使用ssh方式连接linux系统时，发现一直上报这个错误，重启了sshd服务之后，只能登陆一次就无法登录：
# 出现问题的机器执行
[root@localhost ~]# service sshd restart
# 其他机器链接过去，只能连接一次就接着会报错
[root@localhost ~]# ssh root@172.17.0.84
The authenticity of host '172.17.0.84 (172.17.0.84)'can't be established.
RSA key fingerprint is f7:ce:40:fd:7c:d5:c8:64:2a:b9:bb:00:42:6b:25:9f.
Are you sure you want to continue connecting (yes/no)?
再次尝试连接一直报错：
[root@localhost ~] sshroot@172.17.0.84
Connection closed by 172.17.0.84 port 22
[root@localhost ~] sshroot@172.17.0.84
Connection closed by 172.17.0.84 port 22
[root@localhost ~] sshroot@172.17.0.84
Connection closed by 172.17.0.84 port 22
刚开始还以为是端口以及防火墙的问题呢，通过查看和关闭，并没有发现
[root@localhost ~]# nc -w 1 172.17.0.84 22 < /dev/null && echo "tcp port ok"
SSH-2.0-OpenSSH_5.3
tcp port ok
[root@localhost ~]# service iptables status
iptables: Firewall is not running.
什么错误，这就要详细的分析了。到底是哪儿出的问题呢？
根据思路来，先看log：从log可以看出，出错的原因很明显，就是加密文件权限有问题了，接下来就去查看和改动文件权限。
[root@localhost ~]# vim /var/log/messages
Jan 1511:24:53 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:57 localhost sshd[1740]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
Jan 1511:24:57 localhost sshd[1740]: error: Could not load host key: /etc/ssh/ssh_host_dsa_key
Jan 1511:24:58 localhost kernel: __ratelimit: 14450callbacks suppressed
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:24:58 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:01 localhost sshd[1744]: error: Could not load host key: /etc/ssh/ssh_host_rsa_key
Jan 1511:25:01 localhost sshd[1744]: error: Could not load host key: /etc/ssh/ssh_host_dsa_key
Jan 1511:25:03 localhost kernel: __ratelimit: 14573callbacks suppressed
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:03 localhost kernel: TCP: time wait bucket table overflow
Jan 1511:25:05 localhost kernel: atkbd.c: Unknown key pressed (translated set2, code 0x0 on isa0060/serio0).
Jan 1511:25:05 localhost kernel: atkbd.c: Use 'setkeycodes 00 <keycode>'to makeit known.
Jan 1511:25:05 localhost kernel: atkbd.c: Unknown key released (translated set2, code 0x0 on isa0060/serio0).
Jan 1511:25:05 localhost kernel: atkbd.c: Use 'setkeycodes 00 <keycode>'to makeit known.
Jan 1511:25:08 localhost kernel: __ratelimit: 14688callbacks suppressed
在/etc/ssh目录下查看，的确是文件的问题，不知道什么原因，文件秘钥文件被删除了，按照别的系统的文件拷贝过来即可：
[root@localhost ssh]# ls -Zd *
-rw-------. root root system_u:object_r:etc_t:s0 moduli
-rw-r--r--. root root system_u:object_r:etc_t:s0 ssh_config
-rw-------root root ? sshd_config
-rw-------root root ? ssh_host_dsa_key
-rw-r--r--root root ? ssh_host_dsa_key.pub
-rw-------root root ? ssh_host_key
-rw-r--r--root root ? ssh_host_key.pub
-rw-------root root ? ssh_host_rsa_key
-rw-r--r--root root ? ssh_host_rsa_key.pub
总结：
默认情况下密钥对都在/etc/ssh/目录下，包括不同算法的公钥，私钥
ssh 按以下顺序从以下源获取配置数据：
1、命令行选项
2、用户的配置文件（〜/.ssh/config）
3、系统范围的配置文件（/etc/ssh/ssh_config）
