Setting up a VPN Gateway
This example shows an MS Windows 9x/2000 client point-to-site using SSH Communications Security Sentinel 1.1 (Public Beta 3). FreeS/WAN is interoperable with a wide range of IPSec implementations. The ease of implementation and computability will vary depending on the product. Many IPSec products that support 3DES/MD5 encryption through IKE are interoperable with FreeS/WAN. However, I found that legally obtaining fully functional IPSec implementations that support strong encryption can be arduous, especially if you live outside of the United States.
Many vendors offer only limited capabilities in their freely available IPSec implementations. For example, a product may only support weak encryption (DES) or may limit VPN capabilities to transport mode only. It is important to distinguish between the two VPN modes that are offered through IPSec: transport mode and tunnel mode. Transport mode encrypts and authenticates traffic between two fixed end points. Tunnel mode is more useful for connecting subnets and allows tunneling through firewall and router parameters into different subnets. Basically, transport mode restricts traffic to point-to-point communication. Tunnel mode also allows point-to-site (point-to-subnet) or site-to-site communications. At least one vendor does not seem to allow its implementation of IPSec to run over a connection using a static IP address.
The SSH Communications Security Sentinel product (www.ipsec.com) does not seem to suffer from any of these problems, possibly due to the fact that the company is based outside of the US. I downloaded and tested the 30-day trial beta 3 release of Sentinel 1.1 and found it to be very easy to configure on a Windows 98 desktop PC. The Sentinel documentation provides configuration examples for interconnectivity with a FreeS/WAN VPN gateway.
Here is a summary of a roadwarrior configuration that allows remote users with dynamically assigned IP addresses to connect transparently to a LAN behind a firewall. You will need to open ports 50, 51 (TCP) and port 500 (UDP) to the dynamic IP address or the ISP's DHCP address range. Figure 1 shows the basic setup. You will need to edit /etc/network.conf on the DUCLING FreeS/WAN firewall (go into lrcfg, choose 1), then 1) and set
to disable the blocking of tunneled packets. The bundled documentation contains the detailed instructions on how to do these tasks.
The contents of the FreeS/WAN ipsec.conf file are given in Listing 1. The corresponding ipsec.secrets file contains the entry
22.214.171.124 0.0.0.0: PSK "Put your roadwarrior secret string here"
where the phrase in quotes is a shared-secret string. The IP address 0.0.0.0 denotes any IP address, so remember to choose a secure shared-secret string. The rightsubnet and rightnexthop parameters are left blank and imply that the connection is a point-to-subnet connection.
To set up the Sentinel IPSec service:
Download SSH Sentinel from www.ipsec.com and install, following the instructions.
Go into the Sentinel Policy Manager (Figure 2).
Choose the Key Management tab, Authentication Keys and select Add (Figure 3).
Select Create a new preshared key then Next (Figure 4).
Type in your preshared key. It must be identical to the shared-secret string you have inserted in /etc/ipsec.conf (without the quotes). (See Figure 5.)
On the main console of SSH Sentinel Policy Manager, in the Security Policy pane, select VPN connections®Add.
Enter in the IP/hostname of the remote VPN gateway; for our example, it is 126.96.36.199, and choose the preshared secret that you created in step 5 as the Authentication key (Figure 6).
Select 3DES encryption, Main Mode and MODP 1024 for IKE Mode and IKE Group, respectively. The Advanced pane generally can be left with the defaults.
Set the IKE SA lifetime (i.e., the interval between rekeying) to the same value as in the ipsec.conf file, typically 480 minutes (eight hours).
Save all settings and try to ping an internal node behind the firewall (try the internal interface, 192.168.x.254). You should be connected. Try running Sentinel's diagnostics to make sure you are connected. I have found that Sentinel's diagnostic mode can hang the FreeS/WAN-Windows connections sometimes. If this happens, go to the FreeS/WAN gateway and do a restart of IPSec and then bring up the various connections.
Once again, if you need to restart the connection, log in to the LRP box and type
to restart the IPSec components.
I also found in Windows 2000 Professional (but not Windows 98) that I had to add the routing manually to the shared subnet 192.168.0.0/24 from the DOS console:
route ADD 192.168.0.0 MASK 255.255.255.0 188.8.131.52
(refer to the documentation for the Microsoft route command).
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In modern computer systems, privacy and security are mandatory. However, connections from the outside over public networks automatically imply risks. One easily available solution to avoid eavesdroppers’ attempts is SSH. But, its wide adoption during the past 21 years has made it a target for attackers, so hardening your system properly is a must.
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