If all you can do is telnet (because of a telnet proxy), then this solution might be fit for you.
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.
Actually, if your telnet proxy allows connection to an arbitrary port,
and if you can reliably run a daemon on the remote host
(with a cron job to relaunch it in case of breakage),
then you'd better write some program that will just connect
a local port to the remote one through the proxy,
so you can use the above secure solution,
possibly using some variant of ssh -t -o "ProxyCommand ..."
(if you submit it to me, I'll gladly integrate such a solution
to the fwprc distribution).
Note: if you must use the unsecure telnet-based solution, be sure that nothing lies in your target account that you want to keep secret or untampered, since the password will be sent in clear text accross the Internet.
I wrote a very well self-documented script
to pierce firewalls, fwprc,
available from
my site,
together with cotty
(which is required by fwprc.2 0 and later).
At the time of my writing these lines, latest versions are
fwprc 0.3e and cotty 0.4b.
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 indispensable 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 an 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,
and needn't additional firewall configuration to prevent
connections from the outside world into the firewalled network.
The basic functionality in SLiRP works quite well,
but I haven't managed to get some advertised pluses to work
(like run-time controllability).
Of course, since it is free software,
feel free to hack the source
so as to actually implement or fix whichever feature you need.