AOIP - Anything Over IP » Internet

Static NAT overloaded???

Jay — Wed, 27 Jul 2011 05:24:51 +0000

So we have already looked at all 3 possible NAT configurations, however there is one more trick that is always useful to know. How to overload a Static NAT.

Let’s assume the following for this example

We have 2 public IP addresses (192.168.1.1 & 192.168.1.2)
The IP address on the outside interface has been configured to use PAT for all internal IP addresses for Internet access (192.168.1.1)
I have a DMZ with 3 servers, FTP, E-mail, and Web Server (10.0.1.1, 10.0.1.2 & 10.0.1.3 respectively)
I need my 3 DMZ servers to be reachable from the Internet.

The above scenario poses a slight problem. If I have already used one of my public addresses for PAT to allow all internal hosts to access the Internet, I only have one IP address left but I require 3 static NAT entries to be created. In my post on Static NAT we saw that we configure NAT to map on a one-to-one basis, so in this scenario I would require 3 IP addresses, one for each of my DMZ servers.

The nice thing about the above scenario, is that each of the three servers is hosting a totally different service and therefore each requires different ports to be accessible from the Internet. This allows me to create static NAT’s that specify the ports, a type of overload function.

FTP would require ports 20,21 to be allowed
E-mail would require port 25 to be opened, and possibly 143 and 110 if you are using IMAP or POP
Web Server will require port 80, and possibly 443 if there is any SSL been used (https).

The above can be configured in the following way (interfaces would need to be configured as inside and outside as well, as seen here)

AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.1 192.168.1.2 20
AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.1 192.168.1.2 21

AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.2 192.168.1.2 25
AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.2 192.168.1.2 143
AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.2 192.168.1.2 110

AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.3 192.168.1.2 80
AOIP.ORG (config) # ip nat inside source static tcp 10.0.1.3 192.168.1.2 443

Configuring PAT on Cisco Routers (NAT Overload)

Jay — Wed, 20 Jul 2011 14:06:58 +0000

PAT (Port Address Translation) is by far the most common implementation of NAT, and if you have an ADSL router at home there is a 100% chance you are using it.

PAT or otherwise known as NAT overload, allows you to translate IP addresses in a many-to-one method.
In my previous post on Configuring Dynamic NAT we saw that we can NAT many-to-many but this was limited by the amount of public addresses that you have available. In cases such as home ADSL, your ISP will only issue you with a single public IP address but you might have 2 or more devices that need to access the Internet at any given time. This is where PAT takes over and makes this all possible.

As with any NAT configuration we need to first define our inside and outside interfaces. In this example I’ll use FastEthernet 0/0 as my inside, and Serial 0 as my outside.

AOIP.ORG (config) # interface FastEthernet 0/0
AOIP.ORG (config-if) # ip nat inside
AOIP.ORG (config-if) # interface Serial 0
AOIP.ORG (config-if) # ip nat outside

The next step is to define which addresses in my inside network I want to allow to be translated. Let’s assume my inside IP address range is 10.0.1.0 /24

AOIP.ORG (config) # access-list 1 permit 10.0.1.0 0.0.0.255 (Using a standard access-list is the easiest way to achieve this)

Then I need to configure the address that will be used by my internal IP addresses for accessing the outside interface. This can be done in 2 ways.

Option 1:
If I only have 1 public IP address, which is the case with home ADSL, the router will already have that IP address allocated to it by your ISP. The only thing I can do is tell the router to share that address with my internal hosts.

AOIP.ORG (config) # ip nat inside source list 1 Serial 0 overload (This defines my access-list 1 as the source addresses, and tell them to be translated into the same IP address that is configured on Serial 0. The overload command tells the router that it needs to keep track of all the source and destination ports so the IP address can be used multiple times, overloaded)

Option 2:
If I have a second public IP address that I would like to use for Internet browsing, I can configure PAT for that IP address.

AOIP.ORG (config) # ip nat inside source list 1 192.168.1.1 overload (Same as the above command, but I’ve specifically told the router which IP address to translate my internal hosts into)

This option is fantastic if you have multiple public addresses and you want to segment your Internet browsing based on departments or geographic locations. For example
Marketing – 10.1.0.0 /24
Sales – 10.2.0.0 /24
Technical – 10.3.0.0 /24

I can have each of the above departments using their own public IP address, which will make log files easier to read when tracking Internet use and for troubleshooting connection errors.

access-list 2 permit 10.1.0.0 0.0.0.255
access-list 3 permit 10.2.0.0 0.0.0.255
access-list 4 permit 10.3.0.0 0.0.0.255
ip nat inside source list 2 192.168.1.2 overload
ip nat inside source list 3 192.168.1.3 overload
ip nat inside source list 4 192.168.1.4 overload

Configuring Dynamic NAT on Cisco Routers

Jay — Fri, 15 Jul 2011 04:00:58 +0000

In my last post Configuring Static NAT on Cisco Routers we saw how you can translate 1 IP address into another single IP address. This tutorial will cover how to translate many IP addresses into many IP addresses, otherwise referred to as many-to-many translation.

Dynamic NAT allows us to translate many IP addresses into a pool of many IP addresses. The big thing to realize here is that the pool does not need to contain enough IP addresses to translate all the internal addresses at the same time, as would be the case if we used Static NAT. Dynamic NAT allows internal hosts to be translated into an IP address in the pool when it requires a connection. Once the internal host has finished it’s session the NAT entry is removed from the NAT table allowing another internal host to use the external IP address for it’s session.

Assume we have 50 hosts in our inside network but only have 5 public IP addresses available to use. With Dynamic NAT we can allow all 50 internal addresses to share the 5 public addresses as and when they need them. This of course does impose a limit of only 5 simultaneous connections to the outside world and that is where PAT would come in and solve that problem.

On of the benefits of using Dynamic NAT vs Static NAT, is that Dynamic NAT requires the session to originate from the inside network. No outside connections can be established to the inside network. This is obviously a more secure solution as connections from the outside won’t work; only traffic originating from the inside will be translated. Static NAT is different in the fact that the entry is added to the NAT table on a permanent basis and will allow connections in either direction.

Here are the steps to configure Dynamic NAT on a Cisco Router.

Step 1 : I need to define the IP address range that will be translated (my inside IP addresses). I can do this with a standard access-list

AOIP.ORG (config)# access-list 1 permit 10.0.1.0 0.0.0.255 (don’t forget, access-lists use wildcard masks, not subnet masks)

Step 2 : I need to configure the range of addresses that my internal network will be translated into by using a NAT pool.

AOIP.ORG (config) # ip nat pool MY_POOL 10.50.1.1 10.50.1.5 netmask 255.255.255.0 (There are 5 IP addresses that can be used for translation in this example)

Step3 : Define inside and outside interfaces

AOIP.ORG (config) # interface FastEthernet 0/0
AOIP.ORG (config-if) # ip nat inside
AOIP.ORG (config-if) # interface Serial 0
AOIP.ORG (config-if) # ip nat outside

Step 4 : Configure the translation to take place.

AOIP.ORG (config) # ip nat inside source list 1 pool MY_POOL (List 1 is my access-list that defined my inside IP addresses, MY_POOL defined the IP addresses to be used for the translation)

Configuring Static NAT on Cisco Routers

Jay — Thu, 07 Jul 2011 09:00:44 +0000

In my previous post on NAT, I explained the difference between the 3 different types of NAT that can be configured. In this tutorial I’m going to cover the configuration steps to configure static NAT.

Static NAT is a one-to-one mapping. It allows us to translate a single IP address into a different single IP address. This is most commonly found when you have a server inside your DMZ that you would like to allow the outside world (The Internet) to connect to, such as E-mail servers, FTP servers and Web servers (if you’re hosting your own).

The first step in configuration static NAT, is to define which interfaces on your router are involved in the NAT process and then configuring your Cisco router to know which interface is on which side of the network. Your Cisco router needs to know which interface is the inside interface and which is the outside interface to allow the translation to take place.

For example purposes let’s assume that FastEthernet 0/0 is the inside interface, and Serial 0 is my outside.

AOIP.ORG > en
AOIP.ORG # conf t
AOIP.ORG (config)# interface FastEthernet 0/0
AOIP.ORG (config-if)# ip nat inside
AOIP.ORG (config-if)# interface Serial 0
AOIP.ORG (config-if)# ip nat outside

So we have just informed our Cisco router of the inside and the outside, the next step is to tell your Router how to translate and what to translate.

Let’s assume that I have a server in my DMZ that has an IP address of 10.0.1.1 and I have a public IP address of 192.168.1.1 (yes I know this a private range part of RFC 1918, but for example purposes, let’s assume it’s not).

AOIP.ORG (config)# ip nat inside source static 10.0.1.1 192.168.1.1

That’s it, your done. When your server 10.0.1.1 connects to anything on Serial 0 and beyond, the source IP address will be translated into 192.168.1.1. Similarly, when someone from the Internet connects to the IP address 192.168.1.1 it will be translated into a destination IP address of 10.0.1.1 and hence connect to our server in the DMZ (Access-list permitting).

Hosting your own website.

Jay — Mon, 29 Jun 2009 06:00:08 +0000

I have recently been asked quite a few times, “How do you start your own website?”

The first and most important, is to find a domain name that you want, and is still available. This can be done in quite a few places but before you even begin looking you need to decide what extension you want your site to have. .com? .net? .org? etc. Often the choice of the extension is due to the fact that the .com might not be available, but the .org or .net might be. If can be a concern for some, as people might ‘piggy back’ off of your name and branding.

Often it’s not a matter of which one you would like but which domain name can your register ALL of them for. This is a little more tricky as so many domain names are already taken.

The first step is to write down domains names that appeal to you, and decide if which extension you are looking for, and then begin the hunt. The site I use for looking for availability is http://whois.net it’s a quick and easy site to use and gives you the results immediately.

Secondly, once you have found a domain name you would like and it is available, you want to find a hosting company that will host your site. DON’T register your domain name just yet as most hosting companies will include a domain name in the package for free.

I have been with my hosting company – ‘Imhosted’ – since 2004 and so far have been impressed with their offerings and their service. For less than $10 a month – cheaper if you pay for longer time periods -you will get unlimited bandwidth, unlimited disk space, unlimited domain names associated with your account so you can run multiple different website, unlimited e-mails accounts, ftp accounts, SQL databases etc. They also have a very easy to use interface called ‘cpanel’ and they support a large amount of self installing applications like WordPress.

It should not take more than about 5 minutes to set up the account and get your domain name, then about 48 hours for your domain name to register and replicate worldwide, and your site is ready for you to start publishing information.

Apart from learning how to code in php, css, html etc. The quickest way to get a site onto the web is using applications like WordPress. http://wordpress.org