Re: Where are my packets? Was: Unusual routing setup

From: Casey Carter (Casey@Carter.net)
Date: Thu Mar 21 2002 - 14:37:25 EST


Joerg Pommnitz wrote:

[snip]

>tcpdump: listening on ppp1
>15:29:46.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:47.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:48.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:49.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:50.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:51.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:52.120000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>jpo>./tcpdump -i ppp0 udp
>tcpdump: listening on ppp0
>15:29:46.100000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:47.600000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:48.080000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:49.560000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:50.100000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:51.600000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:52.090000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>15:29:53.560000 193.254.167.86.1029 > 193.254.165.18.12345: udp 8 (DF)
>
>So I see the packets going leaving from ppp1 and arriving on ppp0, but
>I do NOT see them in my little program.
>
>Does anybody know where and why they get lost?
>
I'm guessing that your kernel is performing reverse path filtering --
i.e., it "knows" that packets from that source address should be coming
over the loopback interface, and not ppp0. I suggest that you "echo 0 >
/proc/sys/net/ipv4/conf/ppp0/rp_filter" to disable reverse path, and
"echo 1 > /proc/sys/net/ipv4/conf/ppp0/log_martians" so you will get log
messages for similar stuff.

>
>Note that this program works fine without binding to a device:
>
>jpo>./rtt -a 193.254.165.18
>rtt (0:189)
>rtt (0:228)
>rtt (0:148)
>rtt (0:161)
>
>So I'm a little bit lost. Any ideas?
>
>Regards
> Jörg
>
>--- Joerg Pommnitz <pommnitz@yahoo.com> wrote:
>
>>Hi List,
>>I want to do something slightly unusual and need a little push in the
>>right direction. I have a Linux box (2.4.18) with two GPRS devices
>>attached as ppp0 and ppp1. ppp0 has address A, ppp1 has address B. I
>>want
>>to route traffic to address B (e.g. ppp1) through interface ppp0, e.g
>>despite the fact that address B is local I want to sent it through the
>>external net. My first naive try was to
>>route add -host "address B" ppp0
>>
>>This does not do what I want. Traceroute still shows that address B is
>>just a single hop and 1 ms away.
>>
>>How can I get Linux to do what I want (e.g. not to optimize the
>>routing).
>>
>>Regards
>> Jörg
>>
>>P.S.: Just in case you are wondering: This is to make some round trip
>>measurements.
>>
>
>
>=====
>

-- 
Casey Carter
Casey@Carter.net
ccarter@uiuc.edu
AIM: cartec69

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