INTRODUCING THE PACKET WHACKER

THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

INTRODUCING THE PACKET WHACKER

Applications Part 2: Setting a Course with Code
by Fred Eady

Start ı Packet Whacker Utilities ı Receiving Packets ı Who ARP You?ı Ping ı UDP ı TCP ı FIN ı Sources and PDF

PING

On the PGA tour, the right Ping can make you a bunch of money. Our ping makes us no money at all in the financial sense, but itıs worth a million dollars when your Packet Whacker code on the PICDEM.net answers a ping request on the LAN. Donıt go looking for a ping function in the Packet Whacker source code because I didnıt implement a ping utility like the one youıre used to seeing in an operating system like Windows. Looking at the crossroads code in Listing 5 once again, you find that if the incoming packet doesnıt match up to an ARP request, it falls into the it-must-be-IP bucket. After some sifting, the ping request falls into the category of Internet control message protocol (ICMP).

ICMP does a lot of neat things and brings a wealth of functionality to the IP world. It is the simplest of the procedures Iıve had to write yet. Listing 7 lays it out. Most of the work is done in the setipaddrs function. The setipaddrs code swaps the source and destination IP addresses, swaps the source and destination MAC addresses, and calculates the IP header checksum. After the IP side of the packet is secured, the icmp code chunk simply changes the type from Echo (0x08) to Echo reply (0x00) and calculates the ICMP header checksum before sending the ICMP echo reply packet on its way back to the golf course. Because, at this point, Iım only interested in the PICDEM.net code, Photo 7 is the PICDEM.net side of a ping operation as seen by the Sniffer.

Listing 7ıAlthough the ICMP function is sometimes used as a carrier of bad news, the ping process is simply an echo of the incoming ICMP packet with the source and destination addresses swapped, a redefinition of the echo/reply field, and a recalculation of the ICMP checksum.

Photo 7ıThereıs nothing complicated about constructing an ICMP Echo reply packet. The most that has to be done is the calculation of the checksum. The rest of the mind exercise is keeping up with the address swaps.

ARPing and pinging are good things as far as Internet devices go, but itıs really difficult to move data using these processes. So, letıs add some information moving code to our existing "here I am" code.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ıCircuit Cellar, the Magazine for Computer Applications. Posted with permission.