Strongswan: clients can connect to server but no internet access

I've set up an ipsec server and after a while I could connect to it from my android device. but there is no internet connection in client side. I have also added NAT rules to forward traffics from virtual IPs but still problem exists. how can I find and solve the problem? :(

server: /etc/ipsec.conf

conn android
    keyexchange=ikev2
    ike=aes256-sha1-modp1024,aes128-sha256-ecp256,aes256-sha384-ecp384,aes128-sha256-modp2048,aes128-sha1-modp2048,aes256-sha384-modp4096,aes256-sha256-modp4096,aes256-sha1-modp4096,aes128-sha256-modp1536,aes128-sha1-modp1536,aes256-sha384-modp2048,aes256-sha256-modp2048,aes256-sha1-modp2048,aes128-sha256-modp1024,aes128-sha1-modp1024,aes256-sha384-modp1536,aes256-sha256-modp1536,aes256-sha1-modp1536,aes256-sha384-modp1024,aes256-sha256-modp1024!
    esp=aes256-sha1,aes128gcm16-ecp256,aes256gcm16-ecp384,aes128-sha256-ecp256,aes256-sha384-ecp384,aes128-sha256-modp2048,aes128-sha1-modp2048,aes256-sha384-modp4096,aes256-sha256-modp4096,aes256-sha1-modp4096,aes128-sha256-modp1536,aes128-sha1-modp1536,aes256-sha384-modp2048,aes256-sha256-modp2048,aes256-sha1-modp2048,aes128-sha256-modp1024,aes128-sha1-modp1024,aes256-sha384-modp1536,aes256-sha256-modp1536,aes256-sha1-modp1536,aes256-sha384-modp1024,aes256-sha256-modp1024,aes256-sha1-modp1024,aes128gcm16,aes256gcm16,aes128-sha256,aes128-sha1,aes256-sha384,aes256-sha256!
    dpdaction=clear
    dpddelay=300s
    rekey=no
    left=example.com
    leftfirewall=yes
    leftauth=pubkey
    leftsubnet=0.0.0.0/0
    leftcert=example.com.crt
    lefthostaccess=yes
    right=%any
    rightfirewall=yes
    rightauth=eap-mschapv2
    rightsendcert=never
    rightsubnet=192.168.31.0/24
    rightsourceip=192.168.31.0/24
    rightdns=8.8.8.8
    eap_identity=%any
    type=tunnel
    auto=add

ip xfrm policy

src 192.168.31.0/24 dst 0.0.0.0/0 
        dir fwd priority 1955 
        tmpl src x.x.x.x dst y.y.y.y
        proto esp reqid 2 mode tunnel
src 192.168.31.0/24 dst 0.0.0.0/0 
        dir in priority 1955 
        tmpl src x.x.x.x dst y.y.y.y
        proto esp reqid 2 mode tunnel
src 0.0.0.0/0 dst 192.168.31.0/24 
        dir out priority 1955 
        tmpl src y.y.y.y dst x.x.x.x
        proto esp reqid 2 mode tunnel
src 0.0.0.0/0 dst 0.0.0.0/0 
        socket in priority 0 
src 0.0.0.0/0 dst 0.0.0.0/0 
        socket out priority 0 
src 0.0.0.0/0 dst 0.0.0.0/0 
        socket in priority 0 
src 0.0.0.0/0 dst 0.0.0.0/0 
        socket out priority 0 
src ::/0 dst ::/0 
        socket in priority 0 
src ::/0 dst ::/0 
        socket out priority 0 
src ::/0 dst ::/0 
        socket in priority 0 
src ::/0 dst ::/0 
        socket out priority 0

client: strongswan vpn client for android

Solution 1:

Finally I found the problem. adding FORWARD rule to iptables solved my problem.

Solution 2:

You may need to set up NAT, and enable forwarding in the kernel. The following two commands do this:

sudo iptables -t nat -A POSTROUTING -o YOUR_INTERFACE_NAME -j MASQUERADE
sudo sysctl -w net.ipv4.ip_forward=1
sudo sysctl -w net.ipv4.conf.all.forwarding=1

The first adds a NAT rule to the POSTROUTING chain (more info here), the second/third turns on the setting that allows the server to do any forwarding.

Note that these won't persist through a reboot without changes to sysctl.conf. More info here