Hi,

I am new to the linux operating system. The current Linux operation system I am using is Redhat 7.3. I have two nic cards (one eth0 and one tr0) and I trying to use my linx box as a router. Ip forwarding is already enable, but one network is not able to communicate with the other. Here are some of the setting:

[root@linus root]# route
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.11.15.0 * 255.255.255.0 U 0 0 0 eth0
192.168.0.0 * 255.255.255.0 U 0 0 0 tr0
127.0.0.0 * 255.0.0.0 U 0 0 0 lo
default 10.11.15.1 0.0.0.0 UG 0 0 0 eth0

[root@linus root]# cat /proc/sys/net/ipv4/ip_forward
1

Not sure what else I need to do to get it to communicate with each other. Need help.

Your routing table looks correctly. What error messages you get? Try:
1) Ping eth0 address form the router.
2) Ping tr0 address from the router.
3) Ping router (using eth0 address) from network connected to eth0
4) As 3, but for tr0
5) Ping a machine in the network connected to tr0 from a machine in the network connected to eth0.

The ip address of my tr0 is 192.168.0.1 and eth0 is 10.11.15.225 on the linux box and I can ping the tr0 and eth0. All workstations on 192.168.0.0 subnet can ping tr0 192.168.0.1, but can not ping any workstations on 10.11.15.0 subnet. I try using tr0 to ping eth0 and the response is host unreachable.
[root@linus root]# ping -I tr0 10.11.15.225
PING 10.11.15.225 (10.11.15.225) from 192.168.0.1 tr0: 56(84) bytes of data.
From 192.168.0.1 icmp_seq=1 Destination Host Unreachable
From 192.168.0.1 icmp_seq=2 Destination Host Unreachable
From 192.168.0.1 icmp_seq=3 Destination Host Unreachable
And Vice versa from subnet 10.11.15.0 to subnet 192.168.0.0.

Do you have any firewall?

I am not sure I have a firewall. How can I check whether there is a firewall or not and verify the setting is correct or not?

ipchains -L
or
iptables -L
One of them should produce a result.

[root@linus root]# ipchains -L
Chain input (policy ACCEPT):
target prot opt source destination ports
ACCEPT tcp -y---- anywhere anywhere any -> smtp
ACCEPT tcp -y---- anywhere anywhere any -> http
ACCEPT tcp -y---- anywhere anywhere any -> ftp
ACCEPT tcp -y---- anywhere anywhere any -> ssh
ACCEPT tcp -y---- anywhere anywhere any -> teln et
ACCEPT udp ------ anywhere anywhere bootps:bootpc -> bootps:bootpc
ACCEPT udp ------ anywhere anywhere bootps:bootpc -> bootps:bootpc
ACCEPT all ------ anywhere anywhere n/a
ACCEPT all ------ anywhere anywhere n/a
ACCEPT all ------ anywhere anywhere n/a
REJECT tcp -y---- anywhere anywhere any -> 0:10 23
REJECT tcp -y---- anywhere anywhere any -> nfs
REJECT udp ------ anywhere anywhere any -> 0:10 23
REJECT udp ------ anywhere anywhere any -> nfs
REJECT tcp -y---- anywhere anywhere any -> x11: 6009
REJECT tcp -y---- anywhere anywhere any -> xfs
Chain forward (policy ACCEPT):
Chain output (policy ACCEPT):

[root@linus root]# iptables -L
/lib/modules/2.4.18-3/kernel/net/ipv4/netfilter/ip_tables.o: init_module: Device or resource busy
Hint: insmod errors can be caused by incorrect module parameters, including invalid IO or IRQ parameters
/lib/modules/2.4.18-3/kernel/net/ipv4/netfilter/ip_tables.o: insmod /lib/modules/2.4.18-3/kernel/net/ipv4/netfilter/ip_tables.o failed
/lib/modules/2.4.18-3/kernel/net/ipv4/netfilter/ip_tables.o: insmod ip_tables failed
iptables v1.2.5: can't initialize iptables table `filter': iptables who? (do you need to insmod?)
Perhaps iptables or your kernel needs to be upgraded.