The Selftest Command - D-Link NetDefendOS User Manual

• Max TCP Length - Maximum size of a TCP packet (default: 1480 bytes).
However, the MTU value of the individual NetDefendOS interfaces determines how the packet
size is split. For example, the maximum UDP length could be set to 60,000 but the interface MTU
size might be 1500 so packets would be split into 41 fragments (60,000/1500).
Keeping these maximum settings to the lowest possible value is beneficial since unreasonably
large packets can be used as a form of attack and they are immediately rejected by NetDefendOS
when they exceed the set maximum.
TCP Length
With TCP, all normally configured TCP stacks will limit the size of TCP packets to the negotiated
Maximum Segment Size (MSS) value. This MSS value will normally be the MTU value minus the IP
header size of 20 bytes. With an MTU value of 1500 bytes, the MSS will be 1480 bytes and this will
normally never need to be fragmented.

2.6.9. The selftest Command

It may be the case that operational problems are caused by a problem with the hardware
platform and not NetDefendOS. For this reason, the CLI command selftest is provided to perform
tests on various aspects of hardware functioning.
Preparing Hardware
To ensure the complete reliability of any selftest, it is recommended to take a complete backup
of the current configuration and reset the hardware unit to the base configuration as well as
having the unit disconnected from any networks.
This is also true for units in an HA cluster. The cluster should be broken up into two separated
hardware units and they should each be reset to the base configuration.
Resetting to the base configuration can be done through the CLI or Web Interface. Using the CLI,
the command is:
gw-world:/> reset -configuration
A Simple Example
A simple use of selftest is to test the system memory:
gw-world:/> selftest -memory
This writes a one megabyte block of data to memory and reads it back. The number of
megabytes written can be varied using the -num= option. By default, the memory test is
repeated once but could, for example, be repeated 10 times with the command:
gw-world:/> selftest -memory -num=10
Warning: Do NOT conduct tests with live traffic!
It is important to remember that the selftest command should not be used on a system
that is carrying live traffic. The command can cause connections and associated data to
be lost and the test results themselves will be unreliable.
Chapter 2: Management and Maintenance
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