How to generate random IP addresses

Question

$ nmap -n -iR 0 -sL > RANDOM-IPS-TMP.txt
$ grep -o "[0-9]*\.[0-9]*\.[0-9]*\.[0-9]*" RANDOM-IPS-TMP.txt |
      egrep -v "10.*|172.[16-32].*|192.168.*|[224-255].*" > RANDOM-IPS.txt
egrep: Invalid range end
$ 

How can I generate random IP Addresses outside the private IP address range and the multicast IP Address range?

Answer 1

You misunderstand regex syntax. [16-32] does not mean "match 16, 17, ... or 32". It means "match one character which is either 1 or 2 or in the range 6-3" (which is not a valid range, hence the error).

It's possible to write a regex to match a range of integers, but it's complex and error prone. In your case, it would be much easier to use nmap's --exclude option to exclude the ranges you don't want. It understands CIDR notation, which is a much simpler way to describe the ranges you're talking about.

nmap -n -iR 0 --exclude 10.0.0.0/8,172.16.0.0/12,192.168.0.0/16,224-255.-.-.- -sL >RANDOM-IPS.txt

You didn't mention the loopback block (127.0.0.0/8), but you probably ought to exclude that too.

Answer 2

This shell snippet generates an IP address.

ip_address=$(dd if=/dev/urandom bs=4 count=1 2>/dev/null |
             od -An -tu1 |
             sed -e 's/^ *//' -e 's/  */./g')

If you're not happy with it, try again in a loop.

while
  set $(dd if=/dev/urandom bs=4 count=1 2>/dev/null | od -An -tu1)
  [ $1 -lt 224 ] &&
  [ $1 -ne 10 ] &&
  { [ $1 -ne 192 ] || [ $2 -ne 168 ]; } &&
  { [ $1 -ne 172 ] || [ $2 -lt 16 ] || [ $2 -gt 31 ]; }
do :; done
ip_address=$1.$2.$3.$4