Raspberry Pi GSM Surfstick Setup
                --------------------------------
                

Objective:  instead of WiFi or wired  ethernet,  use a surfstick
(USB 3G modem) for access to the net.

1. Install parts and pieces:

sudo apt-get install -y ppp

2. Create peer configurations, one pair (PPP parameters and chat
script) per peers. These go into /etc/ppp/peers:

root@rpi0w-005:/etc/ppp/peers# pwd
/etc/ppp/peers
root@rpi0w-005:/etc/ppp/peers# ls -ls
total 20
4 -rw-r--r-- 1 pi   pi   215 Dec 24  2017 att
4 -rw-r--r-- 1 pi   pi   215 Oct  5  2016 att.chat
4 -rw-r----- 1 root dip 1093 Dec 23  2017 provider
4 -rw-r--r-- 1 pi   pi   217 Jan 15  2018 tmo
4 -rw-r--r-- 1 pi   pi   218 Jan 15  2018 tmo.chat
root@rpi0w-005:/etc/ppp/peers#


For AT&T:

root@rpi0w-005:/etc/ppp/peers# cat att
noauth
debug
connect "/usr/sbin/chat -v -f /etc/ppp/peers/att.chat"
connect-delay 2000
# /dev/ttyU0
/dev/ttyACM0
460800
local
defaultroute
noipdefault
usepeerdns
# novj
user "WAP@CINGULARGPRS.COM"
persist
maxfail 0
root@rpi0w-005:/etc/ppp/peers#


The script  referred to by "connect"  needs to exist. Google the
correct  access point name (APM) for your provider.  There might
be more than one. Here we go with  wap.cingular.  File format is
either directive-and-value, or response-request:

root@rpi0w-005:/etc/ppp/peers# cat att.chat
TIMEOUT 3
ECHO ON
ABORT BUSY ABORT 'NO CARRIER' ABORT VOICE ABORT 'NO DIALTONE'
ABORT 'NO DIAL TONE' ABORT 'NO ANSWER' ABORT DELAYED
'' '\rAT'
'' ATZ
TIMEOUT 6
OK 'AT+CGDCONT=1,"IP","wap.cingular"'
OK 'ATDT*99***1#'
root@rpi0w-005:/etc/ppp/peers#


Same for TMobile:

root@rpi0w-005:/etc/ppp/peers# cat tmo
noauth
debug
connect "/usr/sbin/chat -v -f /etc/ppp/peers/tmo.chat"
connect-delay 2000
# /dev/ttyU0
/dev/ttyUSB0
460800
local
defaultroute
noipdefault
usepeerdns
# novj
# user "WAP@CINGULARGPRS.COM"
persist
maxfail 0
root@rpi0w-005:/etc/ppp/peers#


root@rpi0w-005:/etc/ppp/peers# cat tmo.chat
TIMEOUT 3
ECHO ON
ABORT BUSY ABORT 'NO CARIER' ABORT VOICE ABORT 'NO DIALTONE'
ABORT 'NO DIAL TONE' ABORT 'NO ANSWER' ABORT DELAYED
'' '\rAT'
'' ATZ
TIMEOUT 6
OK 'AT+CGDCONT=1,"IP","epc.tmobile.com"'
OK 'ATDT*99***1#'
root@rpi0w-005:/etc/ppp/peers#


If the configuration (not the chat) script specifies a user, the
password  needs  to  be  provided  in  /etc/ppp/chap-secrets  or
/etc/ppp/pap-secrets:

root@rpi0w-005:/etc/ppp# cat pap-secrets
#
# /etc/ppp/pap-secrets
#
# This is a pap-secrets file to be used with the AUTO_PPP function of
# mgetty. mgetty-0.99 is preconfigured to startup pppd with the login option
#
[... lots of blah omitted ...]
# OUTBOUND connections

# Here you should add your userid password to connect to your providers via
# PAP. The * means that the password is to be used for ANY host you connect
# to. Thus you do not have to worry about the foreign machine name. Just
# replace password with your password.
# If you have different providers with different passwords then you better
# remove the following line.

#       *       password

# *     *       CINGULAR1
*       att     CINGULAR1

root@rpi0w-005:/etc/ppp#


If the  surf-stick-equipped  Pi is supposed to be a  NAT-router,
then IPTables'  configuration  needs to be updated. For that add
this script to /etc/ppp/if-up.d:

root@rpi0w-005:/etc/ppp/ip-up.d# pwd
/etc/ppp/ip-up.d
root@rpi0w-005:/etc/ppp/ip-up.d# cat 123nat
#!/bin/sh

set -x
set -v

iptables-restore /home/pi/IPTables/iptables.ipv4.ppp
sysctl net.ipv4.ip_forward=1

root@rpi0w-005:/etc/ppp/ip-up.d#

The configuration passed to iptables-restore must exist:

root@rpi0w-005:/etc/ppp/ip-up.d# cat /home/pi/IPTables/iptables.ipv4.ppp
# Generated by iptables-save v1.4.21 on Wed Nov 22 19:42:25 2017
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [1:120]
:POSTROUTING ACCEPT [1:120]
-A POSTROUTING -o ppp0 -j MASQUERADE
COMMIT
# Completed on Wed Nov 22 19:42:25 2017
# Generated by iptables-save v1.4.21 on Wed Nov 22 19:42:25 2017
*filter
:INPUT ACCEPT [112:8576]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [91:10408]
-A FORWARD -i ppp0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -i wlan0 -o ppp0 -j ACCEPT
COMMIT
# Completed on Wed Nov 22 19:42:25 2017
root@rpi0w-005:/etc/ppp/ip-up.d#

In case the link is not permanent, means to revert iptables back
to normal must be provided:

root@rpi0w-005:/etc/ppp/ip-down.d# cat 123nat
#!/bin/sh -e

iptables-restore /home/pi/IPTables/iptables.ipv4.takedown
sysctl net.ipv4.ip_forward=0

root@rpi0w-005:/etc/ppp/ip-down.d#

root@rpi0w-005:/etc/ppp/ip-down.d# cat /home/pi/IPTables/iptables.ipv4.takedown
# Generated by iptables-save v1.4.21 on Wed Nov 22 19:55:09 2017
*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [4:304]
:POSTROUTING ACCEPT [4:304]
COMMIT
# Completed on Wed Nov 22 19:55:09 2017
# Generated by iptables-save v1.4.21 on Wed Nov 22 19:55:09 2017
*filter
:INPUT ACCEPT [129:9978]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [114:11942]
COMMIT
# Completed on Wed Nov 22 19:55:09 2017
root@rpi0w-005:/etc/ppp/ip-down.d#

Chances are that (especially in a mobile  configuration) the USB
modem is not  permanently  available. To ease abuse, it might be
worthwhile to check on modem presence from within a cron job:

pi@rpi0w-005:~ $ crontab -l
# Edit this file to introduce tasks to be run by cron.
#
[... blah ...]
# m h  dom mon dow   command

0,2,4,5,6,8 * * * *     /bin/sh /home/pi/bin/cron.tab
10,12,14,16,18 * * * *  /bin/sh /home/pi/bin/cron.tab
20,22,24,26,28 * * * *  /bin/sh /home/pi/bin/cron.tab
30,32,34,36,38 * * * *  /bin/sh /home/pi/bin/cron.tab
40,42,44,46,48 * * * *  /bin/sh /home/pi/bin/cron.tab
50,52,54,56,58 * * * *  /bin/sh /home/pi/bin/cron.tab

pi@rpi0w-005:~ $

pi@rpi0w-005:~ $ cat  /home/pi/bin/cron.tab
#!/bin/sh

if [ -e /dev/ttyACM0 ]
then
    if [ `/bin/ps -ef | /bin/grep "pppd call att" | /bin/grep -v grep | /usr/bin/wc -l` -eq 0 ]
    then
        nohup /usr/sbin/pppd call att > /var/tmp/pppd.log 2>&1 &
    fi
else
    if [ `/bin/ps -ef | /bin/grep "pppd call att" | /bin/grep -v grep | /usr/bin/wc -l` -gt 0 ]
    then
        pkill pppd > /dev/null 2>&1
    fi
fi

if [ -e /dev/ttyUSB0 ]
then
    if [ `/bin/ps -ef | /bin/grep "pppd call tmo" | /bin/grep -v grep | /usr/bin/wc -l` -eq 0 ]
    then
        nohup /usr/sbin/pppd call tmo > /var/tmp/pppd.log 2>&1 &
    fi
else
    if [ `/bin/ps -ef | /bin/grep "pppd call tmo" | /bin/grep -v grep | /usr/bin/wc -l` -gt 0 ]
    then
        pkill pppd > /dev/null 2>&1
    fi
fi

pi@rpi0w-005:~ $

This script supports two different  modems, one (fairly vintage)
Nokia N8 phone  with an AT&T SIM, the other a TMobile  surfstick
with a  TMobile  SIM. They  differ in the way the Pi  recognizes
them: the modem shows up as ttyUSB0,  the phone as ttyACM0. On a
Zero-W,  typically  only one is present,  and the type of device
hints at the provider.