Here is the URL to enable telnet through the web interface:
(Note that your NSLU-2's IP address may be different from the one given here)
The default username and password (with a disk connected) are admin / admin.
Note: the telnet server is part of the Unslung firmware. The original Linksys firmware does not have a telnet server or any known method for remote shell access. Very old Linksys versions, V2.3R24 and earlier, actually did have a telnet daemon executable in the firmware, but this is not pertinent for this article.
In line with other compatibility with the stock Linksys firmware, Unslung boots with telnet disabled by default, and you can visit the URL above to enable it. You will have to do this again after every reboot.
If you would like to always enable telnet - then you probably would like to use ssh rather then telnet. Hence a typical setup of an advanced unslung user would 'ipkg install openssh' and then use putty to connect with the ssh protocol. No need to change other settings, ssh will automatically load up on every reboot. You can find more information about ssh in the OpenSSH HowTo.
If you have a NSLU2 without harddisk, and you really would like to use the built in telnet, because ssh doesn't fit in the flash, then you will have to look up how the unslung diversion scripts can be used to enable telnet with every reboot. The information is here in the wiki.
Enable via Telnet
Just in case you only have telnet available you CAN activate telnet via telnet by simulating a http-post:
telnet <slug-ip> 80 POST /Management/telnet.cgi HTTP/1.1 Host: <slug-ip> Content-Type: application/x-www-form-urlencoded Content-Length: 20 Connection: Keep-Alive Authorization: Basic YWRtaW46YWRtaW4= <empty line> action=Enable+Telnet <empty line>
This only works for admin with password admin. Other Credentials must be encoded base64 "username:password" and inserted after "Basic".
$ perl -e "use MIME::Base64; print encode_base64('admin:admin');"
Note: You can also use an http get by specifying the parameters on the URL
This may even work when the web management pages fail to load.
You should receive some HTML telling you telnet is enabled now. End the telnet-session with CTRL-C and telnet into your slug via port 23.