The practical assignment must be developed in C programming language over
Linux OS and your implementation must work on the Telemática lab computers, where the assessment will take place. Also, there are three hosts available if you want to connect remotely (ssh) from computers external to the university: monitor01.lab.it.uc3m.es, monitor02.lab.it.uc3m.es and monitor03.lab.it.uc3m.es.
It is mandatory the use of the gcc parameter -Wall in every compilation. The code must compile without warnings or errors.
Also, the code must not have memory leaks (valgrind will be used to check this).
The submission instructions will be published during the course. Basically, the source code (.c and .h files), a Makefile file to compile it, and a README file with some data (name, email address and group) have to be submitted. Practical assignments which do no meet these requirements will not be assessed, i.e., implementations containing memory leaks, compilation warnings or compilation errors, will not be assessed.
The DHCPlab environment (manual can be downloaded from here) has been installed in the Telemática labs in order to carry out the practical assignment.
The practical assignment consists of a simplified version of the RFC. Simplifications are listed below. The complete RFC specification can be found in the following link: RFC 2131.
The RFC 2131 standard defines both the client and the server requirements for obtaining bootstrap and configuration parameters. However, in this practical assignment only the client implementation is required.
Due to the fact that it is neccesary to be root in order to carry out the developement, a test environment will be provided, with all the software installed, including a basic DHCP server.
- Basic operation:
Next, the operation of the DHCP protocol will be briefly explained. A deeper explanation of the protocol is out of the scope of this document; it is the student, who should read the complete RFC and understand all the details required for a correct implementation.
DHCP protocol deals with different kinds of packets DHCP maneja distintos tipos de paquetes de los cuales es obligatorio implementar/reconocer estos seis:
DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK, DHCPNACK y DHCPRELEASE.
DHCP works only with UDP, and it is one of the few protocols that use static port numbers not only for servers (port 67), but also for clients (port 68).
DHCP allows to obtain different parameters, such us IP address, subnetwork mask, boot file, DNS, printer server, etc...
As an example, in order to obtain those parameters, the client and server mush exchange the following packets (see figure):
- DHCPDISCOVER, sent by the client. It is an UDP multicast message which is received by DHCP servers.
- DHCPOFFER, where IP addresses are offered by one or several servers. Client should choose one.
- DHCPREQUEST, sent by the client. It is an UDP message (unicast or multicast) to request an specific address.
- DHCPACK, sent by the server to confirm an IP.
There are other messages, like DHCPNACK and DHCPRELEASE, which are not shown in the previous image but whose implementation/support is mandatory.
So it is mandatory to implement a DHCP client taking these considerations into account:
- Executable and parameters: The executable must be named dhcpcl and must support the following input parameters:
dhcpcl interface [-t timeout] [-h hostname] [-a IP address] [-l leasetime] [-d]
- -t timeout: specifies (in seconds) how long the client will try to get an IP address.
The default value is 64 seconds.
- -h hostname: specifies the string that will appear in the hostname option field of
the DHCP datagram.
- -a IP address: indicates the last known IP address, so that it is sent in the DHCPDISCOVER
message.
- -l leasetime: specifies (in seconds and decimal) the value of the leasetime timer
suggested to the server.
Note the server can overwrite this value. If it is not specified, the default value is infinite.
- -d: debugging mode. Developers must use this mode to isolate users from the implemented
debugging output messages.
- Examples and input/output format:
- Obtaining of an IP address. The dhcpcl program must activate the interface first:
; ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:306 errors:0 dropped:0 overruns:0 frame:0
TX packets:306 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)
; dhcpcl eth1 -t 15 -h zorak -a 192.168.100.25
#[2008-10-11 15:30:34+02:00] (0x12345678) pid = 13086
#[2008-10-11 15:30:39+02:00] (0x12345678) DHCPDISCOVER sent.
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPOFFER received from 192.168.100.0 (offered 192.168.100.20).
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPREQUEST sent to 192.168.100.0.
#[2008-10-11 15:30:41+02:00] (0x12345678) DHCPACK received: 192.168.100.20 with leasing 86400 seconds.
#[2008-10-11 15:30:41+02:00] IP 192.168.100.20; leasetime 86400; subnet mask 255.255.255.0; router: 192.168.10.2; server hostname: zorak; primary DNS: 192.168.10.2; secondary DNS: 192.168.10.10
#[2008-10-11 15:32:01+02:00] SIGINT received.
; ifconfig
eth1 Link encap:Ethernet HWaddr FE:FD:00:00:01:01
inet addr:192.168.100.20 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: 0 (0 kb) TX bytes: 0 (0 Kb)
Interrupt:28
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:5124 errors:0 dropped:0 overruns:0 frame:0
TX packets:5124 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:129504 (128.8 Kb) TX bytes:129504 (128.8 Kb)
- Obtaining an IP address. In this case, an error must be shown by the standard error output,
since the interface is already up:
; ifconfig
eth1 Link encap:Ethernet HWaddr FE:FD:00:00:01:01
inet addr:192.168.100.20 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:247 errors:1 dropped:0 overruns:0 frame:0
TX packets:446 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: 176179 (167.98 kb) TX bytes:287587 (273.79 kb)
Interrupt:28
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:5124 errors:0 dropped:0 overruns:0 frame:0
TX packets:5124 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:129504 (128.8 Kb) TX bytes:129504 (128.8 Kb)
; dhcpcl eth1 -t 15 -h zorak -a 192.168.100.25
Error: eth1 is already enabled
; ifconfig
eth1 Link encap:Ethernet HWaddr FE:FD:00:00:01:01
inet addr:192.168.100.20 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: 0 (0 kb) TX bytes: 0 (0 Kb)
Interrupt:28
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:5124 errors:0 dropped:0 overruns:0 frame:0
TX packets:5124 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:129504 (128.8 Kb) TX bytes:129504 (128.8 Kb)
- Cancellation of the IP address. The program must also disable the interface:
; ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:306 errors:0 dropped:0 overruns:0 frame:0
TX packets:306 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)
; dhcpcl eth1 -t 15 -h zorak -a 192.168.100.25
#[2008-10-11 15:30:34+02:00] (0x12345678) pid = 13086.
#[2008-10-11 15:30:39+02:00] (0x12345678) DHCPDISCOVER sent.
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPOFFER received from 192.168.100.0 (offered 192.168.100.20).
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPREQUEST sent to 192.168.100.0.
#[2008-10-11 15:30:41+02:00] (0x12345678) DHCPACK received: 192.168.100.20 with leasing 86400 seconds.
#[2008-10-11 15:30:41+02:00] IP 192.168.100.20; leasetime 86400; subnet mask 255.255.255.0; router: 192.168.10.2; server hostname: zorak; primary DNS: 192.168.10.2; secondary DNS: 192.168.10.10
; ifconfig eth1
eth1 Link encap:Ethernet HWaddr FE:FD:00:00:01:01
inet addr:192.168.100.20 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:247 errors:1 dropped:0 overruns:0 frame:0
TX packets:446 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: 176179 (167.98 kb) TX bytes:287587 (273.79 kb)
Interrupt:28
; kill -SIGUSR2 13086
#[2008-10-11 15:32:01+02:00] SIGUSR2 received.
#[2008-10-11 15:30:41+02:00] (0x12345678) DHCPRELEASE sent 192.168.100.20 released.
; ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:306 errors:0 dropped:0 overruns:0 frame:0
TX packets:306 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)
- Cancellation of the IP address. In this case, an error must be shown by the standard error output,
as the interface is already down:
; ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:306 errors:0 dropped:0 overruns:0 frame:0
TX packets:306 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)
; dhcpcl eth1 -t 15 -h zorak -a 192.168.100.25
#[2008-10-11 15:30:34+02:00] (0x12345678) pid = 13086.
#[2008-10-11 15:30:39+02:00] (0x12345678) DHCPDISCOVER sent.
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPOFFER received from 192.168.100.0 (offered 192.168.100.20).
#[2008-10-11 15:30:40+02:00] (0x12345678) DHCPREQUEST sent to 192.168.100.0.
#[2008-10-11 15:30:41+02:00] (0x12345678) DHCPACK received: 192.168.100.20 with leasing 86400 seconds.
#[2008-10-11 15:30:41+02:00] IP 192.168.100.20; leasetime 86400; subnet mask 255.255.255.0; router: 192.168.10.2; server hostname: zorak; primary DNS: 192.168.10.2; secondary DNS: 192.168.10.10
; ifconfig eth1
eth1 Link encap:Ethernet HWaddr FE:FD:00:00:01:01
inet addr:192.168.100.20 Bcast:192.168.100.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:247 errors:1 dropped:0 overruns:0 frame:0
TX packets:446 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes: 176179 (167.98 kb) TX bytes:287587 (273.79 kb)
Interrupt:28
; ifconfig eth1 down
; ifconfig
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.255.255.0
UP LOOPBACK RUNNING MTU:1500 Metric:1
RX packets:306 errors:0 dropped:0 overruns:0 frame:0
TX packets:306 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:29504 (28.8 Kb) TX bytes:29504 (28.8 Kb)
; kill -SIGUSR2 13086
#[2008-10-11 15:32:01+02:00] SIGUSR2 received.
Error: eth1 is already disabled.
Notes:
- Every message must be shown by the standard output, except error messages, which must be shown
by the error standard output.
- The execution of ifconfig is informative. It is not necessary to develop this command.
- Once the parameters from the server are received, the client will only have to configure
the IP address, the subnetwork mask and the gateway (router) address. Remaining data coming from the DHCP server can be ignored by the client.
- Traces: the program must show traces to allow execution tracking. These traces must be
shown by the standard output. dhcpcl exit code must be 0 if all the
parameters were successfully obtained, 1 if there was no response from the server and -1
if there is any error. Traces corresponding to the DHCP messages (DISCOVER, OFFER, REQUEST, ACK, NACK, RELEASE)
must be the following:
#[timestamp] PID = pid.
#[timestamp] (xid) DHCPDISCOVER sent.
#[timestamp] (xid) DHCPOFFER received from ip_s (offered ip_c).
#[timestamp] (xid) DHCPREQUEST sent to ip_s.
#[timestamp] (xid) DHCPACK received: ip_c with leasing l seconds.
#[timestamp] (xid) DHCPNACK received.
#[timestamp] (xid) DHCPRELEASE sent ip_c released.
#[timestamp] IP ip_c; leasetime l; subnet mask subnet_mask; router: router; server hostname: server_hostname; primary DNS: primary_DNS; secondary DNS: secondary_DNS
#[timestamp] SIGINT received.
#[timestamp] SIGUSR2 received.
Where:
- timestamp: system time, according to the format defined under RFC 3339:
; date --rfc-3339=seconds
2008-10-16 10:08:02+01:00
- xid: transactionID field in the DHCP datagram.
- ip_s: DHCP server IP address.
- ip_c: IP address leased out by the server.
- l: leasetime announced by the server.
- Remaining parameters are self-explanatory.
Simplifications to the RFC:
- It is not necessary to implement DHCPINFORM and DHCPDECLINE messages.
- It is not necessary any implementation related to multi-homed hosts.
- It is not necessary to implement REBINDING and RENEWING states, nor
the use of the T1 and T2 timers (see RFC's Figure 5). According to the RFC, once the client is in BOUND state,
there are three timers (T1 < T2 < leasetime) that start and lead to the transition to other states and the sending
of some messages.
- If T1 expires, the client enters in RENEWING state and sends unicast DHCPREQUEST asking for
a longer leasetime.
- If T2 expires without receiving DHCPACK, the client enters in REBINDING state and sends broadcast DHCPREQUEST.
In this assignment, when in BOUND state, the program will finish once the leasetime is over
(if no DHCPRELEASE is sent before due to SIGUSR2 signal).
- It is not necessary that the client checks whether the obtained IP address is available by means of ARP.
- The only mandatory link level is Ethernet.
- If the client receives more than one answer from the present servers, it has to choose one of them.
How it is selected, is left to the implementors decision.
- Everything that appears in the RFC as MAY or SHOULD is optional.
Everything that appears as MUST is mandatory for the assignment.
Considerations about timers and timeouts: the values will always be those marked as
RECOMMENDED in the RFC.
Considerations about sockets: it is mandatory to use UDP sockets whenever is possible (e.g.
DHCPRELEASE, DHCPDECLINE). When not possible (DHCPDISCOVER, DHCPREQUEST) raw sockets will be used.
In short, the client implementation must make use of both socket types.
A manual page has been created for the proposed implementation of dhcpcl, that resumes the information
about input, output and traces formats. It can also be downloaded
in HTML format.
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