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Introduction to strongSwan: Forwarding and Split-Tunneling

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 this 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, leftfirewall=yes will automatically cause the default updown script to add
rules that allow traffic to be forwarded.

In remote access situations clients will be assigned a virtual IP address from a configured address pool. How a gateway can
assign these to clients is already explained elsewhere. The important part is that in order to respond to requests from hosts
from that virtual subnet, the hosts to which the gateway forwards traffic to have to know that they must send packets for these
hosts to the VPN gateway.

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 NDP proxying (see #1008).
  • 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 it is not, either add a route to 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 other subnets in site-to-site scenarios) can be reached through the VPN gateway, or configure a static route on the
    actual default gateway, which directs 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 clients will look to LAN hosts as if they originated from the
    gateway (see the next section for notes on setting up a NAT).

Hosts on the Internet

If split-tunneling is not used all client traffic will be sent through the IPsec tunnel. In this scenario leftsubnet=0.0.0.0/0 is
configured on the gateway and rightsubnet=0.0.0.0/0 on the client. Now, the gateway could simply ignore/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 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 wouldn't be able respond (they would send their response to the virtual IP address).
What is required, therefore, 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 appears to
originate from the VPN gateway.

By way of example, lets 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 will exempt 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,
that any 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
and make the packets not match the negotiated IPsec policies when IPsec processing of outgoing packets happens (In the graphic here that's "xfrm lookup").
The general solution to that is to except all packets with a matching IPsec policy from NAT 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.

The following rule does that:

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 if traffic selectors (TS) can freely be configured on both peers. In that case you'd simply use
specific values for the rightsubnet and leftsubnet options.

Split-Tunneling with IKEv2

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

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

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 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, for Windows 7 clients the situation is as follows.

The Windows 7 client will always allow access to the host's LAN. So to access, for instance, 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.

With Windows 8.1 (and in 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 has split tunneling enabled by default. So to tunnel all traffic via VPN it 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 (available since 5.0.0)
does support narrowing of traffic selectors similar to how it is implemented for IKEv2. Unfortunately, this is not compatible
with many implementations by other vendors.

On the other hand, such clients may support the Unity extensions developed by Cisco. Since 5.0.1 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). For earlier releases the attr-sql plugin provides the means to manually configure attributes
that enable split-tunneling for Unity-aware clients (since 5.0.1 such attributes can also be provided through the attr plugin).

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. You can work around it 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, 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 it must accommodate for any other overhead introduced by the tunneling protocols in use.
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 may be used, too, but the values set there apply
to the routes that kernel-netlink 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 if it is too low.

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 *mangle POSTROUTING.

Additionally, set net.ipv4.ip_no_pmtu_disc on the server to 0.