I found out my system (as most modern Linux) use
PAM (Pluggable Authentication Modules) and
pam_cracklib module within it.
pam_cracklib enforces minimum length of 6 symbols regardless of parameters, so solution is to turn it off.
One link I've read discussed editing
system-ac files in
/etc/pam.d, the contents of both files were same and no explanation was given of their respective roles.
Upon study of
PAM docs (The Linux-PAM System Administrators' Guide | linux-pam.org), I learned
PAM configs for LINUX services are in that
/etc/pam.d directory in separate files. I saw
passwd file and it in found only
system-ac file mentioned (added by
substack keyword), so on my system I needed to edit only
optional made no difference (I guess that's because
pam_cracklib did not pass entered and rejected pasword to next module in stack -
pam_unix) and commenting line with
pam_cracklib lead to errors during
passwd run. I noted that next line with ordinary
pam_unix had option
use_authtok (docs: enforce the module to set the new password to the one provided by a previously stacked module). After I deleted that option and commented line with
pam_cracklib I now able to set short passwords with