still 400 response code, but so weird this time (2 replies)

In access_log file, huge numbers of log entries like this:

115.85.238.34 1764839163 - 0.242 [01/Aug/2013:11:02:01 +0800] "foo.bar.com" "-" 400 0 "-" "-" "-"


log_format defined in http conf block:

'$remote_addr $connection $remote_user $request_time [$time_local] "$hostname" "$request" $status $body_bytes_sent "$http_referer" "$http_cookie" "$http_user_agent"'


Points I realized:
1) Cannot catch $request, which is full original request line according to Nginx documentation. So can it tell at which phrase the connection was dropped?
2) $body_bytes_sent is zero. So no HTTP response body was generated.
3) $http_refer, $http_cookie and $http_user_agent cannot be caught. So does this indicate any issue?


p.s. this might be caused by HTTPS/SSL connections from mobile client, but I'm not sure.


Could anyone give any words?

Thank you in advance!