If you connect to remote computers over the Internet, it is a pretty good chance you use some form of SSH or secure shell. On Linux or Unix you’ll use the ssh command. Same goes for Linux-like environments on Windows like Cygwin or WSL. For native Windows, you might be using Putty. In its simplest form, ssh is just a terminal program that talks to a server using an encrypted connection. We think it is very hard to eavesdrop on anyone communicating with a remote computer via ssh.
There are several tricks for using ssh — some are pretty straightforward and some are things you might not think of as being in the domain of a terminal program. You probably know that ssh can copy files securely, and there are easy and hard ways to set up logging in with no password.
However, you can also mount a remote filesystem via ssh (actually, there are several ways to do that). You can use ssh to securely browse the web in your favorite browser, or even use it to tunnel specific traffic by port or even use it as a makeshift VPN. In fact, there’s so much ground to cover that this won’t be the last Linux Fu to talk about ssh. But enough setup, let’s get to the tricks.
A Few Good Options
We’ll assume you know the basics: scp and sftp for file copies and ssh-copy-id for setting up password-less login. (If not, you have ten minutes to do a quick web search.) But one of the things ssh is great at is manipulating the network. Keep in mind, though, that the server has to have certain options set to do some of the most interesting things, so if you don’t control the ssh server, some of these tricks might not work for you.
There are a lot of options to remember on the ssh command line, but luckily you don’t have to. You don’t even have to remember your hostname or user name. In the ~/.ssh/config file you can create an alias. For example, suppose you want to connect to your home server:
Host homeserver HostName mih0me.dyndns.org Port 1234 User TheAl IdentityFile ~/.ssh/home_id ForwardX11 yes Compression yes TCPKeepAlive yes
You can have as many aliases as you like. Just keep repeating the Host line and then follow it with options. You can also add more than one alias to a single Host statement. The subsequent options will then apply to any of the aliases. Now to connect just issues ssh homeserver and you are in with all the right options.
Of course, if you are using Putty, your options will mainly be in the host profile and on the ssh panel of the options screen. You might not get as many options, but there are some you can try.
Be Persistent
One really nice set of options to include set the master control file. For example:
ControlMaster auto ControlPath ~/.ssh/master-%r@%h:%p
This lets multiple sessions to the same host share a single TCP socket. Because it takes a some time to set up a secure socket, this can be speed things up if you keep several sessions going between two hosts. You can set it for all hosts by using the Host * line in the config file. You can use that for any global options you might have.
On the other hand, if you shove a lot of data over multiple connections, turning on ControlMaster might not be a great idea. You can add -S none to override the global setting.
One other thing to note is that your first ssh session might appear to hang if you try to exit it before all the other connections are closed. Some people deliberately run a hidden ssh session on login to a host they often connect to which avoids that problem. However, a better way is to set ControlPersist to yes. That will cause the original session to go to the background indefinitely. If you want a little grace period you can set ControlPersist to a number like 180. That would cause the background session to end if there are no connections for three minutes.
Another downside to this approach is that you tend to get orphaned master files. In rc.local I have the following line:
/bin/rm /home/*/.ssh/master-* || true >/dev/null
With Putty, you can click the “Share SSH connections if possible” button in the SSH options panel.
Configuration
There are quite a few configuration options you can use in the config file. For example, BatchMode lets you tell ssh the connection is made for unattended use so don’t bother prompting the user for passwords or anything else. That’s not to say it will just let you in, of course. It just means it will fail if you don’t have things set up to login without a password.
There are some other interesting items. For example, you can run a local or remote command on connection. You can also send an environment variable to the remote host or even just set one. For example, suppose you want to always keep your LS_COLORS the same on your workstation and the server, but you frequently change them and don’t want to use the same profiles. You could add this to the host’s config file entry:
SendEnv LS_COLORS
The server has to be configured to accept this, of course. Putty can handle setting an environment variable from the Data tab in its setup.
On the network side, you can specify TCPKeepAlive to yes if you want the server and client to test their connection during idle periods. This is a two-edged sword. If the connection is idle, you won’t get disconnected. But if the network drops at the right moment for a brief period you might get disconnected where you wouldn’t have if you had stuck with the default. There’s even a way to open a layer 3 or layer 2 tunnel between the machines — a topic for a future Linux Fu.
By Your Command
Don’t forget that ssh can execute a command and send its output to you. As a practical example, I occasionally reflash my 3D printer firmware. The printer is connected to a Raspberry Pi, but I do the firmware build on my main machine. For a long time, I copied my file to the Pi (using scp) and then logged into the Pi to run a script I wrote called flash. The script disables the Reptier server software, flashes the Atmel chip on the printer control board, and then turns the server back on.
Here’s the script that runs on my main computer. Note the ssh commands. One turns off the server. One scp command copies the new firmware over. Then another ssh does the flash and renables the server. There are many other ways to do this, of course. But don’t forget that ssh can run a remote command and then return.
#!/bin/bash if [ -z "$1" ] then echo Usage: flash hexfile [remote_name] echo If omitted, remote_name will have date attached exit 1 fi ONAME="$2" EXTRA=$(date "+%Y%m%d.%H%M") if [ -z "$2" ] then $2="$1-$EXTRA.hex" fi IP=192.168.1.110 echo Stop Server ssh -l pi $IP "sudo systemctl stop RepetierServer" echo Copy... scp "$1" "pi@$IP:a8fw/$ONAME" echo Flashing... ssh pi@$IP "cd a8fw; ./flash $ONAME" echo Restart ssh "pi@$IP" "sudo systemctl start RepetierServer" echo Done exit 0
If you want a more hacker-friendly example, consider using the same idea to run Wireshark locally and analyze a remote packet capture:
ssh root@someserver 'tcpdump -c 1000 -nn -w - not port 22' | wireshark -k -i -
You can do the same trick with tshark, if you prefer.
Speed Test
What to know how fast your ssh connection is? Make sure you have pv installed and try this:
yes | pv | ssh remote_host "cat >/dev/null"
Pretty cool!
If you have good speeds — or even if you don’t — you can try mounting a remote file system using sshfs. This is a FUSE filesystem — that is, a filesystem that lives as a regular user program, not part of the kernel. With nothing on the remote side but the ssh server and standard tools, you can make any host you can connect to look like a local file system.
But Wait…
There’s a lot more you can do with ssh, and I’ll cover more shortly. But for now, hopefully you found at least one ssh trick you can use that was, if not new, at least a reminder for you.
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