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iTunes Home Sharing

A decent into the hell of Bon­jour and black tur­tle-necks

This is just anoth­er short exam­ple in what I’m expect­ing will be a recur­ring theme here on Pack­et Queue: atten­tion to detail.  As a net­work engi­neer, as in so many pro­fes­sions, pay­ing atten­tion to the lit­tle things can mean the dif­fer­ence between 10 min­utes of trou­bleshoot­ing and 3 days of unmit­i­gat­ed, sleep-deprived hell.  Luck­i­ly enough for me, the exam­ple I’m about to give wasn’t 3 days by any means, and since it was per­son­al and not busi­ness the urgency wasn’t the same as if a WAN link had failed.  That said, I want­ed it fixed.

My wife just bought a new computer—her first Mac since the original—and dur­ing the ini­tial mov­ing of files and such, I dis­cov­ered a nifty fea­ture of iTunes: Home Shar­ing.  Now, I have a large iTunes library at home already—something on the order of almost 180 Gigabytes—and want­ed her to be able to share that library on her new Mac.  After all, we’re not pirates; we just want to have access to our shared music library on any com­put­er or device in the house rel­a­tive­ly seam­less­ly.  So I read a quick lit­tle blurb on the how-tos and why-fores of home shar­ing (real men some­times read direc­tions) and turned it on.  Aside from the crick­ets, noth­ing hap­pened.  Sacre­bleu!

Bonjour?

Bon­jour! ¡No Hablo!

No, not a greet­ing but a name giv­en by Apple to their zero­conf imple­men­ta­tion that allows devices (print­ers, stor­age, shares, etc.) to auto-mag­i­cal­ly find one anoth­er.  This is the ser­vice that was sup­posed to make my iTunes library share­able between com­put­ers.  This is the ser­vice that was sup­posed to make every­thing in my dull world shiny again.  Not being over­ly steeped in the Apple world, how­ev­er, has made me nat­u­ral­ly sus­pi­cious of any­thing that “just works” as more often than not, said thing only “just works” if you “just use it in this one way”.  That nat­ur­al sus­pi­cion of mine was proven to be well-found­ed.

Upon read­ing up on Bon­jour, I dis­cov­ered that it uses mDNS (mul­ti­cast DNS) to find ser­vices.  Well, I thought, that would mean that mul­ti­cast rout­ing should work to fix my woes and I set off to work my mag­ic.  Of course, I had missed a crit­i­cal detail that would have saved me some time: the mul­ti­cast DNS imple­men­ta­tion that forms a part of Bon­jour uses the mul­ti­cast group address of 224.0.0.251.  If you haven’t noticed the prob­lem yet, nei­ther did I right away.  Had I noticed said prob­lem, I wouldn’t have com­plete­ly recon­fig­ured my ASA and 2811 for mul­ti­cast rout­ing, and I wouldn’t have start­ed trac­ing pack­ets with Wire­Shark:

The Mul­ti­cast range runs from 224.0.0.0 through 239.255.255.255 as every first-year net­work­ing stu­dent prob­a­bly knows.  But that range is like all oth­er ranges and has cer­tain reserved address­es with­in it.  In our case, the most inter­est­ing range is 224.0.0.0/24 which is known as the Local Net­work Con­trol Block , or some­times just Link-local.  Address­es in this range include the OSPF address­es of 224.0.0.5 and .6, and RIPv2 address of 224.0.0.9, among oth­ers. The salient detail being that these mul­ti­cast address­es are typ­i­cal­ly sourced with a TTL of 1 and are not to be sent off of the broad­cast domain in which they orig­i­nate.

My wire­less net­work, which my wife’s new Mac is on, is a dif­fer­ent VLAN (and hence, dif­fer­ent broad­cast domain) from my wired net­work.  In fact, between my three wire­less net­works and mul­ti­ple lab net­works, my home envi­ron­ment prob­a­bly has some­thing on the order of 25 dif­fer­ent broad­cast domains.  Def­i­nite­ly not the norm for the aver­age user, but also not uncom­mon if you start look­ing at more tech­ni­cal peo­ple or pro­duc­tion envi­ron­ments.  So, the bot­tom line is that Bon­jour and iTunes won’t work in my envi­ron­ment with­out an mDNS proxy or some oth­er trick­ery.

What both­ers me most about this rev­e­la­tion is that a lot of Apple’s soft­ware and periph­er­als work on this same sys­tem.  Air­port (Apple’s wire­less) as well as their print­er set­up, shares, etc. all work using Bon­jour so are, from at least a sim­ple view­point, bro­ken across broad­cast domains.  I’m guess­ing from Google search­es and such that it’s a minor­i­ty of users of iTunes who are con­cerned about this, and so it may not even make sense for Apple to address the prob­lem.  But if you extrap­o­late that out to every­thing else using Bon­jour, and con­sid­er a cor­po­rate net­work envi­ron­ment, I have to won­der how much of this con­tributes to Apple’s lack of pen­e­tra­tion into enter­prise net­works.

As always, if I’ve got­ten details wrong or you’d just like to offer your own opin­ion back and fur­ther the dis­cus­sion, I can be reached here on this blog or via @someclown on Twit­ter.