Dial up server

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 are much better off 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.

Inside is a XECOM XE1414C 14.4 Kbps modem in a single component package. PDF manual is 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.

Matrix MX Modem with top cover removed
MX Modem and RS232 adapters

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:

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

Asterisk set up

Install asterisk

sudo apt-get install asterisk

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

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
Enable the asterisk service so it starts on boot
sudo systemctl enable asterisk
Start asterisk
sudo systemctl start asterisk

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:

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)
Use g711u codec
Disable every echo cancellation option in your ATA (see https://www.voip-info.org/linksys-pap2t)
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

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

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

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"
Install ppp (and getty if your distro doesn’t have it by default)
```
sudo apt-get install ppp mgetty
```

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

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
```
Enable the mgetty service so it starts on boot
sudo systemctl enable mgetty.service
Start mgetty
sudo systemctl start mgetty.service

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

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

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

When using an external modem the choice of USB to RS-232 adapter seems to be crucial. There aren't many requirements but you must use an adapter that supports hardware flow control. If you need to purchase an adapter you can either get one that explicitly says it supports hardware flow control ($$$$) or play the eBay lottery and buy a half dozen different models and hope one of them works.

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

To troubleshoot modem communication and baud rate settings use minicom (or screen) to open a session over serial and try different settings (or read your modem's manual!). Sending the command 'AT' followed by a new line should result in your modem replying 'OK'
If you're getting nothing at all out of your modem perform a serial loopback test

If mgetty is not answering incoming calls it may be having trouble communicating with your modem. Check the logs in /var/log/mgetty/ to determine the problem. You may need to set a modem initialization string in the mgetty device config file so check your modem's manual for help on this. (If this happens you're in for a rough time)

Dial up server

Contents

Prerequisites

Required hardware:

The exact hardware I’ve used

Software used

Choose your modem hardware

The PSTN

Asterisk set up

ATA Configuration

The dial-in-server

Troubleshooting

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