02. [20p] Network Exploration

[5p] Task A - ARP vs ICMP

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

Pro tip #2: yes, ip can also generate color output. Most people don't know this and still use ifconfig, even though it's already deprecated at this point. Add this as an alias to your .bashrc or .zshrc and source it.

# 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.

The Task(s)

Use arp-scan to scan your local network while monitoring ARP traffic with wireshark to get a sense of what's going on. After that, use the following script to identify hosts discoverable via ARP but not ICMP.

Click to display ⇲

Click to hide ⇱

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

# localnet-ping.sh - performs differential ARP / ICMP scan
#   $1 : [required] interface name  
if [ "$#" -ne 1 ]; then
    echo "Usage: ./localnet-ping.sh <interface>"
    exit 1
# 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 [ $? -eq 0 ]; then
        printf '\033[1;32mok\n\033[0m'
        printf '\033[1;31mfail\n\033[0m'
done <<< "$localnet_hosts"

[15p] Task B - nmap vs traceroute

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).

The Task(s)

With as a target, use wireshark to view the traffic generated by both nmap and traceroute. What differences can you find in their default mode of operation?

$ sudo nmap                            \
    -sn     `# disable port scan`      \
    -Pn     `# disable host discovery` \
    -tr     `# perform traceroute`     \
$ traceroute


  • permission denied : make sure that nmap is not installed as a snap; you have two choices:
    • reinstall nmap with apt : sudo snap remove nmap && sudo apt install nmap
    • grant nmap permissions : snap connect nmap:network-control

If we do allow for a port scan by removing -sn (default is a TCP-based scan; use -sU for a UDP scan), this will take place before the actual traceroute. What changes does this bring?

Optional Task (... no, really)

When doing the TCP scan with nmap, you may have noticed a weird field in the TCP header: Options. Generate some TCP traffic with curl and look at the SYN packet in wireshark. What options do you see there?

Here is a quick break down of the more common TCP options and how they are used to overcome protocol limitations and improve throughput. Take a quick look if you want, then move on. We'll dive deeper into protocol options in the next task.

ep/labs/04/contents/tasks/ex2.txt · Last modified: 2021/11/02 14:00 by radu.mantu
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