FAQ: 1st shot

FAQ/Configuration/How fail2ban substitution resp. runtime interpolation works.md

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+For original man-page of fail2ban configuration use `man jail.conf` or e. g. here - https://manned.org/jail.conf.5
+How the default basic interpolation in python config files works you can read in the python documentation (e. g. see https://docs.python.org/3/library/configparser.html#supported-ini-file-structure)
+
+Our implementation is more complex (allows includes, recursive interlaced substitution, etc.).
+
+**[Q]** I'm trying to reference some variables in my configuration, but `cftoken = %(cftoken)s` doesn't work
+
+**[A]**
+This case `cftoken = %(cftoken)s` is not allowed, because it will produce endless cycle (always substitute itself, again and again).
+
+As well as following is not allowed also (for the same reason):
+```
+A = %(B)s 
+B = %(C)s
+C = %(A)s
+```
+Since we support an extensions called "known" with syntax `%(known/option)s`, after some fix (introduced somewhere 0.9.3 or 0.9.4, but some features possible first in 0.10), it allows to use the same last known option...
+
+So try your luck with `cftoken = %(known/cftoken)s`
+
+If you will set resp. deliver some init parameter from jail to filter or action, you can currently use another interpolation extension of us with another syntax `<option>` (Note: this is a run-time substitution):
+```
+# action my-action.conf
+[Definition]
+cftoken = <cftoken>
+[Init]
+cftoken = default value
+```
+```
+# jail.local
+[my-jail]
+banaction = my-action[cftoken="customized value"]
+```
+