How to fix Host key verification failed error on linux servers

When you connect to a server for the first time, the server prompts you to confirm that you are connected to the correct system. 

The following example uses the ssh command to connect to a remote host named redhat007:

[root@redhat001:~]# ssh user02@redhat007

The authenticity of host 'redhat007 (192.168.1.24)' can’t be

established. ECDSA key fingerprint is ...

Are you sure you want to continue connecting (yes/no)? yes

Warning: Permanently added 'redhat007,192.168.1.24' (ECDSA) to the list of known hosts.

The command checks to make sure that you are connecting to the host that you think you are connecting to. 

When you enter yes, the client appends the server’s public host key to the user’s ~/.ssh/known_hosts file and creating the ~/.ssh directory if necessary.

Next time when you connect to the remote server, the client compares this key to the one the server supplies. If the keys match, you are not asked if you want to continue connecting.

If someone tries to trick you into logging in to their machine so that they can sniff your SSH session, you will receive a warning similar to the following:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!

Someone could be eavesdropping on you right now (man-in-the-middle attack)!

It is also possible that the RSA host key has just been changed.

The fingerprint for the RSA key sent by the remote host is

22:cf:23:31:7a:5d:93:13:1s:99:23:c2:5k:19:2a:1c.

Please contact your system administrator.

Add correct host key in /home/readhat001/.ssh/known_hosts to get rid of this message.

Offending key in /home/redhat001/.ssh/known_hosts:7

RSA host key for redhat007 has changed and you have requested strict checking.

Host key verification failed.

To resolve above error, we have two different method.

1. Remove old key manually:

Normally key is stored ~/.ssh/known_hosts file

If root wants to ssh to the server, just removing entry in the /root/.ssh/known_hosts file is all right.

If user01 wants to ssh to the server, then remove the entry in the file /home/user01/.ssh/known_hosts.

I will remove the the key  for the destination server redhat007 from the file /home/user02/.ssh/known_hosts.

# vi /home/user02/.ssh/known_hosts

redhat003 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLrY91bQOihgFZQ2Ay9KiBG0rg51/YxJAK7dvAIopRaWzFEEis3fQJiYZNLzLgQtlz6pIe2tj9m/Za33W6WirN8=

redhat005 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBCrY/m16MdFt/Ym51Cc7kxZW3R2pcHV1jlOclv6sXix1UhMuPdtoboj+b7+NLlTcjfrUccL+1bkg8EblYucymeU=

redhat007 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBCrY/m16MdFt/Ym51Cc7kxZW3R2pcHV1jlOclv6sXix1UhMuPdtoboj+b7+NLlTcjfrUccL+1bkg8EblYucymeU=

2. Removing old key using the ssh-keygen command

[root@redhat001:~]# ssh-keygen -R [hostname|IP address]

[root@redhat001:~]# ssh-keygen -R redhat007

Now once you remove the entry, please login again

[root@redhat001:~]# ssh user02@redhat007

[root@redhat001:~]# ssh user02@redhat007

The authenticity of host 'redhat007 (redhat007)' can't be established.

ECDSA key fingerprint is SHA256:V+iGp3gwSlnpbtYv4Niq6tcMMSZivSnYWQIaJnUvHb4.

Are you sure you want to continue connecting (yes/no)? yes

Warning: Permanently added 'redhat007' (ECDSA) to the list of known hosts.

How to clear cache on Linux

In this article, we will guide you how to clear the memory cache on Linux system by clearing PageCache, dentries, and inodes from the command line.

In linux system basically we have a three different type of caches that need to be clear from linux system.

PageCache is cached files. Files that were recently accessed are stored here so they will not need to be queried from the hard disk again, unless that file changes or the cache is cleared to make room for other data

Dentry, inode cache is directory and file attributes. This information goes hand in hand with PageCache, although it doesn't contain the actual contents of any files.

Please find the below commands to clear the cache from linux device

To clear PageCache only, use this command:-

[root@localhost:~]#sysctl vm.drop_caches=1

To clear dentries and inodes, use this command:-

[root@localhost:~]#sysctl vm.drop_caches=2

To clear PageCache, plus dentries and inodes, use this command:-

[root@localhost:~]#sysctl vm.drop_caches=3

Please use free command or top to check your system's RAM usage and verify that the cache has been cleared.

Also you can use the following commands to accomplish the same thing as the respective systemctl commands:

Clear PageCache:-

[root@localhost:~]# echo 1 > /proc/sys/vm/drop_caches 

Clear dentries and inodes:-

[root@localhost:~]# echo 2 > /proc/sys/vm/drop_caches 

Clear PageCache, dentries and inodes:-

[root@localhost:~]# echo 3 > /proc/sys/vm/drop_caches

Using above commands you can clear the cache from linux system. In case you have any query please comment on this post. Thanks!!

Puppet agent: Exiting; no certificate found and waitforcert is disabled

Puppet agent: Exiting; no certificate found and waitforcert is disabled - 

Such type of error coming when puppet agent connecting to a Puppet master server for a first time will generate a certificate and give it to a Puppet master server to sign.

Basically its depend upon your puppet configuration, a default behavior is that the certificate must by signed manually and thus puppet agent exits with an error.

[root@puppet-client:~]#puppet agent -t

Exiting; no certificate found and waitforcert is disabled

To resolve this issue login to the Puppet master server and run the below command to list all certificates awaiting a signature.

[root@puppet-master ~]# puppet cert list

"puppet-client"      (SHA256)

B3:67:17:66:8E:78:1F:69:4E:11:8E:34:BA:86:A0:E7:07:84:BF:E9:8A:94:A9:41:DD:6C:9D:1B:07:D2:72:6A

From the above output we can see that certificate from a single host puppet-client is waiting for its certificate to be signed. 

Note: Your output may be different and contain multiple certificates awaiting for a signature.

Now we have two options on how to sign the above certificate. 

Option 1: We can sign each certificate individually.

Option 2: We can sign all awaiting certificates at once.

For option 1 , please run the below command 

[root@puppet-master ~]# puppet cert sign puppet-client

For Option 2, please run the below command

[root@puppet-master ~]# puppet cert sign --all

Using above option you can remove such errors. Now login on the puppet-client machine and run the puppet agent again.

[root@puppet-client:~]#puppet agent -t

Now you will not receive certificate error. In case you have any query on above article, please comment on this post. Thanks!!