Sharppcap how to send packets using the route in ubuntu?

I 'm using Sharppcap to convert multicast udp packets to unicast then forward the packet to another network I established a tunnel IPsec connection between two networks like the following scenario IPsec-tools and racoon and it works fine

But in gateway1 I run a simple program using sharppcap that listen to eth1 to capture all multicast udp packets from network A and change the destination address to the eth1 address of gateway2 then resend it then the other gateway change the packet to multicast and forward it to network B. I did this because IPsec tunnel mode doesn't work with multicast For example in gatway1:

private static void device_PcapOnPacketArrival(object sender, PcapCaptureEventArgs e)
        {   
            if(e.Packet is UDPPacket)
            {               
                UDPPacket udp = (UDPPacket)e.Packet;
                System.Net.IPAddress dstIp = udp.DestinationAddress;
              if (dstIp.ToString() == "224.5.6.7")
                {
                    udp.DestinationAddress= IPAddress.Parse("192.168.2.1");
                    udp.SourceHwAddress= System.Net.NetworkInformation.PhysicalAddress.Parse("A0-48-1C-D6-7E-C0");
                    udp.DestinationHwAddress= System.Net.NetworkInformation.PhysicalAddress.Parse("A0-48-1C-D6-D5-90");
                    udp.TimeToLive=20;
                    udp.ipv4.IPChecksum= udp.ComputeIPChecksum();
                    device1.SendPacket(udp);
                }
            }
        }

The modified packet gets to its destination in the other network but still not encrypted in the tunnel between gateway1 and gateway2 I think the solution is to let the operating system handle sending the packets But I don’t know how to do it using sharppcap.

libpcap/winpcap (and thus Sharppcap) sends packets via pcap_sendpacket/pcap_inject(). These are raw packets so if you want to use encryption you'll have to apply that yourself when sending the packet as these calls are bypassing the higher level communication stacks in the OS.

Maybe you could use a hybrid approach of receiving the packet with Sharppcap and then using the IPSec tunnel as one would normally use it as an application, eg. open a socket between the two systems and send the packet through that socket. That would let the OS route the data through the IPSec tunnel.