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I have a C# application that I've written that listens on port 789. It is running on a Windows XP Professional computer. Running

netstat -an | find "789"

TCP                LISTENING

When I run nmap -A -vv ip on a different linux machine (but same subnet) it only reports other ports open on the host (i.e. mysql, vnc, etc), but not the port opened by my application.

However, when I run nmap -p 789 ip I get:

789/tcp  open  unknown

Why is there a difference between the two nmap scans? Is there anything I can do to ensure that nmap detects the open port when doing a full scan?

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Please edit the message title to be relevant to the question being asked. – John Gardeniers Jun 4 '10 at 21:28
up vote 7 down vote accepted

By default, nmap only scans for "common" ports (the 1000 most common ports per protocol I believe). Since 789 is not a common port it isn't found. If you do

nmap -A -vv -p- ip

it should scan ports 1-65535.

Here is the nmap documentation that tells what ports are scanned by default:

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The docs I found disagree with you, as I mention below in my answer. – mfinni Jun 4 '10 at 18:50
@mfinni: Link? I just and looked it up and the documentation I found agrees with me: – MattB Jun 4 '10 at 18:53
You're right - the documentation I found was for an older version. Seems like it was changed sometime after 2006. – mfinni Jun 4 '10 at 18:55
Is there a way to tell what the 1000 most common ports are? It seems I also read the documentation thinking -A was between 1 and 1024. – Andrew Austin Jun 4 '10 at 19:02
@Andrew: there is a file called nmap-services that contains the information. There is a link to a description of the file/etc. in the documentation I linked. – MattB Jun 4 '10 at 19:08

I'm not sure. It is detecting it as "open", which is what you want right? So it is listening. Why it's not being found with -A, I'm not sure.

Edit - this is from old documentation that might not apply to your version. Run man nmap and let us know what version you have.

From the docs, I see :

"The default is to scan all ports between 1 and 1024 as well as any ports listed in the services file which comes with nmap."

So, 789 should be scanned, even if you don't have it in your services file (since it's labeled as "unknown", I'm sure it's not.)

Thus, I dunno. Check your man file for your version of nmap, maybe its behavior for which ports to scan using -A is different from the document I found?

This one is unlikely, but might be worth investigatin - The difference may be in your user ID on the Linux machine. Privileged vs unprivileged users of NMap have different default scan types. Users with root can do TCP SYN scans, users without will default to TCP connect() .

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The version is nmap 5.00 – Andrew Austin Jun 4 '10 at 19:03
-A doesn't add any additional ports, it just runs advanced options, including service detection, OS detection, tracerouting, etc. – Dentrasi Jun 4 '10 at 19:11

The only way you could do that is to modify your nmap's services file, which isn't recommended. The -A option runs advanced options, including service detection, OS detection, tracerouting, etc - it doesn't scan the full range. If you want to scan all 65k ports, use the option -p-. Other than that, you've just got to specify the port with -p 789. Your best option is probably to create an alias that does nmap -p 789, to save yourself time.

If you really need to change the services file (/usr/local/share/nmap/nmap-services by default), find the line that reads unknown 789/tcp 0.000075 and replace it with your program's name, and put the number on the end at 0.9, so it's the top port nmap includes. An alias would be much better though.

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