AWS + StrongSwan

How to tunnel 2 AWS networks and don't use AWS services. Most of manual show how to do it, but without high availability. Here we can have asymmetric routing or any other.

We need 4 instances, 2 from each side (for HA)
AC1 net 10.1.1.0/24
instance A1 EIP 1.1.1.1 IP 10.1.1.11
instance A2 EIP 1.1.1.2 IP 10.1.1.12
AC2 net 10.2.1.0/24
instance B1 EIP 1.1.2.1 IP 10.2.1.11
instance B2 EIP 1.1.2.2 IP 10.2.1.12
Configuratin from both sides same, different only ID's and IP's. We need disable Network > Source/Destination Check for each instance.

Install default or latest versions
apt-get install strongswan keepalived

Strongswan configuration. Remark Left and Right, sometime hard to understood why... L-Local and R-Remote.
# cat /etc/ipsec.conf
# ipsec.conf - strongSwan IPsec configuration file

config setup
  strictcrlpolicy=no
  charondebug=all
conn %default
  ikelifetime=60m
  keylife=20m
  rekeymargin=3m
  keyingtries=1
  keyexchange=ikev2
conn aws2aws
  authby=secret
  auto=start
  type=tunnel
  left=10.1.1.11
  leftid=1.1.1.1
  leftsubnet=10.1.1.0/24
  leftauth=psk
  right=10.1.2.11
  rightsubnet=10.1.2.0/24
  rightauth=psk
  ike=aes128-sha1-modp1024
  esp=aes128-sha1-modp1024

include /var/lib/strongswan/ipsec.conf.inc

and
# cat /etc/ipsec.secrets

1.1.1.1 : PSK "7sMH9Sb5cWh7"
1.1.2.1 : PSK "A4zsR9DtceYY"

include /var/lib/strongswan/ipsec.secrets.inc

Sysctl
# cat /etc/sysctl.conf
net.ipv4.ip_forward=1

Firewall
/sbin/iptables -N SW
/sbin/iptables -A INPUT -j SW
/sbin/iptables -A INPUT -p tcp -j SW
/sbin/iptables -A INPUT -p udp -j SW
/sbin/iptables -A INPUT -p icmp -j SW
/sbin/iptables -A INPUT -p ah -j SW
/sbin/iptables -A INPUT -p esp -j SW
/sbin/iptables -A FORWARD -j SW

/sbin/iptables -A SW -s 1.1.1.1 -j ACCEPT
/sbin/iptables -A SW -s 1.1.2.1 -j ACCEPT

Keepalived forward traffic via A1 or A2 and B1 or B2 from other side.
# cat /etc/keepalived/keepalived.conf

vrrp_script check_sw {
    script "/usr/sbin/ipsec statusall | grep ESTABLISHED"
    interval 2
    timeout 5
    fall 2
    rise 2
}

vrrp_instance VI_vip1 {
    state BACKUP
    interface eth0
    virtual_router_id 1
    priority 1
    advert_int 1

    authentication {
        auth_type PASS
        auth_pass LvTZHnXZDNbk
    }

    unicast_peer {
        10.1.1.11
        10.1.1.12
    }

    track_script {
        check_sw
    }

    notify_master "/etc/keepalived/scripts/t1-1.sh"
}

4 scripts because we need 4 directions
t1-1.sh A1 -> B1 (default)
t1-2.sh A2 -> B2 (failover)
t2-1.sh B1 -> A1 (failover)
t2-2.sh B2 -> A2 (default)

As result we have circle or asymmetric routing. Example how keepalived can manage AWS rout table (or any other, Azure as example).
aws ec2 replace-route --region us-east-1 --route-table-id rtb-00000 --destination-cidr-block 10.1.2.0/24 --instance-id i-00000

And how it's look's when connected propertly.
# /usr/sbin/ipsec statusall
Status of IKE charon daemon (strongSwan 5.2.1, Linux 3.16.0-4-amd64, x86_64):
  uptime: 35 days, since Sep 19 09:39:10 2018
  malloc: sbrk 2539520, mmap 0, used 333632, free 2205888
  worker threads: 11 of 16 idle, 5/0/0/0 working, job queue: 0/0/0/0, scheduled: 3
  loaded plugins: charon aes rc2 sha1 sha2 md5 random nonce x509 revocation constraints pubkey pkcs1 pkcs7 pkcs8 pkcs12 pgp dnskey sshkey pem openssl fips-prf gmp agent xcbc hmac gcm attr kernel-netlink resolve socket-default stroke updown
Listening IP addresses:
  10.1.1.11
Connections:
     aws2aws:  10.1.1.11...1.1.1.1  IKEv2
     aws2aws:   local:  [1.1.1.1] uses pre-shared key authentication
     aws2aws:   remote: [1.1.1.2] uses pre-shared key authentication
     aws2aws:   child:  10.1.1.0/24 === 10.1.2.0/24 TUNNEL
Security Associations (1 up, 0 connecting):
     aws2aws[971]: ESTABLISHED 20 minutes ago, 10.1.1.11[1.1.1.1]...1.1.2.1[1.1.2.1]
...
     aws2aws{971}:   10.1.1.0/24 === 10.1.2.0/24

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