@tech

Well, ‘saga’ may be over stating it a bit, but it was a day of frustration, which began with me just tweaking a few settings and installing some upgrades.  For the record, I’m running OS X 10.8.5.  The next day, after being online for perhaps an hour, I suddenly had no network connection.  After verifying that our connection was still live, a quick check of my network settings revealed an IP address that started with 169.  Okay, that’s simply wrong as our home network is 198.

Time to put on the Sherlock Holmes hat.  For the time being, I opted to ignore trying to figure out why it suddenly changed without warning, and just get the thing connected again to the rest of the world.  First off I tried renewing the DHCP lease, get a new IP from the router, that should fix it.  Nope.  How about a fixed IP address, set manually without DHCP, something I use with a few computers and servers at home.  Nope.  At this point I could wrangle miles of network cable to see if that’s the problem, oh but wait, just turn on AirPort and connect via wireless.  Nope.  Reboot the network hub, maybe it took a power surge.  Nope.  Try different ports in the hub.  No again.  This process went on for a while with no positive results.

At the time this reared it’s ugly head I was in the process of transferring my long running OS X 10.6.8 system and files (from a bootable USB drive I use at work and home so I can leave the laptop at home) into the fresh and new world of OS X 10.8.5.  So I reboot the computer from the 10.6.8 drive and … PRESTO!  Networking that works as it should.  That means this isn’t a cable or hardware failure, but a software issue in the 10.8.5 setup that just occurred in the past day.  ugh.

As I mentioned I was updating some software.  Now I’m generally very careful about these sort of things, not wanting to invite a virus or some other nefarious bit of code into my computing world, but I suppose the inevitable can happen.  And while it’s extremely rare on a Mac, I couldn’t rule that out.  So run the scanner, and while that’s going start nosing around online.  The virus scan can back empty (thankfully) but so did my search efforts, which seemed strange as I can almost always find something.  

During this time I tried all of the following: changed settings and disabled the firewall; disabling TCPBlock and GlimmerBlocker, both preference panes that control what goes in and out of your machine; reset the hosts file; deleted the plist files for network settings; looked at iptables and dns records; rebooted numerous times checking each change, all to no avail.

Eventually through countless rewording of my search terms I one forum that said an IP starting with 169 is a self assigned IP which is only caused when the machine has no network connection.  I already know the connection and hardware is good.  So modifying the search again to add “self assigned IP” I found another forum confirming this and with several possible suggestions, among them a link to an article on cnet: Addressing self-assigned IP addresses after installing OS X 10.6.8.  Not exactly the same version of the OS but here he says the culprit is a corrupt firewall settings file and deleting it will resolve the problem.  This preference file is named com.apple.alf.plist. and can be located in the /Macintosh HD/Library/Preferences/ folder.  Reboot after deleting the file.

SUCCESS!

That was it.  Network restored AND another lesson under my belt on computer troubleshooting. TY Mr. Kessler and cnet!

Quick update: it happened again!  The solution also worked again.  So clearly there is something going on that needs some further examination.

Another update: the problem continued to reappear and so my search for an answer continued.  I eventually discovered in the advanced network settings that there were proxies enabled, of which the web proxy specified a self-assigned IP number.  Could it be…?  I disabled all proxies, and it’s been stable for a week now, even after rebooting a couple of times just to see if I could trip things up again.  So I feel reasonably sure that was the culprit.