Mac OS X Mavericks: DNS-Server required for default route

I ran into an interesting client behavior issues with several Apple Macbook Air Laptops today at work.  I was able to solve the mystery, but thought I would post my findings as I just confirmed it in the home lab with my wife’s computer.

Observed behavior:
Mac OS X clients running 10.9.1 using a DHCP address require the DNS Server to be configured in order for the default route to take affect.  Without the default route, off-network traffic receives a “no route to host” error.

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Now the network engineers at work were mocking up was an isolated network with no DNS or internet access. As far as I can tell

Scenario:
Catalyst 3750 acting as a L3 switch and doing DHCP for local VLANs.

DHCP Configuration:

Vlan Configuration:

For this scenario, I am using a Flexconnect AP with the SSID dropping off to vlan 145.  Also I am using a server upstream to ping.  I haven’t had a chance to verify that this works with other vendors, but I would guess it does.   Assume that routing is working (as it is).  The production scenario was with a 4500, with local mode APs, and the results were the same.

Now for the client.  My client in this case is a 2012 MBA running 10.9.1.

Here is my ifconfig output:

        en0: flags=8863    mtu 1500

Now lets see if we can ping the default gateway:

Success.  Now lets ping my server upstream from the lab:

Hmm, this should work.  I verify in the network control panel that I have the 192.168.145.1 as the default router.  There’s no reason I *shouldn’t* be able to ping the server.  Except that I’m not passing the DNS Server through the DHCP server.

Lets change the DHCP server configuration to the following:

In this example, i’m using 1.1.1.1 which is a totally bogus DNS server.  It doesn’t need to work in order to get routing to work on the MBA.

Same SSID, just the DNS-Server change.

Here is the relavent ifconfig output:

en0: flags=8863 mtu 1500

inet 192.168.145.2 netmask 0xffffff00 broadcast 192.168.145.255
nd6 options=1
media: autoselect
status: active

Now skipping straight to pinging outside the local subnet we find that it works, just by adding the DNS-Server

I have no idea why this client behavior occurs, or if it intentional. I was able to replicate the behavior with a Win2k8r2 dhcp server and an IP Helper.

   *Note that Windows populates a DNS Server of 127.0.0.1 if you do not specify a DNS server.
   You *MUST* delete this option in order to replicate the behavior.

Further Testing:
Configuring the IP address of the client manually without a DNS server configured exhibits the same behavior.

Conclusion:
Mac OS X Mavericks (10.9.1) requires a DNS Server defined in order to use the configured default route.