IPsec Tom Eastep Roberto Sanchez 2004 2005 2006 2009 2016 Thomas M. Eastep 2007 Roberto C. Sanchez 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 article applies to Shorewall 4.3 and later. If you are running a version of Shorewall earlier than Shorewall 4.3.5 then please see the documentation for that release. Shorewall does not configure IPsec for you -- it rather configures netfilter to accommodate your IPsec configuration. The information in this article is only applicable if you plan to have IPsec end-points on the same system where Shorewall is used. While this article shows configuration of IPsec using ipsec-tools, Shorewall configuration is exactly the same when using OpenSwan or any of the other Swan derivatives. When running a Linux kernel prior to 2.6.20, the Netfilter+IPsec and policy match support are broken when used with a bridge device. The problem was corrected in Kernel 2.6.20 as a result of the removal of deferred FORWARD/OUTPUT processing of traffic destined for a bridge. See the "Shorewall-perl and Bridged Firewalls" article.
Shorwall and Kernel 2.6 IPsec This is not a HOWTO for Kernel 2.6 IPsec -- for that, please see http://www.ipsec-howto.org/. The 2.6 Linux Kernel introduced new facilities for defining encrypted communication between hosts in a network. The network administrator defines a set of Security Policies which are stored in the kernel as a Security Policy Database (SPD). Security policies determine which traffic is subject to encryption. Security Associations are created between pairs of hosts in the network (one SA for traffic in each direction); these SAs define how traffic is to be encrypted. Outgoing traffic that is to be encrypted according to the contents of the SPD requires an appropriate SA to exist. SAs may be created manually using setkey(8) but most often, they are created by a cooperative process involving the ISAKMP protocol and a daemon included in your IPsec package (StrongSwan, LibreSwan, ipsec-tools/Racoon, etc.) . Incoming traffic is verified against the SPD to ensure that no unencrypted traffic is accepted in violation of the administrator's policies. There are three ways in which IPsec traffic can interact with Shorewall policies and rules: Traffic that is encrypted on the firewall system. The traffic passes through Netfilter twice -- first as unencrypted then encrypted. Traffic that is decrypted on the firewall system. The traffic passes through Netfilter twice -- first as encrypted then as unencrypted. Encrypted traffic that is passed through the firewall system. The traffic passes through Netfilter once. In cases 1 and 2, the encrypted traffic is handled by entries in /etc/shorewall/tunnels (don't be mislead by the name of the file -- transport mode encrypted traffic is also handled by entries in that file). The unencrypted traffic is handled by normal rules and policies. Under the 2.4 Linux Kernel, the association of unencrypted traffic and zones was made easy by the presence of IPsec pseudo-interfaces with names of the form ipsecN (e.g. ipsec0). Outgoing unencrypted traffic (case 1.) was sent through an ipsecN device while incoming unencrypted traffic (case 2) arrived from an ipsecN device. The 2.6 kernel-based implementation does away with these pseudo-interfaces. Outgoing traffic that is going to be encrypted and incoming traffic that has been decrypted must be matched against policies in the SPD and/or the appropriate SA. Shorewall provides support for policy matching in three ways: In /etc/shorewall/masq (/etc/shorewall/snat when running Shorewall 5.0.14 or later), traffic that will later be encrypted is exempted from MASQUERADE/SNAT using existing entries. If you want to MASQUERADE/SNAT outgoing traffic that will later be encrypted, you must include the appropriate indication in the IPSEC column in that file. The /etc/shorewall/zones file allows you to associate zones with traffic that will be encrypted or that has been decrypted. A new option (ipsec) has been provided for entries in /etc/shorewall/hosts. When an entry has this option specified, traffic to/from the hosts described in the entry is assumed to be encrypted. In summary, Shorewall provides the facilities to replace the use of IPsec pseudo-interfaces in zone and MASQUERADE/SNAT definition. There are two cases to consider: Encrypted communication is used to/from all hosts in a zone. The value ipsec is placed in the TYPE column of the /etc/shorewall/zones entry for the zone. By default, encrypted communication is not used to communicate with the hosts in a zone. The value ipv4 is placed in the TYPE column of the /etc/shorewall/zones entry for the zone and the new ipsec option is specified in /etc/shorewall/hosts for any hosts requiring secure communication. For simple zones such as are shown in the following examples, the two techniques are equivalent and are used interchangeably. It is redundant to have ipsec in the TYPE column of the /etc/shorewall/zones entry for a zone and to also have the ipsec option in /etc/shorewall/hosts entries for that zone. Finally, the OPTIONS, IN OPTIONS and OUT OPTIONS columns in /etc/shorewall/zones can be used to match the zone to a particular (set of) SA(s) used to encrypt and decrypt traffic to/from the zone and the security policies that select which traffic to encrypt/decrypt. This article provides guidance regarding configuring Shorewall to use with IPsec. For configuring IPsec itself, consult your IPsec product's documentation.
IPsec Gateway on the Firewall System Suppose that we have the following situation: 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 protocol 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 /etc/shorewall/tunnels — System B: #TYPE ZONE GATEWAY GATEWAY_ZONE ipsec net 206.162.148.9
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.
/etc/shorewall/zones — Systems A and B: #ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS net ipv4 vpn ipv4
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. The hosts file entries below assume that you want the remote gateway to be part of the vpn zone — If you don't wish the remote gateway included, simply omit its IP address from the HOSTS column.
/etc/shorewall/hosts — System A #ZONE HOSTS OPTIONS vpn eth0:10.0.0.0/8,134.28.54.2 ipsec /etc/shorewall/hosts — System B #ZONE HOSTS OPTIONS vpn eth0:192.168.1.0/24,206.162.148.9 ipsec
If you want to keep things simple, you can simply not restrict the set of addresses in the ipsec zones:
#ZONE HOSTS OPTIONS vpn eth0:0.0.0.0/o ipsec
Assuming that you want to give each local network free access to the remote network and vice versa, you would need the following /etc/shorewall/policy entries on each system:
#SOURCE DEST POLICY LEVEL BURST:LIMIT loc vpn ACCEPT vpn loc ACCEPT
If you need access from each firewall to hosts in the other network, then you could add:
#SOURCE DEST POLICY LEVEL BURST:LIMIT $FW vpn ACCEPT
If you need access between the firewall's, you should describe the access in your /etc/shorewall/rules file. For example, to allow SSH access from System B, add this rule on system A:
#ACTION SOURCE DEST PROTO POLICY ACCEPT vpn:134.28.54.2 $FW
If you have hosts that access the Internet through an IPsec tunnel, then it is a good idea to set the MSS value for traffic from those hosts explicitly in the /etc/shorewall/zones file. For example, if hosts in the vpn zone access the Internet through an ESP tunnel then the following entry would be appropriate: #ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS vpn ipsec mode=tunnel mss=1400 You should also set FASTACCEPT=No in shorewall.conf to ensure that both the SYN and SYN,ACK packets have their MSS field adjusted. Note that CLAMPMSS=Yes in shorewall.conf isn't effective with the 2.6 native IPsec implementation because there is no separate IPsec device with a lower mtu as there was under the 2.4 and earlier kernels.
Mobile System (Road Warrior) 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 — System A #ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS net ipv4 vpn ipsec loc ipv4
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:
#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. The VPN zone is defined using the /etc/shorewall/hosts file:
/etc/shorewall/hosts — System A: #ZONE HOSTS OPTIONS vpn eth0:0.0.0.0/0
You will need to configure your through the tunnel policy as shown under the first example above. On the laptop:
/etc/shorewall/zones - System B: #ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS vpn ipsec net ipv4 loc ipv4 /etc/shorewall/tunnels - System B: #TYPE ZONE GATEWAY GATEWAY_ZONE ipsec net 206.162.148.9 vpn /etc/shorewall/hosts - System B: #ZONE HOSTS OPTIONS vpn eth0:0.0.0.0/0
Mobile System (Road Warrior) with Layer 2 Tunneling Protocol (L2TP) This section is based on the previous section. Please make sure that you read it thoroughly and understand it. The setup described in this section is more complex because you are including an additional layer of tunneling. Again, make sure that you have read the previous section and it is highly recommended to have the IPsec-only configuration working first. Additionally, this section assumes that you are running IPsec, xl2tpd and pppd on the same system that is running shorewall. However, configuration of these additional services is beyond the scope of this document. Getting layer 2 tunneling to work is an endeavour unto itself. However, if you succeed it can be very convenient. Reasons why you might want configure layer 2 tunneling protocol (L2TP): You want to give your road warrior an address that is in the same segment as the other hosts on your network. Your road warriors are using a legacy operating system (such as MS Windows or Mac OS X) and you do not want them to have to install third party software in order to connect to the VPN (both MS Windows and Mac OS X include VPN clients which natively support L2TP over IPsec, but not plain IPsec). You like a challenge. Since the target for a VPN including L2TP will (almost) never be a road warrior running Linux, I will not include the client side of the configuration. The first thing that needs to be done is to create a new zone called l2tp to represent the tunneled layer 2 traffic.
/etc/shorewall/zones — System A #ZONE TYPE OPTIONS IN_OPTIONS OUT_OPTIONS et ipv4 vpn ipsec l2tp ipv4 loc ipv4
Since the L2TP will require the use of pppd, you will end up with one or more ppp interfaces (each representing an individual road warrior connection) for which you will need to account. This can be done by modifying the interfaces file. (Modify with additional options as needed.)
/etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS net eth0 detect routefilter loc eth1 192.168.1.255 l2tp ppp+ -
The next thing that must be done is to adjust the policy so that the traffic can go where it needs to go. First, you need to decide if you want for hosts in your local zone to be able to connect to your road warriors. You may or may not want to allow this. For example, one reason you might want to allow this is so that your support personnel can use ssh, VNC or remote desktop to fix a problem on the road warrior's laptop. Second, you need to decide if you want the road warrior to have access to hosts on the local network. You generally want to allow this. For example, if you have DNS servers on your local network that you want the road warrior to use. Or perhaps the road warrior needs to mount NFS shares or needs to access intranet sites which are not visible from the public Internet. Finally, you need to decide if you want the road warriors to be able to access the public Internet. You probably want to do this, unless you are trying to create a situation where when the road warrior connects to the VPN, it is no longer possible to send traffic from the road warrior's machine to the public Internet. Please note that this not really a strong security measure. The road warrior could trivially modify the routing table on the remote machine to have only traffic destined for systems on the VPN local network go through the secure channel. The rest of the traffic would simply travel over an Ethernet or wireless interface directly to the public Internet. In fact, this latter situation is dangerous, as a simple mistake could easily create a situation where the road warrior's machine is acting as a router between your local network and the public Internet, which you certainly do not want to happen. In short, it is best to allow the road warrior to connect to the public Internet by default.
/etc/shorewall/policy: #SOURCE DEST POLICY LOGLEVEL LIMIT $FW all ACCEPT loc net ACCEPT loc l2tp ACCEPT # Allows local machines to connect to road warriors l2tp loc ACCEPT # Allows road warriors to connect to local machines l2tp net ACCEPT # Allows road warriors to connect to the Internet net all DROP info # The FOLLOWING POLICY MUST BE LAST all all REJECT info
The final step is to modify your rules file. There are three important components. First, you must allow the l2tp traffic to reach the xl2tpd process running on the firewall machine. Second, you must add rules to open up ports on the firewall to the road warrior for services which are running on the firewall. For example, if you are running a webserver on the firewall that must be accessible to road warriors. The reason for the second step is that the policy does not by default allow unrestricted access to the firewall itself. Finally, you should protect an exploit where an attacker can exploit your LT2P server due to a hole in the way that L2TP interacts with UDP connection tracking.
/etc/shorewall/rules: #ACTION SOURCE DEST PROTO DPORT SPORT ?SECTION ESTABLISHED # Prevent IPsec bypass by hosts behind a NAT gateway L2TP(REJECT) net $FW REJECT $FW net udp - 1701 ?SECTION NEW # l2tp over the IPsec VPN ACCEPT vpn $FW udp 1701 # webserver that can only be accessed internally HTTP(ACCEPT) loc $FW HTTP(ACCEPT) l2tp $FW HTTPS(ACCEPT) loc $FW HTTPS(ACCEPT) l2tp $FW
Transport Mode In today's wireless world, it is often the case that individual hosts in a network need to establish secure connections with the other hosts in that network. In that case, IPsec transport mode is an appropriate solution. Shorewall configuration goes as follows:
/etc/shorewall/interfaces: #ZONE INTERFACE OPTIONS net eth0 routefilter,dhcp,tcpflags /etc/shorewall/tunnels: #TYPE ZONE GATEWAY GATEWAY # ZONE ipsec net 192.168.20.0/24 loc /etc/shorewall/zones: #ZONE TYPE OPTIONS IN OUT # OPTIONS OPTIONS loc ipsec mode=transport net ipv4 /etc/shorewall/hosts: #ZONE HOST(S) OPTIONS loc eth0:192.168.20.0/24 It is worth noting that although loc is a sub-zone of net, because loc is an IPsec-only zone it does not need to be defined before net in /etc/shorewall/zones. /etc/shorewall/policy: #SOURCE DEST POLICY LOGLEVEL LIMIT $FW all ACCEPT loc $FW ACCEPT net loc NONE loc net NONE net all DROP info # The FOLLOWING POLICY MUST BE LAST all all REJECT info Since there are no cases where net<->loc traffic should occur, NONE policies are used.
IPCOMP If your IPsec tunnel or transport mode connection fails to work with Shorewall started and you see log messages like the following when you try to use the connection, the problem is that ip compression is being used.Feb 18 23:43:52 vpngw kernel: Shorewall:vpn2fw:REJECT:IN=eth2 OUT= MAC=00:e0:81:32:b3:5e:00:18:de:12:e5:15:08:00 SRC=172.29.59.58 DST=172.29.59.254 LEN=85 TOS=0x00 PREC=0x00 TTL=64 ID=25600 DF PROTO=4The solution is to add an IPCOMP tunnel to /etc/shorewall/tunnels as follows:#TYPE ZONE GATEWAY GATEWAY # ZONE ipip vpn 0.0.0.0/0The above assumes that the name of your IPsec vpn zone is vpn. Note that this protocol 4 (IPIP) traffic appears to originate in the vpn zone, but it's source IP address is that of the remote gateway. As a consequence, that address must be included in the definition of the remote zone. If you haven't done that, the traffic will be dropped in the INPUT chain.
Using SNAT to Force Traffic over an IPsec Tunnel Cases can arise where you need to use an IPsec tunnel to access a remote network, but you have no control over the associated security polices. In such cases, the resulting tunnel is accessible from your firewall but not from your local networks. Let's take an example: Remote gateway 192.0.2.26 Remote subnet 172.22.4.0/24 Your public IP address is 192.0.2.199 Your Internet-facing interface is eth0 Your local network is 192.168.219.0/24 You want to access 172.22.4.0/24 from 192.168.219.0/24 The IPsec tunnel is configured between 172.22.4.0/24 and 192.0.2.199 You need to configure as follows. /etc/shorewall/zones: #ZONE TYPE OPTIONS ... vpn ip # Note that the zone cannot be declared as type ipsec ... /etc/shorewall/interfaces: #ZONE INTERFACE OPTIONS net eth0 nets=(!172.22.4.0/24),... # You must exclude the remote network from the net zone /etc/shorewall/hosts: #ZONE HOSTS OPTIONS vpn eth0:172.22.4.0/24 mss=1380,destonly vpn eth0:0.0.0.0/0 mss=1380,ipsec /etc/shorewall/snat: SNAT(192.0.2.199) 192.168.219.0/24 eth0:172.22.4.0/24 /etc/shorewall/tunnels: #TYPE ZONE GATEWAY GATEWAY_ZONE ipsec net 192.0.2.26 vpn