1 基于busybox的telnetd

  1.    Busybox集成了telnetd功能，有两种选择：第一，使用inetd管理telnetd的启动，当有telnet连接时由inetd启动；第二，直接打开telnetd守护进程，监听telnet连接。本文选择的是直接打开telnetd守护进程，那么在编译busybox时必须做如下选择。
[] telnetd
[] Support standalone telnetd (not inetd only)

2 内核对telnetd的支持

   telnet是一种非常老的远程登录技术，linux内核配置时默认并不支持。要想内核支持telnet，必须选择两项功能。<br />       （1）Unix 98 PTY；<br />（2）dev/pts file system for Unix98 PTYs<br />       根据对linux2.6.20内核配置的经验，只要选择了第一项功能，那么第二项功能就会自动被选中。因此在内核配置项中，找不到第二项。
Device Drivers —->
Character devices —->
[*] Unix98 PTY support		 
   在系统运行是，通过cat /proc/filesystems命令可以看到devpts文件系统是否被支持。

3 根文件系统相关修改

3.1 挂载devpts文件系统

   首先应该确保在根文件系统目录下存在/dev/pts目录；其次，在/etc/fstab文件中添加如下挂载语句。
devpts /dev/pts devpts gid=5,mode=620 0 0 
   这样在系统启动后，会根据fstab文件挂载devpts文件系统的目录。<br />       或者，可以选择系统启动后手动挂载。
mount -t devpts devpts /dev/pts

3.2 添加/dev下ptmx文件

   首先看这样一段话，说明了ptmx文件的作用。
A pseudo terminal (PTY) is a software device consisting of two
halves: a master and a slave. The slave device behaves identical to
a physical terminal; the master device is used by a process to
read data from and write data to the slave, thereby emulating a
terminal. Typical programs for the master side are telnet servers
and xterms.
Linux has traditionally used the BSD-like names /dev/ptyxx for
masters and /dev/ttyxx for slaves of pseudo terminals. This scheme
has a number of problems. The GNU C library glibc 2.1 and later,
however, supports the Unix98 naming standard: in order to acquire a
pseudo terminal, a process opens /dev/ptmx; the number of the pseudo
terminal is then made available to the process and the pseudo
terminal slave can be accessed as /dev/pts/. What was
traditionally /dev/ttyp2 will then be /dev/pts/2, for example.		 
   其次，应该在/dev下建立ptmx设备节点。
mknod -m 666 ptmx c 5 2

4 运行telnetd

/sbin/telnetd -l /sbin/vt100