Date: Wed, 17 Mar 2004 16:58:20 -0800 From: "Sreekanth" <sreekanth@redlinenetworks.com> To: <sbotsford@sjsa.ab.ca>, <freebsd-net@freebsd.org> Subject: RE: tcpdump says errors. Netstat says no. Message-ID: <000001c40c84$1b9a4d10$c204a8c0@SREELAPTOP> In-Reply-To: <200403170738.11990.sbotsford@sjsa.ab.ca>
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Netstat errors are those errors reported by the Hardware(NIC) for e.g, buffer underrun overrun etc,They do not check for errors in tcp layer. Sreekanth > -----Original Message----- > From: owner-freebsd-net@freebsd.org > [mailto:owner-freebsd-net@freebsd.org] On Behalf Of Sherwood Botsford > Sent: Wednesday, March 17, 2004 6:38 AM > To: freebsd-net@freebsd.org > Subject: tcpdump says errors. Netstat says no. > > > Originally posted on FreeBSD-Questions@... > > I'm running mail services (exim, fetchmail, popper) on > FreeBSD 4.5 > > In a session trying to track down a problem related to a bad > cable, I found that: > > tcpdump -i xl0 -v -v -v host pop.incentre.net > > has lots of lines like this: > > 20:20:11.166767 postie.sjsa.internal.net.1126 > > bach.ccinet.ab.ca.pop3: F [bad tcp cksum 4f5e!] > 61:61(0) ack 2075 win 33304 <nop,nop,timestamp 3005321 3937592> > (DF) (ttl 64, id 57145, len 52, bad cksum 0!) > > But netstat -I xl0 -w 10 during the same interval when the > above were coming down at 5-10 per second showed: > > input (xl0) output > packets errs bytes packets errs bytes colls > 32 0 4019 28 0 4573 0 > 24 0 3512 22 0 4934 0 > 444 0 30070 433 0 227132 0 > > Explanations? (I also get them on the local network; > it's not just this destination host.) > > ********************** > > Further testing: > Xterm ssh'd into postie: > netstat -I xl0 -w 10 -d > input (xl0) output > packets errs bytes packets errs bytes colls drops > 4 0 367 4 0 298 0 0 > 27 0 4346 23 0 2368 0 0 > 21 0 1746 10 0 1170 0 0 > 19 0 1472 15 0 1516 0 0 > 4 0 429 1 0 178 0 0 > ... > > Note: NO dropped packets, no errors. > > Separate xterm ssh'd to postie: > tcpdump -i xl0 -c 1000 -v -v -v | grep bad | wc > tcpdump: listening on xl0 > 477 8960 72418 > > Separate xterm on localhost > ping postie. > > So you can see that nearly half of the packets have bad checksums. > > I think I'm missing something here. > > > -- > Sherwood Botsford > St. John's School of Alberta > > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/free>; bsd-net > To > unsubscribe, send any mail to > "freebsd-net-unsubscribe@freebsd.org" > > --- > Incoming mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.573 / Virus Database: 363 - Release Date: 1/28/2004 > >
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