2.7.3. Host-To-Host VPN Using Libreswan

To configure Libreswan to create a host-to-host IPsec VPN, between two hosts referred to as left and right, enter the following commands as root on both of the hosts (left and right) to create new raw RSA key pairs:
~]# ipsec newhostkey --configdir /etc/ipsec.d \
--output /etc/ipsec.d/www.example.com.secrets
Generated RSA key pair using the NSS database
This generates an RSA key pair for the host. The process of generating RSA keys can take many minutes, especially on virtual machines with low entropy.
To view the public key, issue the following command as root on either of the hosts. For example, to view the public key on the left host, run:
~]# ipsec showhostkey --left
ipsec showhostkey loading secrets from "/etc/ipsec.secrets"
ipsec showhostkey loading secrets from "/etc/ipsec.d/www.example.com.secrets"
ipsec showhostkey loaded private key for keyid: PPK_RSA:AQOjAKLlL
	# rsakey AQOjAKLlL
	leftrsasigkey=0sAQOjAKLlL4a7YBv [...]
You will need this key to add to the configuration file as explained below.
The secret part is stored in /etc/ipsec.d/*.db files, also called the NSS database.
To make a configuration file for this host-to-host tunnel, the lines leftrsasigkey= and rightrsasigkey= from above, are added to a custom configuration file placed in the /etc/ipsec.d/ directory.
Using an editor running as root, create a file with a suitable name in the following format:
/etc/ipsec.d/my_host-to-host.conf
Edit the file as follows:
conn mytunnel
    leftid=@west.example.com
    left=192.1.2.23
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightid=@east.example.com
    right=192.1.2.45
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
    authby=rsasig
    # load and initiate automatically
    auto=start
You can use the identical configuration file on both left and right hosts. They will auto-detect if they are left or right. If one of the hosts is a mobile host, which implies the IP address is not known in advance, then on the mobile host use %defaultroute as its IP address. This will pick up the dynamic IP address automatically. On the static host that accepts connections from incoming mobile hosts, specify the mobile host using %any for its IP address.
Ensure the leftrsasigkey value is obtained from the left host and the rightrsasigkey value is obtained from the right host.
Restart ipsec to ensure it reads the new configuration:
~]# service ipsec --full-restart
To check the tunnel is succesfully established, enter the following command:
~]# ipsec whack --trafficstatus
Alternatively, if not using the auto=start option in the /etc/ipsec.d/*.conf file or if a tunnel is not succesfully established, use the following command as root to load the IPsec tunnel:
~]# ipsec auto --add mytunnel
To bring up the tunnel, issue the following command as root, on the left or the right side:
~]# ipsec auto --up mytunnel

2.7.3.1. Verify Host-To-Host VPN Using Libreswan

The IKE negotiation takes place on UDP port 500. IPsec packets show up as Encapsulated Security Payload (ESP) packets. When the VPN connection needs to pass through a NAT router, the ESP packets are encapsulated in UDP packets on port 4500.
To verify that packets are being sent via the VPN tunnel, issue a command as root in the following format:
~]# tcpdump -n -i interface esp or udp port 500 or udp port 4500
00:32:32.632165 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1a), length 132
00:32:32.632592 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1a), length 132
00:32:32.632592 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 7, length 64
00:32:33.632221 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1b), length 132
00:32:33.632731 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1b), length 132
00:32:33.632731 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 8, length 64
00:32:34.632183 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1c), length 132
00:32:34.632607 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1c), length 132
00:32:34.632607 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 9, length 64
00:32:35.632233 IP 192.1.2.45 > 192.1.2.23: ESP(spi=0x63ad7e17,seq=0x1d), length 132
00:32:35.632685 IP 192.1.2.23 > 192.1.2.45: ESP(spi=0x4841b647,seq=0x1d), length 132
00:32:35.632685 IP 192.0.2.254 > 192.0.1.254: ICMP echo reply, id 2489, seq 10, length 64
Where interface is the interface known to carry the traffic. To end the capture with tcpdump, press Ctrl+C.

Note

The tcpdump commands interacts a little unexpectedly with IPsec. It only sees the outgoing encrypted packet, not the outgoing plaintext packet. It does see the encrypted incoming packet, as well as the decrypted incoming packet. If possible, run tcpdump on a router between the two machines and not on one of the endpoints itself.