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 " , and enter the following commands as root on both of the hosts ( " left " and " right " ) to create new raw RSA key pairs: 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: You have to add this key to the configuration file as explained in the following paragraphs. 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 corresponding name in the following format: /etc/ipsec.d/myvpn.conf Edit the file as follows: You can use the identical configuration file on both left and right hosts. They 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 picks 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: To check the tunnel is succesfully established, and additionally see how much traffic has gone through the tunnel, enter the following command as root : 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: To bring up the tunnel, issue the following command as root , on the left or the right side: 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: 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.

Commands:
~]# ipsec newhostkey --configdir /etc/ipsec.d --output /etc/ipsec.d/myvpn.secrets Generated RSA key pair using the NSS database
~]# ipsec showhostkey --left ipsec showhostkey loading secrets from "/etc/ipsec.secrets" ipsec showhostkey loading secrets from "/etc/ipsec.d/myvpn.secrets" ipsec showhostkey loaded private key for keyid: PPK_RSA:AQOjAKLlL # rsakey AQOjAKLlL leftrsasigkey=0sAQOjAKLlL4a7YBv [...]
conn myvpn 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
~]# service ipsec --full-restart
~]# ipsec whack --trafficstatus 006 #2: "myvpn", type=ESP, add_time=1234567890, inBytes=336, outBytes=336, id='@east'
~]# ipsec auto --add myvpn
~]# ipsec auto --up myvpn
~]# 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