The Address Resolution Protocol (ARP) resolves layer 2 addresses (MAC) from layer 3 addresses (e.g.: IP). Normally, all hosts are compelled to reply to ARP requests, but this can be fiddled with using tools such as arptables. You can show the currently known neighbors using iproute2.

$ ip -c neigh show

# alias for iproute2 color output
alias ip='ip -c'

The Internet Control Message Protocol (ICMP) is an ancillary protocol meant mainly to report errors between hosts. Sometimes it can also be used to perform measurements (ping) or to inform network participants of better routes (Redirect Messages). There are many ICMP functionalities, most of which are now deprecated. Note that some network equipment may not be capable of understanding new and officially recognized protocols, while other may not even recognize experimental ICMP codepoints (i.e.: type=253,254) and simply drop the packet. Because ICMP can be used to stage attacks in a network, some operating systems (e.g.: Windows ≥7) went so far as to disable Echo Replies by default.

Hint: click on the file name to download the snippet below.

localnet-ping.sh

#!/bin/bash
 
# localnet-ping.sh - performs differential ARP / ICMP scan
#   $1 : [required] interface name  
 
if [ "$#" -ne 1 ]; then
    echo "Usage: ./localnet-ping.sh <interface>"
    exit 1
fi
 
# generate list of IPs and hostnames in local network for given interface
localnet_hosts=$(sudo arp-scan                                      \
                    --interface=$1           `# scanned network`    \
                    --localnet               `# only local network` \
                | head -n -3                 `# hide footer lines`  \
                | tail -n +3                 `# hide header lines`  \
                | awk '{$2=""; print $0}'    `# hide MAC address`   \
                )
 
# process generated list, one item at a time
while read -r it; do
    # separate IP from hostname
    current_ip=$(awk '{print $1}' <<< $it)
    current_host=$(awk '{$1=""; print $0}' <<< $it)
 
    printf '\033[1;33m%15s   %-35s \033[0;33m==>  \033[0m' \
        $current_ip "$current_host"
 
    # ping current host
    ping -c 1           `# only one ping` \
         -W 1           `# 1s timeout`    \
         $current_ip    `# target host`   \
         1>/dev/null 2>&1
 
    # evaluate ping success
    if [ $? -eq 0 ]; then
        printf '\033[1;32mok\n\033[0m'
    else
        printf '\033[1;31mfail\n\033[0m'
    fi
done <<< "$localnet_hosts"

nmap is a network exploration tool and a port scanner. Today, we will look only at a specific functionality that it shares with the traceroute utility.

Route discovery is simple in principle: IPv4 packets have a Time to Live (TTL) field that is decremented by 1 with each hop, thus ensuring a limited packet lifespan (imagine routing loops without TTL). Even if the TTL is 0, the layer 3 network equipment must process the received packet (the destination host can accept a packet with TTL=0). Routers may check the TTL field only if they are to forward the packet. If the TTL is already 0, the packet is dropped and a ICMP Time-To-Live Exceeded message is issued to the source IP. By sending packets with incrementally larger TTL values, it is possible to obtain the IP of each router on the path (at least in theory).