Staging

Becoming your own dial-up ISP in 2019

Required hardware:

• A hardware modem (not a software modem/winmodem, must be the real deal)
• A computer to install linux on to talk to the a modem (Can be anything that a modern linux distribution will run on. Raspberry Pi, Pi clone, x86 machine, etc)
• A client device (windows 9x PC for example) with a modem
• Some form of PSTN to connect the two modems

The exact hardware I’ve used

• Generic x86_64 PC running Debian 9.5 x86_64
• Matrix “MX Modem” (more on this later)
• USB to RS232 serial adapter (DE-9) to connect to the modem (Must support hardware flow control)
• DE-9 to DB-25 serial adapter
• Linksys PAP2T analog telephone adapter (ATA)
• x86 based Windows 95 PC with a US Robotics Sportster 28800 ISA modem

Software used

• Debian 9.5 x86_64
• PPP
• getty
• Asterisk

Preparing the dial-in-server

1. Install Debian/Ubuntu/Raspbian per the usual methods (not covered here)
2. Update to latest packages and reboot if required
   

sudo apt-get update
sudo apt-get upgrade
sudo reboot

3. Connect USB to RS232 adaptor and confirm it shows up as /dev/ttyUSBXXX (ls /dev/ to check) In my case it presents as /dev/ttyUSB0
   My serial adaptor is a "ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adaptor"
   
   Full lsusb output:

   user@debian:~# sudo lsusb -v
   Bus 004 Device 003: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
   Device Descriptor:
     bLength                18
     bDescriptorType         1
     bcdUSB               1.10
     bDeviceClass          255 Vendor Specific Class
     bDeviceSubClass         0 
     bDeviceProtocol         0 
     bMaxPacketSize0         8
     idVendor           0x1a86 QinHeng Electronics
     idProduct          0x7523 HL-340 USB-Serial adapter
     bcdDevice            2.54
     iManufacturer           0 
     iProduct                2 USB2.0-Ser!
     iSerial                 0 
     bNumConfigurations      1
     Configuration Descriptor:
       bLength                 9
       bDescriptorType         2
       wTotalLength           39
       bNumInterfaces          1
       bConfigurationValue     1
       iConfiguration          0 
       bmAttributes         0x80
         (Bus Powered)
       MaxPower               96mA
       Interface Descriptor:
         bLength                 9
         bDescriptorType         4
         bInterfaceNumber        0
         bAlternateSetting       0
         bNumEndpoints           3
         bInterfaceClass       255 Vendor Specific Class
         bInterfaceSubClass      1 
         bInterfaceProtocol      2 
         iInterface              0 
         Endpoint Descriptor:
           bLength                 7
           bDescriptorType         5
           bEndpointAddress     0x82  EP 2 IN
           bmAttributes            2
             Transfer Type            Bulk
             Synch Type               None
             Usage Type               Data
           wMaxPacketSize     0x0020  1x 32 bytes
           bInterval               0
         Endpoint Descriptor:
           bLength                 7
           bDescriptorType         5
           bEndpointAddress     0x02  EP 2 OUT
           bmAttributes            2
             Transfer Type            Bulk
             Synch Type               None
             Usage Type               Data
           wMaxPacketSize     0x0020  1x 32 bytes
           bInterval               0
         Endpoint Descriptor:
           bLength                 7
           bDescriptorType         5
           bEndpointAddress     0x81  EP 1 IN
           bmAttributes            3
             Transfer Type            Interrupt
             Synch Type               None
             Usage Type               Data
           wMaxPacketSize     0x0008  1x 8 bytes
           bInterval               1
   Device Status:     0x0000
     (Bus Powered)
   

4. Install ppp (and getty if your distro doesn’t have it by default)
   

   sudo apt-get install ppp mgetty

5. Many of the old guides were written when inittab was still around but its 2019 and systemd has taken over.
   We need to create a systemd service for mgetty so edit /lib/systemd/system/mgetty.service with your text editor of choice with elevated privileges (sudo)

   [Unit]
   Description=External Modem
   Documentation=man:mgetty(8)
   Requires=systemd-udev-settle.service
   After=systemd-udev-settle.service
   
   [Service]
   Type=simple
   ExecStart=/sbin/mgetty -x8 /dev/ttyUSB0
   Restart=always
   PIDFile=/var/run/mgetty.pid.ttyUSB0
   
   [Install]
   WantedBy=multi-user.target
   

6. Configure mgetty by editing /etc/mgetty/mgetty.config with your text editor of choice with elevated privileges (sudo)
   Comment out everything except the debug level and append the section for configuring the serial device

   debug 9
   
   port ttyUSB0
    port-owner root
    port-group dialout
    port-mode 0660
    data-only yes
    ignore-carrier no
    toggle-dtr yes
    toggle-dtr-waittime 500
    rings 2
    #autobauding yes
    speed 9600
   

7. Enable the mgetty service so it starts on boot
   sudo systemctl enable mgetty.service
   
8. Start mgetty
   sudo systemctl start mgetty.service
9. Configure ppp by editing /etc/ppp/options
   Like above comment out everything except these settings

   #define the DNS server for the client to use
   ms-dns 8.8.8.8
   #async character map should be 0
   asyncmap 0
   #require authentication
   auth
   #Use hardware flow control
   crtscts
   #We want exclusive access to the modem device
   lock
   #show pap passwords in log files to help with debugging
   show-password
   #require the client to authenticate with pap
   +pap
   #if you are having trouble with auth enable debugging
   debug
   #heartbeat for control messages, used to determine if the client connection has dropped
   lcp-echo-interval 30
   lcp-echo-failure 4
   #cache the client mac address in the arp system table
   proxyarp
   

10. Create a device option file by editing /etc/ppp/options.ttyUSB0
    

    local
    lock
    nocrtscts
    192.168.32.1:192.168.32.105
    netmask 255.255.255.0
    #debug
    noauth
    proxyarp
    lcp-echo-failure 60
    

11. Create the user used for PAP authentication
    useradd -G dialout,dip,users -m -g users -s /usr/sbin/pppd dial
12. Change the password (I set it to dial)
    passwd dial
13. Edit /etc/ppp/pap-secrets and append the username and password (quotes included)
    dial * "dial" *
14. Enable packet forwarding for IP4 by editing /etc/sysctl.conf
net.ipv4.ip_forward=1