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[Translated] tech/20150504 How to access a Linux server behind NAT via reverse SSH tunnel.md
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ictlyh Translating
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How to access a Linux server behind NAT via reverse SSH tunnel
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================================================================================
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You are running a Linux server at home, which is behind a NAT router or restrictive firewall. Now you want to SSH to the home server while you are away from home. How would you set that up? SSH port forwarding will certainly be an option. However, port forwarding can become tricky if you are dealing with multiple nested NAT environment. Besides, it can be interfered with under various ISP-specific conditions, such as restrictive ISP firewalls which block forwarded ports, or carrier-grade NAT which shares IPv4 addresses among users.
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### What is Reverse SSH Tunneling? ###
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One alternative to SSH port forwarding is **reverse SSH tunneling**. The concept of reverse SSH tunneling is simple. For this, you will need another host (so-called "relay host") outside your restrictive home network, which you can connect to via SSH from where you are. You could set up a relay host using a [VPS instance][1] with a public IP address. What you do then is to set up a persistent SSH tunnel from the server in your home network to the public relay host. With that, you can connect "back" to the home server from the relay host (which is why it's called a "reverse" tunnel). As long as the relay host is reachable to you, you can connect to your home server wherever you are, or however restrictive your NAT or firewall is in your home network.
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### Set up a Reverse SSH Tunnel on Linux ###
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Let's see how we can create and use a reverse SSH tunnel. We assume the following. We will be setting up a reverse SSH tunnel from homeserver to relayserver, so that we can SSH to homeserver via relayserver from another computer called clientcomputer. The public IP address of **relayserver** is 1.1.1.1.
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On homeserver, open an SSH connection to relayserver as follows.
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homeserver~$ ssh -fN -R 10022:localhost:22 relayserver_user@1.1.1.1
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Here the port 10022 is any arbitrary port number you can choose. Just make sure that this port is not used by other programs on relayserver.
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The "-R 10022:localhost:22" option defines a reverse tunnel. It forwards traffic on port 10022 of relayserver to port 22 of homeserver.
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With "-fN" option, SSH will go right into the background once you successfully authenticate with an SSH server. This option is useful when you do not want to execute any command on a remote SSH server, and just want to forward ports, like in our case.
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After running the above command, you will be right back to the command prompt of homeserver.
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Log in to relayserver, and verify that 127.0.0.1:10022 is bound to sshd. If so, that means a reverse tunnel is set up correctly.
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relayserver~$ sudo netstat -nap | grep 10022
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----------
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tcp 0 0 127.0.0.1:10022 0.0.0.0:* LISTEN 8493/sshd
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Now from any other computer (e.g., clientcomputer), log in to relayserver. Then access homeserver as follows.
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relayserver~$ ssh -p 10022 homeserver_user@localhost
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One thing to take note is that the SSH login/password you type for localhost should be for homeserver, not for relayserver, since you are logging in to homeserver via the tunnel's local endpoint. So do not type login/password for relayserver. After successful login, you will be on homeserver.
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### Connect Directly to a NATed Server via a Reverse SSH Tunnel ###
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While the above method allows you to reach **homeserver** behind NAT, you need to log in twice: first to **relayserver**, and then to **homeserver**. This is because the end point of an SSH tunnel on relayserver is binding to loopback address (127.0.0.1).
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But in fact, there is a way to reach NATed homeserver directly with a single login to relayserver. For this, you will need to let sshd on relayserver forward a port not only from loopback address, but also from an external host. This is achieved by specifying **GatewayPorts** option in sshd running on relayserver.
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Open /etc/ssh/sshd_conf of **relayserver** and add the following line.
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relayserver~$ vi /etc/ssh/sshd_conf
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----------
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GatewayPorts clientspecified
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Restart sshd.
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Debian-based system:
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relayserver~$ sudo /etc/init.d/ssh restart
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Red Hat-based system:
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relayserver~$ sudo systemctl restart sshd
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Now let's initiate a reverse SSH tunnel from homeserver as follows.
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homeserver~$ ssh -fN -R 1.1.1.1:10022:localhost:22 relayserver_user@1.1.1.1
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Log in to relayserver and confirm with netstat command that a reverse SSH tunnel is established successfully.
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relayserver~$ sudo netstat -nap | grep 10022
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----------
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tcp 0 0 1.1.1.1:10022 0.0.0.0:* LISTEN 1538/sshd: dev
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Unlike a previous case, the end point of a tunnel is now at 1.1.1.1:10022 (relayserver's public IP address), not 127.0.0.1:10022. This means that the end point of the tunnel is reachable from an external host.
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Now from any other computer (e.g., clientcomputer), type the following command to gain access to NATed homeserver.
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clientcomputer~$ ssh -p 10022 homeserver_user@1.1.1.1
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In the above command, while 1.1.1.1 is the public IP address of relayserver, homeserver_user must be the user account associated with homeserver. This is because the real host you are logging in to is homeserver, not relayserver. The latter simply relays your SSH traffic to homeserver.
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### Set up a Persistent Reverse SSH Tunnel on Linux ###
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Now that you understand how to create a reverse SSH tunnel, let's make the tunnel "persistent", so that the tunnel is up and running all the time (regardless of temporary network congestion, SSH timeout, relay host rebooting, etc.). After all, if the tunnel is not always up, you won't be able to connect to your home server reliably.
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For a persistent tunnel, I am going to use a tool called autossh. As the name implies, this program allows you to automatically restart an SSH session should it breaks for any reason. So it is useful to keep a reverse SSH tunnel active.
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As the first step, let's set up [passwordless SSH login][2] from homeserver to relayserver. That way, autossh can restart a broken reverse SSH tunnel without user's involvement.
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Next, [install autossh][3] on homeserver where a tunnel is initiated.
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From homeserver, run autossh with the following arguments to create a persistent SSH tunnel destined to relayserver.
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homeserver~$ autossh -M 10900 -fN -o "PubkeyAuthentication=yes" -o "StrictHostKeyChecking=false" -o "PasswordAuthentication=no" -o "ServerAliveInterval 60" -o "ServerAliveCountMax 3" -R 1.1.1.1:10022:localhost:22 relayserver_user@1.1.1.1
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The "-M 10900" option specifies a monitoring port on relayserver which will be used to exchange test data to monitor an SSH session. This port should not be used by any program on relayserver.
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The "-fN" option is passed to ssh command, which will let the SSH tunnel run in the background.
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The "-o XXXX" options tell ssh to:
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- Use key authentication, not password authentication.
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- Automatically accept (unknown) SSH host keys.
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- Exchange keep-alive messages every 60 seconds.
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- Send up to 3 keep-alive messages without receiving any response back.
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The rest of reverse SSH tunneling related options remain the same as before.
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If you want an SSH tunnel to be automatically up upon boot, you can add the above autossh command in /etc/rc.local.
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### Conclusion ###
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In this post, I talked about how you can use a reverse SSH tunnel to access a Linux server behind a restrictive firewall or NAT gateway from outside world. While I demonstrated its use case for a home network, you must be careful when applying it for corporate networks. Such a tunnel can be considered as a breach of a corporate policy, as it circumvents corporate firewalls and can expose corporate networks to outside attacks. There is a great chance it can be misused or abused. So always remember its implication before setting it up.
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/access-linux-server-behind-nat-reverse-ssh-tunnel.html
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作者：[Dan Nanni][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/nanni
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[1]:http://xmodulo.com/go/digitalocean
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[2]:http://xmodulo.com/how-to-enable-ssh-login-without.html
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[3]:http://ask.xmodulo.com/install-autossh-linux.html
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如何通过反向 SSH 隧道访问 NAT 后面的 Linux 服务器
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================================================================================
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你在家里运行着一台 Linux 服务器，访问它需要先经过 NAT 路由器或者限制性防火墙。现在你想不在家的时候用 SSH 登录到这台服务器。你如何才能做到呢？SSH 端口转发当然是一种选择。但是，如果你需要处理多个嵌套的 NAT 环境，端口转发可能会变得非常棘手。另外，在多种 ISP 特定条件下可能会受到干扰，例如阻塞转发端口的限制性 ISP 防火墙、或者在用户间共享 IPv4 地址的运营商级 NAT。
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### 什么是反向 SSH 隧道？ ###
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SSH 端口转发的一种替代方案是 **反向 SSH 隧道**。反向 SSH 隧道的概念非常简单。对于此，在限制性家庭网络之外你需要另一台主机（所谓的“中继主机”），你能从当前所在地通过 SSH 登录。你可以用有公共 IP 地址的 [VPS 实例][1] 配置一个中继主机。然后要做的就是从你家庭网络服务器中建立一个到公共中继主机的永久 SSH 隧道。有了这个隧道，你就可以从中继主机中连接“回”家庭服务器（这就是为什么称之为 “反向” 隧道）。不管你在哪里、你家庭网络中的 NAT 或 防火墙限制多么严重，只要你可以访问中继主机，你就可以连接到家庭服务器。
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### 在 Linux 上设置反向 SSH 隧道 ###
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让我们来看看怎样创建和使用反向 SSH 隧道。我们有如下假设。我们会设置一个从家庭服务器到中继服务器的反向 SSH 隧道，然后我们可以通过中继服务器从客户端计算机 SSH 登录到家庭服务器。**中继服务器** 的公共 IP 地址是 1.1.1.1。
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在家庭主机上，按照以下方式打开一个到中继服务器的 SSH 连接。
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homeserver~$ ssh -fN -R 10022:localhost:22 relayserver_user@1.1.1.1
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这里端口 10022 是任何你可以使用的端口数字。只需要确保中继服务器上不会有其它程序使用这个端口。
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“-R 10022:localhost:22” 选项定义了一个反向隧道。它转发中继服务器 10022 端口的流量到家庭服务器的 22 号端口。
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用 “-fN” 选项，当你用一个 SSH 服务器成功通过验证时 SSH 会进入后台运行。当你不想在远程 SSH 服务器执行任何命令、就像我们的例子中只想转发端口的时候非常有用。
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运行上面的命令之后，你就会回到家庭主机的命令行提示框中。
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登录到中继服务器，确认 127.0.0.1:10022 绑定到了 sshd。如果是的话就表示已经正确设置了反向隧道。
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relayserver~$ sudo netstat -nap | grep 10022
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tcp 0 0 127.0.0.1:10022 0.0.0.0:* LISTEN 8493/sshd
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现在就可以从任何其它计算机（客户端计算机）登录到中继服务器，然后按照下面的方法访问家庭服务器。
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relayserver~$ ssh -p 10022 homeserver_user@localhost
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需要注意的一点是你在本地输入的 SSH 登录/密码应该是家庭服务器的，而不是中继服务器的，因为你是通过隧道的本地端点登录到家庭服务器。因此不要输入中继服务器的登录/密码。成功登陆后，你就在家庭服务器上了。
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### 通过反向 SSH 隧道直接连接到网络地址变换后的服务器 ###
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上面的方法允许你访问 NAT 后面的 **家庭服务器**，但你需要登录两次：首先登录到 **中继服务器**，然后再登录到**家庭服务器**。这是因为中继服务器上 SSH 隧道的端点绑定到了回环地址（127.0.0.1）。
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事实上，有一种方法可以只需要登录到中继服务器就能直接访问网络地址变换之后的家庭服务器。要做到这点，你需要让中继服务器上的 sshd 不仅转发回环地址上的端口，还要转发外部主机的端口。这通过指定中继服务器上运行的 sshd 的 **网关端口** 实现。
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打开**中继服务器**的 /etc/ssh/sshd_conf 并添加下面的行。
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relayserver~$ vi /etc/ssh/sshd_conf
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----------
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GatewayPorts clientspecified
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重启 sshd。
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基于 Debian 的系统：
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relayserver~$ sudo /etc/init.d/ssh restart
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基于红帽的系统：
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relayserver~$ sudo systemctl restart sshd
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现在在家庭服务器中按照下面方式初始化一个反向 SSH 隧道。
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homeserver~$ ssh -fN -R 1.1.1.1:10022:localhost:22 relayserver_user@1.1.1.1
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登录到中继服务器然后用 netstat 命令确认成功建立的一个反向 SSH 隧道。
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relayserver~$ sudo netstat -nap | grep 10022
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tcp 0 0 1.1.1.1:10022 0.0.0.0:* LISTEN 1538/sshd: dev
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不像之前的情况，现在隧道的端点是 1.1.1.1:10022（中继服务器的公共 IP 地址），而不是 127.0.0.1:10022。这就意味着从外部主机可以访问隧道的端点。
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现在在任何其它计算机（客户端计算机），输入以下命令访问网络地址变换之后的家庭服务器。
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clientcomputer~$ ssh -p 10022 homeserver_user@1.1.1.1
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在上面的命令中，1.1.1.1 是中继服务器的公共 IP 地址，家庭服务器用户必须是和家庭服务器相关联的用户账户。这是因为你真正登录到的主机是家庭服务器，而不是中继服务器。后者只是中继你的 SSH 流量到家庭服务器。
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### 在 Linux 上设置一个永久反向 SSH 隧道 ###
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现在你已经明白了怎样创建一个反向 SSH 隧道，然后把隧道设置为 “永久”，这样隧道启动后就会一直运行（不管临时的网络拥塞、SSH 超时、中继主机重启，等等）。毕竟，如果隧道不是一直有效，你不可能可靠的登录到你的家庭服务器。
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对于永久隧道，我打算使用一个叫 autossh 的工具。正如名字暗示的，这个程序允许你不管任何理由自动重启 SSH 会话。因此对于保存一个反向 SSH 隧道有效非常有用。
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第一步，我们要设置从家庭服务器到中继服务器的[无密码 SSH 登录][2]。这样的话，autossh 可以不需要用户干预就能重启一个损坏的反向 SSH 隧道。
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下一步，在初始化隧道的家庭服务器上[安装 autossh][3]。
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在家庭服务器上，用下面的参数运行 autossh 来创建一个连接到中继服务器的永久 SSH 隧道。
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homeserver~$ autossh -M 10900 -fN -o "PubkeyAuthentication=yes" -o "StrictHostKeyChecking=false" -o "PasswordAuthentication=no" -o "ServerAliveInterval 60" -o "ServerAliveCountMax 3" -R 1.1.1.1:10022:localhost:22 relayserver_user@1.1.1.1
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“-M 10900” 选项指定中继服务器上的监视端口，用于交换监视 SSH 会话的测试数据。中继服务器上的其它程序不能使用这个端口。
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“-fN” 选项传递给 ssh 命令，让 SSH 隧道在后台运行。
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“-o XXXX” 选项让 ssh：
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- 使用密钥验证，而不是密码验证。
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- 自动接受（未知）SSH 主机密钥。
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- 每 60 秒交换 keep-alive 消息。
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- 没有收到任何响应时最多发送 3 条 keep-alive 消息。
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其余 SSH 隧道相关的选项和之前介绍的一样。
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如果你想系统启动时自动运行 SSH 隧道，你可以将上面的 autossh 命令添加到 /etc/rc.local。
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### 总结 ###
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在这篇博文中，我介绍了你如何能从外部中通过反向 SSH 隧道访问限制性防火墙或 NAT 网关之后的 Linux 服务器。尽管我介绍了家庭网络中的一个使用事例，在企业网络中使用时你尤其要小心。这样的一个隧道可能被视为违反公司政策，因为它绕过了企业的防火墙并把企业网络暴露给外部攻击。这很可能被误用或者滥用。因此在使用之前一定要记住它的作用。
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--------------------------------------------------------------------------------
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via: http://xmodulo.com/access-linux-server-behind-nat-reverse-ssh-tunnel.html
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作者：[Dan Nanni][a]
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译者：[ictlyh](https://github.com/ictlyh)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://xmodulo.com/author/nanni
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[1]:http://xmodulo.com/go/digitalocean
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[2]:http://xmodulo.com/how-to-enable-ssh-login-without.html
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[3]:http://ask.xmodulo.com/install-autossh-linux.html
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