Ask SIPB - September 23, 2003

Having problems connecting to the network since Thursday, September 11? Getting deluged with lots of spam? This week, we explain the required re-registration for dynamic IP addresses (DHCP) and e-mail etiquette.

Who was affected by the DHCP reregistration?

DHCP is the network protocol that allows users to automatically obtain an IP address, connect to the network and have various network settings configured automatically. Everyone who was using hostless (dyn-o-reg) DHCP was required to re-register for it as of 8 PM on September 11. People using static IP addresses or host-based (dhreg) DHCP registration were not affected. For more information about the differences between these, please see our August 24 column at

If you are a student and you don't know what a static IP address is, you probably don't need one and are using hostless DHCP. Faculty and staff are likely using a static IP address or host-based DHCP, because hostless DHCP is not available for faculty and staff.

Why do I have to reregister for DHCP?

As of 8 PM on September 11, all DHCP registrations were cleared. Because of the recent slew of Windows vulnerabilities and the number of machines that have been compromised, I/S is trying to ensure that all Windows machines have port filtering turned on, in order to pre-emptively prevent future worms from infecting MIT machines. Unfortunately, reregistration is required for all registered systems, even non-Windows machines.

I tried reregistering for DHCP, but it didn't work!

Because everyone was forced to reregister for DHCP at the same time, the registration server was overloaded. In some cases, it did not respond quickly enough to assign registration (net 10) IP addresses to computers. In other cases, computers received an address, but the registration itself proceeded slowly, taking 15 minutes instead of the usual 30 seconds.

Now that most people are reregistered, things have mostly returned to normal. In some cases, however, students have registered for DHCP properly, but are still unable to get a real net 18 IP address, even after waiting a day after registering.

If you continue to have problems in a dorm, please contact an RCC by submitting a help request at For non-dorm problems, contact the Computing Helpdesk at, x3-1101, or in N42.

How do I reregister for DHCP?

If you have a non-Windows machine, open up a web browser and go to any web site. You will automatically be brought to the DHCP registration page. Simply follow the instructions and enter your username and password when requested.

If you have a Windows 2000 or XP machine, however, you will be instructed to enable TCP port filtering. Instructions will be given when you attempt to reregister. You can also find them online at When using port filtering, please ensure that you are filtering TCP only. Filtering UDP or ICMP will prevent some services from working properly. After enabling filtering, you can finish registering.

I want to send mail to lots of people. Should I do so?

Before doing so, consider whether your email is on-topic for the lists you wish to email. Mass emailing a large number of lists, such as all the dorm lists, is unacceptable if what you are mailing is not relevant for the lists. While your email will certainly be read by a large number of people, they will be annoyed and unlikely to be favorably inclined towards you.

Also, keep in mind that you generally should not send mail to a list you aren't subscribed to, unless it is a list designated as one where others can post, such as reuse.

What should I take note of when replying to mail?

When replying to an email, check to see to whom your reply will be sent. You may be sending email to a lot of people who don't need to see your reply, especially since what looks like a single email address may be a mailing list to which many people subscribe. For example, if a person sends out an invitation to an event and asks the recipients to let him know if they are going, it is inappropriate to CC the whole list in response. In this case you should use the "Reply" command instead of "Reply to All" in your mail program.

On the other hand, when e-mailing lists such as, where you are asking for help, you should continue to keep the list in the To: or CC: line so that other people can continue to make suggestions and help you. You can do so by using "Reply to All."

When replying to a long email, especially on mailing lists, avoid including all of the text of the other person's email only to add "me too" at the bottom.

Should I forward this email?

Do not forward chain letters. This includes email that claims 50,000 email signatures are required to help someone as well as email that claims something good or bad will happen if you do or don't forward the email along. Many so-called "helpful advice" email, especially when written in a sensationalistic tone, are nothing but hoaxes. The Urban Legends Reference Pages,, can help identify common hoaxes and tall tales.

I want to subscribe to this list. I should email it, right?

NO! Emailing lists asking to be added will get the list members annoyed at you, because generally, they don't care that you want to be added and can't do anything anyway. The same is true of unsubscribing.

You can remove yourself from MIT mailing lists using a variety of tools that are part of the Moira database management system. mailmaint is a menu-driven tool; more convenient is blanche. Use

athena% blanche listname -d username
to remove yourself from a list. For more information on how these commands work, please see our November 22, 2002 column at Alternatively, you can use the web interface at

If you're having trouble removing yourself from an MIT mailing list, you can e-mail the administrators of the list at or, who will be able to help you. You should not send your request to the list itself; our servers do not intercept subscribe and unsubscribe requests, and sending a subscribe or unsubscribe request to the list is useless, as most people will be unable to do anything to actually process your request. For non-MIT mailing lists, check for an unsubscribe address in the email text, or try, where is the domain where the list is hosted.

To ask us a question, send email to We'll try to answer you quickly, and we can address your question in our next column. You can also stop by our office in W20-557 or call us at x3-7788 if you need help. Copies of each column and pointers to additional information are posted on our website: