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SSH Public Key Based Authentication – Howto

The SSH protocol is recommended for remote login and remote file transfer which provides confidentiality and security for data exchanged between two computer systems, through the use of public key cryptography. The OpenSSH server provides this kind of setup under Linux. It is installed by default. This how-to covers generating and using ssh keys for automated usage such as:

  1. Automated Login using the shell scripts.
  2. Making backups.
  3. Run commands from the shell prompt etc.

Task: 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 log in, on your local system, AS THE USER you wish to make passwordless ssh connections.

Fig.01 ssh key based authentication

ssh key based authentication

Create the cryptographic Key on FreeBSD / Linux / 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

Use scp to copy the id_rsa.pub (public key) to rh9linux.nixcraft.org server as authorized_keys2 file, this is know as Installing the public key to server.
scp .ssh/id_rsa.pub vivek@rh9linux.nixcraft.org:.ssh/authorized_keys2
From FreeBSD workstation login to server:
ssh rh9linux.nixcraft.org
Changing the pass-phrase on workstation (if needed):
ssh-keygen -p
Use of ssh-agent to avoid continues pass-phrase typing
At freebsd workstation type:
ssh-agent $BASH

Type your pass-phrase

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 delete specific key, enter:
ssh-add -d key

See also:

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{ 39 comments… add one }

  • Ashish May 25, 2004, 4:56 am

    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.

  • Kevin July 12, 2004, 12:57 am

    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,

  • Anonymous October 3, 2004, 11:25 pm

    Hi Vivek

    this is Amit Shiknis here from Pune. Where are you now?How are you? i just gone through SSH docs its really very nice article.

    Hope you will be fine. if possible mail me on amitshiknis@vsnl.net


  • Vivek August 3, 2005, 12:04 am

    See url
    for Local and remote port forwarding techniques on SSH:

  • GV May 2, 2007, 3:15 pm


    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.


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

    C:\Documents and Settings\gvarada\.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 [] 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

  • Jon May 24, 2007, 10:00 am

    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’

  • Jon May 24, 2007, 10:02 am

    to the moderator: I withdraw my previous comment, the post does include ssh-add, but I had not read it thoroughly

  • Gabriel Menini June 26, 2007, 7:59 pm

    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…

  • Arul July 18, 2007, 6:55 am

    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”


  • BusyBecky September 5, 2007, 2:49 pm


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


  • nixCraft September 5, 2007, 7:41 pm

    No it is not required. You can use user name tom on client and username jerry on server.

  • surendra kumar May 21, 2008, 4:36 am

    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

  • Shankar September 17, 2008, 4:39 pm

    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.


  • Brendan October 18, 2008, 12:51 pm

    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

  • Tricky April 15, 2009, 6:13 pm

    Lol. Came back here to figure out how I did that thing ^^ before. :D

    … 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.

  • sandip April 23, 2009, 7:04 am

    i hav did as u mentioned abow but it wont work it is asking for the passwd

  • hari May 29, 2009, 7:49 am


    Please run # passwd -d login_name for each user and
    then check.


  • Rajesh June 12, 2009, 6:28 am

    HI Vivek,

    Your article on SSH is very nice. It very helpfull for us.

    Keep doing the great work


  • sreekar September 8, 2009, 7:05 pm

    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


    • nixCraft September 9, 2009, 4:30 am


      Thanks for feedback!

      I’m glad to know this site helped you to understand Linux and shell scripting.

  • Wanga October 22, 2009, 9:55 am

    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

  • Tricky October 22, 2009, 12:46 pm

    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?

  • crazyswap January 9, 2010, 8:26 am

    I can’t log into my server,it shows network error:connection time out.kindly help.

  • Tricky January 9, 2010, 3:04 pm

    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.

  • Barun May 1, 2010, 7:14 pm

    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.

    • nixCraft May 3, 2010, 9:19 am
      • Tricky May 3, 2010, 8:24 pm

        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. ;)

        • nixCraft May 3, 2010, 8:48 pm

          > 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:

          # 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.


          • Tricky May 3, 2010, 10:39 pm

            > > 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. :)

  • sakthi February 3, 2011, 6:36 am

    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.

  • Allen Cohen July 12, 2011, 1:27 am

    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

  • Daniel January 10, 2012, 1:58 pm

    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,

  • Patch March 30, 2012, 3:53 pm


    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.

  • Nasimuddin Ansari September 5, 2012, 7:47 am

    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

    • Pavel December 11, 2012, 8:45 pm

      But this tool is included to only few distros. Sadly not all…(working daily on Solaris :-()

  • Ritesh September 11, 2012, 2:20 pm


    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.


  • Fred February 6, 2013, 6:58 pm

    authorized_keys2 has been deprecated since 2001. You should just always use authrized_keys

  • Vijay Kanta July 20, 2013, 6:56 am

    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. ;-)

  • mohamed October 18, 2014, 8:42 am

    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 ??

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