Date: Wed, 24 Oct 2012 12:12:06 -0700 From: Jeremy Chadwick <jdc@koitsu.org> To: Harald Schmalzbauer <h.schmalzbauer@omnilan.de> Cc: Adrian Chadd <adrian@freebsd.org>, FreeBSD Stable <freebsd-stable@freebsd.org> Subject: Re: every 2nd echo-request malformed when ping -s >4067 Message-ID: <20121024191206.GA6704@icarus.home.lan> In-Reply-To: <20121024185525.GA6426@icarus.home.lan> References: <20121024154017.GA3167@icarus.home.lan> <5088163E.2090506@omnilan.de> <20121024165148.GA4250@icarus.home.lan> <50881EC7.9030400@omnilan.de> <20121024174425.GA4699@icarus.home.lan> <50882D3B.5050704@omnilan.de> <20121024181239.GA5755@icarus.home.lan> <20121024185525.GA6426@icarus.home.lan>
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On Wed, Oct 24, 2012 at 11:55:25AM -0700, Jeremy Chadwick wrote:
> On Wed, Oct 24, 2012 at 11:12:39AM -0700, Jeremy Chadwick wrote:
> > On Wed, Oct 24, 2012 at 08:02:35PM +0200, Harald Schmalzbauer wrote:
> > > Please find attached the requested info.
> >
> > Thanks, got 'em! I'll reply in a follow-up mail with the decoded
> > results.
>
> As promised, here are the decoded results. Took me longer than I
> expected since I started going down the road of IP options and then was
> like, "no, wait a minute, this is ICMP gah!". Opinions are at the
> bottom. Gosh I hope I didn't botch a copy-paste on this one...
>
> 17:58:08.481888 IP 10.5.49.126 > 10.5.49.65: ICMP echo request, id 49423, seq 0, length 4076
> 0x0000: 4500 1000 1fff 4000 4001 9435 0a05 317e
> 0x0010: 0a05 3141 0800 a352 c10f 0000 5088 2c30
> 0x0020: 0007 5a3b {...snip...}
>
> 0x45 = bits 7-4: IPv4 protocol
> = bits 3-0: header length: 20 bytes
> 0x00 = DSF / RFC 2474 stuff
> 0x1000 = datagram length: 4096 bytes
> 0x1fff = fragment id
> 0x4000 = bits 15-13: %010 = reserved bit (0), DF bit (1), MF bit (0)
> = bits 12-0: fragment offset: 0
> 0x40 = TTL: 64
> 0x01 = protocol: 1 (ICMP)
> 0x9435 = header checksum
> 0x0a05317e = source IP
> 0x0a053141 = destination IP
> 0x08 = ICMP type: 8 = Echo Request
> 0x00 = ICMP code: 0 = always zero for ICMP type 8
> 0xa352 = ICMP header checksum
> 0xc10f = ICMP identifier
> 0x0000 = ICMP sequence number
> 0x5088 = timestamp from ICMP data
> 0x2c30 = timestamp from ICMP data
> 0x0007 = timestamp from ICMP data
> 0x5a3b = timestamp from ICMP data
>
>
> 17:58:09.488461 IP 10.5.49.126 > 10.5.49.65: icmp
> 0x0000: 4500 1000 1fff 0040 4001 d3f5 0a05 317e
> 0x0010: 0a05 3141 0800 8998 c10f 0001 5088 2c31
> 0x0020: 0007 73f3 {...snip...}
>
> 0x45 = bits 7-4: IPv4 protocol
> = bits 3-0: header length: 20 bytes
> 0x00 = DSF / RFC 2474 stuff
> 0x1000 = datagram length: 4096 bytes
> 0x1fff = fragment id
> 0x0040 = bits 15-13: %000 = reserved bit (0), DF bit (0), MF bit (0)
> = bits 12-0: fragment offset: 64
> 0x40 = TTL: 64
> 0x01 = protocol: 1 (ICMP)
> 0xd3f5 = header checksum
> 0x0a05317e = source IP
> 0x0a053141 = destination IP
> 0x08 = ICMP type: 8 = Echo Request
> 0x00 = ICMP code: 0 = always zero for ICMP type 8
> 0x8998 = ICMP header checksum
> 0xc10f = ICMP identifier
> 0x0001 = ICMP sequence number
> 0x5088 = timestamp from ICMP data
> 0x2c31 = timestamp from ICMP data
> 0x0007 = timestamp from ICMP data
> 0x73f3 = timestamp from ICMP data
>
>
> Summary: I don't see anything anomalous EXCEPT the ordeal regarding the
> fragment offset going from 0->64 and the DF bit going from 1->0.
> Possibly this makes tcpdump throw a fit in some way, I'm not sure.
Hmm, question: are you using pf, ipfilter, or ipfw on the machines where
you can reproduce this problem?
On the machine I tested from earlier, I don't use them. I also don't
use jumbo frames (I use stock 1500 bytes). All my ICMP echo packets
look like your 1st one: df=0 and fragoffset=0. I do have a 9.1-PREREL
box that does use pf where I can test from though.
I hate having to ask this question, but pf.conf(5) and the no-df flag
always come to mind whenever I hear the term fragmentation or DF.
--
| Jeremy Chadwick jdc@koitsu.org |
| UNIX Systems Administrator http://jdc.koitsu.org/ |
| Mountain View, CA, US |
| Making life hard for others since 1977. PGP 4BD6C0CB |
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