Issue: Whenever Enter is pressed over Telnet to a Force10 S25 or S50 switch, the new line command is processed twice if the telnet session is from Windows.
I recently hit an odd issue on a number of Force10 S25 switches that have been recently deployed. I’m used to using terminal applications via RS232, telnet, or SSH for device configuration, and understand that are several oddities based on the devices in use. This issue started with a seemingly odd login issue. Here’s the output, after simply typing in the username “switchadmin” and pressing Enter:
Error: Authentication failed.
I spent a while pouring through the AAA and vty settings, nothing out of the ordinary is configured yet here. Left me quite perplexed, but after a break, I tried some different angles, and encountered what I suspected to be a CR+LF issue. If I could remember to type in the username, followed by CTRL+Enter, I’d have a chance to type in the password, and continue along. It’s annoying to see an extra newline processed for every keypress, but at least things were functional again. The biggest detriment to functionality was submitting the blank password upon login.
After kicking the case over to support, things just got frustrating with troubleshooting 101. I had already exhausted a number of different combinations of applications and devices, but always hit the same issue. The problem had to lie with the S25 switch.
I did more research along the lines of ASCII, CR+LF versus CR+NUL, and reviewed RFC854 from 1983 – https://tools.ietf.org/html/rfc854and overall, I didn’t find much to give a good reason for the handling of Telnet New Line codes. I’m still baffled at these switches handling the return in telnet this way, but here are some workarounds:
- Utilize CTRL+Enter instead of Enter
- Utilize CTRL+M instead of Enter
- In Windows telnet, launch telnet by itself, type “unset crlf”, and then “open
” and the issue will be fixed for the session
- In PuTTy, browse to Connection > Telnet, and check “Return key sends Telnet New Line instead of ^M”. I recommend saving this setting into a new profile, something like “CTRL+M Newline Telnet” so it’s easy to retrieve the setting from the main screen.
If I find a way to make the switch behave as-expected, I’ll update this post. Hopefully this helps a couple of people save troubleshooting time.