpn1 zone and the loc zone as well as between vpn2 and the loc zone -- if you simply want to admit all traffic in both directions, you can use the following policy file entries on all three gateways: /etc/shorewall/policy system A SOURCE DEST POLICY LOG LEVEL loc vpn1 ACCEPT vpn1 loc ACCEPT loc vpn2 ACCEPT vpn2 loc ACCEPT
On systems B and C, you will need to allow traffic between the vpn zone and the loc zone -- if you simply want to admit all traffic in both directions, you can use the following policy file entries on all three gateways: /etc/shorewall/policy system B & C SOURCE DEST POLICY LOG LEVEL loc vpn ACCEPT vpn loc ACCEPT
Once you have the Shorewall entries added, restart Shorewall on each gateway (type shorewall restart); you are now ready to configure the tunnels in FreeS/WAN. to allow traffic between the networks attached to systems B and C, it is necessary to simply add two additional entries to the /etc/shorewall/policy file on system A. /etc/shorewall/policy system A SOURCE DEST POLICY LOG LEVEL vpn1 vpn2 ACCEPT vpn2 vpn1 ACCEPT
If you find traffic being rejected/dropped in the OUTPUT chain, place the names of the remote VPN zones as a comma-separated list in the GATEWAY ZONE column of the /etc/shorewall/tunnels file entry.
Mobile System (Road Warrior) Using Kernel 2.4 Suppose that you have a laptop system (B) that you take with you when you travel and you want to be able to establish a secure connection back to your local network. Road Warrior VPN You need to define a zone for the laptop or include it in your local zone. In this example, we'll assume that you have created a zone called vpn to represent the remote host. /etc/shorewall/zones local ZONE DISPLAY COMMENTS vpn VPN Remote Subnet
In this instance, the mobile system (B) has IP address 134.28.54.2 but that cannot be determined in advance. In the /etc/shorewall/tunnels file on system A, the following entry should be made: /etc/shorewall/tunnels system A TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 0.0.0.0/0 vpn
the GATEWAY ZONE column contains the name of the zone corresponding to peer subnetworks. This indicates that the gateway system itself comprises the peer subnetwork; in other words, the remote gateway is a standalone system. You will need to configure /etc/shorewall/interfaces and establish your through the tunnel policy as shown under the first example above.
Dynamic RoadWarrior Zones Beginning with Shorewall release 1.3.10, you can define multiple VPN zones and add and delete remote endpoints dynamically using /sbin/shorewall. With Shorewall 2.0.2 Beta 1 and later versions, this capability must be enabled by setting DYNAMIC_ZONES=Yes in shorewall.conf. In /etc/shorewall/zones: /etc/shorewall/zones ZONE DISPLAY COMMENTS vpn1 VPN-1 First VPN Zone vpn2 VPN-2 Second VPN Zone vpn3 VPN-3 Third VPN Zone
In /etc/shorewall/tunnels: /etc/shorewall/tunnels TYPE ZONE GATEWAY GATEWAY ZONE ipsec net 0.0.0.0/0 vpn1,vpn2,vpn3
When Shorewall is started, the zones vpn[1-3] will all be empty and Shorewall will issue warnings to that effect. These warnings may be safely ignored. FreeS/Wan may now be configured to have three different Road Warrior connections with the choice of connection being based on X-509 certificates or some other means. Each of these connectioins will utilize a different updown script that adds the remote station to the appropriate zone when the connection comes up and that deletes the remote station when the connection comes down. For example, when 134.28.54.2 connects for the vpn2 zone the up part of the script will issue the command: /sbin/shorewall add ipsec0:134.28.54.2 vpn2 and the down part will: /sbin/shorewall delete ipsec0:134.28.54.2 vpn2
Limitations of Dynamic Zones If you include a dynamic zone in the exclude list of a DNAT rule, the dynamically-added hosts are not excluded from the rule. dyn=dynamic zone ACTION SOURCE DESTINATION PROTOCOL PORT(S) CLIENT PORT(S) ORIGINAL DESTINATION DNAT z!dyn loc:192.168.1.3 tcp 80 Dynamic changes to the zone dyn will have no effect on the above rule.