strange routing / nat behavior

[Zhang Weiwu <zhangweiwu@realss.com>]

这个问题很奇怪的。我在兰州家中的电脑使用铁通拨号上网，modem后接了一台路由器（NAT防火墙）， 
路由器上接Linux主机。表现很奇怪。如果连接到北京的一台服务器上，用who命令显示自己IP地址 
为123.138.24.244；如果连接到香港一台服务器上，用who命令显示自己IP地址为123.81.20.79。

如下：

	zhangweiwu@zhangweiwu-laptop:~$ ssh zhangweiwu@www.linuxbanks.cn
	zhangweiwu@www.linuxbanks.cn's password:
	Linux casablanca 2.6.18.1-fl2f-v1.02 #41 Thu Jul 3 10:13:18 CST 2008
	mips64

	The programs included with the Debian GNU/Linux system are free
	software;
	the exact distribution terms for each program are described in the
	individual files in /usr/share/doc/*/copyright.

	Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
	permitted by applicable law.
	You have new mail.
	Last login: Wed Oct  7 22:03:36 2009 from 123.138.24.244
	zhangweiwu@casablanca:~$ date; who
	Wed Oct  7 22:09:06 CST 2009
	zhangweiwu tty1         2009-09-23 20:53
	zhangweiwu tty3         2009-09-24 08:49
	zhangweiwu pts/6        2009-10-07 22:09 (123.138.24.244)
	zhangweiwu@casablanca:~$ logout
	Connection to www.linuxbanks.cn closed.
	zhangweiwu@zhangweiwu-laptop:~$ ssh realss@123.108.108.80
	Enter passphrase for key '/home/zhangweiwu/.ssh/id_dsa':
	Linux www.german-company-directory.com 2.6.18-6-686 #1 SMP Sun Feb 10
	22:11:31 UTC 2008 i686

	The programs included with the Debian GNU/Linux system are free
	software;
	the exact distribution terms for each program are described in the
	individual files in /usr/share/doc/*/copyright.

	Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
	permitted by applicable law.
	No mail.
	Last login: Wed Oct  7 21:26:47 2009 from 123.116.123.210
	realss@www:~$ date; who
	Wed Oct  7 21:29:40 CST 2009
	realss   pts/0        2009-10-07 21:18 (123.116.123.210)
	realss   pts/1        2009-10-07 21:29 (123.81.20.79)
	realss@www:~$ logout
	Connection to www.german-company-directory.com closed.

这种情况，说明从兰州到北京经过了masquerade，到香港则没有。换言之如果在这个兰州 
家中电脑上运行一个网站，香港人可以看到，大陆人看不到。实际使用nc试着做个“网站”
也证实了这个猜测。

问题一：这种奇怪的情况，有谁之前发现过？我在北京家中电脑上无法重现这个问题， 
即如果北京家中电脑上运行一个网站，那么兰州和香港都能访问。

问题二：这种奇怪的设置有什么用意呢？我只能想到这样把国内的amule网络堵上了。