Address (ip:port or unix:socket-path) of the upstream server that handled the request. If multiple upstream addresses were accessed while processing the request, then the addresses are separated by a comma and space, for example: "192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock". If there was an internal redirect from one server group to another using the "X-Accel-Redirect" or error_page, these groups of servers are separated by a colon with a space on each side, for example: "192.168.1.1:80, 192.168.1.2:80, unix:/tmp/sock : 192.168.10.1:80, 192.168.10.2:80". Note the spaces: it's a good idea to enclose this in "" in a log format to make parsing easier.