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Odd behaviour from NMAP behind MX64

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Odd behaviour from NMAP behind MX64

I'm WFH with an MX64 supplying VPN connectivity to the office and I've found a very odd and frustrating behavior that I think is the Meraki.

 

I'm a network engineer on a new (to me) network so ping sweeps are something I do frequently.  I use nmap to do them with "nmap -sn -PE subnet/mask" and the odd behavior is that sometimes, that works and sometimes it doesn't.

 

It always works to the Meraki that's my default gateway but going beyond that is hit or miss. Some days it works, some it doesn't and I haven't been able to find anything that gives me a clue as to why this is happening.

 

  • There are no firewall rules in the Meraki, it's wide open.
  • Ping from the ping command work fine, it's only nmap that doesn't work.
  • If I capture at the MX450 I don't see the nmap echo requests at all, but I do see the ones from ping.exe.
  • If I capture on my desktop I see icmp requests from both apps but replies only for ping.exe.  The only difference I see in the requests is that nmap icmp sequence number is 0 while ping.exe is 1700+.

Anyone got any idea why the Meraki might care of icmp sequence number or how to make NMAP use something besides zero?

Suggestions welcome as this is very annoying and it's making my life more difficult than it needs to be.
That includes suggestions for an nmap alternative that works from CLI and uses CIDR and *.x-y type notation.  None of that crap that makes me type a starting and ending address.

 

Thank you

 

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