Lab 6 - Sample Shell and Telnet Programs

In Lab 5, a "dummy" shell was spawned by your simple daemon since your daemon did not authenticate users. This was done for security reasons to prevent unauthorized users from accessing your files. You also used the telnet command to connect to your daemon. In this lab, we will be looking at code which implements a more realistic shell and which investigates the telnet protocol.

Create a new directory for this lab (important so that the tarball does not overwrite vcrec.c, vcsend.c and other files you have created in previous labs). Download and untar the networking code provided by Dr. Marc Thomas using the commands: 

wget http://www.cs.csubak.edu/~melissa/cs376/cs376_lab_code.tar.gz
tar -xvzf cs376_lab_code.tar.gz
The two files of interest for this lab are s_sh.c (the shell program) and s_tlnt.c (a verbose telnet client). Compile these two programs with the following command: 
make telnet
For the shell program, you can run the shell with the command ./s_sh and then interact with the shell using regular stdin (the keyboard) and stdout (the screen). It is VERY IMPORTANT that you do NOT compile this in the same directory as Lab 5, to prevent simple daemon from using this version of s_sh for its shell. This shell is a multi-platform program which parses commands and command options, runs the commands and displays the results. This program can parse a few commands such as cd, dir and pwd. It also supports one * or ? wildcard per line. The comments section at the top of the file lists more details on the capacity of this shell program.

Some interesting things to look at in the code for s_sh.c is how it parses the typed command, how it spawns the system binaries for certain commands, how it stores a history of typed commands and how it implements internal commands. This mimics much of what a regular shell program, such as bash, would do.

The telnet program s_tlnt is also a multi-platform program. It gives more information about the telnet process than the regular telnet client does. The telnet protocol allows one to have a plaintext (unencrypted, SSH by comparison is encrypted) TCP connection which can support virtual terminals. It was widely used in the early days of the Internet for logging into remote sites and is still widely used for debugging other plaintext protocols such as how we used it for Lab 5.

Some interesting things to look at in the s_tlnt.c code are process_esc() and the commandmode jump target in main(). The process_esc() function interprets basic VT100/ANSI escape sequences. VT100 and ANSI are standards to control the placement of the cursor on a text screen, colors, reverse colors, blinking text, clearing the screen and other such functionality needed to create basic text-based graphical interfaces. If you use Pine, you've seen VT100 and ANSI in action.

The commandmode jump target can be reached by pressing CTRL-T (the "escape character") while the program is running. This will bring up the s_tlnt prompt. Your connection will still be active in the background while you give commands to the s_tlnt program. You can "close" an active connection, "return" to an active connection, "open" a new connection, set s_tlnt parameters, access a "help" screen and see the "status" at the s_tlnt prompt. The parameters can enable features which will give more information about the data being passed across the network, such as VT100 codes.

The command to start s_tlnt is: 

s_tlnt [-v] [-snoopy] hostname port
The v option will turn on verbose, which will give you detailed information about the execution of the program. You can also set "verbose" or "noverbose" at the s_tlnt prompt. The snoopy option activates transparent mode, which gives information about telnet arbitration and shows all control and 8bit characters. Telnet arbitration is a negociation process that occurs between your telnet client and the server to set options such as echoing the text you type or whether to use half-duplex or full-duplex communication. The negociation process consists of one side presenting an option (IAC DO) and the other side either accepting (IAC WILL) or rejecting (IAC WONT) the option. You can set "snoopy" or "nosnoopy" at the s_tlnt prompt.

Lab Writeup

Answer these questions as the writeup for your lab.
  1. Why does s_sh watch for the SIGCLD (SIG_CHLD) signal?
  2. How are the internal commands in s_sh implemented?
  3. Try running the command s_tlnt -v -snoopy helios 23. You must use Helios as Sleipnir does not have a telnet daemon running. Do NOT actually log in to Helios as that will send your password in plaintext. Hit CTRL-T and then type "close" when prompted to log in. What do you see? (NOTE: Helios won't take telnet attempts anymore. Skip this question)
  4. In process_esc in s_tlnt, basic ANSI/VT100 support is provided. Read the comments at the start of this function. How do you think Pine provides a "graphical" text interface using these escape sequences?
  5. Why does s_tlnt also contain code to parse a command line? Hint: Think about when does s_tlnt provides a shell-like prompt to the user.
  6. Notice that both the s_sh and s_tlnt source code files contain references to the code being able to run on multiple platforms, such as Windows or Linux. How is this done within the source code files?