SYN/ACK not forwarded to 2nd NIC

From: Meir (meir@education.gov.il)
Date: Tue Jun 13 2000 - 06:25:47 EST


Hi,

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) !!!

Why ?
Am I missing something ?

                      ___________________
                     | |
       | <- 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 !?!

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

-
To unsubscribe from this list: send the line "unsubscribe linux-net" in
the body of a message to majordomo@vger.rutgers.edu



This archive was generated by hypermail 2b29 : Thu Jun 15 2000 - 21:00:41 EST