Linux Fu: Stupid SSH Tricks

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
  ONAME="$1-$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.

50 thoughts on “Linux Fu: Stupid SSH Tricks

    1. I do that as well a lot. But for some reason vncviewer fails to work when I allow it to use an existing master connection.

      Another “hack” I do is to use the socks proxy built into ssh to connect to web servers on my companies intranet from home. There is a Firefox plugin that allows to toggle the proxy settings with a single click.

      1. Oh, and by the way, master connections are very handy if bitbake fails because your ssh git server starts rate limiting connection attempts.
        What is missing is a way to create a master connection to a server without creating a shell session.

        1. -f Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background. This implies -n. The recommended way to start
          X11 programs at a remote site is with something like ssh -f host xterm.
          -N Do not execute a remote command. This is useful for just forwarding ports.

  1. Also worth mentioning: ssh chaining. If you need to ssh to a gateway host in order to ssh into another machine, you may be tempted to run:

    ssh external ssh internal

    This has the drawback of running the second ssh client on the remote machine, where your private keys, known_hosts and other nice things may not be available (also, the stream is decrypted on the intermediate machine, which may be a trust issue in some cases). You can tunnel the connection instead, like this (in .ssh/config):

    Host internal
    ProxyCommand ssh external nc internal 22

    (one-liner equivalent: ssh internal -o ProxyCommand=’ssh external nc internal 22′)

    Running ‘ssh internal’ on your client machine will execute ‘ssh external nc internal 22’ internally, instead of opening a TCP connection. This requires a netcat client on the remote machine. PuTTY has a way of doing this using the ssh-builtin port forwarding.

    The chain can be extended as long as common sense would allow.

    1. ssh -J external internal

      OpenSSH 7.3 was released on 2016-08-01.

      New Features

      * ssh(1): Add a ProxyJump option and corresponding -J command-line flag to allow simplified indirection through a one or more SSH bastions or “jump hosts”.

      You are welcome.

    1. Good point. I was following directions for installing modified firmwares to minimal linux hardware, it crawled it rather than ran it, and it’s only connection to the outside world is the USB hole I’ve got physical control of, and I’m thinking “Why the merry hell are we doing this with ssh????” because it took like half an hour to write half a megabyte, and I knew it was the crypto slowing it down.

      1. I would also be better to use /dev/zero (instead of /dev/urandom) because that uses even less cpu. Using `dd` (instead of `cat`) is another think worth trying because it allows the buffer size to be changed. If they’re too small they will also limit measured speed.

  2. You still need FUSE support compiled into the kernel for sshfs to function. Most binary distributions will have that already checked, but if you custom roll your own kernel, well, then, you need to double check.

  3. Here’s something I’ve found useful. If you want to give someone access to a service via an ssh tunnel, you can do that via a user that can’t log in.

    Say you have a service on port 6666 of your device fubar, which is only accessible locally [i.e. there’s no internet-facing open port other than ssh]

    You can access that by doing ssh -L 6666:127.0.0.1:6666 me@fubar
    Then localhost:6666 will give you access to port 6666 on fubar.

    If you want to give someone access to the service on port 6666 without giving them login access, create a no-login account like this:

    sudo useradd -mb /home -s /usr/sbin/nologin myservice

    You should see something similar to this in /etc/passwd:

    myservice:x:1002:1002::/home/myservice:/usr/sbin/nologin

    Add the public key of the user you want to access myservice to /home/myservice/.ssh/authorized_keys

    Now they will be able to access ssh -L 6666:127.0.0.1:6666 myservice@fubar without getting shell access.

    If you want to give someone else access, just add their public key to /home/myservice/.ssh/authorized_keys.

  4. Another way to do keep-alive, is to simply set up `screen` to paint a status bar down the bottom of your SSH session.

    In .screenrc (or /etc/screenrc) I have the following:

    caption always "%{yb} %D %Y-%02m-%02d %0c %H %{k}|%{g} %l %{k}|%{w} %-w%{+u}%n %t%{-u}%+w"
    defutf8 on
    bind ^a stuff ^a

    The first line is the key one for keep-alive purposes, it puts a banner down the bottom with the current date/time, the hostname and what screen windows are open (and which one is active).

    The second line turns on UTF-8 mode (a must with modern console applications, IS08859 is for the dinosaurs).

    The third line is useful if you have nested screen sessions, or using applications like minicom that need ^a to mean something, you can type ^a by pressing it twice.

    1. Totally not applicable to any other OS, not:
      Solaris,
      HP-UX,
      AIX,
      OpenVMS,
      FreeBSD,
      NetBSD,
      OpenBSD,
      HaikuOS,
      Gnu Hurd,
      SCO UNIX, a bit dead now, replaced by OpenServer (which was based on FreeBSD),

      Yep, only Linux and no other OS at all, such a shame.

      1. Psst, don‘t feed the troll. The article titled „Linux foo…“ even mentioned Windows alternatives 😉

        Also in your listing of UNIXoid OSses you forgot the most widely used desktop Unix system… macOS

        1. Old ?, Most have a new release every few years. And there is a lot of money in knowing how to raise Lazarus. It is a bit like old school DBA’s or grey beard COBOL programmers, smaller market, much bigger money.

      2. And no way would the above work on DragonFly BSD, just the Linux OS only.

        Anyhow no one uses DragonFly BSD with its filesystem that supports snapshots, checksumming and data deduplication – no one would need that.

      3. Yeah, errm… don’t think SCO OpenServer was FreeBSD based.

        I’ve used both, and I don’t recall ever needing to re-link FreeBSD’s kernel to change IP addresses, whereas SCO OpenServer 5 did require exactly that (followed by a reboot).

        SCO was a primitive OS. Nothing at all like FreeBSD.

  5. Like many of the books in the humble bundle, it’s so old it’s useless – from 2009. A bunch of stuff mentioned in this very article wouldn’t be in that book, because the features came in the last (cough) ten years.

    O’reilly has really slipped; many of their core books that they are most famous for are now hopelessly outdated. The DNS and bind book doesn’t even cover DNSSEC, and who the hell uses bind anymore?

    The humble bundle is possibly still worth it because a few of the books are more recent, but still….don’t get too excited, and check the dates of the editions included.

    1. Yeah, to be more specific, on Windows 10 you go:
      Start -> Settings -> Apps
      Click ‘Optional features’ under Apps & features
      Click ‘Add a feature’
      Scroll down to OpenSSH Server. OpenSSH Client is probably already installed for most people assuming you’re at a high enough build number.

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