Tag Archives: VPS

Replacing a Host Key

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Host keys are used to security log into remote servers (such as Virtual Private Servers – VPS). With Ubuntu if you are using host keys to sign into servers securely and have asked for strict checking, if you make a change (such as rebuilding your VPS) the host key will change and you cannot login and will get a message like:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ 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
…
Please contact your system administrator.
…
RSA host key for 128.0.0.128 has changed and you have requested strict checking.
Host key verification failed.

if that happens you need to remove your local host key. Then you can sign back in and you will be able to save a new copy of the host key. If you don’t know why the key has changed you should figure that out first as it maybe be an indication of an important security problem. To remove you local key, you can use ssh-keygen -R [ip address of server with the bad key] for example: ssh-keygen -R 128.0.0.128

Then when you try to sign in you will get

The authenticity of host '128.0.0.128 (128.0.0.128)' can't be established.

RSA key fingerprint is ed:...:ea.
Are you sure you want to continue connecting (yes/no)?

And if you know why (such as you made changes to the server) you can say yes and connect and save the new known host key.

Basic MySQL Performance Monitoring

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Basic MySQL Performance Monitoring

regular Ubuntu cli tools

  • mysqladmin status – mysqladmin status -uroot -p

MySQL command line interface tools

[bash]mysql -uroot -p[/bash] to open the command line.

[sql]SHOW GLOBAL STATUS;
SHOW ENGINE INNODB STATUS;
SHOW PROCESSLIST;
SHOW GLOBAL STATUS LIKE ‘Qcache_%’;[/sql]

cli tools

  • mytop – top for MySQL. Install using: sudo apt-get install mytop (assuming Ubuntu operating system). There is a very useful setting file that can be used to set parameters instead of having to include them in each command. Save the file as ~/.mytop.
  • MySQLTuner – provides suggestions on performance improvements and my.cnf settings by analyzing data on your mysql database server.

Setting considerations

  • If Open_tables (SHOW GLOBAL STATUS will show this) is equal to your [bash]table_cache size[/bash] (set in /etc/mysql/my.cnf) that means it is being capped by your setting. The more MySQL has to read the table from disk the more IO and slower response, so if you have available RAM increasing the table_cache size may well make a big difference.
  • Key_reads/Key_read_request ratio should normally be < 0.01 (per MySQL manual, this means that nearly all key requests are taken from RAM). You can get both values using SHOW GLOBAL STATUS and then calculate the ratio. If the ratio is too high, consider increasing the key_buffer (in /etc/mysql/my.sql).
  • key_writes/key_writes_request should normally be near 1 (per MySQL manual)