I have configured Ubuntu 12.04 as Gateway machine.its having two interfaces
eth0 with ip 192.168.122.39(Static) and
eth1 connected to modem with ip address 192.168.2.3(through DHCP).
ip-forwarding is enabled in router box.
Client machine is configured as:
ip address 192.168.122.5 and gateway 192.168.122.39
Client machines can ping router box(192.168.122.39).but when pinged 8.8.8.8 reply is not reaching Client machines but in the tcpdump output on gateway i can see echo request for 8.8.8.8 but never echo reply.Is this because of 122.5 not forwarding request to 2.0 network.Can u please help me in fixing this.
edit:STEPS FOLLOWED
This machine is running as XEN HVM.
Router machine:two interfaces eth0:192.168.122.39(static ip)
eth1:192.168.2.3
on router
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -A FORWARD -i eth0 -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
ptables -A FORWARD -i eth0 -o eth1 -j ACCEPT
On the client machine
ping 8.8.8.8
on the gateway tcpdump result
tcpdump -v -i eth0
192.168.122.1 is the virtual bridge
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
14:17:26.352383 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
192.168.122.1 > google-public-dns-a.google.com: ICMP echo request, id 1541, seq 8395,
length 64
14:17:26.938156 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto ICMP (1), length 84)
192.168.122.1 > google-public-dns-a.google.com: ICMP echo request, id 1513, seq 10735, length 64
tracepath 8.8.8.8. from Client machine:
tracepath 8.8.8.8
1: 192.168.122.39 0.046ms
pmtu 15001: 192.168.122.5 0.259ms
1: 192.168.122.5 0.188ms
2: no reply
3: no reply
Best Answer
Your modem doesn't know how to reach 192.168.122.0/24 network. You have to do NAT on your router or you have to tell your modem that 192.168.122.0/24 should be routed through 192.168.2.3.
For NAT try with these rules:
Restart your firewall to flush old rules.