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If you use the captcha cfimage option on a Railo install on linux you may encounter an issue where the captcha text is missing. To fix this you need to install windows fonts. Here is how I did this on Slackware.
Aside: I don't advocate the use of captcha images. I think they are a flawed and outdated solution to the problem of nuisance bots.
Step 1: Copy Windows TTF Files
The first step is obviously to get the standard windows font files. So start up a windows machine and copy all the ttf font files in "c:\windows\fonts\" to a shared location such as a pen drive. Next copy them to your linux box into the fonts directory. On slackware this is located at "/usr/share/fonts/TTF/".
Step 2: Set Permissions
Now run the following commands in this directory to set the required permissions and access settings on these files.
chown root.root *.ttf
chmod 644 *.ttf
Step 3: Load Into Slackware
Next we load the new font files into Slackware with these commands.
Step 4: Restart Railo
With the fonts installed restart Railo and the captcha function of cfimage should now be working.