Forwarding and Split-Tunneling

Overview

In remote access situations clients will usually send all their traffic to the gateway. Below we explain how this traffic can be forwarded and properly routed back to the roadwarriors.

In some situations it might be more desirable to send only specific traffic via the gateway, for instance, to unburden it from forwarding web or even worse, file sharing traffic. Therefore, we also explain how to enable so-called split-tunneling for different clients.

Forwarding Client Traffic

In order to forward traffic to hosts behind the gateway (or hosts on the Internet if split-tunneling is not used), the following option has to be enabled on Linux gateways

sysctl net.ipv4.ip_forward=1
sysctl net.ipv6.conf.all.forwarding=1

This can be added to /etc/sysctl.conf to enable it permanently.

If the firewall on the gateway is rather restrictive, then

connections.<conn>.children.<child>.updown = /usr/libexec/ipsec/_updown iptables

will automatically cause the default _updown script to add rules that allow traffic to be forwarded.

In remote access setups, VPN clients will be assigned a virtual IP address from a configured address pool. The important thing on the gateway side is that all hosts to which the gateway forwards traffic to, have to know that they must route packets to be tunneled back to the remote access clients via the VPN gateway.

Please note that there might be additional considerations when hosting on cloud platforms (e.g. src/dst address checks).

Hosts on the LAN

For hosts on the LAN behind the gateway the following situations are possible:

The virtual IPs are from the subnet behind the gateway

In this situation either the dhcp plugin is used or the gateway assigns virtual IP addresses from a subnet of the whole LAN behind the gateway (distinct from the IP addresses assigned via DHCP to other LAN hosts). If that is the case, the farp plugin must be used so that the hosts behind the gateway may learn that they have to send response packets to the VPN gateway. For IPv6 something similar can be done using Neighbour Discovery Protocol (NDP) proxying.

The virtual IPs are from a distinct subnet / In site-to-site scenarios

If the VPN gateway is the default gateway of the accessed LAN nothing special has to be done. If this is not the case, either add a route on all hosts behind the gateway (manually or e.g. via DHCP option 121), telling them that the subnet from which virtual IP addresses are assigned to roadwarriors or remote subnets in site-to-site scenarios can be reached through the VPN gateway.

Alternatively configure a static route on the actual default gateway that redirects traffic for the virtual subnet to the VPN gateway. It’s also possible to NAT the virtual IPs to the (internal) IP address of the gateway, so that requests from remote clients will look to LAN hosts as if they originated from the gateway (see the next section for notes on setting up a NAT).

If the VPN gateway is not the default gateway of the LAN, ICMP redirects might get returned to hosts if they send traffic destined for the remote hosts/subnets to the VPN gateway, directing them to the default gateway of the LAN (which probably doesn’t work and otherwise might get that traffic out unencrypted). To avoid this, disable sending such ICMP messages by setting

net.ipv4.conf.all.send_redirects=0
net.ipv4.conf.default.send_redirects=0

If the latter option is not set before the network interface comes up, also set the option for the individual interface

net.ipv4.conf.<iface>.send_redirects=0

Hosts on the Internet

If split-tunneling is not used, all client traffic will be sent through the IPsec tunnel. In this scenario 'local_ts = 0.0.0.0/0` is configured on the gateway and remote_ts = 0.0.0.0/0 on the client. Now, the gateway could simply ignore or drop traffic not destined for subnets it doesn’t want the clients to access. But that is probably not what most users expect. It is more likely that they expect being able to continue to surf the web or read their emails while being connected to the VPN.

The situation here is similar to the one for LAN hosts above. If the gateway would simply forward traffic from the virtual subnet to hosts on the Internet, these hosts wouldn’t be able respond because they would send their response to the virtual IP address. What is therefore required are NAT rules so that hosts in the virtual subnet are mapped to at least one IP address of the VPN gateway (which itself could be behind a NAT device too). For hosts on the Internet, traffic from the virtual subnet then would appear to originate from the VPN gateway.

By way of example, let’s assume the gateway assigns virtual IPs from the 10.0.3.0/24 subnet to its roadwarrior clients. The following iptables rules will NAT traffic from that subnet to the gateway’s eth0 interface (this works even for gateways that have only one network interface).

iptables -t nat -A POSTROUTING -s 10.0.3.0/24 -o eth0 -m policy --dir out --pol ipsec -j ACCEPT
iptables -t nat -A POSTROUTING -s 10.0.3.0/24 -o eth0 -j MASQUERADE

The first rule exempts traffic that matches an IPsec policy from the NAT rule. Additional subnets behind the gateway may be listed after -s, like -s 10.0.3.0/24,192.168.88.0/24. The -s option may also be omitted altogether to match all outbound traffic.

General NAT problems

Local firewall stacks generally don’t treat packets with a matching IPsec policy any different from unprotected packets. That means NAT rules also apply to traffic that is supposed to be tunneled.

This often leads to problems, because many hosts have SNAT or MASQUERADE rules set up which change the source IP of the packets, making them not match the negotiated IPsec policies when IPsec processing of outgoing packets happens in the Netfilter packet flow xfrm lookup node. To fix this problem, packets with a matching IPsec policy should skip NAT rules in the POSTROUTING chain of the nat table. This is achieved by inserting a rule that accepts packets with a matching IPsec policy before any NAT rule in the POSTROUTING chain

iptables -t nat -I POSTROUTING -m policy --pol ipsec --dir out -j ACCEPT

Split-Tunneling

With split-tunneling the clients will only send traffic for specific destination subnets to the gateway. For both protocol versions split-tunneling is easy to deploy f traffic selectors (TS) can freely be configured on both peers. In that case you’d simply use specific values for the local_ts and remote_ts options.

Split-Tunneling with IKEv2

With IKEv2, split-tunneling is quite easy to use as the protocol inherently supports the narrowing of the proposed traffic selectors.

For instance if the client proposes 0.0.0.0/0 as a remote traffic selector, i.e. (remote_ts = 0.0.0.0/0), this proposal can be narrowed on the gateway by configuring local_ts = <list of subnets>. Likewise the client may already propose a selective remote TS by configuring a list of subnets with remote_ts which the gateway might simply accept (e.g. if it has configured local_ts = 0.0.0.0/0) or could further narrow the subnets to be tunneled.

While the protocol supports split-tunneling, whether it can actually be used depends on the client. Most remote access clients will propose 0.0.0.0/0 as remote TS, so split-tunneling must be configured explicitly on the gateway. But whether this will actually result in split-tunneling will depend on the client. All strongSwan based clients (Linux, NetworkManager, Android) support this kind of narrowing whereas for Windows clients the situation is as follows:

Windows 7

The client will always allow access to the host’s LAN. So to access e.g. a local printer nothing special has to be done. Since the client always proposes 0.0.0.0/0 as remote TS, the gateway is free to narrow it to a subset. But to make split-tunneling actually work on the client, the Use default gateway on remote network option in the Advanced TCP/IP settings of the VPN connection has to be disabled. Also because a classful route is installed, the virtual IP address has to belong to the remote subnet, otherwise the Disable class based route addition option has to be enabled and routes have to be installed manually.

Windows 8.1 and Windows Server 2012 R2

Microsoft introduced PowerShell cmdlets to configure VPN connections. These provide more options and also allow to configure split tunneling directly (-SplitTunneling option).

Windows 10

Split tunneling is enabled by default but with the same limitations seen since Windows 7, i.e. the virtual IP has to be from the remote subnet or routes have to be added manually, e.g. via the Add-VpnConnectionRoute PowerShell command. To tunnel all traffic via VPN instead, split tunneling has to be disabled explicitly, either by enabling the Use default gateway on remote network setting described above or by using the following PowerShell command

Set-VpnConnection "<Connection Name>" -SplitTunneling 0

Split-Tunneling with IKEv1

IKEv1 does not provide narrowing of traffic selectors by default. That means that the traffic selector configuration usually has to match exactly on both peers. To simplify things, the IKEv1 implementation in the charon daemon does support narrowing of traffic selectors similar to how it is implemented for IKEv2. Unfortunately this is not compatible with many third party implementations.

On the other hand, such clients may support the Unity extensions developed by Cisco. The unity plugin provides strongSwan gateways with a transparent way of assigning narrowed traffic selectors to clients that support these extensions (e.g. racoon as used in Apple products). The attr and attr-sql plugins provides the means to manually configure attributes that enable split-tunneling for Unity-aware clients.

MTU/MSS Issues

It is possible that you encounter MSS/MTU problems when tunneling traffic. This is caused by broken routers dropping ICMP packets and thus breaking PMTUD (Path MTU Discovery). You can work around these issues by lowering the advertised MSS value of TCP with the TCPMSS target in iptables.

Or if you control the router in question, fixing PMTU may be advisable. To do so you need to permit the appropriate ICMP traffic (type 3, destination unreachable, code 4, fragmentation needed - though all of type 3 is usually allowed). In particular one must pay attention to the source address of ICMP messages emitted by the VPN gateway, which will usually be the primary IP address of the gateway’s internal interface, not that of the endpoint experiencing the issue.

The value you set with the TCPMSS target must accommodate for any other overhead introduced by the tunneling protocols in use (e.g. UDP encapsulation of ESP). Google the issue and read the man page of iptables and iptables-extensions if there are any questions about its usage.

The charon.plugins.kernel-netlink.mss and charon.plugins.kernel-netlink.mtu options may be used, too, but the values set there apply to the routes that teh kernel-netlink plugin installs and the impact of them onto the traffic and the behavior of the kernel is currently quite unclear.

Add the following iptables rules on the IKE responder to reduce the MSS (as noted above, the actual values depend on the overhead imposed by the tunneling protocols and the MTU, so it might have to be lower than what is used in the example here):

iptables -t mangle -A FORWARD -m policy --pol ipsec --dir in  -p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360
iptables -t mangle -A FORWARD -m policy --pol ipsec --dir out -p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360

Alternatively you can add the same rules in PREROUTING and POSTROUTING (also in the mangle table). Additionally set net.ipv4.ip_no_pmtu_disc=1 on the server.

In newer kernels the counter XfrmOutStateModeError in /proc/self/net/xfrm_stat is incremented if the kernel detects that a packet would be too large after encapsulation.