一台机器上的 ssh 跳转代理命令失败，但其他机器上却可以运行

Question

~/.ssh/config我的本地机器上有以下文件：

Host head
  User Marry
  HostName xxx.xx.xxx.xx

Host machine1
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine2
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine3
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

Host machine4
  User Marry
  HostName xxx.xxx.xxx.xx
  IdentityFile /home/Marry/ssh_head_keys/id_rsa
  ProxyCommand  ssh head nc %h %p 2> /dev/null

我可以成功执行ssh machine1、、ssh machine2我ssh machine3的本地机器，并转到机器 1-3，但对于机器 4，命令ssh machine4失败：

ssh_exchange_identification: Connection closed by remote host

怎么会这样呢？

更新：按照muru的建议，删除2> /dev/null，当我ssh到machine4时，我收到了消息：

bash: nc: command not found
ssh_exchange_identification: Connection closed by remote host