After upgrade server to 5.1 generally we are getting following error message.

Client does not support authentication protocol requested by server; consider upgrading MySQL client.

The above error message are receiving because latest version of MySql uses a new format for the password in that case to use older client to use older version Mysql with new version Mysql you have to set the passwords on the server to their old format.Refer following steps to set the password.

Login into shell as root user and access mysql with root user and password

[root@admin.com]#mysql -u root -p

Then, copy and paste the following command in shell, editing commands if necessary, to change the password of the user to the old format.

[root@admin.com]#UPDATE mysql.user
[root@admin.com]#SET password=OLD_PASSWORD(‘password’)
[root@admin.com]#WHERE user=’username’
[root@admin.com]#AND host=’host’;

After running above commands flush the tables.

[root@admin.com]#flush privileges;

Then exit the mysql client.

[root@admin.com]#\q