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Wanted Cisco Router 1721 Security polices


I have just installed Cisco 1721 on a public network ( internet connectity ) for a small office i have been given 20 ips from my ISP to use .

As of now i have done a NAT between my office ip and ISP ips .

What security measures I can undertake to gaurd against any untoward incident on my office network. please give security settings.

Can any one inform me on how PAT ( Port Address Translation works ) and how to do it , is PAT more secure than NAT.

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1 Solution
Pat is not more secure than NAT, and it is more difficult to manage, and will break some connections.
Need a few more details from you:
What IOS feature set do you have? IP Only? Firewall?
Do you have internal servers that need to be accessed from the outside? Email? Web? DNS? FTP? POP3?
Do you have internal users that need to make VPN connections to external servers?

If you have the firewall feature set, you can enable Intrusion Detection features also.

Here are some guidelines for securing an external router, not including FW and/or IDS features:
Nat/Pat discussion:
Make sure the IOS is fairly recent General Deployment (GD) version.

Apply a telnet access-list to the router:

access-list 25 permit
line vty 0 15
 access-class 25 in

Switch to PAT if possible (not using VPN, H.323):

ip nat inside source list 1 interface Serial0/0 overload
access-list 1 permit

The reason is, PAT makes it pretty much impossible for Internet devices to create a connection to your internal hosts.  If you are using one-to-one NAT for every device, you have to rely on your access-lists much more to provide security or implement firewall IOS properly which can be confusing if you are inexperienced.  You will still need static NAT translations for any servers like e-mail, but if you only have 1 of each type, they can all be done using the same IP address (port forwarding), like this:

ip nat inside source static tcp 25 interface Serial0/0 25

The interface command in the above statement tells the router to use the IP address assigned to the Serial 0/0 interface for its NAT translations.  Make sure to coordinate your public DNS changes when making these changes, or you will be in trouble.  Have your ISP set cache times for the zone down low.  Or, you could use your current IPs and just do port forwarding for the necessary services by substituting "interface Serial 0/0 25" with "x.x.x.x 25".

There are a few more things to consider, like disabling/restricting SNMP and HTTP access to the router.  You should also setup good password security.  After you get finished with your config, test the router from the internet with a port scanner, like YAPS to make sure it is secure.

If you host an IIS server, there are few more nice things you can do with NBAR to provide more security for it.  The above are just the basics.

Good luck!
Check the following link from Cisco on improving security on Cisco routers.

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