An HP-UX 10.30/11.00 system can be used as an IP traffic amplifier. Small amounts of inbound traffic can result in larger amounts of outbound traffic, using ICMP MTU discovery packets.
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<FONT COLOR="#CC0000">COMMAND</FONT>
PMTU
<FONT COLOR="#CC0000">SYSTEMS AFFECTED</FONT>
HP-UX release 10.30 and 11.00 only
<FONT COLOR="#CC0000">PROBLEM</FONT>
An HP-UX 10.30/11.00 system can be used as an IP traffic
amplifier. Small amounts of inbound traffic can result in larger
amounts of outbound traffic.
HP provides a proprietary method for determining PMTU. When
traffic needs to be routed to a destination for which the optimum
MTU has not been determined, ICMP packets are used to discover
the MTU for that path while data traffic is shipped in parallel.
This is the default strategy for determining PMTU.
<FONT COLOR="#CC0000">SOLUTION</FONT>
Reference ndd manpage (1M), /etc/rc.config.d/nddconf. To ensure
that this parameter is set each time the system is booted, add the
following lines to /etc/rc.config.d/nddconf:
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TRANSPORT_NAME[<index>]=ip
NDD_NAME[<index>]=ip_pmtu_strategy
NDD_VALUE[<index>]=1
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The value of <index> is an integer from 0 to 99. The first
parameter specified in the file should use an <index> of 0, the
second an <index> of 1, and so on. Once these changes have been
made, execute the following command:
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/usr/bin/ndd -c
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This sets all of the options specified in the file
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/etc/rc.config.d/nddconf.
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To verify that the parameter is set correctly, use the following
command:
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/usr/bin/ndd -get /dev/ip ip_pmtu_strategy
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This should report a value of 1. Another way to change the
ip_pmtu_strategy parameter on a running system is to use the
following NDD command:
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/usr/bin/ndd -set /dev/ip ip_pmtu_strategy 1
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This setting will only last until the system is rebooted at which
point the value of the parameter will be determined by the default
value of 2 or whatever value is set in /etc/rc.config.d/nddconf.
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