Mac OS X self-assigned IP address or no IP address

Affected Operating Systems:

• OS X 10.5 Leopard
• OS X 10.6 Snow Leopard

Problem:

Mac is unable to get an IP address via DHCP, reporting either a self-assigned IP address (169.x.x.x) or no IP address. This often happens on the wireless Airport interface, even when Ethernet appears to be working.

Cause:

The OS X firewall is blocking incoming DHCP information, preventing an address from being obtained. This often happens after total battery depletion or improper shutdown which modifies the file creation dates of key network files (configd, mDNSResponder).

Best Solution:

Download and run the Wireless Network Setup Utility from the UVM Software Archive. This installer includes fixes for necessary firewall exceptions.

Quick/Temporary Solution:

Temporarily disable the firewall to allow a DHCP connection, then re-enable the firewall.

OS X 10.5.8 Leopard

1. Open System Preferences by clicking its icon in the Dock or clicking the Apple menu in the upper left corner and choosing System Preferences.
2. Click the Security preference pane.
3. If it is locked, unlock it by clicking the closed padlock icon in the bottom left corner.
4. Select the Firewall tab.
5. Set it to Allow all incoming connections.
6. Click the Show All button at the top of the window.
7. Click the Network preference pane.
8. Select the network interface (Ethernet or Airport) you want to connect from the column on the left.
9. Click the Advanced button.
10. Select the TCP/IP tab.
11. Click the Renew DHCP Lease button.
12. Wait a few seconds.

OS X 10.6.8 Snow Leopard

1. Open System Preferences by clicking its icon in the Dock or clicking the Apple menu in the upper left corner and choosing System Preferences.
2. Click the Security preference pane.
3. If it is locked, unlock it by clicking the closed padlock icon in the bottom left corner.
4. Select the Firewall tab.
5. Click the Stop button.
6. Click the Show All button at the top of the window.
7. Click the Network preference pane.
8. Select the network interface (Ethernet or Airport) you want to connect from the column on the left.
9. Click the Advanced button.
10. Select the TCP/IP tab.
11. Click the Renew DHCP Lease button.
12. Wait a few seconds.

If a DHCP-assigned IP address is obtained, return to the Firewall preference pane and re-enable the firewall. Turning the firewall back on should not reintroduce the problem.

Advanced Solution:

Reset the firewall to its factory default settings. Warning: this will remove any custom firewall rules that may have been added.

1. Open Terminal (located in Applications > Utilities).
2. Type or copy/paste the following:
   

   sudo cp /usr/libexec/ApplicationFirewall/com.apple.alf.plist /Library/Preferences/com.apple.alf.plist

3. Enter the Mac admin password when prompted.
4. Quit Terminal.
5. Restart the computer.

When All Else Fails Solution:

Download and (re)install the latest Combo Update for that version of OS X. This will reinstall the affected network files and correct any date/time discrepancies that may have occurred due to a dead battery.

• Mac OS X 10.5.8 Leopard Combo Update
• Mac OS X 10.6.8 Snow Leopard Combo Update