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“Hosts” Entry

Created: 12 May 2009 • Updated: 14 May 2009 | 1 comment
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Karthikeyan Sundaram's picture
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This article is about “how Hosts file playing a main role in Netbackup Environment”

Here is something about Hosts file from "Wikipedia"

What is Host File?
The hosts file is a computer file used to store information on where to find a node on a computer network. This file maps hostnames to IP addresses. The hosts file is used as a supplement to (or a replacement of) the Domain Name System (DNS) on networks of varying sizes. Unlike DNS, the hosts file is under the control of the local computer's administrator.

Where it is located?
The hosts file is located in different locations in different operating systems and even in different Windows versions:
• Windows NT/2000/XP/2003/Vista/7: %SystemRoot%\system32\drivers\etc\ is the default location, which may be changed. The actual directory is determined by the Registry key \HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\DataBasePath.
• Windows 95/98/Me: %WinDir%\
• Linux, BSD based, and other Unix-like operating systems: /etc/hosts

How IP resolution works on Windows Environment?

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Network can identify the hosts by using the IP Address only. Whenever you to try a reach a particular client with Host name, your operating system will check for the entry in Hosts file to get the relevant IP Address. If it finds the relevant IP in Hosts file then it will pass that IP Address to nearer switch; and switch will re direct the packets to the relevant host. So the Hosts file has high Priority than the DNS because it is controlled by Local system.

How “Hosts” file makes your computer to perform malicious operations?

Normally System Admin guys browse lots of sites for troubleshooting purpose, and from some sites computer viruses, Trojan horses can walk into computer and change “Hosts” file to redirect to an unwanted sites. Normally these issues can happen while doing Symantec Antivirus update. When I was studying in college, we got Symantec antivirus in our Lab Setup. Lab systems get updated from Symantec Antivirus Site. Some viruses changed the “Hosts” file that leads Lab systems to disconnect from Symantec Antivirus update server, because of this our Lab systems not maintained with proper updates.

How is this “Hosts File” playing in Netbackup?

Here is the Play:

Our environment got lot of Windows and UNIX clients and we use Host names to handle those clients. And Our Backup setup uses a separate network for backup usage. All Client servers having 2 NIC Cards; 1 for Public access and other for Backup usage. To take the advantage of Backup network, we have made “Hosts” file Entries pointing to Backup network IP on all clients. So whenever I try to reach a client from master server, master will automatically get into backup network.

On a day I got mail from Windows admin Guys to remove a particular server from regular backup schedule, I opened my master server and searched for that client under Host properties->Clients; by mistake enter key got down and Client properties window opened. My mind got hanged for a second, and then I tried to reach that client by ping command; and reply was “Reply from” and also I was able to telnet that client in bpcd port.

As basic level troubleshooting; I have verified the Original client IP with “Reply from” IP address, and there I found the issue. Issue was My Master is reaching a client other than the client that am trying to remove from backup.

After this, as a next footstep I have verified Hosts file; because Backup network IP entries in “Hosts” may make this to point to a different client. And my assumption got Solution grade :)

The client am trying to reach is pointed with different IP Address other than the Original IP and that was non backup test client; When I was troubleshooting this issue my team lead sitting behind me. With a red-faced smile he asked me to open the BAR window.

That was my Bad day, when I open the BAR Window I could find only the files of Test client for last 1 year.

Then what else; decided to surf into this sites Naukri, timesjobs. But because of my knowledge and team work I got one more chance to stay back in my company.

I should Thank God for this.

So, Guys if you are using Host names to handle clients in your Backup environment please do remember this below things:

 Don’t touch the “Hosts” file in any cause.
 Suppose if you need to edit that file, please be careful in doing that.
 As my environment if you have edited “Hosts” file with backup network IP’s; do verify them in frequent. Because changes happening by mistake can spoil your backup.
 If you have backup network to be used, then change the client properties in way that, it uses a specified network interface; don’t go for “Hosts” file change. (Host Properties->Clients->Client name->Universal settings->use specified network interface);

I hope you guys won’t surf those sites for these kinds of issues.

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Gopinath Subramanian "KSG"'s picture

Very Nice Article

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