Troubleshooting Host Management Showing Hosts as Down

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FOG utilizes the UDP echo port (7) to check if a client is currently up. The proper response to a UDP echo request is generally an ICMP destination port unreachable response from the host if no service is listening, and in general, this is the case. The Windows Firewall by default disallows several of these mechanisms, all of which must be turned on to receive the 'host up' designation in FOG. The following must be working correctly before a green dot will be shown:

  • The FOG server must be able to correctly resolve the target PC's hostname
  • The PC must allow incoming UDP packets on port 7 from the FOG server(s)
  • The PC must be permitted to send outgoing ICMP destination unreachable packets (XP/Vista/7)
  • The PC must have 'stealth mode' disabled in the Windows Firewall with Advanced Security (Vista/7)
  • For testing purposes, enable ICMP echo request packets at the PC as well (XP/Vista/7)

Verify DNS resolution on the FOG server and connectivity to the PC

Log into the FOG server via SSH, and assuming you enabled ICMP echo requests on the PC's firewall, try to ping the hostname listed in the FOG server. For the PC called 'Accounting1' enter:

ping -c 3 Accounting1

If you receive responses, your PC and FOG server are at least able to communicate, and your FOG server is able to determine the proper IP addresses for hostnames. If no response was received, determine if an IP address was correctly resolved. If yes, the issue lies either with the communication between the FOG server and PC (either unroutable on the network or ICMP is blocked at the PC), or the IP address is incorrect in DNS for that hostname. If no resolution occured, verify that the FOG server's /etc/resolv.conf file is configured with the correct name servers and, if necessary, search list. For the domain 'local.domain' with name servers at and, the /etc/resolv.conf will contain:

search local.domain
NOTE: Typically the search option will not be needed if the FOG server's hostname is correctly set, because the system's FQDN is used to determine the search list domain automatically. Manually specifying the search list overrides this behavior. As an example, for the FOG server with the FQDN 'imaging.local.domain' the automatic search domain will be 'local.domain' (anything following the first . in the server's FQDN).

Once any DNS issues are corrected, test again to verify that the PC is able to respond to pings from the FOG server. On the FOG Host Management page, solid red dots are for clients whose name was successfully resolved to an IP address, but are not responding to UDP pings, while red dots with an exclamation point are clients whose name was not able to be resolved. Communication between the FOG server and the PC must be established before the remaining troubleshooting steps will be of any use!

Background on how UDP echo requests work

If you're in a hurry to get things up and running, feel free to skip this section. Often the first advice given when hosts are not responding to UDP pings is to open port 7. Some will find that doing so does not resolve their problem, and this is due to the replys from the PC being blocked by the firewall policy. UDP pings are often sent to the echo port, port 7, which typically does not have a service listening. The response the originating server is looking for is not a UDP response, but the standard ICMP destination (port) unreachable response sent by the target PC. Because of this behavior, both incoming UDP requests on port 7 and outgoing ICMP destination unreachable responses must be permitted at the PC.

Configure PC Firewall

Windows XP

Windows XP contains the original Windows Firewall, which is much simpler in its configuration and administration. Assuming you configure the firewall from Group Policy, configure your firewall policy to contain the following:

  • Allow UDP packets from the FOG server going to the PC's port 7 (the UDP echo port)
  • In the ICMP configuration, ensure that the PC is permitted to send outbound ICMP destination unreachable packets (required even if ICMP echo request/reply packets are later disabled, see note above for more information)

With those configuration options in place, your XP PCs should be able to reply to the FOG server's UDP pings, and show up as green dots in the FOG Host Management page.

Windows Vista/7 (Windows Firewall with Advanced Security)

The firewall integrated into Windows Vitsa and 7 allows more options to be configured, but with options comes additional complexity. The firewall also features a stealth mode which prevents responses on ports where no service is actively listening. In short, this means ICMP destination port unreachable responses will be suppressed by the operating system regardless of the firewall exemption. Windows Vista and 7 must be configured exactly as Windows XP, with one addition: you must disable the stealth mode feature of the Windows Firewall as well. Microsoft does not provide a built-in way to do so, either at the PC or via Group Policy, however it can be disabled via a registry entry (which can be distributed via Group Policy). You will want to create the following registry entry in your firewall policy's registry settings (Computer Settings, Preferences, Windows Settings, Registry):

  • Action: Update
  • Key Path: Software\Policies\Microsoft\WindowsFirewall\DomainProfile (you can use similar key paths for other profiles)
  • Value Name: DisableStealthMode
  • Value Type: REG_DWORD
  • Value Data: 1
  • Base: Hexadecimal

You may need to restart affected machines after applying the policy to ensure stealth mode is disabled. With it disabled and the UDP/ICMP packet allowances configured, Windows Vista and 7 should respond to the FOG server's UDP pings and show as green dots as well.

NOTE: At the time of writing, I cannot confirm whether this works or not with Windows Vista. Originally Microsoft did not intend to allow a way to disable the stealth feature, so it may have been added with Windows 7. It was tested and works on Windows 7 SP1. More info here: