15.9 VPN over IPsec
Written by Nik Clayton.
Creating a VPN between two networks, separated by the Internet, using FreeBSD gateways.
15.9.1 Understanding IPsec
Written by Hiten M. Pandya.This section will guide you through the process of setting up IPsec. In order to set up IPsec, it is necessary that you are familiar with the concepts of building a custom kernel (see Chapter 9).IPsec is a protocol which sits on top of the Internet Protocol (IP) layer. It allows two or more hosts to communicate in a secure manner (hence the name). The FreeBSD IPsec “network stack” is based on the KAME implementation, which has support for both protocol families, IPv4 and IPv6.
IPsec consists of two sub-protocols:
- Encapsulated Security Payload ESP), protects the IP packet data from third party interference, by encrypting the contents using symmetric cryptography algorithms (like Blowfish, 3DES).
- Authentication Header (AH), protects the IP packet header from third party interference and spoofing, by computing a cryptographic checksum and hashing the IP packet header fields with a secure hashing function. This is then followed by an additional header that contains the hash, to allow the information in the packet to be authenticated.
IPsec can either be used to directly encrypt the traffic between two hosts (known as Transport Mode); or to build “virtual tunnels” between two subnets, which could be used for secure communication between two corporate networks (known as Tunnel Mode). The latter is more commonly known as a Virtual Private Network (VPN). The ipsec(4) manual page should be consulted for detailed information on the IPsec subsystem in FreeBSD.
To add IPsec support to your kernel, add the following options to your kernel configuration file:
options IPSEC #IP security device cryptoIf IPsec debugging support is desired, the following kernel option should also be added:
options IPSEC_DEBUG #debug for IP security
15.9.2 The Problem
There is no standard for what constitutes a VPN. VPNs can be implemented using a number of different technologies, each of which have their own strengths and weaknesses. This section presents a scenario, and the strategies used for implementing a VPN for this scenario.15.9.3 The Scenario: Two networks, one home based and one corporate based. Both are connected to the Internet, and expected, via this VPN to behave as one.
The premise is as follows:- You have at least two sites
- Both sites are using IP internally
- Both sites are connected to the Internet, through a gateway that is running FreeBSD.
- The gateway on each network has at least one public IP address.
- The internal addresses of the two networks can be public or private IP addresses, it does not matter. They just may not collide; e.g.: may not both use192.168.1.x.
15.9.4 Configuring IPsec on FreeBSD
Written by Tom Rhodes.To begin, the security/ipsec-tools must be installed from the Ports Collection. This third party software package provides a number of applications which will help support the configuration.The next requirement is to create two gif(4) pseudo-devices which will be used to tunnel packets and allow both networks to communicate properly. As root, run the following commands, replacing the internal and external items with the real internal and external gateways:
# ifconfig gif0 create
# ifconfig gif0 internal1 internal2
# ifconfig gif0 tunnel external1 external2For example, the corporate LAN's public IP is 172.16.5.4 having a private IP of 10.246.38.1. The home LAN's public IP is 192.168.1.12 with an internal private IPof 10.0.0.5.
This may seem confusing, so review the following example output from the ifconfig(8) command:
Gateway 1: gif0: flags=8051 mtu 1280 tunnel inet 172.16.5.4 --> 192.168.1.12 inet6 fe80::2e0:81ff:fe02:5881%gif0 prefixlen 64 scopeid 0x6 inet 10.246.38.1 --> 10.0.0.5 netmask 0xffffff00 Gateway 2: gif0: flags=8051 mtu 1280 tunnel inet 192.168.1.12 --> 172.16.5.4 inet 10.0.0.5 --> 10.246.38.1 netmask 0xffffff00 inet6 fe80::250:bfff:fe3a:c1f%gif0 prefixlen 64 scopeid 0x4Once complete, both private IPs should be reachable using the ping(8) command like the following output suggests:
priv-net# ping 10.0.0.5 PING 10.0.0.5 (10.0.0.5): 56 data bytes 64 bytes from 10.0.0.5: icmp_seq=0 ttl=64 time=42.786 ms 64 bytes from 10.0.0.5: icmp_seq=1 ttl=64 time=19.255 ms 64 bytes from 10.0.0.5: icmp_seq=2 ttl=64 time=20.440 ms 64 bytes from 10.0.0.5: icmp_seq=3 ttl=64 time=21.036 ms --- 10.0.0.5 ping statistics --- 4 packets transmitted, 4 packets received, 0% packet loss round-trip min/avg/max/stddev = 19.255/25.879/42.786/9.782 ms corp-net# ping 10.246.38.1 PING 10.246.38.1 (10.246.38.1): 56 data bytes 64 bytes from 10.246.38.1: icmp_seq=0 ttl=64 time=28.106 ms 64 bytes from 10.246.38.1: icmp_seq=1 ttl=64 time=42.917 ms 64 bytes from 10.246.38.1: icmp_seq=2 ttl=64 time=127.525 ms 64 bytes from 10.246.38.1: icmp_seq=3 ttl=64 time=119.896 ms 64 bytes from 10.246.38.1: icmp_seq=4 ttl=64 time=154.524 ms --- 10.246.38.1 ping statistics --- 5 packets transmitted, 5 packets received, 0% packet loss round-trip min/avg/max/stddev = 28.106/94.594/154.524/49.814 msAs expected, both sides have the ability to send and receive ICMP packets from the privately configured addresses. Next, both gateways must be told how to route packets in order to correctly send traffic from either network. The following command will achieve this goal:
# corp-net# route add 10.0.0.0 10.0.0.5 255.255.255.0
# corp-net# route add net 10.0.0.0: gateway 10.0.0.5
# priv-net# route add 10.246.38.0 10.246.38.1 255.255.255.0
# priv-net# route add host 10.246.38.0: gateway 10.246.38.1At this point, internal machines should be reachable from each gateway as well as from machines behind the gateways. This is easily determined from the following example:
corp-net# ping 10.0.0.8 PING 10.0.0.8 (10.0.0.8): 56 data bytes 64 bytes from 10.0.0.8: icmp_seq=0 ttl=63 time=92.391 ms 64 bytes from 10.0.0.8: icmp_seq=1 ttl=63 time=21.870 ms 64 bytes from 10.0.0.8: icmp_seq=2 ttl=63 time=198.022 ms 64 bytes from 10.0.0.8: icmp_seq=3 ttl=63 time=22.241 ms 64 bytes from 10.0.0.8: icmp_seq=4 ttl=63 time=174.705 ms --- 10.0.0.8 ping statistics --- 5 packets transmitted, 5 packets received, 0% packet loss round-trip min/avg/max/stddev = 21.870/101.846/198.022/74.001 ms priv-net# ping 10.246.38.107 PING 10.246.38.1 (10.246.38.107): 56 data bytes 64 bytes from 10.246.38.107: icmp_seq=0 ttl=64 time=53.491 ms 64 bytes from 10.246.38.107: icmp_seq=1 ttl=64 time=23.395 ms 64 bytes from 10.246.38.107: icmp_seq=2 ttl=64 time=23.865 ms 64 bytes from 10.246.38.107: icmp_seq=3 ttl=64 time=21.145 ms 64 bytes from 10.246.38.107: icmp_seq=4 ttl=64 time=36.708 ms --- 10.246.38.107 ping statistics --- 5 packets transmitted, 5 packets received, 0% packet loss round-trip min/avg/max/stddev = 21.145/31.721/53.491/12.179 msSetting up the tunnels is the easy part. Configuring a secure link is a much more in depth process. The following configuration uses pre-shared (PSK) RSA keys. Aside from the IP addresses, both /usr/local/etc/racoon/racoon.conf files will be identical and look similar to
path pre_shared_key "/usr/local/etc/racoon/psk.txt"; #location of pre-shared key file
log debug; #log verbosity setting: set to 'notify' when testing and debugging is complete
padding # options are not to be changed
{
maximum_length 20;
randomize off;
strict_check off;
exclusive_tail off;
}
timer # timing options. change as needed
{
counter 5;
interval 20 sec;
persend 1;
# natt_keepalive 15 sec;
phase1 30 sec;
phase2 15 sec;
}
listen # address [port] that racoon will listening on
{
isakmp 172.16.5.4 [500];
isakmp_natt 172.16.5.4 [4500];
}
remote 192.168.1.12 [500]
{
exchange_mode main,aggressive;
doi ipsec_doi;
situation identity_only;
my_identifier address 172.16.5.4;
peers_identifier address 192.168.1.12;
lifetime time 8 hour;
passive off;
proposal_check obey;
# nat_traversal off;
generate_policy off;
proposal {
encryption_algorithm blowfish;
hash_algorithm md5;
authentication_method pre_shared_key;
lifetime time 30 sec;
dh_group 1;
}
}
sainfo (address 10.246.38.0/24 any address 10.0.0.0/24 any) # address $network/$netmask $type address $network/$netmask $type ( $type being any or esp)
{ # $network must be the two internal networks you are joining.
pfs_group 1;
lifetime time 36000 sec;
encryption_algorithm blowfish,3des,des;
authentication_algorithm hmac_md5,hmac_sha1;
compression_algorithm deflate;
}
Explaining every available option, along with those listed in these examples is beyond the scope of this document. There is plenty of relevant information in the racoonconfiguration manual page.The SPD policies need to be configured so FreeBSD and racoon is able to encrypt and decrypt network traffic between hosts.
This task may be undertaken with a simple shell script similar to the following which is on the corporate gateway. This file will be used during system initialization and should be saved as /usr/local/etc/racoon/setkey.conf.
flush; spdflush; # To the home network spdadd 10.246.38.0/24 10.0.0.0/24 any -P out ipsec esp/tunnel/172.16.5.4-192.168.1.12/use; spdadd 10.0.0.0/24 10.246.38.0/24 any -P in ipsec esp/tunnel/192.168.1.12-172.16.5.4/use;Once in place, racoon may be started on both gateways using the following command:
# /usr/local/sbin/racoon -F -f /usr/local/etc/racoon/racoon.conf -l /var/log/racoon.logThe output should be similar to the following:
corp-net# /usr/local/sbin/racoon -F -f /usr/local/etc/racoon/racoon.conf Foreground mode. 2006-01-30 01:35:47: INFO: begin Identity Protection mode. 2006-01-30 01:35:48: INFO: received Vendor ID: KAME/racoon 2006-01-30 01:35:55: INFO: received Vendor ID: KAME/racoon 2006-01-30 01:36:04: INFO: ISAKMP-SA established 172.16.5.4[500]-192.168.1.12[500] spi:623b9b3bd2492452:7deab82d54ff704a 2006-01-30 01:36:05: INFO: initiate new phase 2 negotiation: 172.16.5.4[0]192.168.1.12[0] 2006-01-30 01:36:09: INFO: IPsec-SA established: ESP/Tunnel 192.168.1.12[0]->172.16.5.4[0] spi=28496098(0x1b2d0e2) 2006-01-30 01:36:09: INFO: IPsec-SA established: ESP/Tunnel 172.16.5.4[0]->192.168.1.12[0] spi=47784998(0x2d92426) 2006-01-30 01:36:13: INFO: respond new phase 2 negotiation: 172.16.5.4[0]192.168.1.12[0] 2006-01-30 01:36:18: INFO: IPsec-SA established: ESP/Tunnel 192.168.1.12[0]->172.16.5.4[0] spi=124397467(0x76a279b) 2006-01-30 01:36:18: INFO: IPsec-SA established: ESP/Tunnel 172.16.5.4[0]->192.168.1.12[0] spi=175852902(0xa7b4d66)To ensure the tunnel is working properly, switch to another console and use tcpdump(1) to view network traffic using the following command. Replace em0 with the network interface card as required.
# tcpdump -i em0 host 172.16.5.4 and dst 192.168.1.12Data similar to the following should appear on the console. If not, there is an issue, and debugging the returned data will be required.
01:47:32.021683 IP corporatenetwork.com > 192.168.1.12.privatenetwork.com: ESP(spi=0x02acbf9f,seq=0xa) 01:47:33.022442 IP corporatenetwork.com > 192.168.1.12.privatenetwork.com: ESP(spi=0x02acbf9f,seq=0xb) 01:47:34.024218 IP corporatenetwork.com > 192.168.1.12.privatenetwork.com: ESP(spi=0x02acbf9f,seq=0xc)At this point, both networks should be available and seem to be part of the same network. Most likely both networks are protected by a firewall, as they should be. To allow traffic to flow between them, rules need to be added to pass packets back and forth. For the ipfw(8) firewall, add the following lines to the firewall configuration file:
ipfw add 00201 allow log esp from any to any ipfw add 00202 allow log ah from any to any ipfw add 00203 allow log ipencap from any to any ipfw add 00204 allow log udp from any 500 to any
Note: The rule numbers may need to be altered depending on the current host configuration.
pass in quick proto esp from any to any pass in quick proto ah from any to any pass in quick proto ipencap from any to any pass in quick proto udp from any port = 500 to any port = 500 pass in quick on gif0 from any to any pass out quick proto esp from any to any pass out quick proto ah from any to any pass out quick proto ipencap from any to any pass out quick proto udp from any port = 500 to any port = 500 pass out quick on gif0 from any to anyFinally, to allow the machine to start support for the VPN during system initialization, add the following lines to /etc/rc.conf:
ipsec_enable="YES" ipsec_program="/usr/local/sbin/setkey" ipsec_file="/usr/local/etc/racoon/setkey.conf" # allows setting up spd policies on boot racoon_enable="yes"
Note
VPN over IPsec