REF：http://cbonte.github.io/haproxy-dconv/1.5/configuration.html

Proxy configuration can be located in a set of sections :

 - defaults <name>

 - frontend <name>

 - backend  <name>

 - listen   <name>

A "defaults" section sets default parameters for all other sections following

its declaration. Those default parameters are reset by the next "defaults"

section. See below for the list of parameters which can be set in a "defaults"

section. The name is optional but its use is encouraged for better readability.
"defaults" 区块设置默认参数，对其定义后的所有区块有效。所有默认参数在优先级上，会被后面的
“defaults”区块覆盖。"defaults"区块可设置参数清单见下文。“name”不是必须的，但为了有较好的可读性
建议写上。

A "frontend" section describes a set of listening sockets accepting client

connections.
“frontend”区块描述接受客户端连接的监听sockets集。

A "backend" section describes a set of servers to which the proxy will connect

to forward incoming connections.
“backend”区块描述代理服务器集，代理将连接并转发入栈信息给这些服务器。

A "listen" section defines a complete proxy with its frontend and backend

parts combined in one section. It is generally useful for TCP-only traffic.
“listen”区块定义了完整的代理，将“frontend”和“backend”结合在一个区块中。一般用在TCP-only
流量中。

All proxy names must be formed from upper and lower case letters, digits,

'-' (dash), '_' (underscore) , '.' (dot) and ':' (colon). ACL names are

case-sensitive, which means that "www" and "WWW" are two different proxies.
所有的代理名称必须由大小写字符、数字、'-'、'_'、'.'和':'构成。ACL名对大小写敏感，这
意味着"www"和“WWW”代表不同的代理。

Historically, all proxy names could overlap, it just caused troubles in the

logs. Since the introduction of content switching, it is mandatory that two

proxies with overlapping capabilities (frontend/backend) have different names.

However, it is still permitted that a frontend and a backend share the same

name, as this configuration seems to be commonly encountered.

Right now, two major proxy modes are supported : "tcp", also known as layer 4,

and "http", also known as layer 7. In layer 4 mode, HAProxy simply forwards

bidirectional traffic between two sides. In layer 7 mode, HAProxy analyzes the

protocol, and can interact with it by allowing, blocking, switching, adding,

modifying, or removing arbitrary contents in requests or responses, based on

arbitrary criteria.

目前，HAProxy支持两种主要的模式：“tcp”（4层）和“http”（7层）。在4层模式，HAProxy只是
简单的转发双向流量。在7层模式，HAProxy分析协议，并根据任意条件对请求和响应内容进行允许、阻断、
交换和增删改。

In HTTP mode, the processing applied to requests and responses flowing over

a connection depends in the combination of the frontend's HTTP options and

the backend's. HAProxy supports 5 connection modes :
在HTTP模式下，对请求和响应的处理由前端的HTTP option和后端配合决定。HAProxy支持五种
连接模式。

  - KAL : keep alive ("option http-keep-alive") which is the default mode : all

    requests and responses are processed, and connections remain open but idle

    between responses and new requests.

  - TUN: tunnel ("option http-tunnel") : this was the default mode for versions

    1.0 to 1.5-dev21 : only the first request and response are processed, and

    everything else is forwarded with no analysis at all. This mode should not

    be used as it creates lots of trouble with logging and HTTP processing.

  - PCL: passive close ("option httpclose") : exactly the same as tunnel mode,

    but with "Connection: close" appended in both directions to try to make

    both ends close after the first request/response exchange.

  - SCL: server close ("option http-server-close") : the server-facing

    connection is closed after the end of the response is received, but the

    client-facing connection remains open.

  - FCL: forced close ("option forceclose") : the connection is actively closed

    after the end of the response.

The effective mode that will be applied to a connection passing through a

frontend and a backend can be determined by both proxy modes according to the

following matrix, but in short, the modes are symmetric, keep-alive is the

weakest option and force close is the strongest.

                          Backend mode

                | KAL | TUN | PCL | SCL | FCL

            ----+-----+-----+-----+-----+----

            KAL | KAL | TUN | PCL | SCL | FCL

            ----+-----+-----+-----+-----+----

            TUN | TUN | TUN | PCL | SCL | FCL

 Frontend   ----+-----+-----+-----+-----+----

   mode     PCL | PCL | PCL | PCL | FCL | FCL

            ----+-----+-----+-----+-----+----

            SCL | SCL | SCL | FCL | SCL | FCL

            ----+-----+-----+-----+-----+----

            FCL | FCL | FCL | FCL | FCL | FCL

最后附上一段配置文件example供参考

# Simple configuration for an HTTP proxy listening on port 80 on all

    # interfaces and forwarding requests to a single backend "servers" with a

    # single server "server1" listening on 127.0.0.1:8000

    global

        daemon

        maxconn 256

    defaults

        mode http

        timeout connect 5000ms

        timeout client 50000ms

        timeout server 50000ms

    frontend http-in

        bind *:80

        default_backend servers

    backend servers

        server server1 127.0.0.1:8000 maxconn 32

    # The same configuration defined with a single listen block. Shorter but

    # less expressive, especially in HTTP mode.

    global

        daemon

        maxconn 256

    defaults

        mode http

        timeout connect 5000ms

        timeout client 50000ms

        timeout server 50000ms

    listen http-in

        bind *:80

        server server1 127.0.0.1:8000 maxconn 32