COSC 4300 Networks and Internets

Fall 2014

Homework Assignment #2

Due: Saturday, September 13th, 11:59pm CDT
Submit: Turn in your entire Project 2 directory using the turnin command on the Systems Lab machines.
Please run "make clean" in your compile/ subdirectory before submitting. Work is to be completed in teams. Only one team member should turnin, but it would be courteous to notify your teammate(s) when you do this. Names of authors should be included in all work. You may submit multiple times, but only the last turnin will be kept. The automatic submission system will not accept work after the deadline.

DHCP Client

As preparation, make a copy of your Project 1 directory for Project 2.

Add a shell command, "dhcp", that spawns a DHCP protocol daemon.

For a passing grade, your DHCP protocol daemon should at least send a fresh DHCP_DISCOVER packet and receive a DHCP_OFFER packet in order to learn the IP address of the host you are on.

Higher grades will go to more complete implementations that also parse the lease time, subnet mask, and router address in the DHCP payload. Points will be awarded for completing the four steps of the DHCP state machine necessary to reach the "BOUND" state in Figure 18.28 of the text.

Notes

Constructing fresh DHCP packets should not be very hard after completing last week's assignment, but there are a few "gotchas" to be wary of.

Declare an array of unsigned characters of PKTSZ to build your packet, and assign struct pointers to it as in the previous assignment.
Fill in the header fields methodically. The DHCP server has no patience for ill-formed packets and will ignore you. Use your dhcpsnoop helper functions from last week to double-check your constructed packets.
Use the built-in Ethernet driver API to get your own MAC address when needed. Example: control(ETH0, ETH_CTRL_GET_MAC, (ulong)foo, 0); will copy the six bytes of your MAC address to the location pointed to by foo.
Any short or long integer values longer than one byte must be converted from host order to network order, because the MIPS router is little-endian hardware, and most network protocols are big-endian. The macros ntohs() and ntohl() are provided to convert the endianness of integers. The fields we should worry most about are the IP header ID, UDP header source and destination ports, DHCP header ID, and DHCP lease time option.
The IP and UDP headers both have length fields. These should be assigned after you've finished building your DHCP payload, and can calculate the correct length by subtracting the last address you used from the start of the applicable header. (Watch out for unintentional pointer arithmetic!). As a rule of thumb, the IP header's length field will be 20 bytes larger than the UDP header's length field. (Because the IP header is itself 20 bytes.)
There are two checksum fields in the packet. The UDP checksum can be set to zero. (A zero UDP checksum field indicates that checksumming is turned off for this packet.) The IP header checksum must be correctly set after all other fields in the IP header have been determined. A Xinu built-in checksum() function is available. Example: dgram->chksum = checksum((uchar *)dgram, (4 * (dgram->ver_ihl & IPv4_IHL)));
When your packet is ready, send it out of the Ethernet device with write(ETH0, packet, length);.

[Revised 2014 Sep 08 15:25 DWB]