Get ClientIP Address Behind LoadBalancer

You must enable the X-Forwarded-For header in the HTTP profile (Insert X-Forwarded-For) for the LTM to auto-inject the header. Also, depending on how you have the virtual server configured, you may either be seeing the BIG-IP's self-IP (SNAT enabled) address or the client's IP (SNAT disabled). If you require SNAT then you can:

1) Enable the HTTP profile's X-Forwarded-For header option, or

2) Create a simple iRule to push the client's true source as an arbitrary HTTP header:

when HTTP_REQUEST {
     HTTP::header insert CLIENTIP [IP::client_addr]
}

You should then be able to see the client's IP in the incoming "HTTP_CLIENTIP" header.