Using Raspberry Pi as Router

We have a situation. We need to connect a network printer that comes with LAN Ethernet interface [ but, alas there is no WIFI connection]. The printer is quite a distance from our router, and no body is going to help you the lay the LAN cable from the router to the printer.

You can use raspberry pi, as a router. Raspberry has 2 interfaces, the ethernet [eth0] and wifi [wlan0]. We can route the traffic from the ethernet side [ where our printer is connected] to the wifi side [which connected to the router].

1. Enable ip_forward in the kernel

sudo vi /etc/sysctl.conf

add the following line:

net.ipv4.ip_forward = 1

reboot, or reload sysctl:

sudo sysctl -p

2. Install DHCP

Configure eth0 with ip address 192.168.13.1 with /24 subnet. This setting is still not persistent. We need to make it persistent in the following steps below.

sudo ifconfig eth0 192.168.13.1 netmask 255.255.255.0 broadcast 192.168.13.255

Install DHCP

$ sudo apt-get install isc-dhcp-server
$ sudo service isc-dhcp-server stop

Edit /etc/default/isc-dhcp-server. Add the following line:

INTERFACESv4="eth0"

Edit /etc/dhcp/dhcpd.conf, to include the followings:

authoritative; # I will be the single DHCP server on this network, trust me authoritatively

subnet 192.168.13.0 netmask 255.255.255.0{ 
        interface eth0; 
        range 192.168.13.10 192.168.13.250; 
        option broadcast-address 192.168.13.255; 
        option routers 192.168.13.1; 
        option domain-name "local"; 
        option domain-name-servers 8.8.8.8, 8.8.4.4; 
        default-lease-time 600; 
        max-lease-time 7200; 
}                            

Edit /etc/dhcpcd.conf: Here is where we really set the ip address of eth0. It will be persistent after reboot.

# Persist interface configuration when dhcpcd exits. 
persistent 
 
# Rapid commit support. 
# Safe to enable by default because it requires the equivalent option set 
# on the server to actually work. 
option rapid_commit 
 
# A list of options to request from the DHCP server. 
option domain_name_servers, domain_name, domain_search, host_name 
option classless_static_routes 
# Respect the network MTU. This is applied to DHCP routes. 
option interface_mtu 
 
# Most distributions have NTP support. 
#option ntp_servers 
 
# A ServerID is required by RFC2131. 
require dhcp_server_identifier 
 
# Generate SLAAC address using the Hardware Address of the interface 
#slaac hwaddr 
# OR generate Stable Private IPv6 Addresses based from the DUID 
slaac private 
 
# Example static IP configuration: 
#interface eth0 
#static ip_address=192.168.0.10/24 
#static ip6_address=fd51:42f8:caae:d92e::ff/64 
#static routers=192.168.0.1 
#static domain_name_servers=192.168.0.1 8.8.8.8 fd51:42f8:caae:d92e::1 
interface eth0 
static ip_address=192.168.13.1/24 
 
# It is possible to fall back to a static IP if DHCP fails: 
# define static profile 
#profile static_eth0 
#static ip_address=192.168.1.23/24 
#static routers=192.168.1.1 
#static domain_name_servers=192.168.1.1 
profile static_eth0 
static ip_address=192.168.13.1/24 
 
# fallback to static profile on eth0 
#interface eth0 
#fallback static_eth0 
interface eth0 
fallback static_eth0
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

Restart DHCP

sudo service isc-dhcp-server start

3. Routing, NAT and persistence after reboot

Install iptables-persistent

sudo apt-get install iptables-persistent

Set NAT and masquerade. This simply mean to route traffic from source (-s 192.168.13.0/24), output (-o) it to wlan0, and masquerade it, as if it is from wlan0.

sudo iptables -t nat -A POSTROUTING -s 192.168.13.0/24 -o wlan0 -j MASQUERADE

Now the printer should work. However, we’ve got make it persistence after reboot.

sudo dpkg-reconfigure iptables-persistent

At this stage, everything should work. If it does not work, you can check the routing table.

pi@pirouter:/etc $ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         Singtel-ACPlus  0.0.0.0         UG    303    0        0 wlan0
192.168.1.0     0.0.0.0         255.255.255.0   U     303    0        0 wlan0
192.168.13.0    0.0.0.0         255.255.255.0   U     202    0        0 eth0

It should show only one Gateway (UG). If it shows 2 or more gateways, you have to delete it. Eth0 SHOULD NOT be the gateway.

To delete Gateway: [ here we delete eth0 gw with ip address 192.168.13.1]

sudo route del default gw 192.168.13.1