DNS Scavenging

April 27, 2012 Leave a comment

Most of you reading this will already know what DNS Scavenging is. For those of you that don’t it is a feature of Windows DNS servers which allows you to automate the deletion (scavenge) of outdated DNS resource records. It sounds simple enough and it is once you decipher Microsoft’s cryptic descriptions, but to the uninitiated it is just plain confusing and often leads to unpredictable results. This article will attempt to simplify how DNS scavenging works.

Enabling DNS Scavenging on Zones

The first thing we need to do is enable Scavenging. This is done separately for each DNS zone. Read more of this post

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How To Fix Boot/BCD 0xc000000f Error Windows 7

April 17, 2012 96 Comments

This article is specific to Windows 7 but similar errors occur on Vista and Windows 2008/R2.

After rebooting your computer you receive the following error:

Windows Boot Manager
Windows failed to start. A recent hardware or software change might be the cause. To fix the problem:
1.insert windos cd and run a repair your computer option.
File: /boot/bcd
Status: 0xc000000f
Info: an error occurred while attempting to read the boot configuration data.

You may receive a similar error which lists the error code as 0xc0000034 or missing file: \windows\system32\winload.exe instead.

You may also receive further errors after trying to repair the bcd store using bootrec and bcdedit as follows.

The requested system device cannot be found.
The “Windows Boot Configuration Data File is Missing Required Information Read more of this post

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“A Required CD/DVD Device Driver Is Missing” For USB Installs

January 10, 2012 18 Comments

You receive the error “A required CD/DVD device driver is missing” when installing Windows 7 or Windows 2008/R2 from a USB device. Furthermore if you browse to install drivers you will see that your CD/DVD drive is actually installed successfully.

You are using a USB 3 Port

The error is very misleading; the problem is not that the CD/DVD device drivers are missing, it is that USB 3 drivers are missing. Most likely you have plugged your USB drive into a USB 3 port but the problem here is that Windows 7/2008/R2 doesn’t ship with USB 3 support. The solution is very simple; just plug your USB drive into a USB 2 port and start the install again.

Why does this happen?

The initial boot up will work fine but once Windows starts installing drivers it will fail for the USB 3 hardware. At this point the installation can no longer see the USB pen resulting in the error. By booting up on a USB 2 port Windows has the required drivers to continue the installation successfully. As for the “A required CD/DVD device driver is missing” error I can only guess this is because the install expects to be installed from the CD/DVD drive. Once it can’t load the drivers for the USB device it is as if the CD/DVD drive has disappeared resulting in the “A required CD/DVD device driver is missing” error.

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VPN Passthrough and How It Works

September 28, 2011 Leave a comment

VPN Passthrough is a feature of routers which allows computers on a private network to establish outbound VPNs unhindered. VPN passthrough has nothing to do with inbound VPNs, only outbound ones. The term comes from allowing the VPN traffic to “passthrough” the router. NO ports need opening to enable VPN passthrough, it will automatically work.

There are different types of VPNs but the most dominant ones are PPTP VPNs and IPsec VPNs. When a router states it supports VPN passthrough it actually means it supports both these types of VPNs. To be exact VPN passthrough is just the combination of PPTP passthrough and IPsec passthrough. This is important as you will see next because PPTP passthrough is handled differently to IPsec Passthrough. Read more of this post

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Symmetric NAT and It’s Problems

September 19, 2011 29 Comments

This article continues on from where Types of NAT left off. A symmetric NAT applies restrictions exactly the same way as a port restricted cone NAT but handles the NAT translation differently. All types of NAT discussed so far don’t change the source port when NATing connections. For example when a client accesses the Internet using IP 192.168.0.1 and source port 56723 NAT changes the source IP to say 56.35.67.35 but keeps the port number the same; this is known as port preservation. Read more of this post

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Types Of NAT Explained (Port Restricted NAT, etc)

September 16, 2011 Leave a comment

All types of NAT fall into two categories; Static NAT and Dynamic NAT. Static NAT is where administrators manually create and maintain the NAT mappings and is usually associated with inbound types of NAT. Dynamic NAT is where the router creates and maintains mappings automatically on demand and is usually associated with outbound types of NAT. Read more of this post

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How IP Routing Works

August 24, 2011 18 Comments

Welcome to the final part in a 3 part series of articles about routers and routing . Previously I wrote about how routers work and the routing table in parts 1 and 2 respectively. I am now going to talk about IP routing and how you can manipulate routes to “direct” traffic. Read more of this post

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The Routing Table

August 24, 2011 3 Comments

Welcome to part 2 in a 3 part series of articles about routers and routing . If you have arrived here directly through a search engine you may wish to read Part 1 – How Does a Router Work? first. Here we are going to look at the routing table.

All network devices that use the TCP/IP protocol have a routing table, even your Windows PC has one. ALL devices use their routing table to determine where to send packets. Without a routing table your PC wouldn’t even be able to communicate with computers on the same subnet. Here is a screenshot of the routing table of my PC. To see your own routing table open a command prompt by typing CMD in the run or search box. Then at the command prompt type “”route print” and press enter. Read more of this post

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Network Address Translation

August 19, 2011 5 Comments

NAT RouterThis article assumes you have an understanding of computer networking basics.

Network Address Translation has several advantages but its primary goal is to allow a single Internet IP address to be shared on a network by multiple devices. Your home router has built in NAT capabilities and does all this automatically. It works by your ISP assigning you ONE IP address to your router, NAT then allows multiple computers to access the Internet through this shared IP address. Read more of this post

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Why Multiple VPN Connections To The Same IP Fail

August 9, 2011 7 Comments

In a previous article I explained what PPTP passthrough is and how it works. In this article I will explain why multiple VPN connections fail with certain routers. This issue only affects PPTP connections and it is directly related to PPTP passthrough.

Here is a brief comparison of how NAT handles PPTP VPN connections differently to normal connections. Read the PPTP passthrough link above for more details:

  • When computers make normal outbound connections the source IP address is NATed to the public IP. Source ports are used  to uniquely identify the multiple connections.
  • When PPTP clients make outbound connections the same thing happens but the call ID AND destination IP is used instead of source ports to uniquely identify the VPN connections. Read more of this post

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