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When I scan an external IP address with Nmap my router always reboots itself, sometimes I even have to manually restart it by using the power switch.

The command I usually use is:

nmap -p 1-65535 -T4 -A -v -Pn *IP address*

I'm using an Asus RT-AC66U with the latest firmware. I have also tried using the Asuswrt-Merlin firmware but the result is still the same.

Has anyone else experienced this problem and solved it?

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2 Answers 2

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You are probably exhausting the router's resources, primarily the NAT table. Every outgoing connection maps your scanning machine's private IP address and source port onto a new source port on the router's public IP address. Since there are more IP address + source port combinations than there are source ports on the single public IP, the router has to do extra work to avoid collisions. Ideally, this would not cause a reboot or a crash, but home routers are not ideal pieces of equipment.

The easiest way to slow your scan down to something your router can handle would be to change -T4 to -T3 (the default). If that doesn't work, continue reducing the timing options until you get a good result. The other thing that can help if you are scanning more than one known-alive IP address is to not use -Pn.

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Personally haven't used Nmap in a while but it sounds like you're unintentionally DoSing yourself? Does the aggressive (-A) parameter not overwrite -T4 and just scan as fast as possible? If that's the case, it's pretty easy to UDP flood home routers.

Definitely sounds like a timing issue though. Look up the aggressive parameter and take what you need from it (it uses 4-5 flags that you can set manually anyway) and from there, play with rate limiting and possibly maximum transmission unit.

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  • -A has nothing to do with timing. It stands for "advanced" and is equivalent of -sV -sC -O --traceroute but skips -O and --traceroute if you do not have sufficient privileges. You are confusing it with -T aggressive, which is equivalent to -T4. Oct 23, 2015 at 19:08
  • Ah that makes sense. For some reason I recalled -A using -T4/5 automatically for whatever reason. Thanks for clearing that up.
    – Sevaara
    Oct 24, 2015 at 20:00

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