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Old 19th July 2010
Frothingdog Frothingdog is offline
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Default Determine packet sizes

Is there any way to determine the size of the packets passing through an interface?

We run a wireless network, and the size of the packets is a big factor in determining the amount of data we can through a wireless link.

Cheers
Brad
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Old 19th July 2010
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jggimi jggimi is offline
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This is defined by the Maximum Transmission Unit (MTU).

See the ifconfig(8) man page, and the "mtu" option. You can see the current setting in the output from the ifconfig command.
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Old 19th July 2010
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MTU = 1500, so does that mean all packets passing through that interface are 1500 bytes?
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Old 19th July 2010
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No, that is the maximum.

(Ethernet is limited to 1500 byte packets. See http://en.wikipedia.org/wiki/Maximum_transmission_unit for details.)

MTU is not generally considered a "tuning" knob. In general, one reduces it from 1500 downward, in Ethernet networks, in order to manage special purpose network configurations, such as PPPoE.

For more guidance on OpenBSD network knobs, please read www.openbsd.net/faq/faq6.html#Tuning
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Old 19th July 2010
Frothingdog Frothingdog is offline
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I don't want to modify anything right now, I would just like the ability to see the size of the packets that passing through an interface. I'm looking something to give me a snap shot of packet sizes, or even on average packet size would be ok.
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Old 19th July 2010
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See the tcpdump(8) man page, and the netstat(1) man page.
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Old 24th July 2010
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Perhaps you want to look at netflow records with (for example) pflow and flow-tools. The default flow-report gives you a "Average IP packet size distribution".

Code:
$ flow-cat /var/log/netflow/ | flow-report

[some crap removed]

Average IP packet size distribution:

   1-32   64   96  128  160  192  224  256  288  320  352  384  416  448  480
   .000 .193 .261 .138 .117 .058 .025 .016 .010 .011 .010 .008 .004 .004 .005

    512  544  576 1024 1536 2048 2560 3072 3584 4096 4608
   .004 .006 .005 .068 .055 .000 .000 .000 .000 .000 .000

[more crap removed]
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Old 26th July 2010
Frothingdog Frothingdog is offline
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Great thanks...I'll look into that.

Cheers
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