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Tobias Brunner, 09.10.2012 10:56

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h1. Virtual IP
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{{>toc}}
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IKEv1 and IKEv2 both know the concept of _virtual IPs_. This means that the initiator  requests an additional IP address from the peer to use as inner IPsec tunnel address.
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In IKEv1, virtual IPs are exchanged using the _mode config_ extension. IKEv2 has full support for virtual IPs in the core standard using _configuration payloads_.
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strongSwan currently implements one scenario with configuration payloads, where an IP address is assigned to the initiator (since [[5.0.1]] multiple addresses can be assigned from multiple pools). The opposite is possible by the protocol, but is an uncommon setup and therefore not supported.
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h2. Initiator Configuration
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The client needs an additional parameter called _leftsourceip_.
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<pre>
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    leftsourceip=%config
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</pre> 
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_%config_ means to request an address from the responder and is an alias for the IKEv1 specific _%modecfg_. But you may specify an address explicitly by setting:
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<pre>
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    leftsourceip=10.3.0.5
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</pre>
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This will include _10.3.0.5_ into the configuration payload request. However, the responder may return a different address, or may not return one at all.
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Since [[5.0.1]] a client may request multiple IP addresses by listing a comma-separated combination of _%config4_, _%config6_ or fixed IP addresses in _leftsourceip_. Configuring _%config_ (or one of its aliases) will request an address of the tunnel address family.
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The main use case is for dualstack hosts to request a virtual IP of each address family:
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<pre>
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    leftsourceip=%config4,%config6
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</pre> This is also illustrated in the {{tc(ikev2/ip-two-pools-v4v6)}} test scenario.
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The @leftsubnet@ option should not be included when requesting a virtual IP, as the subnet may not match the received address. Without the _leftsubnet_ option, the subnet is narrowed to the assigned virtual IP automatically.
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h3. DNS servers
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Before [[5.0.1]] the client couldn't explicitly request other attributes, but it may still have processed the DNS attribute. With current releases DNS servers may explicitly be requested with the _leftdns_ option. Received DNS servers are handled, for instance, by the [[resolveplugin|resolve plugin]] which writes them to _/etc/resolv.conf_, or an other file specified with the _--with-resolve-conf_ configure directive.
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h3. Implementation
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On Linux, the virtual IP addresses will be installed on the outbound interface by default (may be changed, since [[5.0.1]], with the _charon.install_virtual_ip_on_ option) and source routes will be installed in the routing table configured with _charon.routing_table_ in [[strongswan.conf]] (or [[Autoconf|./configured]] with _--with-routing-table_). The source routes force the use of the virtual IP when sending packets to the subnets in _rightsubnet_.
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h2. Responder Configuration
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The responder configuration uses the _rightsourceip_ option:
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<pre>
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    rightsourceip=10.3.0.6
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</pre>
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This will serve the IP _10.3.0.6_ to the client, even if the initiator requested another address. Additionally, the responder may define:
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<pre>
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    rightsourceip=%config
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</pre>
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to let the client choose an address. This is not recommended if you do not trust the client completely.
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You may define an address pool in CIDR notation, e.g.
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<pre>
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    rightsourceip=10.3.0.0/24
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</pre> to serve addresses from that pool.
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You may also use an external pool implemented as a plugin where you can specify a pool name to select addresses from. The definition
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<pre>
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    rightsourceip=%poolname
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</pre> 
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queries registered plugins for an IP from a pool named _poolname_. This can also be the name of another connection in [[IpsecConf|ipsec.conf]] which defines a pool in CIDR notation with _rightsourceip_, as a pool with that connection's name is created implicitly. Since [[5.0.1]] multiple connections (i.e. [[ConnSection|conn sections]]) can share the same pool implicitly if they use the same definition in _rightsourceip_ (previously each connection would use it's own copy and the same virtual IP may have been handed out to different clients).
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Multiple pools can be listed in _rightsourceip_ since [[5.0.1]], e.g.
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<pre>
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    rightsourceip=10.3.0.0/28,fec3::/120
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</pre>
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As used in the {{tc(ikev2/ip-two-pools-v4v6)}} test scenario.
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h3. DNS servers
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DNS servers and other attributes can be assigned by plugins (e.g. the [[AttrPlugin|attr plugin]]) or since [[5.0.1]] directly in [[ipsec.conf]] by use of the _rightdns_ option.
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h3. Database backend
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The [[IpsecPool|ipsec pool]] utility allows to easily manage IP address pools and other attributes, like DNS servers, stored in an SQL database using the [[attrsql|attr-sql plugin]].
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h3. DHCP backend
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With the [[DHCPPlugin|dhcp plugin]] the responder can request virtual IP addresses for clients from a DHCP server using broadcasts, or a designated server.
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DNS/WINS server information is additionally served to clients if the DHCP server provides such information.
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The plugin is used in [[ipsec.conf]] configurations by setting
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<pre>
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    rightsourceip=%dhcp
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</pre>
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The [[FARPPlugin|farp plugin]] might also be of use when using the [[DHCPPlugin|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.
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h2. Versions before 5.0.0
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The description above covers the features of the charon daemon, which handled only IKEv2 connections in earlier releases.  The pluto daemon that handled IKEv1 connections provided a similar feature set but not everything was supported (for instance, the dhcp and farp plugins).
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The pluto daemon did not request virtual IP addresses from the responder if they were explicitly configured with _leftsourceip_. In that case, that is, if the virtual IP was not assigned by the responder with _rightsourceip_ one may had to use the _rightsubnetwithin_ directive (refer to "this example":http://www.strongswan.org/uml/testresults4/ikev1/virtual-ip/).
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The IKEv2 daemon charon supports address pools since version [[4.2.1]], the IKEv1 daemon pluto gained support for this in [[4.4.0]].