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Cloud Platforms

Running strongSwan on a cloud platform is usually relatively painless, because only the hardware is virtualized.
The kernel- and userspace is untouched.

General Warnings And Problems

Generally, in a cloud environment, the underlying network checks the source IP address of the sent IP packets.

You need to be able to deactivate this check in order to use strongSwan as a VPN gateway, either in a roadwarrior or a
site-to-site scenario. Except if you masquerade the source address of all packets that are transmitted over the network.
However, it is advisable to avoid NAT at any cost, because it prevents you from addressing remote endpoints. This only
applies to the outermost IP packet though, so it does not affect the inner IP packet of any tunneling protocol.

As an alternative, the packets from and to the VPN gateway can be transported to the hosts on the local network using
other means than directly on top of Ethernet, like by using IPsec in tunnel mode, GRE, IPinIP or other protocols.

Container Virtualization

Container virtualized environments often do not offer a working IPsec stack to software in the container. Therefore,
kernel-libipsec may have to be used instead.

To use kernel-libipsec, tun devices have to be available. Keep in mind that using kernel-libipsec has drawbacks and
is generally discouraged. Change to a hardware virtualized virtual machine, if possible.

OpenVZ uses container virtualization. On OpenVZ, support for IPsec seems to have been broken at some point (#592).
Virtuozzo can also be a container. With Virtuozzo, it depends on the VM type.

If the daemon complains about not being able to open an XFRM socket or the installation of SAs fails, then it's very likely
container virtualized.

strongSwan in a Docker container works with kernel IPsec if the host operating system has a working IPsec stack and if
the docker container, as well as strongSwan, have the necessary privileges to access it (CAP_NET_ADMIN). You do not have
to enable host networking.

An incomplete list of container virtualization technologies:

AWS EC2

On AWS, the source check of a machine can be disabled in the EC2 management menu. Disabling it is required if the host
running strongSwan is supposed to make the whole Internet available to the hosts in the VPC. Otherwise, add the routes to
the remote network in the VPC's route settings.

This is not required if you protect the traffic between the other hosts and the "central gateway" in the VPC (not the AWS-
provided Internet Gateway) using IPsec in tunnel mode.

Generally, the source check only has to be disabled if the routing table of the VPC disagrees with the direction the traffic goes
into and comes from (the return path check fails). That is the same mechanism as the rp_filter on Linux.

Azure

Each virtual NIC attached to a VM has an option to enable IP forwarding. If that option is not enabled, then Azure does a src/dst check.

Google Cloud Engine

No specialties about GCE are known at this point.

DigitalOcean Droplets

On DigitalOcean, the source check can not be disabled.