My mother has a great cottage on a lake. There is no cell phone service for a few kilometers around. There is a landline. There is a form of rural data wireless that is often down. Thus the need for a dialup wifi server as a backup, for when rural data wireless service is unavailable. Too slow for web surfing, but enough for email and texting. This seems like a perfect task for Linux. After all, Linux is usually a good foundation for web servers, email servers, vpn servers, file share servers, voip servers, and more.
I started this project last year, and found that although Windows could drive the built-in 56K modem on a circa 2009 laptop, Linux could not. Fortunately, I have a US Robotics USB 56K modem, which is recognized by Linux. I got as far as a dialup connection without DNS.
This year I learned how to override some settings in wvdial to enable dns. So i was able to surf on fedora 28 via dialup. However, I was unable to share the connection via the GUI networkmanager.
To explain why, let me tell you a few stories:
I should point out that sharing a connection like this is something that Windows can do without breaking a sweat. I first installed such a gateway, albeit dialup shared as a NAT over wired Ethernet, on a Windows 98 box in 1999.
Last year (2017), while on vacation at a hotel with a 2 device limit, I confronted the dilemma: how does one choose between a laptop, an iPad, and 2 cell phones? Answer: use a laptop with a second wifi adapter to provide a repeated hotspot under my control.
NetworkManager on Gnome 3 on Fedora offered to share a wired Ethernet connection as a local WIFI hotspot, but was unable to do other permutations — like a second wifi on usb sharing a first wifi connection to hotel wifi. Fortunately, the machine was able to dual-boot into Windows and share the hotel wifi via a second wifi usb adapter.
Back to the Linux dialup wifi server project.
It would seem that I must configure hostapd, in order to create a hotspot that can share the dialup connection. I have followed the instructions, but have not succeeded so far — there are some things i can try. I find myself consulting blog posts from 2002-2011. I suppose that is the kind of deep time audience that reads these blog posts, a few at a time, in the future.
More to follow at the beginning of October when I return to the cottage to take another run at the problem. for now i will simulate the hostapd problem with a test machine here, then bring that solution to the cottage and test the integrated solution with the already-working dialup connection.
I suppose I could try to test the dialup connection via a voip analog telephone adapter device, but that seems wrong.
[edit 20180924] My brother had a somewhat similar but more successful experience in 2015: http://www.malak.ca/blog/index.php/2015/03/05/having-to-find-multiple-levels-of-internet-access-oh-fun/