Multiple passowrdless ssh logins

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We have discussed the passwordless authentication or passwordless logins in our previous post. But what if you have to allow more than one hosts to login to a server without password? Then you have to add the dsa/rsa keys of the initiator servers to the destination server authorized_keys file.
Suppose we have three systems A,B abd C. And we want to login to system C without password from A and B.

All we have to do is

1. Generage dsa/rsa key in system A and copy that to authorized_keys file of C.
2. Generate dsa/rsa key in system B and APPEND that key to the authorized_key file of C.

Generating the key in system A:
[root@nagios ~]# ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Created directory '/root/.ssh'.
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
14:13:25:f1:c6:ed:51:c6:08:a4:3f:af:eb:2c:80:97 root@nagios.lap.work

Copying the key to the authorized_keys of system C:
[root@nagios ~]# scp /root/.ssh/id_rsa.pub 192.168.137.85:/root/.ssh/authorized_keys
The authenticity of host '192.168.137.85 (192.168.137.85)' can't be established.
RSA key fingerprint is 63:6d:4a:08:b4:b4:19:3c:d0:58:f3:60:8a:ec:7a:a0.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.137.85' (RSA) to the list of known hosts.
root@192.168.137.85's password:
id_rsa.pub                                                                              100%  402     0.4KB/s   00:00
[root@nagios ~]#

Checking the key from the system C:
[root@test ~]# cat .ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAydSsh5wlG/lvWFeZcI+Rlxr2hTWJ4diU7b1/OsDWE72goA72eIx+tfzg6/aT4vPbWA8GC8arK6XxLOWJbv2Y5tFRGmXwn+Trw3RzWOHFT76NTv6NP+SCvBciwTr55Tt6jIgGrVu6f/pBvU8tIgctu/5efH611w/pToIJbezlooJ/1GGWaydEc3eTJernwzia5UMEsRGIztT6GN8zqkVtKIRhql3y2lQjgg3jA4ceAXwJ8h49xFuo8ZIEo4mWmEwW8Kn2VaTnJVh/YsO7tMRs8KsWXonbTm0vtD2OQv59Lswjs5fMmBv0EGZJvZ3uDypQw/IH33MWKbAotwQ1fewbiw== root@nagios.lap.work
[root@test ~]#

Now creating the key in system B:
[root@server ~]# ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_rsa.
Your public key has been saved in /root/.ssh/id_rsa.pub.
The key fingerprint is:
5e:7f:e6:bc:3e:bc:9f:65:2f:b3:95:89:d6:0e:9d:5f root@server.lap.work
[root@server ~]#

Now APPENDING (Do not copy it will overwrite the key of the system A) the key of system B to the authorized_keys of system C:
First we will copy the key to a file abc.txt in system C.
Then we will append the file abc.txt to athorized_keys of system C.

[root@server ~]# scp /root/.ssh/id_rsa.pub 192.168.137.85:/root/.ssh/abc.txt
The authenticity of host '192.168.137.85 (192.168.137.85)' can't be established.
RSA key fingerprint is 63:6d:4a:08:b4:b4:19:3c:d0:58:f3:60:8a:ec:7a:a0.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.137.85' (RSA) to the list of known hosts.
root@192.168.137.85's password:
id_rsa.pub                                                                              100%  402     0.4KB/s   00:01
[root@server ~]#

Now in system C:
[root@test ~]# cat .ssh/abc.txt
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAys2LlRFyQZay+9QWaCT6mS7gmM6qN0GzCGM7AXAMlEDWHUSmXSC9EPih4uOAGH6IWGqRk7EVerVEMq39vVchDAE5B3nMofQkc2fAlC9Ct/5+TirQaQxmHCN0If6O+RlO4F3hVhqX7d0ZNjJhvWLezRXsXkZY+g0215nd+qeZSz39N8NtkKBuuYW7LFdEU8dmiUaFrUjkBpZYuP5THaGqD/wZr8Pxf7t/MIpRbkuleP7b6S8kEreR9AdDX5DWJOy3qqxZzJVfXgYH6wq/MDuY14X+p1zJjzqQRV8cD7rA2Q8WQy4R7oBAJvZk9Q5gkyt50rDfiMXLPYF1myrfo/kDpQ== root@server.lap.work
[root@test ~]#

Appending the key in the file abc.txt to authorized_keys
[root@test ~]# cat .ssh/abc.txt >> .ssh/authorized_keys

Now checking the authorized_keys:
[root@test ~]# cat .ssh/authorized_keys
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAydSsh5wlG/lvWFeZcI+Rlxr2hTWJ4diU7b1/OsDWE72goA72eIx+tfzg6/aT4vPbWA8GC8arK6XxLOWJbv2Y5tFRGmXwn+Trw3RzWOHFT76NTv6NP+SCvBciwTr55Tt6jIgGrVu6f/pBvU8tIgctu/5efH611w/pToIJbezlooJ/1GGWaydEc3eTJernwzia5UMEsRGIztT6GN8zqkVtKIRhql3y2lQjgg3jA4ceAXwJ8h49xFuo8ZIEo4mWmEwW8Kn2VaTnJVh/YsO7tMRs8KsWXonbTm0vtD2OQv59Lswjs5fMmBv0EGZJvZ3uDypQw/IH33MWKbAotwQ1fewbiw== root@nagios.lap.work
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAys2LlRFyQZay+9QWaCT6mS7gmM6qN0GzCGM7AXAMlEDWHUSmXSC9EPih4uOAGH6IWGqRk7EVerVEMq39vVchDAE5B3nMofQkc2fAlC9Ct/5+TirQaQxmHCN0If6O+RlO4F3hVhqX7d0ZNjJhvWLezRXsXkZY+g0215nd+qeZSz39N8NtkKBuuYW7LFdEU8dmiUaFrUjkBpZYuP5THaGqD/wZr8Pxf7t/MIpRbkuleP7b6S8kEreR9AdDX5DWJOy3qqxZzJVfXgYH6wq/MDuY14X+p1zJjzqQRV8cD7rA2Q8WQy4R7oBAJvZk9Q5gkyt50rDfiMXLPYF1myrfo/kDpQ== root@server.lap.work
[root@test ~]#

Now checking the passwordless login from A to C
[root@nagios ~]# ssh 192.168.137.85 ls
anaconda-ks.cfg
Desktop
install.log
install.log.syslog
[root@nagios ~]#

Now checking the passwordless login from B to C
[root@server ~]# ssh 192.168.137.85 ls
anaconda-ks.cfg
Desktop
install.log
install.log.syslog
[root@server ~]#

You should not expose the keys to others. My system is for testing use and the domain is private. That is why I don't mind to share them.