Sometimes, only one side of the firewall can launch telnet sessions into the other side; however, some means of communication is possible (typically, through e-mail). Piercing the firewall is still possible, by triggering with whatever messaging capability is available a telnet connection from the ``right'' side of the firewall to the other.
fwprc
includes code to trigger such connections
from a PGP-authentified e-mail message;
all you need is add fwprc
as a procmail(1)
filter
to messages using the protocol,
(instructions included in fwprc
itself).
Note however, that if you are to launch pppd with appropriate priviledges,
you might need create your own suid wrapper to become root.
Instructions enclosed in fwprc
.
If you are firewalled, your mail may as well be in a central server
that doesn't do procmail filtering or allow external telnet.
No problem! You can use fetchmail(1)
to run in daemon mode
to poll and get mail to your client linux system,
and/or add a cron-job to automatically poll for mail every 1-5 minutes,
which will forward mail to a local address through sendmail(8)
,
which itself will have been configured to use procmail(1)
for delivery
(or you can run fetchmail(1)
in the background as a daemon,
only it will lock away another fetchmail that you'd like to run
only at times when you open you fwprc
connection).
Too frequent a poll won't be nice to either the server or your host.
Too unfrequent a poll means you'll have to wait before the message gets read
and the reverse connection gets established.
I use two-minute poll frequency.