Embedded devices connected to the internet are growing every day. In many cases, these devices will be installed somewhere without a wire or wireless network connection. A good way to keep the devices connected to the internet in these situations is by seeking mobile network coverage. Therefore, this blog post will present the necessary configurations to connect a device running Linux to the internet through the PPP (Point-to-Point Protocol) link.

In this article we will be using the Colibri iMX6S with the base board called “Colibri Evaluation Board”, both of which, are from Toradex. The Colibri Evaluation Board is recommended specifically for project evaluation and development. This product comes with a great number of interfaces like USB, Ethernet, I2C, SPI, RS242, RS485, CAN and many others. We can also find some multimedia interfaces like HDMI, LVDS, VGA and LCD already supported on the Evaluation Board.

To follow this tutorial, you can use any USB or Serial modem to connect to the internet. You just have to adapt the interface created at “/dev/” and check the “AT” commands supported and implemented for your modem because each modem has specific AT commands. In this case, we have used for this tutorial a Huawei E173s 3G modem with a compatible SIM Card with data plan activated.

To use the 3G USB modem through the PPP protocol, it is necessary to select some features in the kernel configurations and then compile the Kernel. To understand how to compile the kernel, I suggest following the link below.
http://developer.toradex.com/software-resources/arm-family/linux/board-support-package/build-u-boot-and-linux-kernel-from-source-code

Basically, the process to compile the Kernel can be summarized in the following steps:

• Toolchain configuration
• Download the kernel from the correct repository and branch
• Configure the standard processor features
• Change the features for your application
• Compile

When you are configuring the kernel features, go into menuconfig and be sure to enable the following options:

CONFIG_PPP:
PPP (Point to Point Protocol) is a newer and better SLIP.  It serves the same purpose: sending Internet traffic over telephone (and other serial) lines.  Ask your access provider if they support it, because otherwise you can't use it; most Internet access providers these days support PPP rather than SLIP. 
Device Drivers  ---> 
   [*] Network device support  --->
          PPP (point-to-point protocol) support
            PPP BSD-Compress compression
             PPP Deflate compression 
       [*]     PPP filtering
            PPP MPPE compression (encryption)
       [*]     PPP multilink support 
            PPP over Ethernet  
            PPP support for async serial ports
            PPP support for sync tty ports

CONFIG_USB_ACM:
This driver supports USB modems and ISDN adapters which support the
Communication Device Class Abstract Control Model interface.
Please read <file:Documentation/usb/acm.txt> for details.      
Device Drivers  --->
    [*] USB support  --->
            USB Modem (CDC ACM) support 

To configure and connect the 3G modem to the internet, Linux uses, in addition to the kernel drivers, a software which manages the connection. This software is PPP. Usually Toradex images already have the PPP software installed by default.

In case you do not have PPP installed, just add the following to your local.conf file for build environments based on Yocto/OpenEmbedded:

IMAGE_INSTALL_append = " ppp"

After all the necessary drivers and software are installed, it is possible to check if the module recognizes the 3G modem. The modem interface should appear in /dev. Basically the driver simulates a USB connection as many serial connections.

Execute the following command to search for the interfaces /dev/ttyUSB

$ ls -l /dev/ttyUSB*

If the files ttyUSB0, ttyUSB1 … e ttyUSB2 were found, the modem was succesfully recognized by the system.

In case you want to conduct in-depth testing, it is possible to use a serial terminal like “minicom” to start communicating with the modem and check if it is working well. In my case the modem had a baudrate of 115200.

$ minicom -D /dev/ttyACM0
___________________________
AT
OK

After validating the communication with the modem, it is time to create the configuration files responsible for connecting to the internet.

You should be able to find a folder inside /etc/ called ppp. Some files need to be created inside this folder.

First we create the PPP options file:

$ vi /etc/ppp/options
_____________________________________
auth
crtscts
lock
hide-password
modem
mru 296
mtu 296
lcp-echo-interval 30
lcp-echo-failure 4
noipx
persist
asyncmap 0xa0000
mru 1500
refuse-chap
ipcp-max-failure 30
logfile /home/root/ppp

In my case we use the PAP authentication file (may vary depending on Network Provider).

$ vi /etc/ppp/pap-secrets
_____________________________________
#    *      password
vivo vivo vivo

The next step is to create one more file inside /etc/ppp/peer In this file we have configurations for our network provider and the path for the chat file:

$ vi /etc/ppp/peers/vivo-3g.provider
_____________________________________
hide-password
noauth
debug
defaultroute
noipdefault
user vivo
remotename vivo
ipparam vivo
persist
usepeerdns
/dev/ttyUSB0 115200 crtscts
replacedefaultroute
connect 'chat -v -f /etc/ppp/chat/vivo-3g.chat'

Now we create the file responsible for sending the AT commands to the modem so it can connect to the internet. This file varies according to modem models/brands.

$ vi /etc/ppp/chat/vivo-3g.chat
_____________________________________
ECHO
ON
ABORT
'BUSY'
ABORT
'NO CARRIER'
ABORT
'ERROR'
""
ATZ OK
\d\dAT+CGDCONT=1,"IP","zap.vivo.com.br" OK
\d\d\dATDT*99# CONNECT

Next we run ppp and analyze the log files. In the log files we can see when each AT command has been executed.

$ pon vivo-3g.provider

After running ppp, we open the log file:

$ tail -f "/home/root/ppp"

In my case, I received the following output:

tail -f ppp.log 
Sent 3940 bytes, received 2843 bytes.
restoring old default route to eth0 [192.168.10.1]
restore default route ioctl(SIOCADDRT): Network is unreachable(101)
Script /etc/ppp/ip-down started (pid 778)
sent [LCP TermReq id=0x2 "User request"]
rcvd [LCP TermAck id=0x2]
Connection terminated.
Script /etc/ppp/ip-down finished (pid 778), status = 0x0
ATZ
OK
AT+CGDCONT=1,"IP","zap.vivo.com.br"
OK
ATDT*99#
CONNECT
Script chat -v -f /etc/ppp/chat/vivo-3g.chat finished (pid 795), status = 0x0
Serial connection established.
using channel 2
Using interface ppp0
Connect: ppp0  /dev/ttyACM0
rcvd [LCP ConfReq id=0x1     ]
sent [LCP ConfReq id=0x1    ]
sent [LCP ConfAck id=0x1     ]
rcvd [LCP ConfAck id=0x1    ]
sent [LCP EchoReq id=0x0 magic=0xa068db01]
sent [PAP AuthReq id=0x1 user="vivo" password=]
rcvd [LCP EchoRep id=0x0 magic=0x96baf40f]
rcvd [PAP AuthAck id=0x1 ""]
PAP authentication succeeded
sent [CCP ConfReq id=0x1   ]
sent [IPCP ConfReq id=0x1    ]
rcvd [LCP ProtRej id=0x2 80 fd 01 01 00 0f 1a 04 78 00 18 04 78 00 15]
Protocol-Reject for 'Compression Control Protocol' (0x80fd) received
rcvd [IPCP ConfNak id=0x1  ]
sent [IPCP ConfReq id=0x2    ]
rcvd [IPCP ConfNak id=0x2  ]
sent [IPCP ConfReq id=0x3    ]
rcvd [IPCP ConfNak id=0x3  ]
sent [IPCP ConfReq id=0x4    ]
rcvd [IPCP ConfNak id=0x4  ]
sent [IPCP ConfReq id=0x5    ]
rcvd [IPCP ConfNak id=0x5  ]
sent [IPCP ConfReq id=0x6    ]
rcvd [IPCP ConfReq id=0x1]
sent [IPCP ConfNak id=0x1 ]
rcvd [IPCP ConfRej id=0x6 ]
sent [IPCP ConfReq id=0x7   ]
rcvd [IPCP ConfReq id=0x2 ]
sent [IPCP ConfAck id=0x2 ]
rcvd [IPCP ConfNak id=0x7   ]
sent [IPCP ConfReq id=0x8   ]
rcvd [IPCP ConfAck id=0x8   ]
local  IP address 179.133.47.109
remote IP address 179.133.47.109
primary   DNS address 187.100.246.254
secondary DNS address 187.100.246.251
Script /etc/ppp/ip-up started (pid 805)
Script /etc/ppp/ip-up finished (pid 805), status = 0x0

As seen in the log file, the chat script is executed step-by-step until we get the following:

ATDT*99#
CONNECT

You can validate the necessary command sequence to get connected using the serial terminal. Once it is validated, you can create your own customized chat script.

After checking for an IP address from the network provider, look for whether the ppp0 network interface is active:

root@apalis-imx6:/etc/ppp# ifconfig 
eth0      Link encap:Ethernet  HWaddr 00:14:2D:C0:00:4C  
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:1181 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1223 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
         RX bytes:107025 (104.5 KiB)  TX bytes:149841 (146.3 KiB)
 
lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:22 errors:0 dropped:0 overruns:0 frame:0
          TX packets:22 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
         RX bytes:1536 (1.5 KiB)  TX bytes:1536 (1.5 KiB)
 
ppp0      Link encap:Point-to-Point Protocol  
          inet addr:179.133.47.109  P-t-P:179.133.47.109  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:296  Metric:1
          RX packets:13 errors:0 dropped:0 overruns:0 frame:0
          TX packets:35 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3 
         RX bytes:1330 (1.2 KiB)  TX bytes:2131 (2.0 KiB)

To test if the connection has been succesfully established, ping an external ip address:

root@apalis-imx6:/etc/ppp# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: seq=0 ttl=45 time=182.304 ms
64 bytes from 8.8.8.8: seq=1 ttl=45 time=34164.126 ms
64 bytes from 8.8.8.8: seq=2 ttl=45 time=33164.085 ms

To configure DNS, we can use the following command and then ping a URL, for example:

$ echo nameserver 8.8.8.8 > /etc/resolv.conf
$ ping google.com

We can avoid the file being rewritten when system is restarted with the following command:

$ chattr +i /etc/resolv.conf

Now it is possible to connect using SSH from an external computer:

$ ssh root@179.133.47.109

Another thing we can do is to associate our IP address to a host name. For this purpose, we used the NoIP services (www.noip.com). We then created a free account and added a Host. In our case, we chose toradex.noip.me.

After the Host name is created we can again connect to the module using SSH:

$ ssh root@toradex.noip.me

In our case, when the 3G modem connection is restarted, we always receive a new IP adress. If we try to reconnect to the module, we no longer have access. A workaround was to use a method called Dynamic DNS(Dynamic Domain Name System). NoIP also offers Dynamic DNS services. Further information for using this method can be found here. Basically we send an http request to NoIP with information like IP, hostname, username and password.

To accomplish this request, a small Python program was made. It gets executed when we start the 3G modem connection and follows the sequence as listed below:

#!/usr/bin/python
 
import sys
import requests
import netifaces as ni
 
user = 'xxxxxxx'
pswd = 'xxxxxxx'
 
ni.ifaddresses('ppp0')
ip = ni.ifaddresses('ppp0')[2][0]['addr']
myhostname = 'toradex.noip.me'
 
payload = {'hostname' : myhostname , 'myip' : ip}
r = requests.get("http://dynupdate.no-ip.com/nic/update", params=payload, auth=(user,pswd))
 
print " " 
if "good" in r.text:
print "Hello", user, "!"
print "Your IP was successfully updated to:", ip
print myhostname, "is up and running!"
if "nochg" in r.text:
print "Hello", user, "!"
print "Your IP", ip, "is still active, no change needed"
if "nohost" in r.text:
print "The given Host name", myhostname, "does not exist under specified account"
print "Please review your Host name and try again"
if "badauth" in r.text:
print "Login and/or Username incorrect"
print "Please correct your credentials and try again"
if "911" in r.text:
print "Sorry for the incovenience but we are experiencing some problems right now"
print "Please try again later"
print "noip.com says:", r.text
print " "

After creating the Python program and making it executable with chmod +x, we can move it to /etc/ppp/ip-up.d/, which is a folder where scripts are executed after ppp connection or create a small script which calls our Python program.

Inside /etc/ppp/ we have the files that we created at the beginning of the article as well as other files like ip-down, ip-up, ppp_on_boot. Feel free to explore each one of these files.

ip-up is the script which calls another script or program found inside ip-up.d folder.

In this case, we created a small script inside ip-up.d

#!/bin/bash
python /home/root/noipReq.py > /home/root/noipLog.log

Note: the script is not terminated with .sh

Note that all the print commands inside our Python program can be seen in the log file:

root@colibri-imx6:~# cat noipLog.log 
Hello giovannibauer !
Your IP was successfully updated to: 179.92.172.193
toradex.noip.me is up and running!
noip.com says: good 179.92.172.193

If everything is configured correctly, when we receive a new IP from our network provider, the system automatically updates our host created at NoIP, so it will be possible to connect to the module despite the IP address being changed.

Based on all the information provided in this article, it is possible to achieve many other network activities. One of them is to share the 3G modem network with another computer via Ethernet. For this we configured a basic LAN network between two machines and ran the following command:

iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE

Apart from internet sharing, it is also possible to conduct port forwarding and many other network features.

As seen in this article, a Linux system has many network functionalities. We just need to properly configure them and have access to ppp0 network. Another important point, is that regardless of the used interface, the programming method remains the same. The task of choosing the interface to use is made automatically by Linux depending on the routes. For those who require network connection in their projects and also need system flexibility, the use of Linux in embedded systems is the way to go.

• http://metodiew.com/huawei-e173s-and-ubuntu/
• http://developer.toradex.com/products/colibri-imx6
• http://developer.toradex.com/products/colibri-evaluation-board

This blog post was originally featured on Embarcados.com in Portuguese. See here.

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