Enumerating network services and protocols using Bash

I perform a network packet capture on every internal network pentest. I’m looking for the default Hot Standby Router Protocol (HSRP) default password of 'cisco', DHCPv6 discovering broadcasts without a corresponding offer, and broadcast or multicast protocols such as LLMNR, NBT-NS, and MDNS, which can yield password hashes or be relayed to crack into other systems.

The following code can be found on this chapter’s GitHub page as packetcap.sh:

#!/usr/bin/env bash
if [ "$#" -ne 1 ]; then
  echo "You must specify a network adapter as an argument."
  echo "Usage: $0 [network adapter]"
  exit 1
fi

The first block of code is the familiar shebang, followed by an if statement that prints usage information and exits if exactly one argument is not provided.

echo "[+] Please wait; capturing network traffic on $1 for 2.5 minutes."
sudo timeout...