Linux Tips, Tricks, Tutorials & Fixes

strace can be seen as a useful diagnostic and instructional debugger. It allows a programmer/user to quickly find out how a program is interacting with the OS. It does this by monitoring system calls and signals. Syntax: strace -tf -s 1000 <command> The above syntax include timestamps, follow forks, increase string size to 1000 characters. You can reduce the set of calls returned to just those dealing with network, file access, reads, and writes as follows: strace -tf -s 1000 -e trace=file,network,write,read <command> Reading strace output: Each line in the output represents a system call. They follow the format: system_call(argument1, argument2, ... ) = return_value execve - the arguments show the path to the command being run followed by a list of arguments (the command itself is argument 0) brk - the process requests memory access - check to see if the file is able to be read and/or written to open - open the file specified read - read the conten...

1. rndc: connect failed: 127.0.0.1#953: connection refused This error may appear on Centos 6.3+ when named is unable to bind to port 953 for rndc. This is caused by the named init script not being configured properly for "portreserve" and can be fixed using the following steps: # mv -v /etc/init.d/named /etc/init.d/named.prfixbak # yum -y reinstall bind # service named stop # service portreserve restart # service named start 2. Using named-checkzone to check for errors named-checkzone will check a given DNS zone for errors. # named-checkzone avinash.com /var/named/avinash.com.db 3. Flush local DNS cache The local DNS cache can be flushed, thereby forcing an actual domain name lookup. You can do it using following steps: In Linux : Open up a root terminal window (ctrl T in gnome). Type the following command and hit enter. # /etc/init.d/nscd restart Restart your application (e.g. browser or email). In Windows: Run the following on command promp...

What is SQL Injection SQL injection refers to the act of someone inserting a MySQL statement to be run on your database without your knowledge. Injection usually occurs when you ask a user for input, like their name, and instead of a name they give you a MySQL statement that you will unknowingly run on your database. SQL Injection Example: Below is a sample string that has been gathered from a normal user and a bad user trying to use SQL Injection. We asked the users for their login, which will be used to run a SELECT statement to get their information. MySQL & PHP Code: // a good user's name $name = "avi";  $query = "SELECT * FROM customers WHERE username = '$name'"; echo "Normal: " . $query . "<br />"; // user input that uses SQL Injection $name_bad = "' OR 1'";  // our MySQL query builder, however, not a very safe one $query_bad = "SELECT * FROM customers WHERE username = ...

Replication enables data from one MySQL server, called the master to be replicated to one or more MySQL servers, called slaves. MySQL database replication is used to spread the load among multiple slaves to improve performance. MySQL allows slave nodes to have read access on the replicated databases. This means, that you can load balance the requests so that the master will handle the write requests while the slave(s) will manage the reading. I suppose MySQL is already installed in the Master as well as slave server. Now, follow the steps to configure Master Slave replication: Master server Setup: 1. Login to the MySQL shell and create a new user for replication using following queries: [root@master-node~] # mysql mysql> GRANT REPLICATION SLAVE ON *.* to 'replication_user'@'%' IDENTIFIED BY 'repl_password'; mysql> FLUSH PRIVILEGES; mysql> quit; 2. Dump the data from the Master server and copy it to the Slave server using mysqldump: [ro...

SSH Keying through Linux, Mac OS X SSH keys are fairly simple to setup and can be done so even simpler when using a native terminal application, such as the terminal in OSX. Here's how! In terminal, type the following command: ssh-keygen -t dsa This will ask you a few questions, the defaults for which are just fine, no passcode is necessary. This will generate a key in the ~/.ssh/ directory. Now we just need to get that file up to the server. You can do this using scp or rsync, I'll give rsync as an example here. rsync -av -e "ssh" ~/.ssh/id_dsa.pub root@IP_address:.ssh/authorized_keys In the event your server uses a non-standard port for ssh, you can specify this inside the quotes around ssh, an example for port 2222 is below. rsync -av -e "ssh -p 2222" ~/.ssh/id_dsa.pub root@ip.add.ress.here:.ssh/authorized_keys Once running this command you will be prompted for your root password as rsync creates an SSH connection to tra...

To copy data from a table to another table in two MySQL databases, both the databases need to exist on the same account. Method 1: Using Cpanel From cPanel, click on phpMyAdmin icon. Click on the SQL tab at the top. You will see where it says, 'Run SQL query/queries on server "localhost":' In the text box below that, insert the following code, but replace DB1 and DB2 with the database names. Also, replace TABLE1 with the table name you are trying to merge. INSERT INTO DB1.TABLE1 SELECT * FROM DB2.TABLE1 Click on Go button. Repeat for any other tables you want to merge. Method 2. Through Shell (SSH) From SSH, you need to type the command to access mysql. Here is the format, but replace MYNAME with your username and PASS with your password. mysql -u MYNAME -pPASS Now type the following code, but replace DB1 and DB2 with the database names. Also, replace TABLE1 with the table name you are trying to merge. INSERT IN...

cPanel/WHM Initial Installation Errors: »» /var/log/cpanel*install* These log files contain verbose logs of the cPanel installation, and should be the first point of reference for any issues which might occur ‘out of the box’ with new cPanel installations. cPanel/WHM Requests and Errors: »» /usr/local/cpanel/logs/error_log cPanel logs any error it incurs here. This should always be the first place you look when you encounter errors or strange behavior in cPanel/WHM. »» /usr/local/cpanel/logs/license_log All license update attempts are logged here. If you run into any license errors when logging in, check here. »» /usr/local/cpanel/logs/stats_log The stats daemon (cpanellogd) logs the output from all stats generators (Awstats, Webalizer, Analog) here. »» /usr/local/cpanel/logs/access_log General information pertaining to cPanel requests is logged here(Client Information, Request URI) cPanel/WHM Update Logs »» /var/cpanel/updatelogs/up...

top → The top program provides a dynamic real-time view of a running system activity vmstat → System Activity, Hardware and System Information w → Find Out Who Is Logged on And What They Are Doing uptime → Tell How Long The System Has Been Running ps → Displays The Processes free → Memory Usage iostat → Average CPU Load, Disk Activity sar → Collect and Report System Activity mpstat → Multiprocessor Usage ss → Command is used to dump socket statistics netstat → Network Statistics strace → System Calls mtr → Combines the functionality of the traceroute and ping programs in a single network diagnostic tool. lsof → list open files, network connections and much more. psacct → to keep a detailed audit trail of what’s being done on your Linux systems ac → The ac command displays statistics about how long users have been logged on. lastcomm → The lastcomm command displays information about previous executed commands. ac...

adduser:- Command used to add user accounts. chage:- Used to change the time the user's password will expire. chfn:- Change a user's finger information chsh:- Change a user's shell chgrp: -Changes the group ownership of files. chown:- Change the owner of file(s ) to another user. gpasswd:- Used to administer the /etc/group file. groupadd:- Create a new group. grpconv:- Creates /etc/gshadow from the file /etc/group which converts to shadow passwords. grpunconv:- Uses the files /etc/passwd and /etc/shadow to create /etc/passwd, then deletes /etc/shadow which converts from shadow groupdel:- Delete a group groupmod:- Modify a group groups:- print the groups a user is in grpck:- Verify the integrity of group files. id:- Print group or user ID numbers for the specified user. newgrp:- Allows a user to log in to a new group. newusers:- Update and create new users in batch form. passwd:- Used to update a user's password. The command "pass...