Re: UDP checksumming on SunOS 4.1.x

From: Casper H.S. Dik (
Date: Wed Aug 30 1995 - 14:38:28 CDT

Andy McCammont <> writes:

>I understand that the default for SunOS 4.1.x fo udp checksumming is off

>I've found from the sun-managers archives that I can turn this on using adb

(And reboot, thjis changing only /vmunix, not the in kernel value.
or also do adb -k -w /vmunix /dev/mem and udp_cksum/W1)

>but - what does this give me? Will UDP packets that don't meet the checksum be
>ignored? What happens if that occurs half way through an NFS file operation -
>will NFS handle this correctly, and resend - if so how does it know, if UDP
>doesn't ACK packets?

Yes. The bug in SunOS 4 is that with udp_cksum not set, incoming checksums
are ignored. It is up to the protocol implemented on top of RPC
to do the retransmissions (NFS, DNS, ONC RPC will all retransmit after a time

It buys you that bad udp packets (no silent NFS file corruption, DNS corruption)
will be ignored.

>Also, any indication of what sort of performance hit this will incur per

Not much.


Casper Dik - Network Security Engineer - Sun Microsystems
This article is posted from my guest account at the University
of Amsterdam.  My real-life e-mail address is: Casper.Dik@Holland.Sun.COM
Opinions expressed here are mine (but you're welcome to share them with me)

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