Re: nmap scans on FreeBSD showing incorrect results

Vincent,

Thanks for reporting this! Filtered port state can be caused by dropped
packets, though Nmap usually slows down and tries again if it determines
some packets are being dropped. I noticed that the two examples you gave of
incorrect results actually took less time to complete than the correct
ones. It's likely that Nmap just isn't slowing down quickly enough to catch
the replies it ought to.

Here's some diagnostic stuff I'd like to see from you, if you can:

1. Debug output with -d2 for an incorrect scan. Also add -n to skip the
reverse-DNS phase which can add noise to the total scan time.

2. Does slowing the scan down "fix" the incorrect results? Add -T2 to slow
it down. If this works, then it's most likely a timing or missed packets
issue.

3. Let us know if there's anything special about the network: virtual
machine (bridged, NAT, etc)? WiFi? Gigabit Ethernet? It's already very
helpful to know this affects multiple versions of Nmap and FreeBSD, but if
you find a version combination that *does* work, that's useful info as well.

Thanks. Hopefully we can fix it soon!
Dan

On Tue, Sep 19, 2017 at 8:11 PM, Vincent Stemen <[email protected]>
wrote:

> Hi.
>
> On FreeBSD 11.1 release I am getting inconsistent results from nmap version
> 7.40.  It is randomly showing some ports as filtered even though they are
> not.
> I am wondering if this could be a bug in nmap when running on FreeBSD.
>
> For comparison, I ran nmap version 7.40 on Linux Debian 4.9.30 and I do not
> have the problem.  It consistently correctly shows all unfiltered ports.
>
> The host being scanned is running a packet filter firewall on FreeBSD 11.1.
>
> I also ran a few of the same tests from a FreeBSD 10.3-RELEASE-p11 machine,
> running nmap-7.12 and got similar inconsistent results.
>
> On these tests, there are 5 unfiltered ports.
> If it has been at least a minute or so since the last scan, it seems to
> output
> the correct results.
>
> # nmap  -p 1000-1040  pt02
>
> Starting Nmap 7.40 ( https://nmap.org ) at 2017-09-19 18:21 CDT
> Nmap scan report for pt02 (xx.xx.xx.xx)
> Host is up (0.026s latency).
> Not shown: 36 filtered ports
> PORT     STATE  SERVICE
> 1000/tcp open   cadlock
> 1001/tcp open   webpush
> 1002/tcp closed windows-icfw
> 1003/tcp closed unknown
> 1004/tcp closed unknown
>
> Nmap done: 1 IP address (1 host up) scanned in 4.89 seconds
>
> -------------------------------------
>
> But if I run the scan again, I get random wrong results.
>
> # nmap  -p 1000-1040  pt02
>
> Starting Nmap 7.40 ( https://nmap.org ) at 2017-09-19 18:21 CDT
> Nmap scan report for pt02 (xx.xx.xx.xx)
> Host is up (0.024s latency).
> Not shown: 39 filtered ports
> PORT     STATE  SERVICE
> 1000/tcp open   cadlock
> 1004/tcp closed unknown
>
> Nmap done: 1 IP address (1 host up) scanned in 1.79 seconds
>
> ????
> This is outright wrong.
> Why does it only show 2 unfiltered ports?
> ????
>
> -------------------------------------
>
> It is not consistant about which ports it shows as being unfiltered.
>
> # nmap  -p 1000-1030  pt02
>
> Starting Nmap 7.40 ( https://nmap.org ) at 2017-09-19 18:29 CDT
> Nmap scan report for pt02 (xx.xx.xx.xx)
> Host is up (0.024s latency).
> Not shown: 29 filtered ports
> PORT     STATE  SERVICE
> 1001/tcp open   webpush
> 1002/tcp closed windows-icfw
>
> Nmap done: 1 IP address (1 host up) scanned in 1.77 seconds
>
> -------------------------------------
>
> If I scan *no more* than 10 ports, it seems to always be correct.
> From 15 on up it appears to get more and more inconsistant.
>
> # nmap  -p 1000-1010  pt02
>
> Starting Nmap 7.40 ( https://nmap.org ) at 2017-09-19 18:32 CDT
> Nmap scan report for pt02 (xx.xx.xx.xx)
> Host is up (0.025s latency).
> PORT     STATE    SERVICE
> 1000/tcp open     cadlock
> 1001/tcp open     webpush
> 1002/tcp closed   windows-icfw
> 1003/tcp closed   unknown
> 1004/tcp closed   unknown
> 1005/tcp filtered unknown
> 1006/tcp filtered unknown
> 1007/tcp filtered unknown
> 1008/tcp filtered ufsd
> 1009/tcp filtered unknown
> 1010/tcp filtered surf
>
> Nmap done: 1 IP address (1 host up) scanned in 3.99 seconds
>
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>

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