TweetFollow Us on Twitter

OSX IP Failover, Part 2

Volume Number: 23 (2007)
Issue Number: 04
Column Tag: Network Administration

OSX IP Failover, A Beginners Guide (Part 2)

Giving IP Failover a Real Job

By Ben Greisler

Introduction

In the previous article, we looked at the general concepts involved with IP failover on OSX and how to set it up. At this point if you set up a pair of machines and tried out the failover process you would have simply had one machine take the IP address of the other. While that is certainly nice, it doesn't really do anything productive for us. In this article we will look at making IP failover do some real work and start to become productive.

NOTE: In Part 1 of this article, there was a small terminology error: The line, "Within that folder can be 4 subfolders: PreAcq, PostAcq, PreRel and PostRel." Should read, "Within that folder can be 4 scripts: PreAcq, PostAcq, PreRel and PostRel." Instead of 'subfolders', it's 'scripts'. I apologize for any confusion.

IP Failover Considerations

At its core, IP failover describes one machine taking the IP of another. Unless we tell the machines otherwise, this is all that will happen. We need to take a few additional steps for the secondary machine to be able to take on the personality of the failed primary server.

We first need to determine what services need to be running on the primary machine so we can make sure they are available on the backup machine when failover occurs. Is it a web server? A file server? An application server? Are there ancillary services running that support the primary function such as a database that feeds a web server? Everything that is running on the primary server needs to run on the secondary.

If you need authentication services for the secondary server in an Open Directory system, consider making the secondary server a replica of the OD master. This gives you built-in and automatic failover for OD.

Another consideration is how do we get the data that the primary server uses over to the secondary server. For static content such as web pages, it may be appropriate to make a copy of the data on the secondary server and be done with it. The problem with that is if the data changes at any point, you have to remember to update it on both machines. If you don't,

you will end up with old data being served by the secondary server. This indicates the need for shared data between the two servers where if it is updated or changed, it is automatically updated on both.

So what are our choices for synchronized data? If the amount of data is relatively small, we could write a script to copy the data from the primary server to the secondary server on a regular basis. This may be appropriate for web pages or content that doesn't change that much. If we have data that is constantly changing, we may want to look into shared storage such as an XSan volume. Another choice is to use external storage that gets mounted on demand when the failover occurs.

Action Scripts

Part of the process of IP failover is the server checking the /Library/IPFailover/<IP address> folder on the secondary server for various scripts. The scripts need to be prefaced with PreAcq, PostAcq, PreRel or PostRel. If there are multiple scripts of one of the 4 types, they will be performed in alphabetical order (i.e.: PostAcq_a, PostAcq_b, etc.).

A script with the PreAcq prefix is run before the network interface on the secondary server takes on the IP of the primary server during failover.

A script with the PostAcq prefix is run after the network interface on the secondary server takes on the IP of the primary server during failover.

A script with the PreRel prefix is run before the network interface on the secondary server releases the IP of the primary server during failback. A script with the PostRel prefix is run after the network interface on the secondary server releases the IP of the primary server during failback. The scripts can be written to perform just about any action you may need to occur during each part of the process. You may want to include writing out to a log or maybe send informational emails. You may need the scripts to start or stop services and mount or unmount volumes.

For example, we might need to mount a volume on the secondary server since we don't want to have the same volume mounted on both servers at the same time, assuming we don't have them as part of a SAN. By the way, a SAN is a great idea for this situation as we can avoid many of the pitfalls of having to mount and unmount volumes. The fstab entry for preventing the mounting of a volume will look something like this:

UUID=2B228FFC-B727-2910-A3B9-917CBAD7134F       none   hfs   rw,noauto

We will need a PreAcq script that may look like this:

diskutil list | grep webstuff | awk ' {print $6 } ' >> /tmp/XRAID_1
diskutil mount $(cat /tmp/XRAID_1)
mount -uw /Volumes/webstuff

This PreAcq script mounts the volume named "webstuff" and mounting the volume would be required if we had edited fstab to prevent mounting of the volume on startup of the secondary server. Once we have mounted the volume and the secondary server has taken over the IP of the primary server, we need to start the services on the secondary server. We might use a PostAcq script that looks like this:

sudo serveradmin start web

or

sudo serveradmin start afp

Once we have the secondary server up and running we are in good shape. While I recommend not directly failing a server back, you might decide that this is fine for your needs. In this case we need to look into the Pre and Post Release scripts. As we know that the services are about to be reassigned to the primary server, we need to stop the services on the secondary server. The PreRel script might look like this:

sudo serveradmin stop web

or

sudo serveradmin stop afp

Then, as part of the same script or possibly another PreRel script, remembering to name them in the order you want them acted on, you will need to unmount the volume that will be picked back up by the primary server:

sudo diskutil unmount /Volumes/webstuff

Once the secondary server has relinquished the primary servers IP the final script is run and that is the PostRel script or scripts. You might want to put an email notification script together or simply log the event:

logger "Completed failback"

AFP Special Considerations

One of the obvious uses of IP failover is to provide seamless protection for file sharing. While IP failover can be used for AFP file servers, AFP has an interesting twist; AFP uses a special cache at /etc/AFP.conf to determine if incoming connections are new or reconnects. The Finder on a client machine disconnected during failover will try to reconnect and is expecting a reconnect. However, if the secondary server has picked up AFP duties from the primary server, it won't know that the client is trying for a reconnect and not a new connection.

Hmm, so what do we do about this? By providing a place for AFP.conf to live on some shared storage is the key. This might be an Xserve RAID volume that both machines can see at the same time. I know that we try to avoid the situation where more than one server can write to a volume, but we have to do that in this case. We also need to point the reconnectKeyLocation key that lives in the /Library/Preferences/com.apple.AppleFileServer.plist to point to the shared location:

<key>reconnectKeyLocation</key>
<string>/Volumes/AFPToken/AFP.conf</string>

And while it is tempting to do so, I can't recommend using IP failover for network home folders. Try it if you want, but I suspect you will find that the clients will have issues with it, even with automatic reconnect.

Conclusion

IP failover can be a powerful tool when it is well thought out and matched to the appropriate need. It is not the end all, be all of high availability, but it can help us add an occasional extra "9" to our uptime percentages. The intention of this article was to give you the most basic tools needed to implement IP failover and to give you a jumping off point. Take these tools and examples and try it out on your own.


Ben has worked Apple based technology integration projects from Maine to Japan while learning all the way. When not collecting frequent flyer miles he spends his favorite time with his wife and 2.5 year old daughter at their home outside of Philadelphia. He can be reached at magikben@mac.com.

 
AAPL
$98.63
Apple Inc.
-0.39
MSFT
$43.92
Microsoft Corpora
-0.05
GOOG
$585.19
Google Inc.
-5.41

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Yosemite Wallpaper 1.0 - Desktop im...
OS X Yosemite Wallpaper is the gorgeous new background image for Apple's upcoming OS X 10.10 Yosemite. This wallpaper is available for all screen resolutions with a source file that measures 5,418... Read more
Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
Vienna 3.0.0 RC 2 :be5265e: - RSS and At...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
VLC Media Player 2.1.5 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Default Folder X 4.6.7 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
TinkerTool 5.3 - Expanded preference set...
TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows to activate hidden features in the operating system and in some of the... Read more
Audio Hijack Pro 2.11.0 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Intermission 1.1.1 - Pause and rewind li...
Intermission allows you to pause and rewind live audio from any application on your Mac. Intermission will buffer up to 3 hours of audio, allowing users to skip through any assortment of audio... Read more

Latest Forum Discussions

See All

Note Review
Note Review By Jennifer Allen on July 29th, 2014 Our Rating: :: TOO SIMPLEiPhone App - Designed for the iPhone, compatible with the iPad Note is a note taking app that’s a little too short on features to be worth its asking price... | Read more »
Chainsaw Warrior Goes on Sale & Ther...
Chainsaw Warrior Goes on Sale & There’s a Chance to Win a Copy of the Original Board Game Posted by Jennifer Allen on July 29th, 2014 [ permalink | Read more »
It Came From Canada: Tiny Tower Vegas
If you go to a casino, you might make a lot of money. If you run a casino, you’re guaranteed to make a lot of money. The choice seems pretty obvious. So while waiting for your shady real estate deals to move forward, get prepared with Tiny Tower... | Read more »
Z Hunter Review
Z Hunter Review By Lee Hamlet on July 29th, 2014 Our Rating: :: RIGHT ON TARGETUniversal App - Designed for iPhone and iPad While it might not necessarily break new ground, Z Hunter has enough tricks up its sleeve to ensure that... | Read more »
Huge Update Comes To Duet, Adding 48 New...
Huge Update Comes To Duet, Adding 48 New Stages Posted by Jennifer Allen on July 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Sharknado: The Video Game Available Now....
Sharknado: The Video Game Available Now. Seriously. Posted by Rob Rich on July 29th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Frog Orbs 2 Review
Frog Orbs 2 Review By Nadia Oxford on July 29th, 2014 Our Rating: :: THIS MAGIC IS A TAD MONOTONOUS Universal App - Designed for iPhone and iPad Frog Orbs 2 is repetitive, but younger players should enjoy it nonetheless.   | Read more »
Puzzix Review
Puzzix Review By Jennifer Allen on July 29th, 2014 Our Rating: :: NICE IDEAUniversal App - Designed for iPhone and iPad A little like Tetris, Puzzix is all about piecing together blocks and watching them vanish. It could do with... | Read more »
Cannonball eMail is Now Live – Works Wit...
Cannonball eMail is Now Live – Works With Gmail, Yahoo, Outlook, Hotmail, and AOL Posted by Jessica Fisher on July 29th, 2014 [ permalink ] | Read more »
To The End Review
To The End Review By Lee Hamlet on July 29th, 2014 Our Rating: :: A VICIOUS CYCLEUniversal App - Designed for iPhone and iPad To The End will test players’ patience, timing, and dedication as they try to navigate all 13 levels in... | Read more »

Price Scanner via MacPrices.net

Updated MacBook Pro Price Trackers
We’ve updated our MacBook Pro Price Trackers with the latest information on prices, bundles, and availability on the new 2014 models from Apple’s authorized internet/catalog resellers as well as... Read more
Apple updates MacBook Pros with slightly fast...
Apple updated 13″ and 15″ Retina MacBook Pros today with slightly faster Haswell processors. 13″ models now ship with 8GB of RAM standard, while 15″ MacBook Pros ship with 16GB across the board. Most... Read more
Apple drops price on 13″ 2.5GHz MacBook Pro b...
The Apple Store has dropped their price for the 13″ 2.5GHz MacBook Pro by $100 to $1099 including free shipping. Read more
Apple drops prices on refurbished 2013 MacBoo...
The Apple Store has dropped prices on Apple Certified Refurbished 13″ and 15″ 2013 MacBook Pros, with model now available starting at $929. Apple’s one-year warranty is standard, and shipping is free... Read more
iOS 8 and OS X 10.10 To Support DuckDuckGo As...
Writing for Quartz, Dan Frommer reports that Apple’s forthcoming iOS 8 and OS X 10.10 operating systems version updates will allow users to select DuckDuckGo as their default search engine. He notes... Read more
U.K. Hospital Using iPods and iPads To Record...
British news journal GazetteLive’s. Ian McNeal notes that the old “an apple a day keeps the doctor away” proverb is being turned on its head at http://southtees.nhs.uk/hospitals/james-cook/ James... Read more
13-inch 2.5GHz MacBook Pro on sale for $1099,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $1099.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $100 off MSRP. Price is... Read more
Roundup of Apple refurbished MacBook Pros, th...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available for up to $400 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. Their prices... Read more
Record Mac Shipments In Q2/14 Confound Analys...
A Seeking Alpha Trefis commentary notes that Apple’s fiscal Q3 2014 results released July 22, beat market predictions on earnings, although revenues were slightly lower than anticipated. Apple’s Mac’... Read more
Intel To Launch Core M Silicon For Use In Not...
Digitimes’ Monica Chen and Joseph Tsai, report that Intel will launch 14nm-based Core M series processors specifically for use in fanless notebook/tablet 2-in-1 models in Q4 2014, with many models to... Read more

Jobs Board

Sr Software Lead Engineer, *Apple* Online S...
Sr Software Lead Engineer, Apple Online Store Publishing Systems Keywords: Company: Apple Job Code: E3PCAK8MgYYkw Location (City or ZIP): Santa Clara Status: Full Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.