• doc»
• xvm»

This document aims to explain everything you should need to know about XVM's IP renumbering.

What is this "renumbering" all about? Didn't we already do this last year?

If you were an XVM user one year ago, you may remember having your VM(s) renumbered from IP addresses beginning with 18.181 to IP addresses beginning with 18.49 back then. We are being required to renumber again by IS&T. Please contact IS&T if you have questions about why we are being required to renumber. The process this time will be very similar, except that this time, addresses will be changing to ones beginning with 18.25.

Do I need to take any action?

That depends on your VM. In many cases, you will not need to worry about anything at all, and everything will be taken care of automatically. Here are the most common examples of reasons that you will need to take special action and perhaps coordinate with us. If any of the following apply, please look through the rest of this FAQ to find out what you should do:

• Your VM's network is statically configured (this is not the default; most VMs are configured with DHCP)
• You have an external (non-.mit.edu) DNS hostname configured to point to your VM's IP address
• Your VM runs an AFS client
• Your VM runs daemons that have your IP address hardcoded in their configurations

I've already handled a server that had to be migrated in my dorm/office. Will XVM's migrations work the same way?

No. We have done a great deal of work to attempt to make VM migrations as smooth as possible; please keep reading to learn what to expect.

If my VM does not require special coordination, how will my automatic IP migration work?

Once we set your VM to be migrated, the next time it renews its DHCP lease (which should happen within an hour), your VM will be given its new IP address. At the same time that this happens, the XVM infrastructure will set up a temporary NAT to accept traffic at your VM's old IP address and forward it to the new address. This means that, regardless of whether users try to contact your VM at its old address or its new address, they will successfully reach it. There will be no period where your VM will be unreachable.

At the same time as your VM receives its new IP address, the XVM DNS server, which answers lookups for the hostname $VMNAME.xvm.mit.edu, will be updated (previous lookups will only be cached for at most 15 minutes). Sometime during the following day, we will update the record for your VM's IP address in MIT's Moira database, which controls lookups for any custom .mit.edu hostname you may have requested (these default to names like xvm-number-xyz.mit.edu if you have not requested a custom hostname).

Some time after all cached lookups of the .mit.edu hostnames have expired, we will remove the temporary NAT, and your VM's IP migration will be complete.

If I have active connections when my IP address is migrated, will they continue to work?

No, unfortunately, there are limits to what we can do to maintain your VM's connectivity. If you have active connections (for example, ssh) when your IP address is migrated, those connections will be broken. However, you will be able to immediately reestablish those connections at either the new or old IP address.

If you are running something like a webserver, however, and you have users interacting with your site, they will be able to continue clicking around your website as your VM migrates IP addresses.

When will my IP addresses change? Can I postpone this? Can I migrate early?

We plan to begin automated migrations of VMs to their new IP addresses on Monday, August 5, 2019. This process is expected to take somewhere between a few days and one week.

There are several forces at play that we do not control, so there is very limited flexibility that we can offer.

If your VM requires special coordination in order to be migrated, we will work with you to find a time that works well, possibly earlier than the automated migrations. You should see other questions in this FAQ to determine if your VM will require special handling, and get in touch with us if so by sending e-mail to xvm@mit.edu. If you would like your VM to be migrated early, perhaps because you will be away during the week we plan to perform most automated migrations and you would like to be available to ensure that all has gone well once your machine has migrated, we will be happy to migrate your machine early - once again, please contact us at xvm@mit.edu and be sure to tell us the name of the VM and when you will be ready for it to be migrated.

What will my new IP address be? How can I tell if my migration has happened?

Your old IP address was of the form 18.49.xxx.yyy. Your new IP address will be of the form 18.25.zzz.yyy where yyy will remain the same and zzz will be xxx + 128. If your VM's networking is statically configured and you will be updating this info manually because you cannot switch to using DHCP, it is critical that you note that the gateway will also be changing. Please see the question on what to do if you use static addressing below.

Before your migration, you can see what your new network parameters will be by visiting XVM's website at https://xvm.mit.edu/ and clicking the link for your VM to go to its info page. On that page, look for a line that begins with "Other Address:".

Once your migration has occurred, the "IP:" line on the info page and the IP address on the line for your VM on XVM's main webpage will show your new address, which begins with 18.25.

My VM's networking is statically configured. What should I do?

The simplest and best way to handle this is to reconfigure your VM to use DHCP, if at all possible, as soon as you can. If you can change your VM to use DHCP, then you needn't do anything else, and you don't need to contact us to let us know that you've done so - everything will just proceed automatically.

If there is a reason that you cannot reconfigure your VM to use DHCP, then once we have begun migrating VM IPs, you should visit our website at https://xvm.mit.edu/ and click the link for your VM to go to its info page. On that page, you will be able to see both the current and new IP address, netmask, and gateway. There will also be a new button on the page with a message above it saying that you can reconfigure your VM with the new IP parameters (make sure you get the gateway correct; it will not be the same as the old one) and then click the button. If your VM is running, clicking the button will powercycle your VM. If your VM is not running, clicking the button will update XVM's configuration for your VM and tell you to turn it on to complete the transition. You need to edit the configuration files for your VM's networking on the VM before you click the button.

If the above works fine for you, you do not need to contact us.

I have an external (non-.mit.edu) DNS hostname pointed at my VM. What should I do?

You should contact us at xvm@mit.edu to coordinate. Once your VM's IP address has been migrated, we will delay removing the NAT that allows both the old and new addresses to work until you have updated your external DNS. You should not update your external DNS before your machine's IP address has been migrated, because the new address will not work before that point.

My VM runs an AFS client. What should I do?

You should wait for your VM's IP address to be migrated. Once that happens, you will need to either restart your AFS client, or (perhaps more easily in some cases) reboot your VM. If you are curious about the technical reasons for this, see the next paragraph. You do not need to contact us about this.

Restarting the AFS client after you are renumbered is necessary because after your AFS client starts up, the first time it talks to each of the fileservers in the AFS cells you use, it registers a list of the IP addresses it has with those fileservers. That list will not be updated when your VM's IP address changes (there is a command fs setclientaddrs to change the list of IP addresses that will be sent to fileservers your client has not yet communicated with, but it will not update the addresses on file with servers your VM has already communicated with). This will make the fileservers unable to inform your VM when changes have been made to files once the temporary NAT is removed, and you will begin to experience mysterious problems with AFS.

My VM runs daemons that have my IP address hardcoded in their configuration files. What should I do?

If at all possible, you should reconfigure them not to have IP addresses hardcoded. If you're able to do that, you won't need to get in touch with us.

If any of your daemons cannot work without a hardcoded IP address, you will need to get in touch with us at xvm@mit.edu to coordinate a time to migrate your VM's IP address. Even though we will continue to send traffic for the old IP address to your VM for a period of time after its migration, we will do this with a NAT (essentially, we will take packets destined for your VM's old IP address, and we will change the destination address on them to the new address); your VM will only have an interface set up for the new address, and any daemons that try to make use of the old address in their configurations will encounter errors.

We will work out a time with you when your IP address can be migrated. At that time, we will set your VM to migrate, and you will change the configuration files for your daemons. Once the machine's IP address migrates, you will restart the daemons or reboot your server.

Should I contact you about my VM's renumbering? When do I need to contact you by?

If you have questions that are not answered by this FAQ, you should contact us and we'll do our best to answer them. If your VM requires special coordination (for example, due to any of the reasons listed in the "Do I need to take any action?" question near the top of this page), you should definitely contact us no later than Sunday, August 4. If anything goes wrong during your VM's migration, again, we need to hear from you. Finally, if you have feedback about the process that you would like to share, we welcome you to contact us.

If none of the above applies to you, we hope that your migration experience is very smooth, and there is no need for you to get in touch with us. Please keep in mind that, while we will do our best to answer messages that we receive, there are approximately 1,200 VMs that we need to get migrated in a very short time, and our resources are limited, so there is no need to contact us just to acknowledge receipt of the announcement or to report that your VM's migration was successful.