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Getting the primary IP address (plain and CIDR) on Linux and OS X, then nmap scan on the associated subnet

Posted by jpluimers on 2021/12/13

The below answer works on my Linux and OS X systems (each having multiple network adapters configured):

[WayBack] bash – How to get the primary IP address of the local machine on Linux and OS X? – Stack Overflow

ip route get 1 | awk '{print $NF;exit}'

For Linux, I have this bash function:

# note the ";exit" lists the first default route interface, as there can be multiple
function nmap-fingerprint_network_of_default_interface() {
        default_if=$(ip route list | awk '/^default/ {print $5;exit}')
        default_if_cidr=$(ip -o -f inet addr show $default_if | awk '{print $4}')
        nmap -sP $default_if_cidr
}

And for OS X this one:

# requires ipcalc
function nmap-fingerprint_network_of_default_interface() {
        default_if=$(route -q -n get default | awk '/interface:/ {print $2;exit}')
        default_if_address=$(ifconfig $default_if | awk '/inet / {print $2;exit}')
        default_if_netmask_hex=$(ifconfig $default_if | awk '/inet / {print $4;exit}')
        default_if_network_bit_count=$(ipcalc --nocolor --class $default_if_address $default_if_netmask_hex)
        default_if_cidr=$(echo "$default_if_address/$default_if_network_bit_count")
        nmap -sP $default_if_cidr
}

These are the variables used:

  • default_if: network interface of the default route
  • default_if_cidr: IPv4 CIDR of the network interface of the default route (see Classless Inter-Domain Routing: CIDR notation – Wikipedia)
  • default_if_address: IPv4 address of network interface of the default route
  • default_if_netmask_hex: hexadecimal IPv4 network mask of network interface of the default route
  • default_if_network_bit_count: number of set bits in the IPv4 network mask of the network interface of the default route

Links used to get the above functions:

I might have gotten away with a pure bash solution (see [WayBack] Bash script for calculating network and broadcast addresses from ip and netmask or CIDR Notation · GitHub or my post Getting your local IPv4 addresses, netmasks and CIDRs), but the above works and is way shorter, and easier to maintain.

In stead of ipcalc, subnetcalc can do the same calculations and also supports IPv6, so that is something for a future try:

–jeroen

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