Home Wifi With CDMA Modem

In case CDMA technology is still offered in your area, this post might be useful to you in setting up a simple Home WiFi using a CDMA modem, WiFi router and a Linux machine. This could be a good excercise to revive your old Linux box and that old WiFi router which had been laying useless :-).

What you need

• Linux machine - An old Debian / Ubuntu Linux pc will do too.
• WiFi router - An old 2.4GHz router from DLink would work.
• CDMA-1x USB modem - Typically available as CDMA USB-Dongle.
• Ethernet cable to connect the Router to the Linux machine.

Hardware setup

• Configure the WiFi router to act as an access point using the Router’s web interface.
• Connect the Ethernet cable to one of the 4 ports of the Router and the other end to the Linux machine’s Ethernet port.
• Connect the CDMA USB modem to one of the USB port of the Linux machine.

Software setup

• Install the following packages using the package manager.

  1apt-get install -y pppd wvdial iptables dnsmasq
  

• Ensure that the Linux device driver modules usbserial and cdc_acm are loaded. You may use the command insmod in case they are not already loaded.

Setting up the WiFi

This process involves the following main steps.

1. Configuring kernel modules for auto-loading at bootup
2. Configuring and running “wvdial” for dialup (ppp) connection
3. Setting up “iptables” for NAT forwarding
4. Setting up “dnsmasq” for resolving DNS and as DHCP server

Configure Kernel modules

1. Open a terminal and the command sudo bash in order to enter BASH as super-user.

2. Edit the file /etc/modules and append the following lines if they do not already exist. Check and use the appropriate values for vendor and product ids for the USB modem, by referring to the file /proc/bus/usb/devices.

   1usbserial vendor=0x19d2 product=0xfffd
   2cdc_acm
   

3. Save the file and exit the terminal.

4. Reboot the machine for automatic driver loading to happen.

Configure Dial-Up connection

1. Open a terminal and the command sudo bash in order to enter BASH as super-user.

2. Take a backup of the file /etc/wvdial.conf and then replace it’s content with the following configuration.

    1[Dialer Defaults]
    2Init1 = ATZ
    3Init2 = AT+CRM=1
    4Modem Type = Analog Modem
    5SetVolume = 0
    6Baud = 115200
    7New PPPD = yes
    8Modem = /dev/ttyUSB0
    9Carrier Check = no
   10Stupid Mode = 1
   11ISDN = 0
   12Phone = <dial-number>
   13Password = <dial-password>
   14Username = <dial-username>
   

   Use appropriate values for dial-number, dial-password and dial-username. For Reliance CDMA modem, dial-number will be #777 and, dial-password and dial-username will be the phone number itself.

3. Save and close the file.

4. Take a backup of /etc/resolv.conf then, clear all the content of /etc/resolv.conf and save it.

5. Run wvdial to initiate the PPP connection.

NAT forwarding with iptables

In this step, we will be configuring the NAT table to masquerade ppp0 network interface and configure forwarding rule for the ethernet interface.

1. Open a terminal and the command sudo bash in order to enter BASH as super-user.

2. Create or open the file /usr/local/sbin/ppp_eth_route and enter the following content. If the file altready exists, first take a backup and then replace its content with the following.

    1#!/bin/bash
    2
    3## Flush the nat table
    4iptables --flush
    5iptables --table nat --flush
    6
    7iptables --delete-chain
    8iptables --table nat --delete-chain
    9
   10## Set up IP forwarding and masquerading
   11iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
   12iptables --append FORWARD --in-interface eth0 -j ACCEPT
   13
   14echo 1 > /proc/sys/net/ipv4/ip_forward

3. Save the file and change its permission to 0755 using the command;

   1chown 755 /usr/local/sbin/ppp_eth_route
   

4. Run the ppp_eth_route script created above.

Setting up local DNS and DHCP server

We will be using the dnsmasq tool to setup a DHCP server and configure the DNS host resolution.

1. Open a terminal and the command sudo bash in order to enter BASH as super-user.

2. Edit the file /etc/network/interfaces and replace it’s content with the following. It is recommended to take a backup of the file before changing it’s content.

   1auth eth0
   2iface eth0 inet static
   3address 10.10.1.1
   4netmask 255.255.255.0
   

   Save and close the file.

3. Edit the file /etc/dnsmasq.conf as follows. Then, save and close the file.

   1. Uncomment the line containing interface=eth0
   2. Uncomment the line that starts with dhcp-range to enable integrated DHCP server. The line must be edited as dhcp-range=10.10.1.10,10.10.1.200,12h

   Here, the dnsmasq tool is allowed to provide IPs in the range 10.10.1.10 to 10.10.1.200 with a lease time of 12 hours.

4. Restart the system services - network-manager and dnsmasq.

1. Restart the system networking / network-manager service.
2. Run wvdial &. Wait till it fetches IP address and DNS entries. Otherwise, there is no use in proceeding further since, there is likely a PPP connection issue.
3. Run /usr/local/sbin/ppp_eth_route.
4. Restart the system dnsmasq service.

Of course, you can put the above commands in a shell script and run them as super-user. That’s it!