Strongswan forwarding traffic between two IPsec tunnels

Question

Given a remote (centralized) VPN server with a StrongSwan IKEv2 setup with external static IP:

• centralized server internal IP 192.168.1.65, network 192.168.1.64/27

And two gateways doing NAT for internal networks:

• first gateway internal IP 192.168.1.33, network: 192.168.1.32/27;
• second gateway internal IP 192.168.1.1, network: 192.168.1.0/27;

Gateways are connected to centralized server also using StrongSwan.

Connection works perfectly from gateways to server and from server to gateways: ping works, access to services on the server/gateways. Devices behind gateways also can access server without problems.

The problem begins when I'm trying to access computers between different gateways.

Configurations:

centralized:

conn base
    keyexchange = ikev2
    keyingtries = %forever
    forceencaps = yes
    compress = no

    left = centralized
    leftid = @centralized
    leftauth = pubkey
    leftca = "..."
    leftcert = centralized.crt
    leftupdown = sudo -E ipsec _updown iptables
    leftsubnet = 192.168.1.65

    right = %any
    rightauth = pubkey
    rightauth2 = psk
    rightca = %same

conn gateway-first
    auto = add
    rightid = @gateway-first
    rightcert = gateway-first.crt
    rightsubnet = 192.168.1.32/27
    rightsourceip = 192.168.1.66
    also = base

conn gateway-second
    auto = add
    rightid = @gateway-second
    rightcert = gateway-second.crt
    rightsubnet = 192.168.1.0/27
    rightsourceip = 192.168.1.67
    also = base

gateway-first:

conn gateway-first
    auto = route
    dpdaction = restart
    closeaction = restart
    keyexchange = ikev2
    keyingtries = %forever
    forceencaps = yes
    compress = no

    rightid = @centralized
    right = centralized
    rightauth = pubkey
    rightca = "..."
    rightcert = centralized.crt
    rightsubnet = 192.168.1.65,192.168.1.0/27

    leftid = @gateway-first
    left = %defaultroute
    leftauth = pubkey
    leftauth2 = psk
    leftca = %same
    leftcert = gateway-first.crt
    leftupdown = sudo -E ipsec _updown iptables
    leftsubnet = 192.168.1.32/27
    leftsourceip = %config4

gateway-second:

conn gateway-second
    auto = route
    dpdaction = restart
    closeaction = restart
    keyexchange = ikev2
    keyingtries = %forever
    forceencaps = yes
    compress = no

    rightid = @centralized
    right = centralized
    rightauth = pubkey
    rightca = "..."
    rightcert = centralized.crt
    rightsubnet = 192.168.1.65,192.168.1.32/27

    leftid = @gateway-second
    left = %defaultroute
    leftauth = pubkey
    leftauth2 = psk
    leftca = %same
    leftcert = gateway-second.crt
    leftupdown = sudo -E ipsec _updown iptables
    leftsubnet = 192.168.1.1/27
    leftsourceip = %config4

When trying to ping gateway-second from computer behind gateway-first (source computer IP is 192.168.1.40) and running tcpdump on centralized server in the same time shows:

tcpdump -i eth0 host 192.168.1.1 -n
error : ret -1
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 262144 bytes
21:27:49.030650 IP 192.168.1.40 > 192.168.1.1: ICMP echo request, id 9721, seq 35, length 64
21:27:50.026652 IP 192.168.1.40 > 192.168.1.1: ICMP echo request, id 9721, seq 36, length 64
21:27:51.031805 IP 192.168.1.40 > 192.168.1.1: ICMP echo request, id 9721, seq 37, length 64
21:27:52.041165 IP 192.168.1.40 > 192.168.1.1: ICMP echo request, id 9721, seq 38, length 64
21:27:53.029530 IP 192.168.1.40 > 192.168.1.1: ICMP echo request, id 9721, seq 39, length 64

So, according to this log packets are arrived to the centralized server, but never forwarded to the 192.168.1.1.

On the centralized server and both gateways I've enabled forwarding:

net.ipv4.ip_forward = 1

Routing table on centralized server:

# ip route
default via yy.yy.yy.yy dev eth0  proto static 
zz.zz.zz.zz dev eth0  proto kernel  scope link  src xx.xx.xx.xx
192.168.1.64/27 via 192.168.1.65 dev eth1  proto static

And also route table #220 (VPN):

# ip route show table 220
192.168.1.0/27 via 5.189.141.1 dev eth0  proto static  src 192.168.1.65 
192.168.1.32/27 via 5.189.141.1 dev eth0  proto static  src 192.168.1.65 

Any ideas how to enable forwarding between two different tunnels?

Answer 1

Try adding the subnets of the two gateways to leftsubnet on the central server. Even though rightsubnet on each gateway includes the respective opposite subnet, the traffic selector will be narrowed to what's configured on the central server as leftsubnet (i.e. 192.168.1.65). You should see that in the output of ipsec statusall. You may also configure leftsubnet=0.0.0.0/0 on the central server, then it will accept anything the gateways propose as their rightsubnet.