Middlebury

Network Troubleshooting - Mac OS X

Revision as of 10:53, 14 July 2008 by Christopher Hunt (talk | contribs) (Reorganized to make style more similar to other pages)

Typical troubleshooting steps

Typical troubleshooting steps on Macs include three main approaches:

  • Renewing the IP address
  • Turning the network card off and on again
  • Creating a new location, getting the IP, MAC address and running “arp -a".

Release and renew DHCP lease (renew IP address)

  1. Open System Preferences => Network.
  2. For Tiger: Click on the menu next to “Show” and select “Built-in Ethernet”. For Leopard: Click on “Ethernet” in the list on the left, then click the button “Advanced” in the lower right corner.
  3. Under the “TCP/IP” tab click the button “Renew DHCP Lease”. Click the button “Apply” (located in lower right corner).

Create a new location

  1. Open System Preferences => Network.
  2. Click on the menu next to “Location”.
  3. For Tiger: Select “New Location”. For Leopard: Select “Edit Locations”, then click on the plus “+” button.
  4. Type in any name for the new location (e.g. Middlebury), click OK or Done. Click the button “Apply” (located in lower right corner).
  5. Creating a new location changes all network settings to their defaults (automatically obtain an IP, no proxy servers, no manual DNS servers, etc.). It also saves the customer’s custom settings.

Disable and re-enable the Network Card (NIC)

  1. Open System Preferences => Network.
  2. For Leopard: Click on “Ethernet” in the list on the left. Click on the gear button [[Image:]] (it’s below the list on the left).
  3. To disable the NIC, elect “Make Service Inactive”. Click the button “Apply”.
  4. Wait 5 seconds, then enable the NIC by clicking on the gear button again and selecting “Make Service Active” this time. Click the button “Apply”.
  5. For Tiger: Click on the menu next to “Show” and select “Network Port Configurations”. In the list of ports:
  6. To disable the NIC, remove the checkmark next to “Built-in Ethernet”. Click the button “Apply Now” (located in lower right corner).
  7. Wait 5 seconds, then enable the NIC, by placing a checkmark next to “Built-in Ethernet”. Click the button “Apply Now” (lower right corner).

Getting the IP address and MAC address and running "arp -a"

To get the IP address and MAC address:

  1. Open System Preferences => Network.
  2. For Tiger: Click on the menu next to “Show” and select “Built-in Ethernet”. For Leopard: Select “Ethernet” in the list on the left and click “Advanced”.
  3. Under the TCP/IP tab, record the IP address (if it’s blank, leave a note saying that it’s blank).
  4. Under the Ethernet tab record the Ethernet ID (this is the MAC address).

Arp is a Unix command used to identify network-connected computers, routers, etc. To run "arp -a":

  1. Open Macintosh HD => Applications => Utilities. Find “Terminal” and double-click it.
  2. Type in arp -a
  3. Press enter (or return) on the keyboard.
  4. Record the result. This will give the IP address and MAC address of the router, and may be used by SR or LNS to determine if there’s a Rogue DHCP server in the customer’s building.

Other troubleshooting steps

  1. Find out what’s the OS[1]. E.g. Tiger (10.4) or Leopard (10.5). Older OSs can be troubleshot like Tiger.
  2. Find the status of the NIC: Open System Preferences => Network. NOTE: In 10.4 you need to select “Network Status” from the 2nd drop-down menu to be able to look at the status of the NIC. In 10.5 the status is displayed when you select “Ethernet” from the list on the left.
    1. “Built-in Ethernet is currently active and has the IP address [see list of IPs below]”Possible causes: Should have a live connection. If the customer still cannot access the internet, there may be a proxy server, manually entered DNS address or a 3rd party firewall. Solutions: Quit and re-open browser. Try visiting www.middlebury.edu (if the registration page appears, register the computer). Renew lease. Create a new location. Disable NIC / enable NIC. Try different browser. Reboot. Disable 3rd party firewalls (they are rare on Macs but do exist, e.g. Little Snitch, NetBarrier, Norton etc.). Invite customer to LIB202.
    2. “Built-in Ethernet is currently active but has a self-assigned IP address” OR“The cable for Built-in Ethernet is connected but your computer does not have an IP”Possible causes: The computer detected a cable, a jack, and some connection, however, it cannot talk to the DHCP server. Loose cable. Bad cable. Bad card.Solutions: Ensure that the cable is firmly plugged into the network card and a live jack. Create a new location. Try a different jack. Try a different cable. Reboot. Invite customer to LIB202.
    3. “The cable for Built-in Ethernet is not plugged in”Possible causes: The cable may be unplugged/loose on one end. The cable may be bad. The network card may have failed. Solutions: Ensure that the cable is firmly plugged into the network card and a live jack. Create a new location. Try a different jack. Try a different cable. Reboot. Invite customer to LIB202.
  3. Check the IP address. Refer to section below to see what you can learn from the IP address.

If you still haven’t resolved the issue, get some information from the customer and create a ticket:

  • Name and location (building, room #). Make sure we have their extension or cell #.
  • Jack ID (it’s on a silver or white tag around or ON the jack, e.g. “A-0-12”).
  • Open System Preferences => Network. In
  • IP address, MAC address and the result of running the “arp -a" command.
  1. Rogue Servers and entire building losing connectivity:
    1. Talk to someone in SR at once. Still make tickets assigned to SR.
    2. If this happens after 5pm, check “SNS After Hours” (it’s a Public Folder/Calendar in Outlook) and call the person listed there. Be prepared to give all the information you would put in a ticket. Still make tickets and indicate that you have contacted someone, if that’s true.
    3. Note that one person calling with connection issues doesn’t mean the whole building is down. Ask the customer if anyone else is having the same issue. Generally we need a few calls to consider the issue to be building wide.

Decoding IP addresses

We can get a little information about the computer's network status by looking at the first two sections of its IP address.

  • 140.233... Computer should have a normal connection. Not working? Try a), above.
  • 172.17... is the registration subnet. If you see 172.19… or 172.18… that’s the penalty or quarantine subnet – search http://kb for steps to take.
  • 169... Computer can’t find our DHCP server, try all the steps under a) and b)
  • 192... or 10… (BAD addresses for on-campus, may be OK for off-campus, see footnote [2]). May indicate a manual IP address or rogue DHCP server. Create a new location, reboot. If it still gets a 192… or 10… IP address, there may be a rogue DHCP server in that subnet/area. See 4. below.
    1. Other messages or unclear symptoms - Invite the customer to LIB202 to see if the problems persist in another location.




  1. Find OS version: Click on apple logo in top left corner of screen, select “About this Mac” and look in middle of the pop-up.
  2. Some off-campus college houses are supposed to have 192 or 10 addresses. Search http://kb for a list of off-campus locations.