nbtscan - NETBIOS nameserver scanner

Monday, March 31, 2014

This is a command-line tool that scans for open NETBIOS nameservers on a local or remote TCP/IP network, and this is a first step in finding of open shares. It is based on the functionality of the standard Windows tool nbtstat, but it operates on a range of addresses instead of just one. I wrote this tool because the existing tools either didn't do what I wanted or ran only on the Windows platforms: mine runs on just about everything.

NETBIOS is commonly known as the Windows "Network Neighborhood" protocol, and (among other things), it provides a nameservice that listens on UDP port 137. When it receives a query on this port, it responds with a list of all services it offers. Windows ships with a standard tool nbtstat which queries a single IP address when given the -A parameter. When run against a machine on the local network (a development box), it shows:

C:\> nbtstat -A
       NetBIOS Remote Machine Name Table

   Name               Type         Status
XPDEV          <00>  UNIQUE      Registered
UNIXWIX        <00>  GROUP       Registered
XPDEV          <03>  UNIQUE      Registered
XPDEV          <20>  UNIQUE      Registered
UNIXWIX        <1E>  GROUP       Registered

MAC Address = 00-50-04-6D-50-37

The numeric code (in hexadecimal) and the type serve to identify the service being offered, and (for instance) a UNIQUE code of <20> indicates that the machine is running the file-sharing service. Unfortunately, nbtstat only reports the codes, and it requires looking up the meanings elsewhere. The References section at the end of this document lists some resources to learn what all the codes mean.

Machines participating in NETBIOS listen on UDP port 137 for these queries and respond accordingly. Simple configurations might only have a few resource records (as above), but an NT server supporting a large enterprise could easily have more than a dozen. Though it's sometimes useful to examine the full set of resource records for a given machine, in practice it's more useful to summarize them into the key "interesting" services.

Our tool has taken this approach. Not only does it scan ranges of addresses -- instead of just one machine -- but it can fully decode most of the resource record types and can summarize the interesting data on a one-line display.

On our network we have quite a few machines, but it appears that only three respond to our queries:
C:\> nbtscan     MTNDEW\WINDEV               SHARING DC     MTNDEW\TESTING     MTNDEW\WIZ                  SHARING U=STEVE    MTNDEW\XPDEV                SHARING

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