1. 安装Ubuntu之后，使用sudo apt-get install ssh提示：
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package ssh is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
openssh-client ssh-askpass-gnome
E: Package 'ssh' has no installation candidate
经过网上查询找到了原因及解决方法：
这个问题的原因是ubuntu的/etc/apt/source.list中的源比较旧了，需要更新一下，更新方法：
$ sudo apt-get -y update
更新完毕之后，在使用apt-get就没有问题了。
2.使用ssh localhost时，出现如下提示：
The authenticity of host 'localhost (127.0.0.1)' can't be established.
ECDSA key fingerprint is 8e:8c:a7:ff:a4:1a:38:a0:a9:53:fa:9a:48:7a:9d:7c.
Are you sure you want to continue connecting (yes/no)?
最后的解决方法：
ssh -o StrictHostKeyChecking=no localhost
3.执行start-dfs时提示：
The authenticity of host '0.0.0.0 (0.0.0.0)' can't be established.
解决方法：
关闭防火墙：sudo ufw disable
4.Hadoop在初次执行mapreduce任务时卡死
解决方法：Hadoop在开启时默认是安全模式，所以使用以下命令关闭安全模式：
bin/hadoop dfsadmin -safemode leave
