Might be missing this, from an old Apache config:
# Configuration for mod_rpaf
<IfModule mod_rpaf.c>
RPAFenable On
RPAFproxy_ips 192.168.2.123
# RPAFsethostname host.your.domain
</IfModule>
# End of mod_rpaf.
Posted at Nginx Forum: http://forum.nginx.org/read.php?2,249698,249710#msg-249710