https://en.wikipedia.org/wiki/Extended_Log_Format

相比 Common Log Format (通用日志格式)，ELF 是名副其实的可拓展：在头部声明了版本，以及字段。
例如：

#Version: 1.0
#Date: 12-Jan-1996 00:00:00
#Fields: time cs-method cs-uri
00:34:23 GET /foo/bar.html
12:21:16 GET /foo/bar.html
12:45:52 GET /foo/bar.html
12:57:34 GET /foo/bar.html

• Version: <integer>.<integer>
  The version of the extended log file format used. This draft defines version 1.0.
• Fields: [<specifier>...]
  Specifies the fields recorded in the log.
• Software: string
  Identifies the software which generated the log.
• Start-Date: <date> <time>
  The date and time at which the log was started.
• End-Date: <date> <time>
  The date and time at which the log was finished.
• Date: <date> <time>
  The date and time at which the entry was added.
• Remark: <text>
  Comment information. Data recorded in this field should be ignored by analysis tools.

然后这个字段的声明又有一套规则，可以参考