Preliminaries

Specification

You are responsible for the following requirements. Most of them are the same as in Lab 1. New ones are in bold:

• Your MTA is able to properly receive multiple, simultaneous messages from well-behaved MUAs like Netscape, Outlook Express, or Mozilla. This will require you to support the following SMTP commands: HELO, MAIL, DATA, RCPT, QUIT, and RSET.
• You may not use the fork() system call.
• You do not need to handle messages with more than 255 recipients.
• You do not need to handle mail messages larger than 65535 bytes.
• Your MTA should deliver each message to every specified recipient if possible. If an error occurs, and it is possible for your MTA to continue processing additional messages, it should do so. If it encounters an unrecoverable error, i.e., socket(), bind(), or listen() fails, it should print an error message and call exit(1).
• If message delivery for any particular recipient fails at all the available mail exchangers, the message should be queued for an additional attempt. The amount of time to wait before attempting a second delivery should be specified on the command line. If, after two attempts at every listed mail exchanger, the message cannot be delivered to a particular recipient, an "undeliverable mail" message should be generated.

• You don't need to support local destinations or source-routed messages. If the argument to the RCPT TO: command is not of the form user@domain.name, you should refuse to accept the message.
• Your MTA may block (and not process any other messages or clients) during a call to gethostbyname()/getmxrr() or the other provided MX record retrieval functions.
• You do not need to support the use of dotted-quad IP addresses instead of domain names in both the HELO, MAIL, and RCPT commands.
• You don't need to implement any other SMTP commands like EXPN, VRFY, HELP, ETRN, NOOP, or VERB. You may simply return an error message if your MTA receives one of these commands.
• ESMTP extensions. You do not need to recognize EHLO or any of the additional parameters to standard SMTP commands.
• You do not need to implement the data transparency procedure described in Section 4.5.2 of the RFC. Basically, you can assume your MTA will never receive a message with a period as the first character of a line.
• You should ignore all headers in the DATA portion of the message. Further, you do not need to implement any sort of loop detection.

Building your MTA (C/C++)

% tar xzf lab2.tgz
% cd lab2
% gmake

% ./mta2 30 2225

The provided code has the makings of a select-based event loop, as discussed in class. I have also included an example of how to set a timer, although you will likely need to do something more sophisticated since you may need keep track of timers for multiple messages at once. As you can see, each connection is managed by its own object, which defines callbacks to be called from the event loop. Feel free to change our object structure around as you see fit.

One final note: by default, connect() is a blocking system call. In order to prevent your MTA from hanging while attempting to contact a remote MTA, you'll need to set the O_NONBLOCK flag on any socket you use to connect to another server. I suggest you do a man on the fcntl() system call. With the socket set non-blocking, the connect() call will return immediately, whether or not it was successful. In order to determine when the socket has successfully connected, you can check for writability in the select() loop. Once select() indicates the socket is writable, it has either connected or failed. You may find the getpeername() function useful to determine whether the connection succeeded or not.

Building your MTA (Java)

MXDomain.java provides a simple method for obtaining DNS MX records, similar to what's provided in domain.c for the C/C++ skeleton code. It exports a single function called getMXRecords which, when given domain name string, returns a Java ArrayList of MXRecord objects from the DNSJava library. Be sure to include the DNSJava jar file (/usr/share/java/dnsjava.jar) in your classpath before using this class!

MTA2.java contains the skeleton MTA code. Like the C version, it provides an example of a select loop and a simple timer. Client.java and TimeHandler.java contain skeleton client and timer handlers, respectively. Feel free to change the object structure or define new objects as you see fit. You can build it by running:

% javac MTA2.java

% java MTA 30 2225

Building your MTA (Python)

Looking up MX records in Python is fairly straightforward:

import dns.resolver

dns_result = dns.resolver.query('cs.ucsd.edu', 'MX')

for mx_record in dns_result:
    print mx_record

10 oec-vmmx.ucsd.edu
5 inbound.ucsd.edu

You can run the code by executing:

python mta2.py 30 2225

Testing your MTA

In order to test the ability of your MTA to continue to process messages while waiting for a slow peer MTA, I have provided an infinitely slow MTA running on sysnet41.ucsd.edu; it will never accept messages, just accept connections and hang. If you'd like to test your MX handling as well, there is an MX record for tarpit.sysnet.ucsd.edu that points to sysnet41.ucsd.edu. So, if you attempt to send a message to anyone@tarpit.sysnet.ucsd.edu, your MTA should never succeed, but continue processing other mail. If you find this infinite wait unsatisfying, you are welcome to add an additional timeout to your MTA that aborts open connections if they stall for longer than the specified delay.

Once you think your MTA is working, you can hook it up to a real MUA and give it a whirl. The simplest way is to use your normal mail reader (Eudora, Netscape, Mozilla, Outlook Express, etc.) and just configure it to use your MTA rather than your default one.

For Mozilla, you can do this by going to Edit->Mail and Newsgroup Account Settings and selecting the Outgoing Server (SMTP) tab. Enter the name of the machine your MTA is running on (e.g., sysnet30.ucsd.edu) and port 2225. Make sure you turn off SSL (Never) and uncheck the user name and password option.

Similar changes can be made to other mailers. I'll post them here if anyone wants to email me how to do this in your own mail reader.

Collaboration policy

Turnin procedure

% gmake turnin