Virtual IP Addresses

IKEv1 and IKEv2 both know the concept of virtual IP addresses. This means that the initiator requests an additional IP address from the responder to use as the inner IPsec tunnel address.

In IKEv1, virtual IPs are exchanged using the ModeConfig extension. IKEv2 has full support for virtual IPs in the core standard using CP Configuration Payloads.

strongSwan currently implements one scenario with IKEv2 configuration payloads, where a virtual IP address is requested by the initiator and one or several IPv4 and/or IPv6 addresses are assigned from multiple pools by the responder.

An alterantive approach is possible by the IKEv2 protocol where the responder would propose virtual IP addresses to the initiator but this is an uncommon setup and therefore not supported. It is supported for IKEv1, though, with ModeConfig in push mode.

Initiator Configuration

To request an arbitrary virtual IP from the responder the client configures the following in swanctl.conf:

IPv4 IPv6

connections.<conn>.vips =

connections.<conn>.vips = ::

It is possible to request a specific address by setting:

IPv4 IPv6

connections.<conn>.vips =

connections.<conn>.vips = 2001:db8::1

However the responder may return a different address, or may not return one at all.

To use a specific and static virtual IP (i.e. without exchanging any configuration payloads) it may simply be added to any local interface (even lo) and referenced in the client’s local traffic selector (local_ts). Configuring such an IP as in the example above will not have the intended effect because the IP won’t get installed on the system unless the server actually assigns that IP to the client with a configuration payload.

The traffic selectors in local_ts and remote_ts default to the value dynamic. If virtual IPs are used this value gets dynamically replaced by the received or assigned virtual IP, respectively. Therefore, no local traffic selector must be configured on the client and no remote traffic selector on the server when using virtual IPs. This ensures the client’s traffic selector is correctly narrowed to the assigned virtual IP.

A client may request multiple IP addresses by listing multiple IP addresses in connections.<conn>.vips. The main use case is for dual-stack hosts to request a virtual IP of each address family:

connections.<conn>.vips =, ::


Figure 1. strongSwan example illustrates the use dual IPv4/IPv6 address pools

DNS servers

VPN clients cannot explicitly request DNS servers via a special DNS option in swanctl.conf. But DNS server information received from the VPN gateway through the IKEv2 CP or IKEv1 ModeConfig payloads are handled for instance by the resolve plugin which in turn uses either the resolvconf(8) utility to add the DNS server information on the host or write it directly to /etc/resolv.conf or to an alternative file specified with the --with-resolve-conf ./configure directive.


On Linux the virtual IP addresses will be installed on the outbound interface by default. The interface may be changed with the charon.install_virtual_ip_on option. Source routes will be installed in the routing table configured with charon.routing_table in strongswan.conf or via the ./configure option --with-routing-table. The source routes force the use of the virtual IP when sending packets to the subnets defined as remote traffic selector (if the physical IP were used, the IPsec policies wouldn’t match and the packets wouldn’t get tunneled).

Responder Configuration

As already documented above, the remote traffic selector remote_ts defaults to the value dynamic. This placeholder gets dynamically replaced by the virtual IP assigned to a client. Therefore no remote traffic selector must be configured on the server when using virtual IPs. This ensures the client’s traffic selector is correctly narrowed to the assigned virtual IP (otherwise, every client would get the same traffic selector assigned, resulting in conflicting policies).

To serve a specific IP address (even if the initiator requests a different address) to a single client, the following may be configured:

IPv4 IPv6

connections.<conn>.pools = <name>

connections.<conn>.pools = <name>

pools.<name>.addrs =

pools.<name>.addrs = 2001:db8::1

To serve multiple clients, define an address pool in CIDR notation

IPv4 IPv6

connections.<conn>.pools = <name>

connections.<conn>.pools = <name>

pools.<name>.addrs =

pools.<name>.addrs = 2001:db8::3.0/24

or as an IP address range

IPv4 IPv6

connections.<conn>.pools = <name>

connections.<conn>.pools = <name>

pools.<name>.addrs =

pools.<name>.addrs = 2001:db8::3.1-2001:db8::3.100

As address pools are explicitly assigned to connections defined in swanctl.conf, multiple connections can easily share the same pool.

It’s also possible to use an external pool provided by a plugin by specifying a pool name to request addresses from (see further below for details on these backends):

connections.<conn>.pools = poolname

This queries registered plugins for an IP address from a pool named poolname (below are two examples that use the dhcp and eap-radius plugins, respectively).

Multiple pools may be defined. Note that the order in which they are queried primarily depends on the plugin order. Only if pools are provided by the same backend, does the order defined in swanctl.conf matter:

connections.<conn>.pools = v4pool, v6pool
pools.v4pool.addrs =
pools.v6pool.addrs = 2001:db8::3:0/24

DNS servers

DNS servers and other attributes can be assigned by plugins (e.g. by the attr plugin. In swanctl.conf each pool in the pools section may define a list of attributes to assign to clients.

In-memory backend

A non-persistent memory-based backend is the default as described for swanctl.conf. Such a backend is suitable for environments where pools are not shared across multiple gateways, server restarts are rare and direct administration of the pools is not needed (though there are some provisions for non-destructively reloading pool configurations in a running server when using swanctl.conf). To preserve IP address affinity, the remote IKE identity (as defined for the connection in question) is used. This maintains the same IP for a client (most of the time) even across brief teardowns of connections, approximating the behavior of a DHCP server without the stricture of lease expiry timers but prevents clients from intentionally attempting to get a different IP address as there is no corresponding release mechanism.

Database backend

The pool utility allows easy management of IP address pools and other attributes, like DNS servers, stored in an SQL database using the attr-sql plugin.

DHCP backend

With the dhcp plugin the responder can request virtual IP addresses for clients from a DHCP server using broadcasts or from a designated server.

DNS/WINS server information is additionally served to clients if the DHCP server provides such information.

The plugin is used by setting

connections.<conn>.pools = dhcp

The farp plugin might also be of use when using the dhcp plugin. It allows the responder to fake ARP responses for virtual IP addresses handed out to clients. This lets a road-warrior act as a client on the local LAN of the responder.

RADIUS backend

The eap-radius plugin can provide virtual IP addresses assigned to RADIUS clients via the Framed-IP-Address attribute. Forwarding of other RADIUS attributes is also supported. The plugin is used by setting

connections.<conn>.pools = radius

Multiple pools, Different Backends

If multiple pools are defined from different backends, for instance

connections.<conn>.pools = radius, v4pool
pools.v4pool.addrs =

the order in which they are queried for virtual IPs depends on the plugin load order. In-memory pools are provided by the vici plugins. The order pools is irrelevant unless multiple in-memory pools from the same backend are defined.