使用curl进行网站测速

网站访问可以分为下面几个阶段：

• DNS 域名解析
• 通过SSL协议交换密钥（HTTPS)
• 与服务端创建 TCP 访问连接
• 客户端发起请求
• 服务端收到请求并准备回应内容
• 服务端回复回应内容

当我们优化网站速度时通常要现确定哪个阶段是耗时大头。而令我感到惊奇的是，原来 curl 的 -w 选项可以让 curl 完成访问网站的操作后将各阶段的耗时情况（以及其他一些访问信息）输出到标准输出。

man curl 中关于 -w 选项的说明如下：

-w, --write-out <format>
       Make curl display information on stdout after a completed trans‐
       fer. The format is a string that may contain  plain  text  mixed
       with  any  number of variables. The format can be specified as a
       literal "string", or you can have curl read the  format  from  a
       file  with  "@filename" and to tell curl to read the format from
       stdin you write "@-".

       The variables present in the output format will  be  substituted
       by  the  value or text that curl thinks fit, as described below.
       All variables are specified as %{variable_name} and to output  a
       normal  % you just write them as %%. You can output a newline by
       using \n, a carriage return with \r and a tab space with \t.

       NOTE: The %-symbol is a special symbol in the win32-environment,
       where  all  occurrences  of  %  must  be doubled when using this
       option.

       The variables available are:

       content_type   The Content-Type of the  requested  document,  if
                      there was any.

       filename_effective
                      The  ultimate  filename  that curl writes out to.
                      This is only meaningful if curl is told to  write
                      to  a  file  with  the  -O,  --remote-name or -o,
                      --output option. It's most useful in  combination
                      with  the -J, --remote-header-name option. (Added
                      in 7.26.0)

       ftp_entry_path The initial path curl ended up in when logging on
                      to the remote FTP server. (Added in 7.15.4)

       http_code      The numerical response code that was found in the
                      last retrieved HTTP(S)  or  FTP(s)  transfer.  In
                      7.18.2  the alias response_code was added to show
                      the same info.

       http_connect   The numerical code that was  found  in  the  last
                      response   (from  a  proxy)  to  a  curl  CONNECT
                      request. (Added in 7.12.4)

       http_version   The  http  version  that  was  effectively  used.
                      (Added in 7.50.0)

       local_ip       The  IP  address  of  the  local  end of the most
                      recently done connection - can be either IPv4  or
                      IPv6 (Added in 7.29.0)

       local_port     The  local  port number of the most recently done
                      connection (Added in 7.29.0)

       num_connects   Number of new connects made in the recent  trans‐
                      fer. (Added in 7.12.3)

       num_redirects  Number  of  redirects  that  were followed in the
                      request. (Added in 7.12.3)

       proxy_ssl_verify_result
                      The result of the HTTPS proxy's SSL peer certifi‐
                      cate verification that was requested. 0 means the
                      verification was successful. (Added in 7.52.0)

       redirect_url   When an HTTP request was made without -L, --loca‐
                      tion  to follow redirects (or when --max-redir is
                      met), this variable will show the  actual  URL  a
                      redirect would have gone to. (Added in 7.18.2)

       remote_ip      The  remote  IP address of the most recently done
                      connection - can be either IPv4 or IPv6 (Added in
                      7.29.0)

       remote_port    The  remote port number of the most recently done
                      connection (Added in 7.29.0)

       scheme         The URL scheme (sometimes called  protocol)  that
                      was effectively used (Added in 7.52.0)

       size_download  The total amount of bytes that were downloaded.

       size_header    The total amount of bytes of the downloaded head‐
                      ers.

       size_request   The total amount of bytes that were sent  in  the
                      HTTP request.

       size_upload    The total amount of bytes that were uploaded.

       speed_download The average download speed that curl measured for
                      the complete download. Bytes per second.

       speed_upload   The average upload speed that curl  measured  for
                      the complete upload. Bytes per second.

       ssl_verify_result
                      The  result of the SSL peer certificate verifica‐
                      tion that was requested. 0 means the verification
                      was successful. (Added in 7.19.0)

       time_appconnect
                      The  time,  in  seconds,  it  took from the start
                      until the SSL/SSH/etc  connect/handshake  to  the
                      remote host was completed. (Added in 7.19.0)

       time_connect   The  time,  in  seconds,  it  took from the start
                      until the TCP connect  to  the  remote  host  (or
                      proxy) was completed.

       time_namelookup
                      The  time,  in  seconds,  it  took from the start
                      until the name resolving was completed.

       time_pretransfer
                      The time, in seconds,  it  took  from  the  start
                      until  the file transfer was just about to begin.
                      This includes all pre-transfer commands and nego‐
                      tiations that are specific to the particular pro‐
                      tocol(s) involved.

       time_redirect  The time, in seconds, it took for all redirection
                      steps including name lookup, connect, pretransfer
                      and transfer before  the  final  transaction  was
                      started.  time_redirect shows the complete execu‐
                      tion time for multiple  redirections.  (Added  in
                      7.12.3)

       time_starttransfer
                      The  time,  in  seconds,  it  took from the start
                      until the first byte was just about to be  trans‐
                      ferred.  This  includes time_pretransfer and also
                      the time  the  server  needed  to  calculate  the
                      result.

       time_total     The  total time, in seconds, that the full opera‐
                      tion lasted.

       url_effective  The URL that was fetched last. This is most mean‐
                      ingful  if  you've  told curl to follow location:
                      headers.

       If this option is used several times, the last one will be used.

其中与时间相关的变量包括：

time_namelookup

DNS 解析时间，可以与 --resolve 选项配合寻找最快的DNS

time_connect

与服务端创建好 TCP 连接的时间，严格来说是客户端回复 ACK 的时间。我们可以通过 time_connect - time_namelookup 来大致推断网络延时。

time_appconnect

完成 SSL/TLS 设置的时间，此时客户端与服务端完成密钥交换，客户端准备发起请求

time_pretransfer

服务端收到请求的时间

time_starttransfer

服务端准备好回应内容的时间。

time_total

完成整个请求的所有时间

time_redirect

若请求经过多次重定向，那么这个包含直到最后一次请求开始所耗的时间。

下面这张从 cloudflare 偷来的例子可以很直观的看出每个变量的对应关系

curl -so /dev/null -w "dnslookup: %{time_namelookup} | connect: %{time_connect} | appconnect: %{time_appconnect} | pretransfer: %{time_pretransfer} | starttransfer: %{time_starttransfer} | total: %{time_total} | size: %{size_download}\n" https://www.zasag.mn

其中 -s 表示进入 silent 模式， -o /dev/null 表示不显示获取到的文件内容

结果为：

dnslookup: 1.510 | connect: 1.757 | appconnect: 2.256 | pretransfer: 2.259 | starttransfer: 2.506 | total: 3.001 | size: 53107

图示如下：