Archive for November, 2010

A Word about Network Interface Port Speed and Duplex Settings

November 10, 2010 3 comments

When configuring network interface settings on a Forefront Threat Management Gateway (TMG) 2010 firewall, I strongly recommend that the port speed and duplex settings on all active network interfaces be manually configured. Although autonegotiation is typically enabled by default, and in most cases works without issue, I prefer to eliminate any possibility that a slower link speed or incorrect duplex setting is negotiated in error by configuring these settings explicitly.

Some believe you can configure one side of a link (either the host or the switch) manually and leave the other side set to autonegotiate. The theory is that the host set to autonegotiate will determine what settings the other side has configured and automatically choose those. This is not completely true and doesn’t work as expected. When one side of a link is set for autonegotiation and the other side is not (or doesn’t support it), a process called parallel detection takes place whereby the device that is configured to autonegotiate can determine the port speed of the other device, but it cannot determine the duplex settings and defaults to half duplex. Often this results in a duplex mismatch, which will cause extremely poor performance.

So, when configuring port speed and duplex settings, always remember that BOTH sides of the link should be configured identically. That is, if the switch is configured for 100Mbps/full duplex operation, the network interface on the TMG firewall should be configured the same. On some systems (mostly older ones), when attempting to change these settings, you might notice that there is no option to enable 1000Mbps (gigabit) at full duplex operation. The only available option is autonegotiate 1000Mbps.

Why is there no option to select 1000Mbps and full duplex? Because the designers of the 1000BASE-T specification (IEEE 802.3ab) made duplex autonegotiation mandatory when operating at 1000Mbps port speed over copper cabling. For this reason some network interface management software may limit your choices and not allow you to select this option. In my experience it appears that adherence to this mandate has been relaxed as most newer systems I have worked with now give you the option of configuring 1000Mbps and full duplex operation with copper cabling. If you have an older system and don’t have the option to specify 1000Mbps/full duplex on the network interface, I would recommend updating your network interface device drivers. If you still don’t have the option to specify 1000Mbps/full duplex you may need to replace the network interface card itself.

Fwengmon.exe and Forefront Threat Management Gateway (TMG) 2010

November 2, 2010 2 comments

For engineers performing advanced troubleshooting on TMG, you have likely noticed that fwengmon.exe, a utility that you used with previous versions of ISA, no longer functions with TMG.

Not to worry! This detailed information is readily accessible using netsh.exe in the tmg context. The following is a list of common commands and their fwengmon.exe equivalents (where applicable):

To view creation objects, active sessions, NLB hook rules, NLB server assigned ranges, and dynamic and persistent allowed ranges:

netsh tmg show all

To view connections only (fwengmon.exe /session or /s):

netsh tmg show connections

To view detailed information about a specific connection (fwengmon.exe /s <ID>):

netsh tmg show connections <connection_number>

To view firewall creation elements (fwengmon.exe /creations or /c):

netsh tmg show creations

Note: You can sort and filter output from show connections or show creations by source IP address, source port, destination IP address, destination port, or protocol using the sort and filter parameters (fwengmon.exe /organize or /o, or fwengnmon.exe /filter or /f). You can also limit the number of connections or creations displayed using the display parameter. Type netsh tmg show connections ? or netsh tmg show creations ? for more information.

To show NLB hook rules (fwengmon.exe /querynlb or /n):

netsh tmg show nlbhookrules

To view packets held in kernel mode:

netsh tmg show holdpackets

To view packets held in user mode:

netsh tmg show usermodepackets

To view global firewall engine driver settings:

netsh tmg show global

To specify a temporary address range to exempt from firewall filtering (fwengmon.exe /allow or /a):

netsh tmg add allowedrange <beginning_ip> <ending_ip>

To specify a permanent address range to exempt from firewall filtering (fwengmon.exe /allow or /a):

netsh tmg add allowedrange <beginning_ip> <ending_ip> persistent

Note: netsh tmg add allowedrange allows all traffic to and from hosts within the IP address range specified to bypass stateful firewall inspection completely. It should be used for troubleshooting purposes only.

To delete a temporary address range (fwengmon.exe /noallow):

netsh tmg delete allowedrange id=<id>

To delete a permanent address range (fwengmon.exe /noallow):

netsh tmg delete allowedrange id=<id> persistent