Re: v4/v6 packet length printing inconsistency

[Gert Doering <gert () greenie muc de>]

Hi,

On Wed, Jul 02, 2025 at 03:28:59PM -0400, Michael Richardson wrote:
> Gert Doering <gert () greenie muc de> wrote:
>     > I've been writing a document explaining inside/outside fragmentation in
>     > an OpenVPN context, explaining my points with "tcpdump", like this:
>
>     > $ tcpdump -vv -i any host ... or host ...
>
> Note also that you are comparing the ICMPv6 printer vs the UDP printer.

True, but the UDP printer is not the one printing the "real packet length"
and introducing the newline - that comes from print-ip.c.

-v on an IPv4 ICMP looks the same...

gert@blueNUC2:/home/gert/src/tcpdump.git$ SU ./tcpdump -v -n -s0 -i tun1

existing output for IPv4 ICMP ("ping -s 64"):

23:18:12.121037 IP (tos 0x0, ttl 64, id 36165, offset 0, flags [DF], proto ICMP (1), length 92)
    10.231.203.11 > 10.231.203.1: ICMP echo request, id 28546, seq 1, length 72
23:18:12.135331 IP (tos 0x0, ttl 64, id 26157, offset 0, flags [none], proto ICMP (1), length 92)
    10.231.203.1 > 10.231.203.11: ICMP echo reply, id 28546, seq 1, length 72

new output format for IPv6 ICMP ("ping6 -s 64"):

23:19:12.042337 IP6 (flowlabel 0x381c8, hlim 64, next-header ICMPv6 (58), length 112)
    fd48:82aa:99e6::1009 > fd48:82aa:99e6::: [icmp6 sum ok] ICMP6, echo request, id 28661, seq 1, length 72
23:19:12.056159 IP6 (flowlabel 0x6ff2e, hlim 64, next-header ICMPv6 (58), length 112)
    fd48:82aa:99e6::1 > fd48:82aa:99e6::1009: [icmp6 sum ok] ICMP6, echo reply, id 28661, seq 1, length 72


> If anything, I'd want to fix the UDP printer to not use multiple lines,
> because it breaks people who are trying to grep.

Use "tcpdump -g" then :-)

gert
-- 
"If was one thing all people took for granted, was conviction that if you 
 feed honest figures into a computer, honest figures come out. Never doubted 
 it myself till I met a computer with a sense of humor."
                             Robert A. Heinlein, The Moon is a Harsh Mistress

Gert Doering - Munich, Germany                             gert () greenie muc de
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