Staging

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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 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

  • Ubuntu server 18.04
  • PPP
  • getty
  • Asterisk

Choose your hardware

Modem hardware varies greatly but thankfully nothing special is required. This tutorial should work with any dial-up modem that presents itself as a serial device to the operating system.

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
    2. 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


Whether you want to call this a Public Switched Telephone Network or a Private Switched Telephone Network is up to you (Thanks @itsybitsydots)


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
    3. 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: 
    # 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 distros 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


Debugging

Choice of USB to RS232 adapter seems to be crucial and very finicky. The USB adapter Ive 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)

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