how can I get client's real IP behind F5

[wuzhez]

Hi all,
I use F5 for load balance, and how can I get client's Real IP, I had
tried the NginxHttpRealIpModule, but it doesn't work.
And my log format: $remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent "$http_referer" '
'"$http_user_agent" '