Openswan implements the IPSec Internet Standard for Linux. not the only solution but it is based on the oldest implementation of IPSec for Linux called FreeSwan. The FreeSwan project ended some years ago and their code base was used to create openswan. The feature list

May 25, 2016 Openswan - Wikipedia In the field of computer security, Openswan provides a complete IPsec implementation for Linux 2.0, 2.2, 2.4 and 2.6 kernels. Openswan, begun as a fork of the now-defunct FreeS/WAN project, continues to use the GNU General Public License. Unlike the FreeS/WAN project, it does not exclusively target the GNU/Linux operating system. OpenWrt Project: IPsec Site-to-Site DNS fowarding through VPN tunnels is almost the same as normal DNS forwarding with one exception. Dnsmasq must use the correct source interface. By default it will use the OpenWrt internet IP for it's requests but this cannot be tunneled. So just expand the Dnsmasq forward settings in LuCI with the OpenWrt internal IP address. VPN solutions (Openswan + xl2tp [Windows] or Openswan VPN solutions (Openswan + xl2tp [Windows] or Openswan + Shrew VPN Client [Linux/Windows])

Build an IPSEC VPN Without Losing Your Mind

Creating VPNs with IPsec and SSL/TLS | Linux Journal Jan 01, 2008 How to setup a Gateway to Gateway VPN using IPSec on CentOS May 20, 2017

On the VPN server on the other side, apply the same above command with the source address of 172.16.2.0/24 (its iternal network) Now we are set to install and configure openswan ipsec server on both the VPN servers. Openswan IPSec VPN configuration in Linux

IPsec site to site virtual private network (VPN) tunnel use to interconnect two different location network securely over the internet. IPsec objective is to provide security communication for IP packets such as data encrypting, authentication, protection against replay and data confidentiality. Azure VPN – Linux Openswan VPN – Mobireme Jul 19, 2017