Enable T-NAT for client to site VPN via conn mark

### The required feature described as a wish

Dear cloudstack community,

I was surprised to see that the current implementation of client to site VPN does not enable to open multiple client to site VPN when computers share the same public IPs.

I read #8566 and found that another people have the same experience.
Charon is aware of this NAT client situation as per my logs :

```
Apr 2 20:27:35 r-4188-VM charon: 06[ENC] parsed ID_PROT request 0 [ KE No NAT-D NAT-D ]
Apr 2 20:27:35 r-4188-VM charon: 06[IKE] remote host is behind NAT
Apr 2 20:27:35 r-4188-VM charon: 06[ENC] generating ID_PROT response 0 [ KE No NAT-D NAT-D ]
Apr 2 20:27:35 r-4188-VM charon: 06[NET] sending packet: from 182.234.27.275[500] to 36.35.10.124[57049] (372 bytes)
Apr 2 20:27:35 r-4188-VM charon: 12[NET] received packet: from 36.35.10.124[55814] to 182.234.27.275[4500] (76 bytes)
Apr 2 20:27:35 r-4188-VM charon: 12[ENC] parsed ID_PROT request 0 [ ID HASH ]
Apr 2 20:27:35 r-4188-VM charon: 12[CFG] looking for pre-shared key peer configs matching 182.234.27.275...36.35.10.124[192.168.1.93]
Apr 2 20:27:35 r-4188-VM charon: 12[CFG] selected peer config "L2TP-PSK"
Apr 2 20:27:35 r-4188-VM charon: 12[IKE] IKE_SA L2TP-PSK[150] established between 182.234.27.275[182.234.27.275]...36.35.10.124[192.168.1.93]
Apr 2 20:27:35 r-4188-VM charon: 12[ENC] generating ID_PROT response 0 [ ID HASH ]
[...]
Apr 2 20:27:35 r-4188-VM ipsec[6798]: 15[CFG] looking for pre-shared key peer configs matching 182.234.27.275...36.35.10.124[192.168.1.14]
Apr 2 20:27:35 r-4188-VM ipsec[6798]: 15[CFG] selected peer config "L2TP-PSK"
Apr 2 20:27:35 r-4188-VM ipsec[6798]: 15[IKE] IKE_SA L2TP-PSK[149] established between 182.234.27.275[182.234.27.275]...36.35.10.124[192.168.1.14]
```
--> Client host is behind NAT + private IP addresses discovered !

But like described in #8566 only one client is able to login at a time. The second one disconnect the first one...

[Strongswan's input ](https://docs.strongswan.org/docs/latest/plugins/connmark.html
)on that is to compile with --enable-connmark flag.

I am researching to fix this myself and then to make a PR to make this available for the community. Are the system vm images closed source or can I find a repo somewhere to improve this behavior?

Fred

Start by locating the system VM image source and reviewing #8566 alongside the StrongSwan connmark documentation referenced in the issue. Done means multiple client-to-site VPN clients behind the same public IP can connect concurrently without disconnecting one another.

Written by the indexing model from the issue text.