Hosts and LMHosts files

Though rare, there still is an occational need or advantage to using Hosts and LMHosts files.  Though simple text files, the syntax used is critical.  The following is a repost from an older blog of mine.  I have done so in hopes that it may be helpful to some folk, and also it will be useful in and upcoming post regarding Hyper-V configuration and management.

There are two files in the %systemroot%\system32\drivers\etc directory that can be used for name resolution. The Hosts file, used for DNS name resolution, and the LMHosts.sam file used for NetBIOS name resolution. In an age where DNS dominates your network both locally and throughout the Internet, these two files are seldom ever used, but they can be very useful in a few situations. Both are simple text files that match names to IP addresses, and are very easy to create and implement. Most people are familiar with these files, but are often frustrated when they do not work as expected. This is usually due to the fact that they have some very simple, but specific requirements, for them to work at all.

LMHosts (NetBIOS names):

The primary use today for an LMHosts file, is for name resolution over a VPN. If DNS is configured on the host and client machines there should be no need of static text files for resolving names, but it does work well, and many folks uses them as a dependable simple solution. The catch is there are a few gotcha’s you need to be aware of:

• The LMHosts file in it’s default form, is named lmhosts.sam The .sam represents sample. If you are planning on using this file it needs to be saved without a file extension – lmhosts. Be careful if using a text editor like NotePad as it will add a .txt file extension to the name. The safest method is to save the file using quotes, “lmhosts” to assure no extensions are added.
• Text following a # in the LMHosts file is a comment and can be ignored or deleted.
• A typical entry would look like:

192.168.100.101 COMPUTERNAME #PRE #my notes

• When making a new entry you must hit enter at the end of the line, which adds a “carriage return”
• Use the Tab key between items in each line rather than spaces (recommended but not necessary), but there must be a space.
• Though it is not needed, adding the #PRE parameter or extension will load the entry into the local NetBIOS name cache when the computer boots. This allows for a slightly faster resolving of the name, before it has been added to the cache.
• Most entries such as #PRE, #DOM, DOMAIN names and such are case sensitive. Always use uppercase to be safe.
• It is a good idea to add the domain name as well. This requires two lines, and uses extremely critical formatting. A sample would be:

192.168.100.10 DCNAME #PRE #DOM:YOUR-DOMAIN
192.168.100.10 “YOUR-DOMAIN x1b” #PRE

There must be exactly 20 characters, including spaces, between the quotes in “YOUR-DOMAIN x1b”, and the spaces need to be between the domain name and the x1b. The domain name used is the NetBIOS name, not the FQDN. If your domain name exceeds 15 characters, you must truncate it at the 15th character, it will still work.

If you find this last step tedious, the University Of Sait Louis has a little Java script that will create these two lines for you: http://bflinux.slu.edu/LSI/tools/lmhosts.html

Hosts (DNS names):

The Hosts file today seems to be more used for blocking unwanted web sites. This is done by simply entering the website address and substituting the IP address with the localhost IP address such as:

127.0.0.1 www.unwantedsite.com#advertising site

There are subscription services that will actually update your Hosts file, according to a schedule, with a list of known unwanted sites.

Another handy use of the Hosts file is to create abbreviations for your own use. For example quick access to a website like Google can be achieved adding an abbreviation like ‘G”, or your firewall with ‘F”, or I frequently uses it for accessing client sites with remote desktop using 3 letter acronyms such as ACL for Acme Corp Ltd. :

64.233.187.99 G #Google

192.168.100.254 F #my firewall

123.123.123.123 ACL #Acme Corp Ltd

Most of the rules that apply to the LMHosts file, apply to the Hosts file as well:

• The Hosts file already has no file extension, so there is no need to change it like the LMHosts file. Just be careful when editing you don’t accidentally add one such as .txt when using NotePad or a similar editor.
• Text following a # in the Hosts file is a comment, and can be ignored or deleted
• A typical entry would look like:

123.123.123.123 ftp.acme.com#company FTP site

• When making a new entry you must hit enter at the end of the line, which adds a “carriage return”
• Use the Tab key between items in each line rather than spaces (recommended but not necessary), but there must be a space.

Keep in mind when editing these files you can run into conflicts, or ineffective changes if you do not reboot or purge the local name caches. To clear the NetBIOS cache ( and PREload), at a command line enter (R is case sensitive):

nbtstat -R

To clear the DNS cache, at a command line enter:

ipconfig /flushdns

For the record, the Hosts file can also be used for IPv6 addressing. Vista for example includes the IPV6 localhost entry, by default:

::1 localhost