In the right (or wrong) hands ssh is a powerful tool for the remote management of a Unix system. Most desktop, or workstation distributions of Linux disable remote access over ssh by default. The simplest method to check if you have ssh server running on your machine is to run
$ ssh localhost
If ssh is not installed or running this will print out a message
ssh: connect to host localhost port 22: Connection refused
most likely indicating that the ssh server is not running.
Before heading any further in enabling remote access over ssh, it is a good idea to ensure that you have a strong password for all of the accounts on the device, in particular the root account (even better is disabling it). By enabling remote access for yourself you are also enabling remote access for anyone else that might want to try and log in.
The package required for installation is in most distributions named
So for Ubuntu running the command;
$ sudo apt install openssh-server
will install the package as appropriate.
Other Linux distributions have different package managers so
use the one for your distribution.
So for fedora, replace
dnf and for CentOS replace
On macOS everything is already installed and just needs to be enabled
in System Preferences > Sharing then ensure Remote Login is checked.
Once the package is installed, it needs to be both added to the list of packages to run at boot, and started now to test.
To start the openssh server, run the below command
$ sudo systemctl start ssh
which is using the
systemd init system to start the openssh server instance.
For fedora and CentOS, the ssh service instead has the name sshd so run
$ sudo systemctl start sshd
to start the server instance. Since this is probably a service you want running automatically on boot, running the command
$ sudo systemctl enable ssh
will put files in the appropriate places to enable the service on boot.
Other useful commands for
Once the ssh server is running our command to connect at localhost
$ ssh localhost
should present a new message like the one below.
The authenticity of host 'localhost (::1)' can't be established. ECDSA key fingerprint is ba:ba:b1:4c:5a:e2:aa:40:aa:7f:aa:01:aa:df:aa:47. Are you sure you want to continue connecting (yes/no)?
In some ways this is like the terms and conditions when you sign up for a website
in that you should care about and understand it,
however, typically you click through (or type yes) without reading.
Since we are connecting to localhost,
which has an IPV6 address of
or IPV4 address of
we can trust the key fingerprint.
This is a method of checking that whenever we log into a host over ssh,
the host hasn’t changed which if unexpected
could be indicative of a man-in-the-middle attack or other nefarious actions.
Now we are able to connect to the machine locally, we need to work out how to connect from a remote machine.
How do I Find My Device?
With the previous commands we have been passing
localhost to the ssh command,
in the same way we might type
google.com into a web browser.
These names are a human readable format for addressing devices,
which map to IPV6 or IPV4 addresses computers read to initiate a connection.
To find the name of the computer we are working on,
also known as the hostname,
we can use the
$ hostname -f lovelace.staff.sydney.edu.au
-f gives the full hostname of the device you are using,
-s flag also supported,
however we need to full hostname.
With the full hostname,
we know the name of our device,
and we know that our device is aware of it’s hostname.
This doesn’t necessarily mean that any other devices know what ours is called.
The process of taking a hostname and turning it into an address
the computer can use is known as DNS,
which is provided by name servers.
Typically in organisations there are multiple levels of name servers,
which will handle both internal names,
like the hostname of our device,
and external names
Where a device is querying an external name server,
it will not be able to resolve local names.
To test this we can use the name server lookup
$ nslookup lovelace.staff.sydney.edu.au Server: 172.18.240.210 Address: 172.18.240.210#53 Name: lovelace.staff.sydney.edu.au Address: 10.65.205.200
This tells us that the server at the ip address
which my laptop is using as the default name server,
lovelace.staff.sydney.edu.au exists at the address
If we instead used one of the large public DNS servers
provided by Google (
188.8.131.52) or CloudFlare (
we will instead get an error message.
$ nslookup lovelace.staff.sydney.edu.au 184.108.40.206 Server: 220.127.116.11 Address: 18.104.22.168#53 ** server can't find lovelace.staff.sydney.edu.au: SERVFAIL
Setting the hostname
When going through the initial setup of macOS or linux machines,
there is a step where you give the machine the hostname.
To later change the hostname requires the editing a couple of files.
/etc/hostname stores the hostname of the machine,
which will typically be the Fully Qualified Domain Name (FQDN) of the host.
To actually change the hostname a reboot is required,
as this file is read during the boot sequence.
Before you do reboot,
there is another file
/etc/hosts that requires editing.
/etc/hosts file is the fist place that is checked
when looking to resolve a domain name
and is sometimes used as a way to block websites
or just to resolve hostname.
We want to our domain name to resolve to
so that services like web servers don’t get confused.
Ensure the following lines (with the appropriate hostname) are in the
127.0.0.1 lovelace lovelace.malramsay.com ::1 lovelace lovelace.malramsay.com
This sets the ip address we want the hostname to resolve to,
which on the first line is
followed by a list of hostnames,
being the short version
lovelace and the FQDN
What do I do if I can’t lookup my hostname?
The reason we use domain names and hostnames is that they are far easier to remember than a string of numbers. It is possible to navigate to Google by navigating to the ip address 22.214.171.124 which is the address returned for google.com.
$ nslookup google.com Server: 172.18.240.210 Address: 172.18.240.210#53 Non-authoritative answer: Name: google.com Address: 126.96.36.199
While using the IP address is possible, it is far more cumbersome to remember and to type. For a single IP address which we can put in a configuration file, this isn’t too problematic.
To find the ip address of the current host
run the command
which will give output like that below.
$ ip addr lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 inet 127.0.0.1/8 lo0 inet6 ::1/128 inet6 fe80::1/64 scopeid 0x1 en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 ether 78:4f:43:4d:cc:53 inet6 fe80::18c5:93d4:e972:b798/64 secured scopeid 0x7 > inet 10.16.249.156/21 brd 10.16.255.255 en0 awdl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1484 ether b6:54:4e:bd:a7:d7 inet6 fe80::b454:4eff:febd:a7d7/64 scopeid 0xa utun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 2000 inet6 fe80::1c95:35f4:acea:1034/64 scopeid 0x10 en5: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 ether ac:df:48:00:11:22 inet6 fe80::aede:48ff:fe00:1122/64 scopeid 0x9 en7: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500 ether 00:0e:c6:e2:bb:69 inet6 fe80::1871:5dc7:1f58:2127/64 secured scopeid 0x12 > inet 10.65.205.200/24 brd 10.65.205.255 en7
The lines we are looking for are those with
inet at the start
which I have denoted with a
This gives me two IP addresses,
one for the wifi and another for the wired ethernet connection.
You may have noticed that the
10.65.205.200 address matched that obtained
nslookup of my hostname above.
This tells me that the name server is directing
queries of my hostname back to my device.
Now we have an ip address, how do we make the computer remember it when the name server doesn’t.
Remembering IP addresses
The DNS lookup of IP addresses from hostnames works on the institutional network I have access to at the University of Sydney. You will likely not have this same configuration, so there are some alternate methods for getting your computer to remember ip addresses.
SSH configuration file
The simplest method of getting the computer
to remember an ip address for you is the ssh config file
This approach only works when connecting using ssh,
however, it is useful for setting all the parameters for an ssh connection.
By putting the code snippet below in
Host lovelace Hostname 10.65.205.200 User malcolm
running the command
is equivalent to running
ssh [email protected]
and is much easier to both type and remember (and tab-complete).
Additional useful parameters for the configuration are
RemoteForwardfor port forwarding,
Portwhen using a non-standard port, and
IdentityFilefor using specific private keys.
ssh_config man pages (
man ssh_config) also contain
all the parameters available and information about each of them.
Earlier we modified the
/etc/hosts file to change the hostname of our system.
We can also use it to remember the ip address of other systems.
This is not recommended for managing many devices, use DNS for that,
for a small number of systems we can use use this file for the translation of
domain names to ip addresses.
Instead of using the ip address of localhost,
we use the ip address we want the hostname to resolve to as shown below.
The advantage this approach has over the ssh configuration is that it will also resolve the hostname in the web browser. This also makes it much easier to access web services like jupyter notebooks on the remote host.
This should be sufficient to get started using SSH within the same network. To access the service over the internet, where possible use the institutional VPN, or set up your own.