hosts file ignored, how to troubleshoot?

Question

The hosts file on Windows computers is used to bind certain name strings to specific IP addresses to override other name resolution methods.

Often, one decides to change the hosts file, and discovers that the changes refuse to take effect, or that even old entries of the hosts file are ignored thereafter. A number of "gotcha" mistakes can cause this, and it can be frustrating to figure out which one.

When faced with the problem of Windows ignoring a hosts file, what is a comprehensive troubleshoot protocol that may be followed?

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This question has duplicates on SO, such as hosts file seems to be ignored, HOSTS file being ignored, /etc/hosts file being ignored as well as numerous discussions elsewhere.

However, these tend to deal with a specific case, and once whatever mistake the OP made is found out, the discussion is over. If you don't happen to have made the same error, such a discussion isn't very useful. So I thought it would be more helpful to have a general protocol for resolving all hosts-related issues that would cover all cases.

Answer 1

Based on my own experience and what I encountered while Googling, here are some things to try:

1. Did you check that it works correctly?

Changes to hosts should take effect immediately, but Windows caches name resolution data so for some time the old records may be used. Open a command line (Windows+R, cmd, Enter) and type:

ipconfig /flushdns

To drop the old data. To check if it works, use (assuming you have an entry in your hosts for www.example.com):

ping www.example.com -n 1

And see if it uses the correct IP. If yes, your hosts file is fine and the problem is elsewhere.

Also, you can reset the BetBios cache with (open the console as an admin or it will fail):

nbtstat -R

You can check the current data in the DNS cache with:

ipconfig /displaydns | more

2. Basics

• Is your hosts file named correctly? It should be hosts and not host, etc.
• Is the extension correct? It should have no extension (hosts not hosts.txt) - be careful if you have configured windows to hide known extensions, check the properties to be sure: The correct hosts file's type will show up as just "File".
• Did you follow the correct syntax? Did you accidentally prefix lines with a hash (#) which indicates comments?
• Did you take care of all variants (www.example.com vs. example.com - safest to just add both)?

3. Whitespace

The format for each line is IP address, then a tab (\t or ASCII HT/0x09) or a single space (ASCII 0x20), then "CRLF" - a carriage return character followed by a line feed character, so each line terminates with 0x0d0a.

The individual bytes may be viewed in Notepad++ with the hex editor plugin. Notepad++ will also show special characters (View -> Show Symbol) so you can easily inspect the number and kind of whitespace characters.

If you copied and pasted hosts entries from somewhere, you may end up with multiple spaces. In theory hosts supports multiple spaces separating the two columns, but it's another thing to try if nothing else works.

To be on the safe side, make sure all lines in your hosts file either use tabs or spaces, not both.

Lastly, terminate the file with a blank line.

4. Registry Key

There is a registry key specifying the location of the hosts file. Supposedly, Windows doesn't actually support putting the hosts file in other locations, but you might want to check. The key is:

\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\DataBasePath

The entry should be:

%SystemRoot%\System32\drivers\etc

5. Permissions

Sometimes there are issues with permissions on the file, the file attributes, and similar things. The easy way to deal with this is:

1. Create a new text file on your desktop.
2. Copy and paste the contents of your current hosts file into this file in Notepad.
3. Save the new text file and rename it to hosts.
4. Copy (do not move) the file to your drivers/etc directory, and overwrite the old file.

Last point is important: Copying works, moving doesn't.

6. Encoding

The hosts file should encoded in ANSI or UTF-8 without BOM. You can do this with File -> Save As.

7. Proxies

If you have a proxy configured, it may bypass the hosts file. The solution is to not use the proxy, or configure it to not do this.

To check, go to your Internet Explorer -> Internet Options -> Connections -> LAN settings. If everything is blank and "Automatically detect settings" is checked, you aren't using a proxy.

8. DNS address

(This may also resolve proxy issues.)

Go to your network connections properties, then TCP/IP settings, and change the first DNS server to 127.0.0.1 (localhost). The second should probably be your actual DNS's IP.

This is not necessary for the hosts file to work, but it may help in your case if something is configured strangely.