Staging

Prerequisites

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 Ubuntu Server 18.04
• Matrix “MX Modem” (more on this later)
• USB to RS-232 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

• Ubuntu server 18.04
• PPP
• getty
• Asterisk

Choose your Modem hardware

Modem hardware varies greatly but thankfully nothing special is required beyond compatible protocols between your 'ISP' and 'client' modems such as V.22, V.32, etc.
This tutorial should work with any dial-up modem that presents itself as a serial device to the operating system including cheap USB modems, ISA modems, PCI modems and of course external rs232 serial modems.
Note: You will have a lot of trouble using a 'Softmodem\winmodem'! You're much using a 'real' hardware based modem.

I'll be using an external serial modem + USB to RS-232 adapter for this tutorial. Using dedicated serial hardware has the advantage of being easy to troubleshoot and can also scale up to dozens of lines if you have enough desk space and USB ports.
My modem is a "MX Modem" made by MATRIX but I was unable to find any information about it. Obviously the next step was to crack it open to try and figure out its capabilities.
(insert pic here) Inside is a XECOM XE1414C 14.4 Kbps modem in a single component package. PDF manual is here. (insert PDF link here) To connect the modem to the computer running the ISP side of things I'm using a generic USB to RS-232 adapter and a DE-9 to DB-25 adapter. (insert pic here)

The PSTN

We need a way to connect our modem (serving as an ISP) to clients. There are many ways to approach this:

• Use the actual PSTN via real phone lines
• Use a PBX to provide local connectivity
• Build your own circuity (not covered here, would require extra config as you would not have dial tone)
• Fake PSTN using VoIP ATAs and a software PBX

I've gone with the fourth option. Here's the breakdown:

1. Asterisk, a VoIP PBX, is configured on the dial-in server to accept connections from two SIP client accounts and route calls between them.
2. A Linksys PAP2T ATA, which supports two phone lines, is set up as both of those SIP clients connected to the PBX.
3. The ISP side modem is connected to the first line and the client device is on the second line.

Asterisk set up

1. Install asterisk
   

sudo apt-get install asterisk

2. Append configuration for the two SIP clients to the end of /etc/asterisk/sip.conf

   [pap2t-ispmodem]
   context=default
   type=friend
   secret=password
   qualify=200                     ; Qualify peer is no more than 200ms away
   host=dynamic                    ; This device registers with us
   directmedia=yes                 ; Send RTP directly to the peer to reduce latency and jitter
   regexten=881
   nat=no
   
   [pap2t-client]
   context=default
   type=friend
   secret=password
   qualify=200                     ; Qualify peer is no more than 200ms away
   host=dynamic                    ; This device registers with us
   directmedia=yes                 ; Send RTP directly to the peer to reduce latency and jitter
   regexten=882
   nat=no
   

3. Edit /etc/asterisk/extensions.conf and make two changes:
   Search for [default] (should be around line 672) and comment out
   include => demo
   After that commented out line add a new line with
   exten => _X!,1,Dial(SIP/pap2t-ispmodem, 20)
   The _X! tells this dial plan rule to match any number a client dials and send the call to the pap2t-ispmodem client
4. Enable the asterisk service so it starts on boot
   sudo systemctl enable asterisk
   
5. Start asterisk
   sudo systemctl start asterisk
6. Open the asterisk console to confirm your ATA lines are registered
   sudo asterisk -rvvvv
   

   dialupserver*CLI> sip show peers
   Name/username             Host                                    Dyn Forcerport Comedia    ACL Port     Status      Description                      
   pap2t-client/pap2t-client 10.1.8.112                               D  No         No             5061     OK (7 ms)                                    
   pap2t-ispmodem/pap2t-ispm 10.1.8.112                               D  No         No             5060     OK (6 ms)                                    
   2 sip peers [Monitored: 2 online, 0 offline Unmonitored: 0 online, 0 offline]

   If you make changes to your configuration after starting asterisk you can use the reload command in the console to reload the configuration.

ATA SIP Registration example:

    -- Registered SIP 'pap2t-ispmodem' at 10.1.8.112:5060
       > Saved useragent "Linksys/SPA2102-5.2.5" for peer pap2t-ispmodem
       NOTICE[4253]: chan_sip.c:24592 handle_response_peerpoke: Peer 'pap2t-ispmodem' is now Reachable. (7ms / 200ms)
    -- Registered SIP 'pap2t-client' at 10.1.8.112:5061
       > Saved useragent "Linksys/SPA2102-5.2.5" for peer pap2t-client
       NOTICE[4253]: chan_sip.c:24592 handle_response_peerpoke: Peer 'pap2t-client' is now Reachable. (5ms / 200ms)

Successful call example:

  == Using SIP RTP CoS mark 5
       > 0x7fa234059e30 -- Strict RTP learning after remote address set to: 10.1.8.112:16482
    -- Executing [88567682@default:1] Dial("SIP/pap2t-client-00000002", "SIP/pap2t-ispmodem, 20") in new stack
  == Using SIP RTP CoS mark 5
    -- Called SIP/pap2t-ispmodem
    -- SIP/pap2t-ispmodem-00000003 is ringing
       > 0x7fa1f8252350 -- Strict RTP learning after remote address set to: 10.1.8.112:16384
    -- SIP/pap2t-ispmodem-00000003 answered SIP/pap2t-client-00000002
    -- Channel SIP/pap2t-ispmodem-00000003 joined 'simple_bridge' basic-bridge <5f9983eb-3f93-49d1-90b3-c91c545c2d38>
    -- Channel SIP/pap2t-client-00000002 joined 'simple_bridge' basic-bridge <5f9983eb-3f93-49d1-90b3-c91c545c2d38>
       > Bridge 5f9983eb-3f93-49d1-90b3-c91c545c2d38: switching from simple_bridge technology to native_rtp
       > Remotely bridged 'SIP/pap2t-client-00000002' and 'SIP/pap2t-ispmodem-00000003' - media will flow directly between them
       > 0x7fa1f8252350 -- Strict RTP learning after remote address set to: 10.1.8.112:16384
       > 0x7fa234059e30 -- Strict RTP learning after remote address set to: 10.1.8.112:16482
    -- Channel SIP/pap2t-client-00000002 left 'native_rtp' basic-bridge <5f9983eb-3f93-49d1-90b3-c91c545c2d38>
    -- Channel SIP/pap2t-ispmodem-00000003 left 'native_rtp' basic-bridge <5f9983eb-3f93-49d1-90b3-c91c545c2d38>
  == Spawn extension (default, 88567682, 1) exited non-zero on 'SIP/pap2t-client-00000002'
       > 0x7fa1f8252350 -- Strict RTP learning after remote address set to: 10.1.8.112:16384

ATA Configuration

I wont go into much detail on ATA configuration since the topic has been beat to death on various forums but for modem communications here is the gist:

1. Set up both lines on the ATA to register with the PBX with the usernames 'pap2t-ispmodem' and 'pap2t-client' with the password 'password' (so secure)
2. Use g711u codec
3. Disable every echo cancellation option in your ATA (see https://www.voip-info.org/linksys-pap2t)
4. Set the jitter buffer to be as small as possible (you want voice data on the wire ASAP)

Congrats! You now have your own voice network!

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 RS-232 adapter and confirm it shows up as /dev/ttyUSBXXX (ls /dev/ to check) In my case it presents as /dev/ttyUSB0
   My serial adapter is a "ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adaptor"
   
   Full lsusb output:

   # 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 /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
   # Disable the IPXCP and IPX protocols.
   noipx
   

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
    noauth
    proxyarp
    lcp-echo-failure 60
    

11. Create the user used for PAP authentication
    sudo useradd -G dialout,dip,users -m -g users -s /usr/sbin/pppd dial
12. Change the password (I set it to dial)
    sudo 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
16. Last step for the dial-up server is to configure the firewall to allow forwarding of traffic from PPP out onto the network (and off to the internet)
    
    1. On Linux distributions with iptables you need to add a line to /etc/rc.local to enable masquerading. If your ethernet interface is named eth0 you would add this line:
       iptables -t nat -A POSTROUTING -s 192.168.32.0/24 -o eth0 -j MASQUERADE
    2. On modern Ubuntu installs ufw is used instead of iptables so the procedure is a bit different. Follow this guide but you can omit -o eth0 and use -s 192.168.32.0/24
       https://help.ubuntu.com/lts/serverguide/firewall.html.en#ip-masquerading

Troubleshooting

Choice of USB to RS-232 adapter seems to be crucial and very finicky. The USB adapter I've been using on a raspberry pi, which worked perfectly would not work a properly on a x86_64 Debian system. To debug modem communication and baud rate use either minicom (or your editor of choice!) to open a session over serial. Sending the command AT followed by enter should result in your modem replying OK

I actually ran into a bug in Debian 9.5 with my ch341 based usb to serial where setting the baud rate was not working. (Seems to be this: https://bugzilla.redhat.com/show_bug.cgi?id=1235715)