The firewall-piercing program, fwprc,
will use a "tty proxy", cotty,
that opens two pseudo-tty devices,
launches some command on each of those devices' slaves,
and stubbornly copies every character that one outputs
to the tty that serves as input of the other command.
One command will be telnet connection to remote site,
and the other will be the local pppd.
pppd can then open and control the telnet session
with a chat script as usual.
I wrote a very well self-documented script
to pierce firewalls, fwprc,
available from my site
http://www.tunes.org/~fare/files/fwprc/,
together with cotty
(which is required by fwprc 0.2 and later).
At the time of my writing these lines, latest versions are
fwprc 0.3a and cotty 0.3a.
The name "fwprc" is voluntarily made unreadable and unpronounceable, so that it will confuse the incompetent paranoid sysadm who might be the cause of the firewall that annoys you (of course, there can be legitimate firewalls, too, and even indispensible ones; security is all a matter of correct configuration). If you must read it aloud, choose the worst way you can imagine.
CONTEST! CONTEST! Send me a .au audio file
with a digital audio recording of how you pronounce "fwprc".
The worst entry will win a free upgrade and his name on the fwprc 1.0 page!
I tested the program in several settings, by configuring it through resource files. But of course, by Murphy's law, it will break for you. Feel free to contribute enhancements that will make life easier to other people who'll configure it after you.
fwprc can be customized through a file .fwprcrc
meant to be the same on both sides of the firewall.
Having several alternate configurations to choose from is sure possible
(for instance, I do it),
and is left as an exercise to the reader.
To begin with, copy the appropriate section of fwprc
(the previous to last) into a file named .fwprcrc
in your home directory.
Then replace variable values with stuff that fits your configuration.
Finally, copy to the other host, and test.
Default behavior is to use pppd locally, and slirp remotely.
To modify that, you can redefine the appropriate function
in your .fwprcrc with such a line as:
remote_IP_emu () { remote_pppd }
Note that SLiRP is safer than pppd, and easier to have access to, since it does not require being root on the remote machine. Anoter safe feature is that it will drop packets not directly coming from the connected machine (which feature becomes a misfeature if you attempt to route a subnetwork onto it with masquerading). The basic functionality in SLiRP works quite well, but I've found advertised pluses (like run-time controllability) to be deficient; of course, since it is free software, feel free to hack the source so as to actually implement whichever feature you need.