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sal
Asked: 2009-05-02 08:14:33 +0800 CST

List of IPs that are not in DNS

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What's the easiest way to get a list of windows desktops that are on a subnet but do not have names listed in the dns server?

domain-name-system networking tcpip

5 Answers

  1. Best Answer

    You could use nmap

    nmap 192.168.1.0/24

    would give you a listing of all the IPs being used on the subnet, along with their open ports.

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  2. The nmap tool will do both. However, get permission in writing to run it before you do so. Most organizations would classify it a security tool (it is) and most have explicit policies about getting permission before executing such tools.

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  3. The following is my follow up to the selected answer.

    If you don't need the port scan info, -sP will just check if the host is up. You might want to specify which DNS server with --dns-servers. If a machine is offline but has a DNS entry, -R might be useful. My suggestion for a fast scan is:

    nmap 192.168.0.1/24 -sL -R | grep -v '(.*)' | awk '{print $2}'

    There are better ways to grep the output, but this does the job quite well. -sL doesn't even check that the machine is up, it just runs through the list of IPs, -R sends DNS queries for all IPs, not just ones that are up - nmap won't know which are up and does not do DNS queries on down hosts by default. The grep outputs only those lines without prens -- those without DNS entries; remove -v to get only those with DNS entries, but you'll have to change the the grep/awk to parse the IPs it that case.

    Output example:

    $ nmap 192.168.0.1/24 -sL -R | grep -v '(.*)' | awk '{print $2}'
192.168.0.0
192.168.0.1
192.168.0.2
192.168.0.3
# ...

    Those are the IPs that have no DNS entries.

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    1. get a list of your Windows desktops and their names
    2. for each one, ask the DNS server if they're listed...

    If you can't do (1), you've got bigger problems...

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  5. Advanced ip scanner might help.

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