SSH Public Key Based Authentication on a Linux/Unix server

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The SSH protocol recommended a method for remote login and remote file transfer which provides confidentiality and security for data exchanged between two server systems. The SSH depends upon the use of public key cryptography. The OpenSSH server offers this kind of setup under Linux or Unix-like system. This how-to covers generating and using ssh public keys for automated usage such as:

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  1. Automated Login using the shell scripts
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  4. Login without password

How to configure SSH Public key-based authentication for a Linux/Unix

The steps and commands are as follows:

  1. On your local system type: ssh-keygen
  2. Install public key into remote server: ssh-copy-id user@remote-server-ip-name
  3. Use ssh for password less login: ssh user@remote-server-ip-name

Let us see all commands in details.

Generating SSH Keys

First, log on to your workstation. For example, log on to workstation called admin.fbsd.nixcraft.org as vivek user. Please refer the following sample setup. You will be logged in, on your local system, AS THE USER you wish to make passwordless ssh connections.

How To Configure SSH Key-Based Authentication on a Linux/Unix Server
ssh key based authentication

To create the cryptographic keys on your local system powered by FreeBSD/Linux/macOS/ UNIX workstation, enter:
ssh-keygen -t rsa
Assign the pass phrase (press [enter] key twice if you don’t want a passphrase). It will create 2 files in ~/.ssh directory as follows:

  • ~/.ssh/id_rsa : identification (private) key
  • ~/.ssh/id_rsa.pub : public key

How to copy a public ley (~/.ssh/id_rsa.pub) to your server

Use the scp command to copy the id_rsa.pub (public key) from your local system to rh9linux.nixcraft.org remote server as authorized_keys file, this is know as, “installing the public key to server”:
scp ~/.ssh/id_rsa.pub vivek@rh9linux.nixcraft.org:~/.ssh/authorized_keys
Another option is to use the ssh-copy-id command as follows from your local workstation:
ssh-copy-id user@remote-box
ssh-copy-id -i ~/.ssh/id_rsa.pub vivek@rh9linux.nixcraft.org

How to login to your remote server using SSH keys

From your local system (e.g. FreeBSD/macOS/Linux/Unix workstation) type the following command:
ssh user@remote-box
ssh vivek@rh9linux.nixcraft.org

Changing the pass-phrase on workstation

To change a passphrase for your ssh keys, use the ssh-keygen command as follows:
ssh-keygen -p
OR
cd ~/.ssh/
ssh-keygen -f id_rsa -p

How to use ssh-agen command

You can use the ssh-agent command to avoid continues passphrase typing at the CLI:
ssh-agent $SHELL
ssh-add
Now ssh server will not use prompt for the password. Above two commands can be added to your ~/.bash_profile file so that as soon as you login into workstation you can set the agent.

Deleting the keys hold by ssh-agent

To list keys, enter:
ssh-add -l
To delete all keys, enter:
ssh-add -D
To remove specific key, enter:
ssh-add -d key

See also:

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Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.


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40 comment

  2. Hi Vivek Sir,

    This is Ashish here. Yes… I caught you..
    It very nice to see you once again. Where are you right now?? In india?? Wanted to meet you..
    Now we are expecting some good technical documents from you as usual. Just now finished LLST written by you.

    If possible please mail me at ashish_r_pathak@yahoo.com

    Thanks and Regards,
    /Ashish Pathak.
    Pune, India.

    Reply

  3. Hi Vivek,

    I am kevin here from mumbai. Thanks for this article on SSH. Also i liked your Shell programming tutorial. If possible can you give me some examples on Local and remote port forwarding techniques on SSH.

    Best regards,
    Kevin

    Reply

  6. Hello,

    I installed openSSH client on windows.

    Create a public key using the command

    ssh-keygen -t rsa

    Copied the key to the unix box using the command

    scp .ssh/id_rsa.pub user@hostname:.ssh/authorized_keys

    changed the permissions on authorized_keys
    chmod 600

    The USERNAME on the windows and unix box are the same.

    When I try to run the remote script using ssh

    ssh user@hostname scriptname

    It Prompts me for the PASSWORD. I am not sure what am I doing wrong here. Any help on this is much appreciated.

    Thanks
    GV

    I am including the client side trace when I used ssh below.

    C:Documents and Settingsgvarada.ssh>ssh -v stlap08d whoami
    OpenSSH_3.8.1p1, OpenSSL 0.9.7d 17 Mar 2004
    debug1: Reading configuration data /etc/ssh_config
    debug1: Connecting to stlap08d [172.19.1.24] port 22.
    debug1: Connection established.
    debug1: identity file /home/gvarada/.ssh/identity type -1
    debug1: identity file /home/gvarada/.ssh/id_rsa type 1
    debug1: identity file /home/gvarada/.ssh/id_dsa type -1
    debug1: Remote protocol version 2.0, remote software version OpenSSH_4.1
    debug1: match: OpenSSH_4.1 pat OpenSSH*
    debug1: Enabling compatibility mode for protocol 2.0
    debug1: Local version string SSH-2.0-OpenSSH_3.8.1p1
    debug1: SSH2_MSG_KEXINIT sent
    debug1: SSH2_MSG_KEXINIT received
    debug1: kex: server->client aes128-cbc hmac-md5 none
    debug1: kex: client->server aes128-cbc hmac-md5 none
    debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024

    Reply

  7. Your method of not giving a pass phrase is convenient but not really secure (IMHO). Much better to create a key with a pass phrase, and use ssh-add to enter the pass phrase ONCE PER SESSION. That is, before you ssh to the remote machine, run ssh-add which will prompt you for the passphrase. For the rest of the session, ssh-add will authenticate for future ssh connections, which are in effect ‘password-less’

    Reply

  9. Nice tip. Thanks.

    Now I did the same for an OpenSSH server which listen on port 22000. Actually, there’s a firewall listening no that port, which redirects the incoming traffic to a LAN’s OpenSSH server.

    I’ve copied the key to the /home/user/.ssh/authorizedkeys but the client doesn’t connect without prompting for the password…

    Reply

  10. Hi, I am new to SSH. Can you tell me how can I automate connecting to remote unix boxes using SSH through a shell script and the SSH connections should take the password at runtime possibly using a config file

    something like

    cat server_repo.txt:
    abc_server abc/def

    where abc_server – unix box
    abc – username
    def – password

    Note – I dont want to use “Passwordless Connectivity”

    Thanks,
    Arul.

    Reply

  11. Hi,

    Why is it mandatory to have the same username on both source and target servers?
    Is there any workaround to this limitation?

    Thnx.

    Reply

  13. hi vivek,
    i think this method will not work for different users
    ie what i want to say is user1 can not login to user2 account with out password in to server can u conform it and revert back again?
    thanks in advance
    surendra

    Reply

  14. Hi Vivek,

    In your step 3 as below. It will prompt for the password of user vivek on rh9linux.nixcraft.org to complete the copying of the public key.

    $ scp .ssh/id_rsa.pub vivek@rh9linux.nixcraft.org:.ssh/authorized_keys2

    Is there any method by which I can pass this value non-interactively.

    Thanks
    Shankar

    Reply

  15. Regarding using scp to copy is_rsa.pub into authorized_keys2, I don’t believe this to be a good idea if there is any chance that you need more than one user or public key to have access to the server.

    In this case, rather do the following:
    ssh vivek@rh9linux.nixcraft.org “cat >> .ssh/authorized_keys2” < .ssh/id_rsa.pub

    This will pipe the public key through the ssh session and append it to the existing file if it exists. Otherwise it will create the file with the contents of your id_rsa.pub

    Reply

  16. Lol. Came back here to figure out how I did that thing ^^ before. 😀

    … and realised I hadn’t explained properly:
    the authorized_keys2 file can contain multiple keys. By using scp, you might overwrite any previously-placed keys with a single key. By appending (using the >>) you specifically add your key to the end of the authorized_keys2 file and you won’t lose any previous keys.

    Reply

  20. sir,
    your article is very educational. i also referred your tutorial on shell scripting. The way you write in simple language makes a difficult concept also understandable. I think this is a trait of all Indian writers.

    thank you for the good work

    sincerely,
    sreekar

    Reply

  21. Am not able to login into another computer even after installing ssh on both computers.
    It tells me the permission denied ,please try again and when i try again it doesnt log in.
    And yet other people are able to use ssh comfortably. My computer is also uptodate

    Reply

  22. Hi Wanga

    Likely you have not got the ssh daemon running on the computer you want to connect to, though there could be many other reasons it is not working. Could you paste any error messages you might be getting when you try to connect?

    Reply

  24. Hi crazyswap

    Try running a tcptraceroute (http://en.wikipedia.org/wiki/Tcptraceroute) to your server to confirm that the problem is not the network:
    tcptraceroute server.name.or.ip 22

    You may need to install tcptraceroute.

    If tcptraceroute fails only on the last step then it is likely that the ssh service is not running on the server. If your server is under paid hosting, contact your hosting provider to find out what the cause is.

    Reply

  25. Hi Vivek,

    Is there any way to skip typing in the passphrase while login through ssh? For example, some cron jobs run daily, which open ssh sessions to remote machines to do something. Even to have ‘ssh-add’ executed, we need to provide the passphrase.

    ~ Barun.

    Reply

      1. I’m not sure if keychain would work for ssh sessions created by cronjob while you’re not logged in. A passphraseless key would work in that case except that passphraseless keys are not so good. What you could do is limit a separate passphraseless key to only be able to execute a single command:

        Add a separate key to the authorized_keys file but start the line of the key with the command that will be run remotely. For example if you want to remotely execute a script called /usr/local/bin/cronjob1, put the key in as:
        command=”/usr/local/bin/cronjob1″ ssh-rsa AF899EDC23…..rest-of-key…..A3C== cronjob_description@my-desktop

        Then in the cronjob, ensure that the ssh session specifies that you want to use a non-default ssh key with “-i”:
        0 22 * * * /usr/bin/ssh -i /home/user/.ssh/cronjob1id_rsa user@server “/usr/local/bin/cronjob1”

        When the new key is used, the server will always execute the cronjob1 script even if you specify a different command. This can be useful in other ways however I think this is getting towards tutorial territory. 😉

        Reply

        1. > I’m not sure if keychain would work for ssh sessions created by cronjob while you’re not logged in.
          Why not? We have live backup server that pulls data from 20 Linux servers using rsnapshots. rsnapshots is called from cronjobs, all you’ve to do is in your backup script:

          #!/bin/bash
# get keys for ssh, rsync, rsnapshot
/usr/bin/keychain /root/.ssh/id_dsa

# start backup
rsync source dest...

          All my backup server ssh keys are protected and server generally don’t go offline. I’ve the following in /root/.bash_profile

          /usr/bin/keychain --clear $HOME/.ssh/id_dsa

          The –clear option is very handy as it allows cron job to do password less login but all users including an intruder must provide a passphrase-key for interactive login.

          HTH

          Reply

          1. > > I’m not sure if keychain would work for ssh sessions created by cronjob while you’re not logged in.
            > Why not?
            Maybe should have been more specific – I’m referring to keys which have a passphrase as these keys cannot be used non-interactively.

            I do like the –clear now that you’ve made me aware of it. 🙂

            Reply

  26. WE have a script which tries to scp to the same machine
    machine1>> scp -r user@machine1:fromdir todir
    As the keys are not inplace it is prompting for password. Is there any way we could automate this part by generating keys?. I would appreciate if you could give me the steps to perform the ssh.

    Reply

  27. I’ve used your method to ssh without a password for a non-root user, say “user”. This works as long as I’m logged in as “user”.
    But if I run as root, the following still asks for “user”‘s password.
    i.e.: the following works w/o a password:
    su – user
    ssh host date
    But the following asks to the password of “user”:
    su – root
    ssh user@host date

    Reply

  28. Hi all,

    Can perhaps anybody give me a hint for the following ssh issue?

    I have machine A and machine B. (AIX machines). I’m logged in as root and wants to check/create ssh keys for some users. For example user STAFF1 has ssh keys on machine A but not on machine B I would like to create ssh keys (ssh-keygen -t rsa …)
    To check if keys are already there I just would check if id_rsa and id_rsa.pub files are existing in machineb:/home/STAFF1.
    The main problem is how to generate keys for / as user STAFF1 on the remote machine? My understanding is that I need to be the user when I create the keys, otherwise I would need to use su in a way that it works on a remote machine like
    > ssh machineb ‘su STAFF1; ssh-keygen -t rsa…’ which doesn’t work.
    Is there a command where I (as root) can create keys for another user????

    I’m looking extreeeeeeeeeemly forward to here something from you 🙂

    Best regards from Germany,
    Daniel

    Reply

  29. @Daniel

    When creating a key it is only user specific because of where it is located, create your key and then move to the appropriate users home directory usually under a sub directory .ssh.

    ssh someuser@machineb ls -al /home/STAFF1/
    ssh someuser@machineb ssh-keygen -t rsa -C “User STAFF1 key” -f /home/STAFF1/.ssh/id_rsa

    -C is a comment you can associate with the key to know who its for and -f places the key in the correct location.

    Reply

  30. ssh-copy-id command is better way to copy identity (keys) on remote system. It is basically a shell script – /usr/bin/ssh-copy-id

    $ man ssh-copy-id

    Reply

  31. Hi,

    I am trying to connect to b@M1 from a@M1.
    Upto step to is fine but after that the key fingerprint is generated.
    and when i try to copy *.pub file onto b@M1, it prompts for password.

    Kindly help.

    Thanks,
    Ritesh

    Reply

  33. This website and the author never cease to amaze me. You have taught me a lot in my Linux journey. Kudos for the very helpful article. 😉

    Reply

  34. while installing oracle grind infra structure ssh cat work through the forms but it work manually fine without password …ssh node2 date & ssh node1 date works whats the problem plz ??

    Reply

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