ServerBuddies Support Blog

Using myisamchk for Crash Recovery in cPnael

If you run mysqld with external locking disabled (which is the default), you cannot reliably use myisamchk to check a table when mysqld is using the same table.

If the server is run with external locking enabled, you can use myisamchk to check tables at any time. In this case, if the server tries to update a table that myisamchk is using, the server will wait for myisamchk to finish before it continues.

If you use myisamchk to repair or optimize tables, you must always ensure that the mysqld server is not using the table (this also applies if external locking is disabled). If you do not stop mysqld, you should at least do a mysqladmin flush-tables before you run myisamchk. Your tables may become corrupted if the server and myisamchk access the tables simultaneously.

When performing crash recovery, it is important to understand that each MyISAM table tbl_name in a database corresponds to the three files in the database directory shown in the following table.

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

myisamchk works by creating a copy of the .MYD data file row by row. It ends the repair stage by removing the old .MYD file and renaming the new file to the original file name.

This does a complete and thorough check of all data (-e means “extended check”). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a long time for a large table that has many indexes. Normally, myisamchk stops after the first error it finds. If you want to obtain more information, you can add the -v (verbose) option. This causes myisamchk to keep going, up through a maximum of 20 errors.

Reference: http://dev.mysql.com/

Tags: cpanel, crash, myisamchk, Recovery

Block IP Addresses With IPtables:

This command will simply drop any packet coming from the address 25.55.55.55. To list the chains:

The -n sticks with just IP addresses, rather than resolving the name. This is useful if you have a lot of IP addresses. It can take a lot of time to resolve all of the addresses.

If you later decide that you don’t want to drop packets from a particular host, use the -D option instead of -I:

Tags: addresses, Block, IP, iptables

SSH Security On cPanel Servers.

1. Change SSH port number.

Edit your ssh configuration file under /etc/ssh/sshd_config and add/replace this line:

2. Allow only the IP’s that you would like to have access to SSH through your firewall.

3. Use a utility like BFD, BlockHosts and DenyHosts

4. Use iptables to limit the rate of incoming connections to SSH.

This will limit incoming connections to port 22 to no more than 3 attempts in a minute. Any more will be dropped.

Tags: cpanel, Security, Servers, SSH

Managing an Exim 4 server.

Remove mails by ID.

List queded mails.

Output the number of queded mails.

Delete frozen mails.

Deliver forcefully emails.

Freeze Mails from the sender.

Remove mails from the sender.

Files in /var/spool/exim/msglog contain logging information for each message and are named the same as the message-id.

Exim includes a utility that is quite nice for grepping through the queue, called exiqgrep.
http://www.exim.org/exim-html-4.50/doc/html/spec_49.html#IX2895

Reference: http://www.exim.org/

Tags: Exim4, Managing, server

Testing Link Status from the Command Line

mii-tool and ethtool commands command will provide reports on the link status and duplex settings for supported NICs.

ethtool - Display or change ethernet card settings.
ethtool is used for querying settings of an ethernet device and changing them. ethX is the name of the ethernet device on which ethtool should operate. ethtool with a single argument specifying the device name prints current settings of the specified device.

Tags: Command Line, link, status, testing