PuTTY inserts random characters during a session

I recently started renting space on a remote server so that I could work on a project. I found that a relatively painless way to access it on a windows machine is through PuTTY. However, there is one thing that has always irked me when using it: for seemingly no reason random characters are sometimes inserted at the cursor.

Most of the time it is just a single tilde, but rarely it spits out what looks like some escape sequence ([[^8 or the like). It will only occur when I am focused on the window, whether I am typing or 20 feet away from the keyboard. If left for long enough, it will spit tildes at random intervals (average is about 1 minute).

Finally, this behavior seems to be inconsistant when running programs such as nano or the mysql interface: in nano, instead of inserting tildes, it will set marks (ctrl-^); in mysql, lines will become un-editable.

My question is this: Has anyone else experienced this sort of behavior in PuTTY? And if so, what can be done to prevent/correct this behavior?

Solution 1:

I had the same problem with PuTTY and found that is was being caused by an application I have called "caffeine" which effectively presses the F13 key every minute to stop the screen saver from being activated (group policies prevent me from changing screen saver timeout on my machine). Disabling the application solved me problem.

Solution 2:

If you prefer to have Putty and Caffeine running then follow these steps:

1. Stop Caffeine
2. Check the Caffeine readme.txt for the default key simulated - mine was F15
3. Login to your remote terminal via Putty
4. Start Caffeine (Tip: start Caffeine with short interval e.g 3 seconds).
5. Bring the Putty session in focus.
6. On your bash command prompt press Ctrl+V (This is required to get the special code for the key. e.g try Ctrl+V followed by F12)
7. Wait for Caffeine to simulate the key
8. The key code will be displayed on the command line (e.g F15 is ^[[28~, where ^[ is escape). Stop Caffeine
9. Now you can bind this key to blank so that when Caffeine simulates it, it won't type ~
   • edit ~/.inputrc (create a new file if it doesn't exist)
   • add the line "\e[28~":""
10. Start Caffeine
11. Restart your terminal session
    • There should be no more random ~ when using the Putty terminal with that linux server

Solution 3:

I found shreyansp's solution to be the most (but not quite) satisfactory one :)
Here's my attempt on improving that (of course YMMV). Hope this may be of use to those looking for a solution :)

My solution behaves as follows:

• caffeine sends to Windows an appropriate Virtual-Key Code which:
  • prevents Windows from going to sleep or idle
  • does not otherwise generate any side-effects on Windows neither alone nor in combination (no Ctrl, Shift, Alt, Alt-Gr, Win, F1-F5, F10, etc.)
  • either is not sent to Putty or is ignored by Putty

TL;DR: My solution is applied between steps 2 and 3 (see below) with the -key:0E caffeine param:
Exit Caffeine & re-launch it with:
caffeine.exe 5 -key:0E (for easy testing)
caffeine.exe 50 -key:0E (for a mandatory screen saver set at 1 minute)

• Launch read program on the remote host and watch how no keystrokes are received every 5 or 50 seconds.
• Exit read with Ctrl+C

  ---

Shreyansp proposed a solution where a fix would be applied between steps 5 and 10 (see below).
The side-effect of that (on my config) was that, with each keystroke that putty forwarded from caffeine to the remote host:

• it triggered a 'Reset scrollbar on keypress' (setting on Putty/Window page)
  which I would normally want, but only when I (the human) is pressing the key but not regularly by caffeine :)
• readline/bash translated version of it (from '"\e[28~"' to '""' (blank key ?) caused the remote session interaction to hang for several seconds

In order to easily test the above, exit Caffeine & re-launch it with 5 seconds interval and Virtual-Key Code 07:
caffeine.exe 5 -key:07

• Launch read program on the remote host and watch how keystrokes are received every 5 or 50 seconds.
• Exit read with Ctrl+C

  ---

The keystroke 'pipeline', as I understand it:

1. Caffeine sends a Virtual-Key Code to Windows
2. Windows sends that Virtual-Key Code to Putty
3. Putty does some 'translations'/'mappings' based on some session settings in:
   • Terminal/*
   • Window/*
4. Putty sends the 'translated'/'mapped' key code to the remote host
5. On the remote host, the 'terminal' program (e.g: $TERM=xterm, vt100, vt102, vt220, etc.) translates from the 'line protocol' into key codes.
6. the readline library does some translations/mappings based on ~/.inputrc
7. readline sends the Key Code to bash
8. bash does some translations/mappings based on ~/.bashrc (based on the builtin bind command)
9. bash or readline (not sure which one) sends the translated Key Code to nano (my text editor)
10. This pipeline can get even longer by adding the screen program (which includes a $TERM=screen for step 5. and loops back again at step 6. to 10.)

Note: Once it gets to step 4., it becomes very difficult to precisely control the different layers of 'translations'/'mappings'. I would recommend avoiding that if you can.

Background:
I used caffeine.exe -key:07 for years before having to deal with a pfsense 2.3.3-RELEASE-p1 (based on FreeBSD 10.3-RELEASE).
Then, caffeine.exe -key:07 was received on the other side as ^[[28~ ... which seems to be mapped to Ctrl+^ (Set Mark) in Nano.
This was quite annoying (imagine someone pressing and keeping Shift Key while you move the text caret around in Notepad).

Previously, I did numerous customizations in Putty Settings, ~/.tcshrc, ~/.inputrc, ~/.bashrc, ~/.nanorc, ~/.screenrc to get what I consider basic functionality (Backspace, Delete, Home, End, PgUp, PgDown, Ctrl+Left, Ctrl+Right, Numpad 0-9, Numpad ./*-+) working consistently between bash/nano/screen.

Once I discovered this caffeine.exe -key:07 'bug', I didn't want to retrace that all over again :)

Tested on:
Windows 8.1 64-bit Enterprise (6.3.9600) / Putty 0.66 / pfsense 2.3.3-RELEASE-p1 (based on FreeBSD 10.3-RELEASE) / bash 4.4.12-release / nano 2.7.3 / screen 4.04.00

References:

• Caffeine:

  • Virtual-Key Codes sent by caffeine to Windows (I would choose the 1st Undefined code that works for your setup)

• Readline/inputrc/bash/bashrc/(builtin) bind:

  • http://www.softpanorama.org/Scripting/Shellorama/inputrc.shtml
  • http://cnswww.cns.cwru.edu/php/chet/readline/rluserman.html
  • https://docs.freebsd.org/info/readline/readline.pdf
  • Summary:
    • unbinding with 'bind -r keyseq' only forwards the non-translated keys to the application (nano in my case)
    • remapping from 'keyseq' to '""' does only mean that an empty key (whatever that may mean) is sent to the application (causing some lockups)

• Putty:

  • PuTTY wish portable-keyboard (handler)
  • PuTTY wish key-mapping
  • Summary:
    • Putty has no provisions to make it ignore one or more specific keys (it's not even listed as a needed/wanted feature)
    • However, Putty seems to filter out Virtual-Key codes that it doesn't recognizes/supports (which is good for us :)

Solution 4:

This really does look like an escape sequence, or control character, which might explain why you don't see it in other programs like notepad.

I'm not quite sure which keys have to be pressed to get this output, but I'll play around a bit and try to find out. Can you name any other sequence shown? The behaviour of mysql and nano might just be another interpretation of the escape sequences that seem to get entered.

Finally, the question is what causes these key inputs. Do you, by any chance, use a laptop which e.g. automatically controls the backlight using some proprietary software? I've seen some weird preinstalled software actually emulating the keypresses instead of using the API. You might want to use something like inputlog on yourself.