DNS – Resolving DNS Lookups Issue with Ping vs Host

dns

I'm using pfSense 2.0rc3, and I've set it up as a DNS forwarder and enabled "Register DHCP leases in DNS forwarder" and what I understand are all the appropriate settings to get DNS server for local lookups.

It works as expected with Linux and in particular I can run host abc and ping abc (and other applications) and they all work as expected.

However in Mac OS X Lion 10.7 it does not work as expected. In particular, only lookups with the host command seem to work, i.e.

$ ping abc
ping: cannot resolve abc: Unknown host

$ host abc
abc.local has address 192.168.1.128

$ ping abc.local
ping: cannot resolve abc.local: Unknown host

$ host abc.local
abc.local has address 192.168.1.128

Why does the lookup for abc work when using the host command but fail with ping (and other applications)?

Thanks for reading.

Best Answer

Why they made this change, I don't know, but it's driven me crazy for a while.

I don't know why things work for host, but not ping, but I think it has to do with the nature of these two utilities. Ping is a simple (although very helpful) diagnostic utility for dropping packets on the wire that should get echoed back to you. The hostname lookup functionality is just a side effect of the job and handed off to the system's recursive resolver (I believe -- I haven't verified by checking linked libraries or anything of that sort). Host's main job is to do DNS name resolution, so it implements its own recursive resolver.

Apple's recursive resolver is mDNSResponder. For some reason, the version of mDNSResponder in Lion needs the "-AlwaysAppendSearchDomains" command line option to behave as it did in Snow Leopard (at least).

Here's a quick way to fix it:

sudo sed -i .orig '/ProgramArguments/,/<\/array>/ {
s/\(<string>-launchd<\/string>\)/\1\
                <string>-AlwaysAppendSearchDomains<\/string>/
}' /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

(There should be two tab characters at the start of the second-to-last line above, but I couldn't figure out how to get this little editor to insert tabs, so I added 16 spaces. Either should work, but the tabs fit the spacing of the original file better.)

This will add the "-AlwaysAppendSearchDomains" argument to the mDNSResponder startup plist file (and save a backup copy), but since this is controlled by launchd, that system needs to be told to restart mDNSResponder.

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

Now, if you check your running mDNSResponder process, you should see it running with your new argument:

ps auxww | grep mDNSResponder

(Props to http://www.makingitscale.com/2011/fix-for-broken-search-domain-resolution-in-osx-lion.html and http://kavassalis.com/2011/07/wtf-bug-in-os-x-10-7/, where I found my answers to this problem.)

Related Question