原因：
在执行scp id_rsa.pub root@hostname:/root/.ssh这一步时，没在本机的/root/.ssh下生成known_hosts文件。

解决方案：
vi /etc/ssh/ssh_config
最后有两行是
         StrictHostKeyChecking no 
         UserKnownHostsFile /dev/null
把这两行注释掉并保存文件
再次执行scp id_rsa.pub root@host141:/root/.ssh
出现提示
The authenticity of host 'host141 (172.20.0.141)' can't be established.
RSA key fingerprint is a0:09:89:d2:3b:aa:91:60:59:7f:7f:bc:03:31:b0:09.
Are you sure you want to continue connecting (yes/no)? 
输入yes后，则可看到在/root/.ssh目录下生成了known_hosts文件：
[root@host100 .ssh]# ls
id_rsa  id_rsa.pub  known_hosts