Advantages Over Nmap

You might wonder why you would want to use hping to look for open ports when you already have Nmap. In some situations, hping offers advantages over Nmap.

First, hping is a lightweight application; if you've got it installed and ready to go, why worry about installing anything more?

Second, it's always good to know how to do the same thing with more than one application. Hping's creator, for example, still maintains the tool even though he's collaborated for years with Fyodor, the creator of Nmap.

Third, you can also conduct incremental scans, which means each scan will climb up one port on a system:

sudo hping -S targethost-p ++0

This command creates a report that tells you what ports are open on the system.

A Better Traceroute?

One interesting feature of hping3 is that you can generate a more revealing trace-route report using any protocol. For example, suppose you want to determine exactly what happens on each hop of a traceroute. To do this, you can specify the use of a TCP SYN packet. The -T option allows you to enable hping3's traceroute function. In the command shown above, the --ttl option allows you to specify the number of routers (i.e., hops) you want to transmit.

If you want to issue a traceroute command using UDP, the command shown in Listing 2 will suffice. The output shows how each router processes the UDP packet.

Why would you want to do such a thing? Because many routers block traditional ICMP packets, even if your latest system used UDP.

To analyze one particular hop of a traceroute packet, you can use the --tr-keep-ttl option:

sudo hping3 -S 12.119.80.1 -p 80 -T --ttl 3 --tr-keep-ttl -n

The -n option ensures that numbers aren't resolved.

The preceding command issues TCP-based packets to the target host, but then reports only the third hop. The output is shown in Listing 3. The information in Listing 3 can help you determine exactly if and how a particular router is altering packets in transit.

Discovering the MTU

To determine the MTU (Maximum Transmission Unit – the largest datagram allowed for the network), you could issue the following command:

hping3 -D -V -I em1 --icmp targethost

Replace targethost with the host name or IP address of the system on the network where you want to test the MTU.

Why is it important to discover the MTU? First, VPN connections and other network transmissions sometimes encounter problems if the MTU on a system or a network is set strangely.

In convergence networks (for example, where you're implementing a VoIP SIP or H.323 system), you might need to determine the MTU to avoid problems with jitter and traffic congestion. By determining the MTU and adjusting it properly at the router or your individual hosts, you can reduce latency and resolve call quality issues that would otherwise prove elusive.