Ephemeral port

一、"ephemeral port" 即 "临时端口"

二、ephemeral port range是一种system resource

三、可以认为: Linux中，每个socket都有一个对应的port:

server会bind到一个指定的port

client会被分配一个临时的port

四、参考文章:

1、wanweibaike Ephemeral port

2、idea.popcount Bind before connect

3、linuxperf 临时端口号(EPHEMERAL PORT)的动态分配

这篇文章，描述了kernel如何分配 ephemeral port

wanweibaike Ephemeral port

An ephemeral port is a short-lived transport protocol port for Internet Protocol (IP) communications. Ephemeral ports are allocated automatically from a predefined range by the IP stack software. An ephemeral port is typically used by the Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or the Stream Control Transmission Protocol (SCTP) as the port assignment for the client end of a client–server communication to a particular port (usually a well-known port) on a server.

On servers, ephemeral ports may also be used as the port assignment on the server end of a communication. This is done to continue communications with a client that initially connected to one of the server's well-known service listening ports. File Transfer Protocol (FTP) and Remote Procedure Call (RPC) applications are two protocols that can behave in this manner. Note that the term "server" here includes workstations running network services that receive connections initiated from other clients (e.g. Remote Desktop Protocol).

The allocations are temporary and only valid for the duration of the communication session. After completion (or timeout) of the communication session, the ports become available for reuse.[note 1] Since the ports are used on a per request basis they are also called dynamic ports.

csdn linux临时端口（port）以及最大值

在一个Linux上跑一个服务，需要指定一个监听端口（系统调用listen），当client请求服务时，系统为该请求建立 tcp连接，该连接的port值是一个随机数字。该数字的取值范围配置在/proc/sys/net/ipv4/ip_local_port_range 文件中，我的机器上的内容如下：

32768   61000

port的最大值其实可以达到65535（2^16 - 1)。

65535这个数字与操作系统无关，不管Linux系统是32位的还是64位的。

这个数字是由网络tcp协议决定的，tcp协议头部中的16位表示端口号，这就决定了其最大值65535，操作系统只能跟着这个限制。