[渗透测试] msf中的正向连接与反向连接

reverse_tcp和bind_tcp这两者的执行过程可以大致理解为以下内容：
[reverse_tcp]
attacker -> [contact me at the port 4444] -> victim

after the payload is executed
attacker <-> [port 4444] <-> victim

[bind_tcp]
attacker -> [open the way for me in the port 4444] -> victim

after execution
attacker <-> [port 4444] <-> victim

reverse_tcp：攻击机设置一个端口（LPORT）和IP（LHOST），Payload在测试机执行连接攻击机IP的端口，这时如果在攻击机监听该端口会发现测试机已经连接。
bind_tcp：攻击机设置一个端口（LPORT），Payload在测试机执行打开该端口，以便攻击机可以接入。

采用reverse的方法一般较为安全，因为是在测试机连接攻击机，所以一般不会被防火墙发现；而bind在测试机打开端口时很容易被安全软件和防火墙发现。

官方文档对bind_tcp和reverse_tcp的说明：

Reverse TCP
The reverse TCP handler is provided by the Msf::Handler::ReverseTcp class. It will listen on a port for incoming connections and will make a call into handle connection with the client sockets as they do.

Bind TCP
The bind TCP handler is provided through Msf::Handler::BindTcp. It will attempt to establish a connection to a target machine on a given port (specified in LPORT). If a connection is established, a call is made into handle connection passing along the socket associated with the connection.