(thx glen)
Regression in lighttpd 1.4.56 - 1.4.58 when extforward.headers is
not set in lighttpd.conf.
Workaround: extforward.headers = ("X-Forwarded-For")
(or appropriate value depending on header sent by upstream proxy)
x-ref:
"mod_extforward regression 1.4.56"
https://redmine.lighttpd.net/issues/3051