client.conf

# basic configuration

config setup
    # nat_traversal=yes

# Connections into AWS VPC
conn %default
    ikelifetime=60m
    keylife=20m
    rekeymargin=3m
    keyingtries=1
    keyexchange=ikev2
    authby=secret

conn us-east-1-vpc
    left=%any
    leftid=aep
    leftfirewall=yes
    right=54.72.251.51
    rightsubnet=172.31.6.212/20
    rightid=@us-east-gw.example.com
    auto=start

server log

Apr 29 14:44:12 ip-172-31-6-212 charon: 12[CFG] looking for peer configs matching 172.31.6.212[us-east-gw.example.com]...78.55.52.47[aep]
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[CFG] selected peer config 'client'
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] authentication of 'aep' with pre-shared key successful
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] peer supports MOBIKE
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] authentication of 'us-east-gw.example.com' (myself) with pre-shared key
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] IKE_SA client[1] established between 172.31.6.212[us-east-gw.example.com]...78.55.52.47[aep]
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] scheduling reauthentication in 9831s
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] maximum IKE_SA lifetime 10371s
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] traffic selectors 172.31.0.0/20 === 192.168.1.130/32  inacceptable
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[IKE] failed to establish CHILD_SA, keeping IKE_SA
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[ENC] generating IKE_AUTH response 1 [ IDr AUTH N(AUTH_LFT) N(MOBIKE_SUP) N(ADD_4_ADDR) N(ADD_4_ADDR) N(TS_UNACCEPT) ]
Apr 29 14:44:12 ip-172-31-6-212 charon: 12[NET] sending packet: from 172.31.6.212[4500] to 78.55.52.47[4500] (172 bytes)
Apr 29 14:44:32 ip-172-31-6-212 charon: 04[IKE] sending keep alive to 78.55.52.47[4500]
Apr 29 14:44:52 ip-172-31-6-212 charon: 15[IKE] sending keep alive to 78.55.52.47[4500]
Apr 29 14:45:12 ip-172-31-6-212 charon: 16[IKE] sending keep alive to 78.55.52.47[4500]
Apr 29 14:45:32 ip-172-31-6-212 charon: 02[IKE] sending keep alive to 78.55.52.47[4500]

server.conf

config setup
	# strictcrlpolicy=yes
	# uniqueids = no

conn client
	authby=secret
	# The leftid parameter is not a real DNS name
	leftid=us-east-gw.example.com
	# The "left" parameter is the gateway's private IP 
	left=172.31.6.212
        leftsubnet=172.31.6.212/20
	# We are protecting the entire VPC, not just this subnet
	leftfirewall=yes
	right=%any
	# The virtual IP pool is outside the VPC!
	auto=add