Linux.redhat AS5 下telnet的配置和使用

This has to be one of the more common support calls that I get. The telnet daemon is no longer usually installed by default, so people are surprised when their newly installed Linux system won't answer telnets.

By the way, I am always careful to explain why telnet isn't there anymore; that they probably should be thinking about ssh. But sometimes there are perfectly reasonable reasons: it's only going to be used for a few internal users, there's no path from the outside world, or their firewall is going to tightly control access. I'd still rather see them use ssh, but if they understand its weaknesses and still want to go ahead, so be it.

The first task is to install telnetd. That's the Telnet Daemon that will respond to connection requests from telnet clients. The "telnet" client is installed by default; the telntd daemon is not.

If you have a system that is connected to the internet and that has a package manager like up2date, yum or apt-get, you can use that to load this. For example, on a RedHat system, you might do "up2date telnetd" and that would go fetch the appropriate rpm, download it and install it. You still need to get it running (see below) but it's now installed.

If you don't have that ability, you need to find the software on your CD's. I'll lead you through an example from a RedHat 9 install.

We have to start by putting one of the CD's into the machine. I happen to know that what we want is on CD 3 for RedHat 9, but we're going to pretend that I have forgotten that, so I'll first insert CD 2.

Your system may mount the CD automatically. Give it a second, and type "mount". If you see /dev/cdrom in one of the output lines, you can proceed to the next step. If not, you need to mount it manually. For RedHat 9, "mount -r /dev/cdrom /mnt" will work.

Now you need to find the rpm that has telnetd. This command will search the appropriate place:

ls /mnt/RedHat/RPMS | grep telnet

If you don't have RedHat 9, you are going to have to search around for the proper drectory yourself. You can "cd /mnt" and do "ls" and continue until you find what you want. Remember, you are only looking for rpm's if you are using an rpm based system. You need to know what your OS uses.

If you can't find anything that looks like telnet (and you won't on CD 2 of RedHat 9), you have to move on to the next CD. If you have cd'd into the mounted directory, you have to cd off that before you can unmount and eject the cd. If not, you'll get "device is busy" and won't be able to remove the cd. In our sample case, we'll do:

cd /
umount /dev/cdrom

I could have also done:

cd / 
eject cdrom

Now mount the next CD and repeat your search. On RedHat 9, the grep will turn up something like "telnet-server-0.17-20.i386.rpm". I don't remember the exact numbers from the cd - the important thing is that it's "telnet-server". However, on other systems, it may not be so obvious. For example, from a RedHat ES system:

# rpm  -q --whatprovides `which telnetd`
krb5-workstation-1.2.7-44

http://www.rpmfind.net/ can sometimes help you find what you need. Once found, it needs to be installed. For the RedHat 9:

rpm -iv /mnt/RedHat/RPMS/telnet-server*

Now telnetd is installed (and "which telnetd" will show you where if you are using Bash). It's not running yet, though. For RedHat 9, do:

chkconfig telnetd on

Confirm that telnet now works by "telnet localhost" or "telnet 127.0.0.1".

However, if you installed the RedHat firewall, telnet won't be accepting outside connections, not even from your LAN. For RedHat, you can correct that by running "setup", choosing "Firewall Configuration", select "Customize", and "Allow Incoming Telnet".

Other systems have other methods for enabling telnet. Most likely, they use Xinetd. For example, it might look like one of these examples from two different systems:

root# cat /etc/xinetd.d/telnet
service telnet
{
        disable         = yes
        socket_type     = stream
        wait            = no
        user            = root
        server          = /usr/libexec/telnetd
        groups          = yes
        flags           = REUSE
}

# cat /etc/xinetd.d/krb5-telnet
# default: off
# description: The kerberized telnet server accepts normal telnet sessions, /
#              but can also use Kerberos 5 authentication.
service telnet
{
        disable = no
        flags           = REUSE
        socket_type     = stream        
        wait            = no
        user            = root
        server          = /usr/kerberos/sbin/telnetd
        server_args     = -n
        log_on_failure  += USERID
}

That's it. The details will vary, but the basic outline is find it, install it, and get it running. You really do need to think about security - not just because it's telnet, but telnet does raise more flags than other protocols and may even invite more break in attempts if it's on the internet: the fact that something responds on port 23 may be seen as weakness and cause more attention than otherwise. There are numerous articles here about general security and ssh; I really suggest you at least brush through them.

---

 一、安装telnet软件包(通常要两个）
1、 telnet-client (或 telnet)，这个软件包提供的是 telnet 客户端程序；
2、 telnet-server ，这个软件包提供的是 telnet 服务器端程序；

安装之前先检测是否这些软件包已安装，方法如下：
[root@root]#rpm –qa telnet或
[root@root]#rpm –qa telnet-client
[root@root]#rpm –qa telnet-server

如果没有检测到软件包，需要进行安装，red hat 默认已安装了telnet软件包，一般只要安装telnet-server软件包。
1、在red hat linux 9的安装盘中获取telnet-server-0.17-25.i386.rpm软件包。
2、安装软件包
[root@root]#rpm –ivh /mnt/dvdrom/Server/telnet-server-0.17-38.el5.i386.rpm

------------可能遇到的问题：
尚未安装 xinetd:
浏览光盘位置
[root@localhost ~]# find /mnt/dvdrom/Server/xinetd*
/mnt/dvdrom/Server/xinetd-2.3.14-10.el5.i386.rpm

二、启动telnet服务
1、开启服务
方法一：使用ntsysv,在出现的窗口之中，将 telnet前面*加上，然后按下 OK 。
方法二：编辑 /etc/xinetd.d/telnet
[root@root]# vi /etc/xinetd.d/telnet
找到 disable = yes 将 yes 改成 no 。

2、激活服务
[root@root]# service xinetd restart

三、测试服务
[root@wljs root]#telnet ip(或者hostname）
如果配置正确，系统提示输入远程机器的用户名和密码
Login:
Password:
注：默认只允许普通用户

四、设置telnet端口
# vi /etc/services
进入编辑模式后查找telnet(vi编辑方式下输入/telnet)
会找到如下内容：
telnet 23/tcp
telnet 23/udp
将23修改成未使用的端口号(如：2000)，退出vi，重启telnet服务，telnet默认端口号就被修改了。

五、Telnet服务限制
telnet是明文传送口令和数据的，如果你对其默认的设置不满意，有必要对其服务范围进行限制。假设你的主机的ip是210.45.160.17，就可以按如下方式设置了，^_^!

#vi /etc/xinetd.d/telnet
service telnet
{
disable = no #激活 telnet 服务,no
bind = 210.45.160.17 #your ip
only_from = 210.45.0.0/16 #只允许 210.45.0.0 ~ 210.45.255.255 这个网段进入
only_from = .edu.cn #只有教育网才能进入！
no_access = 210.45.160.{115,116} #这两个ip不可登陆
access_times= 8:00-12:00 20:00-23:59 # 每天只有这两个时间段开放服务
......
}

六、Telnet root用户的登入
telnet 不是很安全，默认的情况之下不允许 root 以 telnet 进入 Linux 主机 。若要允许root用户登入，可用下列方法：
[root @root]# vi /etc/pam.d/login
#auth required pam_securetty.so #将这一行加上注释！
或
[root@wljs root]# mv /etc/securetty /etc/securetty.bak
这样， root 就可以直接进入 Linux 主机了。不过，建议不要这样做。也可以在普通用户进入后，切换到root用户,拥有root的权限。