iRule add host IP address for Logstashes

You can use the command [IP::client_addr] to log the client IP address in the originating request. This will log to /var/log/ltm and if you have configured a syslog, will also log to the configured server.

E.g.

when CLIENT_CONNECTED {
    log local0. "Client IP:[IP::client_addr]"
}

Alternatively you may want to look at X-Forwarded-For headers if it is appropriate for your environment.

IP::addr Wiki

X-Forwarded-For