Hi,
Sorry to bother you with this, but I try linux-net and get no
answer (for now).
I have a curious problem.
My box (linux) is trying to send mail to a target.
Between my box and the target there is a linux box 2.2.12
with 4 NICs but (for now) _without_ any filtering rules
at all (all default to ACCEPT).
Only 2 NICs are up: eth0 to external net and eth1 to internal net.
The problem is that when I tcpdump the 2 NICs from this middle-box,
I can see a SYN getting out from eth1 and then passed to eth0
(ip forwarding is enabled), and then I receive a SYN/ACK from the
target box via eth0, but this SYN/ACK _never_ reach
eth1 (which point to internal net) !!!
The figure describe what's happen:
___________________
| |
| <- SYN |<- SYN <- SYN | |
Target | |eth0 eth1 |----------| My box
| SYN/ACK -> | -> | | telnet target 25
|_____________ ____|
^
|
|_____ SYN/ACK never reach eth1 !!!
The same thing occures when telneting Target on ports 7/9/79 etc...
_But_ when I telnet Target 80 or 21 from My Box, it works !
Why ?
What am I missing ?
I try with kernel 2.2.5, 2.2.12, 2.2.14.
/proc/sys/net/ipv4/conf/{all,eth*}/rp_filter are set to 1
/proc/sys/net/ipv4/ip_forward is set to 1
Thanks in advance,
-- Meir
ps: thanks to cc to meir@education.gov.il
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