前一段时间装了一个RHEL。但是发现不管是telnet 还是ssh都登陆很慢，这让我很郁闷，在网上找了很久，终于找到了，将文件/etc/resolv.conf里面的DNS服务注释了就好了。这也可能是在虚拟机上没有DNS的原因吧。

[root@leshy ~]# cat /etc/resolv.conf
#nameserver 200.1.1.1
[root@leshy ~]#

网上高人的回复原因是：

Its your /etc/resolv.conf
i bet if u go to the comp in a web browser its instant, yeah? why? because your PC doesnt do a reverse DNS lookup on ur IP then... when u telnet/ftp into ur linux box, it'll do a dns lookup on you, to see who you are, if u either correct the settings in the /etc/resolve.conf file, or just back it up, then delete the contents all together, it should be fine !

我猜原因就是当有登陆就去DNS去找相应信息，直至超时返回。将DNS信息删了，就可以正常。