The first programming project is meant to introduce you to socket programming, as well as the Unix software development environment (gcc, make, etc.) and pthread programming. You may find it useful to refer to this document for socket programming (Beej's Guide to Network Programming).
Your task will be to write a TCP Proxy. You'll learn how to write both client and server code in this mini-project.
A TCP proxy server is a server that acts as an intermediary between a client and another server, called the destination server. Clients establish connections to the TCP proxy server, which then establishes a connection to the destination server. The proxy server sends data received from the client to the destination server and forwards data received from the destination server to the client. Interestingly, the TCP proxy server is actually both a server and a client. It is a server to its client and a client to its destination server.
A TCP proxy server can be useful to get around services which restrict connections based on the network addresses. For example, the web page http://fireless.cs.cornell.edu/courses/2016fa/cs6410/restricted/ is only accessible from Fractus hosts. If you try to access it from elsewhere, you will receive an access denied error. However, you can view this page from a web browser anywhere on the Internet by running a proxy server on one of the Fractus instance machines. The web server will think it is serving the data to a web client on the machine running the proxy. However, the proxy is forwarding the data out of the class network, thus subverting the protection mechanism.
The proxy server you will build for this mini-project will be invoked at the command line as follows:
# ./tcp-proxy destination-host destination-port listen-port
For example, to redirect all connections to port 3000 on your local machine to yahoo's web server, run:
# ./tcp-proxy www.yahoo.com 80 3000
As another example, to view the restricted web page mentioned above, you might run the following command on your Fractus VM:
# ./tcp-proxy fireless.cs.cornell.edu 80 4000Then in another terminal of your Fractus VM, you can view the restricted web page by running
# curl http://localhost:4000/courses/2016fa/cs6410/restricted/You can also type the URL http://128.84.105.XXX:4000/courses/2016fa/cs6410/restricted/ into your browser window, provided that
128.84.105.XXXis the public IP address of your Fractus instance, and that you have authorized network access on the proxy listen-port (-p 4000).
Note: Fractus is behind the firewall of the CS Department. You need to use the Cisco VPN if you are outside the Cornell network.
Submit a single threaded version, which accepts a single connection from a client and forwards it using a single connection to the server. During the connection, the proxy might not accept any other connections. This version will not be graded, but we will give you some feedback.
The proxy server will accept connections from multiple clients and forward them using multiple connections to the server. No client or server should be able to hang the proxy server by refusing to read or write data on its connection. For instance, if one client suddenly stops reading from the socket to the proxy, other clients should not notice interruptions of service through the proxy. In order to do so, you will need to use multiple
pthreads. However, in this mini-project, the maximum number of threads you can use is limited by five (One main thread that accepts connections, and four worker threads that handle active connections). Then, you will need to use
poll within each thread to handle multiple connections.
semaphorefor this. Also, you must use
pthread_mutex_tfor accessing shared data structures. But, you should not abuse your mutexes. Minimize criticial sections as much as possible to achieve better performance.
Before booting up the VM again, we need to first change its instance type to
euca-describe-instances --filter key-name=kp-netid-xxx
m3.xlarge(with 4 CPUs and 2GB memory), because by-default its instance type is
m1.small(with 1 CPU, 4GB memory). To change its instance type, use the following command:
euca-modify-instance-attribute -t m3.xlarge i-xxxxxxx
i-xxxxxxxis your instance ID, which can be obtained by using the
euca-describe-instancescommand. After chaing its type, you can now start the VM again:
Remember to mark on the shared Google doc that you are now using your VM.Start by downloading the skeletal code from CMS (tcp-proxy.tar.gz), then copy it (using scp/rsync) to your home directory on your Fractus VM (recall that
prompt>denotes your own machine, while
#denotes the Fractus instance). You should be able to get the files from your local box to your instance and build like this:
prompt> scp -i ~/.euca/id-rsa-kp-zs272-test tcp-proxy.tar.gz firstname.lastname@example.org.XXX:~/ prompt> ssh -i ~/.euca/id-rsa-kp-zs272-test email@example.com.XXX # tar --no-same-owner -xzf tcp-proxy.tar.gz # cd tcp-proxy # make gcc -pthread -o tcp-proxy tcp-proxy.cIf you work as a root, it is very important you use the
--no-same-ownerflag for tar, since you are acting as
rooton the Fractus machine, and otherwise tar will change permissions (sadly propagating up the entire /root/tcp-proxy path) to the user ID of your local machine (the machine the scp command was initiated from), which does not necessarily exist on the Fractus instance (unless you fancy being
rooton your box). As a result, subsequent SSH connections will fail, since the
/roothome folder of user
roothas just been owned by a rogue user ID.
# cd ~/tcp-proxy # make dist prompt> scp -i ~/.euca/id-rsa-kp-zs272-test firstname.lastname@example.org.XXX:tcp-proxy/tcp-proxy.tar.gz .
tcp-proxy. To test it, type, for example:
# ./tcp-proxy www.yahoo.com 80 1234Now you should test your program using telnet. In the new window, run:
# telnet localhost 1234 Trying ::1... telnet: connect to address ::1: Connection refused Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Connection closed by foreign host. #The message "Connected to localhost" says that your proxy accepted a TCP connection, but then immediately closed it, since the proxy is not fully implemented. You must finish implementing the proxy. You are free to use any basic C library, or you can design your own data structures. It is possible to complete the assignment without using any other external libraries or data structures.
You should test your proxy to make sure that it continues to forward data even when some connections aren't responding. Here's one test you should be able to pass.
First, run the proxy and point it at fireless.cs.cornell.edu's HTTP port:
# ./tcp-proxy fireless.cs.cornell.edu 80 1234Now, in another window, use telnet to fetch /big through the proxy:
# telnet localhost 1234 Trying ::1... telnet: connect to address ::1: Connection refused Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. GET /courses/2016fa/cs6410/mps/bigWatch the data go by for a while, then interrupt the output by typing control-], after which telnet should stop and print telnet>. Now check that the proxy hasn't been hung because telnet isn't reading data; suspend your telnet by typing ``
zRETURN'', wait for 10 seconds, and fetch something else:
telnet> z Suspended # telnet localhost 1234 Trying ::1... telnet: connect to address ::1: Connection refused Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. GET /courses/2016fa/cs6410/mps/small You got it! Connection closed by foreign host.If you see "You got it!," your program passes the test.
Now try to access the restricted page from your web browser with a URL like
http://128.84.105.XXX:1234/courses/2016fa/cs6410/restricted/. Again, make sure
128.84.105.XXX is the IP address of your Fractus instance running your tcp-proxy, and that you have
authorized access on port 1234 to it.
Next, lower the maximum number of allowed concurrent proxied connections to something like 2, and test by pointing your proxy to fireless.cs.cornell.edu, port 80, just like in the first test. Start by opening 3 telnet connections, but without issuing the
HTTP GET. The third connection should not be accepted. Now issue
GET /small in one of your two connected telnet prompts, and once you received the responce from the server your third connection should be accepted --- HTTP web servers orderly terminate
the connection to indicating that the end of file was reached.
iperf, a benchmarking tool for TCP and UDP protocols. You can checkout this website for more details. Installing
iperfis as easy as running the following commands:
# apt-get update # apt-get install -y iperfStart the
# iperf -s -p 5001And then in another terminal of your VM, start your TCP proxy:
# ./tcp-proxy localhost 5001 80Now open another terminal, start the
iperfclient and test the performance:
# iperf -c localhost -p 80 -t 10 -P <thread_number>An example with 4 threads:
# iperf -c localhost -p 80 -t 10 -P 4 ------------------------------------------------------------ Client connecting to localhost, TCP port 80 TCP window size: 648 KByte (default) ------------------------------------------------------------ [ 6] local 127.0.0.1 port 50190 connected with 127.0.0.1 port 80 [ 5] local 127.0.0.1 port 50188 connected with 127.0.0.1 port 80 [ 4] local 127.0.0.1 port 50186 connected with 127.0.0.1 port 80 [ 3] local 127.0.0.1 port 50184 connected with 127.0.0.1 port 80 [ ID] Interval Transfer Bandwidth [ 6] 0.0-10.0 sec 10.2 GBytes 8.73 Gbits/sec [ 5] 0.0-10.0 sec 10.6 GBytes 9.14 Gbits/sec [ 4] 0.0-10.0 sec 10.9 GBytes 9.36 Gbits/sec [ 3] 0.0-10.0 sec 10.7 GBytes 9.23 Gbits/sec [SUM] 0.0-10.0 sec 42.4 GBytes 36.5 Gbits/secOn the server side, we can see this output:
# iperf -s -p 5001 ------------------------------------------------------------ Server listening on TCP port 5001 TCP window size: 85.3 KByte (default) ------------------------------------------------------------ [ 8] local 127.0.0.1 port 5001 connected with 127.0.0.1 port 40240 [ 4] local 127.0.0.1 port 5001 connected with 127.0.0.1 port 40242 [ 5] local 127.0.0.1 port 5001 connected with 127.0.0.1 port 40244 [ 6] local 127.0.0.1 port 5001 connected with 127.0.0.1 port 40246 [ 4] 0.0-10.0 sec 10.9 GBytes 9.34 Gbits/sec [ 6] 0.0-10.0 sec 10.2 GBytes 8.71 Gbits/sec [ 8] 0.0-10.0 sec 10.7 GBytes 9.21 Gbits/sec [ 5] 0.0-10.0 sec 10.6 GBytes 9.13 Gbits/sec [SUM] 0.0-10.0 sec 42.4 GBytes 36.4 Gbits/sec
We can see that
iperf reports the throughput for each thread, and the total throughput. You may notice that the throughput reported on the server side is different with that on the client side. This is because we have a TCP proxy in between which has some buffering mechanism.
Play with different numbers of threads to see if your TCP proxy can handle it efficiently.
# cd tcp-proxy # make dist rm -fr .DS_Store *.tar.gz *.ps *.pdf *.o *.dSYM *~ tcp-proxy test-tcpproxy tar -czf tcp-proxy.tar.gz ../tcp-proxy --exclude=tcp-proxy.tar.gz --exclude=".svn" tar: Removing leading `../' from member names # tar -tzf tcp-proxy.tar.gz tcp-proxy/ tcp-proxy/list.h tcp-proxy/Makefile tcp-proxy/tcp-proxy.c # md5sum tcp-proxy.tar.gz 0cedb78a282b2543cdd412061cac6894 tcp-proxy.tar.gzThe last command computes the MD5 checksum of your tar file, which can be used to verify your submission (CMS provides the MD5 checksum after you submit a file). To turn in your distribution, upload the
readme.txtfile on CMS.
If you have any problems about submission, please contact the TAs.
man socketgives you the man page for the socket system call.
ulimitcommand before you will see the core files being created (e.g.
ulimit -c unlimited). You can examine the core files with
gdbin order to learn what went wrong --- this is an invaluable tool. You can start by typing
gdb program program.core, and then typing the gdb command
backtrace. GDB will in turn return a trace pointing to where your program has crashed.
This page was originally created by Tudor Marian.