VPN server for remote clients using IKEv2 split VPN

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Split VPN is the term used to indicate you only want to use the VPN connection to reach one or more remote subnets. In order words, this type of VPN disables the "send all traffic through the VPN".

The configuration is basically the same as for VPN server for remote clients using IKEv2 except now we indicate with leftsubnet= on the VPN server what the subnet is that we are giving access to. The easiest way to ensure that the clients can reach the subnet involved is to give them an address from that subnet and enable proxy arp. For example, say you want clients to reach the remote 10.10.0.0/16 network via the VPN server at IP 1.2.3.4. You grab 10.10.20.0/24 as the range for the VPN clients. Then you configure the connection as follows (this example assumes certificates):

conn access-vpn
	authby=rsasig
	ikev2=insist
	# support Apple and Windows at the same time
	ike=aes256-sha2_512;modp2048,aes128-sha2_512;modp2048,aes256-sha1;modp1024,aes128-sha1;modp1024
	esp=aes_gcm256-null,aes_gcm128-null,aes256-sha2_512,aes128-sha2_512
	auto=add
	rekey=no
	# fill in with your VPN server IP
	left=1.2.3.4
	leftcert=yourcert
	leftsendcert=always
	leftid=@yourFQDN
	leftsubnet=10.10.0.0/16
	rightaddresspool=10.10.20.1-10.10.20.254
	right=%any
	rightca=%same
	# make cisco clients happy
	cisco-unity=yes
	# address of your internal DNS server
	modecfgdns=10.10.10.10
	leftxauthserver=yes
	rightxauthclient=yes
	leftmodecfgserver=yes
	rightmodecfgclient=yes
	modecfgpull=yes
	dpddelay=9m
	dpdtimeout=30m
	dpdaction=clear
	fragmentation=yes
	# if you want an ipsec0 interface using VTI
	vti-interface=ipsec0
	vti-shared=yes
	vti-routing=yes
	mark=20/0xffffffff

And don't forget to enable proxyarp on the VPN server's internal interface. If this is eth1, add to /etc/sysctl.conf (or equivalent file in /etc/sysctl.d/)

# eth1 is the internal interface with a 10.10.X.Y/16 IP address
net.ipv4.conf.eth1.proxy_arp=1