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Hi,

  We are unable to ping to machines on the internet from
our machines on the intranet. But traceroute from intranet
machine to a machine on the internet works. All the rest
of the services (DNS, telnet, ftp, traceroute, www) work
fine through our masq. gateway.

To explain the problem...

We have following n/w topology
2 LANs, 1 intranet, 1 LAN has valid IP addresses


   .--(A)----.              .--(B)----.
   |         |              |         |   ei    si
   |  n/w 1  +------(G)-----+  n/w 2  +------(R)----// internet
file://---(C)
   |         | eth1    eth0 |         |
   .---------.              .---------.
 192.168.x.x LAN            Valid IP addr LAN

Legends :
  A - an internal m/c          (say 192.168.5.32)
  B - a m/c with valid IP addr (say 204.56.33.1)
  G - gateway (masquerades)  2 ethernet interfaces
                               (say eth1 : 192.168.5.254)
                               (say eth0 : 204.56.33.5)
  R - Router 2 interfaces  (ethernet lan port, wan serial port)
                               (say ei : 204.56.33.17)
                               (say si : 204.56.33.18)


Ping test from src to dest

        Dest
        ---------------------------------------------------------
Src     |  B  |  G(eth1)  |  G(eth1)  |  R(ei)  |  R(si)  |  C  |
--------|-------------------------------------------------------|
A       |  Y  |     Y     |     Y     |    Y    |    N    |  N  |
-----------------------------------------------------------------

Ping from B to A = Yes
Ping from G to A = Yes

An ipchains forwarding rule masquerades all traffic from
source as -S 192.168.0.0/16 -D 0/0 with -J MASQ

Running  tcpdump on the gateway m/c G we observe
- for ping from A to B
  --> both ping request and ping reply are seen

- for ping from A to C (or anything with and after R(si)
  --> only ping request are seen (no replies)

Why is only ping failing (while it works for other ICMP
type packets) when we leave our network and enter the
internet ? Is the router playing some role here ?


With regards
 Nikhil

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