It's definitely possible. There's an article here:
http://www.kinox.org/articles/linuxdc.html
about it. The way he outlines is not the most straightforward way to go at it since it involves setting up mgetty to handle arbitrary dial-in ppp connections and then sending it a USR1 signal to tell it to pick up. This I'm sure will work, but this isn't the only way.
I'm actually a Mac OS X user who happened upon your question in a web search. I had some trouble getting mgetty working correctly on OS X and most of the recommended mgetty guides from the web page I mentioned did not exist. (And I don't mean that OS X versions didn't exist, I mean that the guides themselves had disappeared from the web). So I've figured out the other way to do it. I'll explain what I do below. You may have to adapt this some to linux, but it ought to work. The only thing that I know for certain that you'll have to change is the device. OS X uses /dev/cu.modem. There's no such device under Linux. I don't know what the appropriate serial device is under Linux, but I'm sure that you can figure it out or that someone else here knows. So replace /dev/cu.modem in my instructions with something else. Probably /dev/modem
On the Dreamcast side, you're on the right track with telling the modem to blind dial. That's the only real configuration needed on that end. It's also usually worth shortening or removing the number to be dialed just so that you don't have to wait for that. I know that I also removed the username and password on mine, but I don't think that you have to. On the computer end, there are two basic steps:
1) Once you've tried to connect on the DC, tell the modem to answer. This must be done by issuing an ATA command to the modem. Telling it to answer after any number of rings won't work since there are no rings, and telling it to answer after zero rings just turns off the auto-answer stuff.
2) Hand off to pppd.
I found two different ways to accomplish step 1:
a) cu
If you install UUCP it comes with a utility called CU which can send commands to the modem. It can be envoked with "sudo cu -l /dev/cu.modem -s 57600" (the sudo may not be needed under Linux). The 57600 is just the speed I used. It can be left out, but it will connect at a slower speed.
Then you type "ATA" and hit return. The modem should then pick up and you should hear them handshaking. Then funny characters should start appearing. Goto step 2.
b) minicom
You'll need to configure minicom to make sure it uses the right device. You may also need to run sudo minicom -s if you haven't ever run it before. Then run minicom, type "ATA" and hit return. Then funny characters should start appearing. Goto step 2.
In all truth, there are probably several other options. I suspect that chat and wvDial can accomplish this (although I'm not certain since there's no OS X version of chat or wvDial) and it wouldn't surprise me if there were a way to do it with shell scripting and a pipe.
Step 2:
Leave cu or minicom running. Just switch to another terminal. Then launch pppd. Pppd will then take over the device from the dialer. The argument set which worked for me is:
sudo pppd -detach /dev/cu.modem 57600 noauth 192.168.0.1:192.168.0.2
Is all of this necessary? Probably not. The sudo is needed on OS X, may not be on Linux. The -detach is definitely a good idea because it means that you'll actually see the error messages right then and there if any occur. The 57600 is just the speed. When I used minicom, it seemed to mostly connect at 38400 instead, so I used that. Noauth removes the need for a login (and is what requires the sudo). The IP numbers are just the computer and dreamcast IP addresses across the interface. They may or may not be needed depending on your set-up. I know that if I leave them out, the connection comes up fine, but then the Dreamcast can't find the rest of the internet.
I should also note that I have an /etc/ppp/options file which contains "debug" "default-asyncmap" and "ms-dns aaa.bbb.ccc.ddd" where aaa.bbb.ccc.ddd is the nameserver which my computer uses. The debug option is not needed. The ms-dns option could probably be done without, but according to the site mentioned, the default-asyncmap is definitely needed. You could put this on the command line instead if you'd like.
For the Dreamcast to use the computer's internet connection, you'll obviously also have to make sure that you have whatever NAT stuff you want to use set-up too. Under OS X, it means pushing the "share my internet connection" button in the control panel. I'm sure the linux one is a little trickier to set up, but undoubtedly much, much more flexible. Our ease of set-up is nice, but there are no options to tweak at all.
I'm sure that clever people can find all sorts of ways to improve on this, but it should be a good jumping-off point. Good luck.
Keith
