How to make Ubuntu/Debian as a router

Here is your Ubuntu serve box with two interfaces,
eth0-------------Internet
eth2-------------Internal
Note: Your Internet is running using eth0.
Step1: Install DHCP Server

#apt-get install dhcp3-server

Step 2: Configure the DHCP server

Edit the /etc/dhcp3/dhcpd.conf file and add your domain, dns, ip range and other options.

option domain-name "oslohosting.com";
option domain-name-servers ns1.yourdns.com, xx.xx.xx.xx;
#
# Internal network
#
subnet 192.168.50.0 netmask 255.255.255.0 {
range 192.168.50.100 192.168.50.200;
option broadcast-address 192.168.50.255;
option routers 192.168.50.1;
default-lease-time 600;
max-lease-time 7200;
}

Step 3: Configure the Internal interface (eth2) with static IP.
Edit the /etc/network/interfaces file and add following

iface eth2 inet static
address 192.168.50.1
netmask 255.255.255.0
network 192.168.50.0
broadcast 192.168.50.255
gateway 192.168.50.1

Step 4: Restart network and verify the eth2 interface's IP.

#/etc/init.d/network restart

check ip by ifconfig eth2, it will have 192.168.50.1 ip, if not please restart the interface/netwrok service, you can also restart your machine if it is not in production environment.


Step 5: Restart the DHCP server.

#/etc/init.d/dhcp3-server restart

If every thing is ok, it should run successfully,
Note: If your interface does not have any IP it might give error and does not restart, first configure you internal interface.

Step 6: Test the DHCP server.

connect the cable on interface eth2 and other side to your second pc, you will get the IP from 192.168.50.x range.
Open the syslog messeges with
#tail -f /var/log/syslog
of your debian box, it will also notify with leased ip and detail of requested machine.

Step 7: Enable forwarding

# cat /proc/sys/net/ipv4/ip_forward
0

# echo 1 > /proc/sys/net/ipv4/ip_forward

or open the file manually and uncomment

# nano /etc/sysctl.conf

net.ipv4.ip_forward = 1

Step 8: Add IPTABLES rule for NAT

Type following at command line

#iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Step 9: Final Testing

Your second Pc attached to LAN have internal ip, ping to www.google.com, it should get reply, if not review and trouble shoot carefully.

imran@imran-laptop:~$ ping www.google.com
PING www.l.google.com (74.125.77.103) 56(84) bytes of data.
64 bytes from ew-in-f103.google.com (74.125.77.103): icmp_seq=1 ttl=52 time=30.7 ms
64 bytes from ew-in-f103.google.com (74.125.77.103): icmp_seq=2 ttl=52 time=30.2 ms
64 bytes from ew-in-f103.google.com (74.125.77.103): icmp_seq=3 ttl=52 time=30.3 ms
^C
--- www.l.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 30.298/30.451/30.729/0.242 ms

Huraaaa!