Silver Bullet for booting Linux on legacy hardware

My friend, Jake ( aka the |-,@[V](V)3|2 ), gave me this tip for booting newer linux kernels on older hardware:

Enter this at the boot prompt, instead of pressing to boot:

linux acpi=off noapic nolapic

Selinux tips

Here are some really good SElinux resources:

http://www.linuxsecurity.com/content/view/120567/49/
http://www.linuxsecurity.com/content/view/120622/49/
http://www.linuxsecurity.com/content/view/120700/49/
http://www.linuxsecurity.com/content/view/120837/169/
http://www.it-observer.com/articles.php?id=1013

Touch up a folder for apache to access (change file context labels recursively):
chcon -R -t httpd_sys_content_t

Touch up a folder for samba to access (change file context labels recursively):
chcon -R -t samba_share_t /backups


To reset the labels on all mounted filesystems:
restorecon -Rv /

If you need the filesystems to be relabeled on bootup, do this and then reboot:
touch /.autorelabel

To boot once with SELinux enforcement disabled, add the enforcing=0 flag to the kernel boot line.

To get the kernel's enforcement status:
getenforce

To change the kernel's enforcement status to "permissive" (aka log-only):
setenforce 0
To change the kernel's enforcement status to "Enforcing" (aka block access):
setenforce 1
(edit /etc/selinux/config to make the changes persistent across reboots)



How to send mail manually through a smtp telnet session

First, connect to the mail server.
telnet myserver.com 25
A successful connection will produce
...
Connected to myserver.com
Escape character is '^]'.
220 myserver.com ESMTP Sendmail
Next, specify the email source domain:
HELO local.domain.com
This should result in:
250 myserver.com Hello local.domain.com [1.2.3.4]
Specify the email source address:
MAIL FROM: myaddress@domain.com
This should result in:
250 2.1.0 myaddress@domain.com... Sender ok
Specify the recipient address:
RCPT TO: otheraddress@myserver.com
This should result in:
250 2.1.0 otheraddress@myserver.com... Recipient ok
Then start the message with this command
DATA
To specify a Subject, CC: list, etc., add for example:
Subject:my subject
followed by the enter key twice, to conform to RFC 882.
Then type the rest of your message:
Hello, world
Then, close your message with .
.
To close the SMTP connection:
QUIT

MySQL recipes

To perform any of these, log in to the server with credentials that are authorized to perform the commands on the databases you want to look at:
mysql -u username -p

• show databases on a server:
  

show databases;

• show status of a database server:

show status;

• get user account info for a database server:

select User,Host,Grant_Priv,Super_Priv from mysql.user;

• show tables in a database:

show tables in database_name;

• get information about a table in a database:

describe database_name.table_name;

• get all contents of a table in a database:

select * from database_name.table_name;

• get the number of rows of a table in a database:

select count(*) from database_name.table_name;

• create an account on database with a given password:

grant all on database_name.* to username@hostname identified by 'password';

• Change user's password

SET PASSWORD FOR foo@localhost=PASSWORD('rubberchicken');
SET PASSWORD FOR foo=PASSWORD('rubberchicken');

• Flush / reload grant tables (after changing table permissions):

flush privileges;

• Conditionally drop a table, if it exists:

drop table if exists database_name.table_name;

• Create a database:

create database database_name;

• Copy a table from one database to another:

CREATE TABLE new_table_name LIKE old_database_name.table_name;
INSERT new_table_name SELECT * FROM old_database_name.table_name;

• Rename a table name:

RENAME TABLE old_table_name TO new_table_name;

• Eliminate duplicate results from a SELECT:

SELECT DISTINCT party FROM presidents;

• Use Regular Expressions in a SELECT:

SELECT selection FROM table_name WHERE column_name REGEXP 'regularexpression'

• Drop a column from a table:

ALTER TABLE 'table_name' DROP 'column_name'

• Check consistency of MyISAM database tables (must be in data dir), first flush the tables, then make sure nobody is accessing the tables, then run the command below (see http://www.lampspot.net/linux/how-to-guide-database-table-corruption-recognise-repair-and-prevent-by-jeng/):
  

myisamchk --update-state --key_buffer_size=256M --sort_buffer_size=256M --read_buffer_size=1M --write_buffer_size=1M *.MYI

• Flush database tables:

flush tables

• Check database table status:

check table database_name.table_name

• Switch MyISAM tables to InnoDB tables (or switch to MyISAM if you have problems with InnoDB). (You must of course start mysqld with InnoDB support, which can be done by adding innodb options from the distributed my-medium.cnf file to your my.cnf file. This can be done with your table as long as you are not using two part autoincrement or FULLTEXT indexes.):
  

ALTER TABLE problem_table TYPE=INNODB

Get mysql version:
mysql -V

Dump all contents of a database to a .sql file; will prompt for password:
mysqldump -u username -p database_name > /tmp/backup.sql

Restore all contents of a database from a .sql file (user must have access to do so; database must exist but be empty, I think; if tables already exist, this will not overwrite them):
mysql -u username -p database_name < /tmp/backup.sql

Change your mysql password (hostname matters?), will prompt for old password:
mysqladm -u username -p password newpassword

Firefox settings to change

In Firefox 2, type "about:config" in the location bar, and press enter. You'll get some settings. Here are the ones I change:

Old New Setting/Affect
3 1 browser.cache.check_doc_frequency - check page every load.
true false browser.cache.disk.enable - don't cache to disk
140 140 browser.tabs.tabClipWidth - don't shot close button on tabs narrower than this
100 30 browser.tabs.tabMinWidth - let tabs become this narrow
1 3 browser.startup.page - restore your previous browsing session

MS SQL Tips

Here are some MS SQL Tips, Tricks, Recipes, etc.:

• Find out who's using what databases:

1. Open SQL Query Analyzer
2. type "exec sp_who" into the Query window.
3. Click "Execute Query" or press F5.

• From cryptosid at experts-exchange:
• before using DBCC REBUILD_LOG better check this article and read it through
  http://www.sqlmag.com/Articles/Index.cfm?ArticleID=9011
  
  a safe alternative would be
  Backup the consistent Database.
  and RESTORE the database with the MOVE option using which u can move the database files to whichever drive u want...

• Get information on a DB's logfile:

1. Open Query Analyzer
2. Type this in the query window, then click Execute Query:
• use db_name
• dbcc LOGINFO

• Detach and re-attach a database:
1. confirm that nothing has the db open. Open QA and run "exec sp_who"
2. If nobody is using the db, detach the database with the QA command "EXEC sp_detach_db 'db_name' ", or right-click the DB in the enterprise mgr and select "detach database".
   
3. Re-attach the DB by using the QA command "EXEC sp_attach_db 'db_name', 'x:\full\path\to\db\file\db_filename.MDF' " or by right-clicking the databases in the enterprise mgr, and select "attach database".
   

• How to set up SQL mail
• http://support.microsoft.com/?id=263556

• Safely delete a transaction log:

1. make sure nobody's using the database, as above under "Detach..."
2. Backup the database.
3. make sure nobody's using the database still.
4. detach the database, as above.
5. rename the log file (.LDF file)
6. re-attach the database, as above.
7. logfile should be automatically re-created.

• From rsrm at experts-exchange, http://www.experts-exchange.com/Databases/Microsoft_SQL_Server/Q_20823435.html

• Comment from rsrsm Date: 12/11/2003 07:29PM PST	Comment
  If the log file is deleted for the database and SQL server is not able to locate it..you can rebuild the log. try the following steps (But if the log file is not deleted then you can ignore this comment): REBUILD LOG and recover a suspect database: 1. Back up the .mdf/.ndf and .ldf files. 2. Change the database context to Master and allow updates to system tables: Use Master Go sp_configure 'allow updates', 1 reconfigure with override Go 3. Set the database in Emergency (bypass recovery) mode: select * from sysdatabases where name = '' -- note the value of the status column for later use begin tran update sysdatabases set status = 32768 where name = '' -- Verify one row is updated before committing commit tran If you run DBCC REBUILD_LOG without setting the database in Emergency mode, the command does not work. You do not receive an error, but the log is not rebuilt either. 4. Stop and restart SQL server. If you run DBCC REBUILD_LOG without recycling the server, the following message displays: Server: Msg 5023, Level 16, State 2, Line 1 Database must be put in bypass recovery mode to rebuild the log. DBCC execution completed. If DBCC printed error messages, contact your system administrator. 5. The syntax for DBCC REBUILD_LOG is as follows: DBCC REBUILD_LOG('','') where is the name of the database and is the physical path to the new log file, not a logical file name. If you do not specify the full path, the new log is created in the Windows NT system root directory (by default, this is the Winnt\System32 directory). If the file is placed in the Winnt\System32 folder, you can move it to a different folder by running the following commands from the SQL Query Analyzer: sp_detach_db '' Go Move the log file over from the Winnt\System32 folder to the preferred folder. sp_attach_db '', '', '' Go If a log file with the same name as specified in DBCC REBUILD_LOG already exists in that directory, then the following message occurs: Server: Msg 5025, Level 16, State 1, Line 1 The file 'C:\MSSQL7\Data\' already exists. It should be renamed or deleted so that a new log file can be created. DBCC execution completed. If DBCC printed error messages, contact your system administrator. You will need to specify a different filename or rename or delete the existing one. Rebuild the log with this code: DBCC TRACEON (3604) DBCC REBUILD_LOG('','') Go If the command is successful, the following message appears: Warning: The log for database '' has been rebuilt. Transactional consistency has been lost. DBCC CHECKDB should be run to validate physical consistency. Database options will have to be reset, and extra log files may need to be deleted. After the log is successfully rebuilt, the database is placed in DBO Use Only mode. That is, the status of the database is 2048 irrespective of what the status was previously. You must reset the status using sp_dboption or through the SEM. 6. Set the database in single-user mode and run DBCC CHECKDB to validate physical consistency: sp_dboption '', 'single user', 'true' DBCC CHECKDB('') Go begin tran update sysdatabases set status = where name = '' -- verify one row is updated before committing commit tran Go 7. Turn off the updates to system tables by using: sp_configure 'allow updates', 0 reconfigure with override Go WARNING: After verifying the consistency of the database by running DBCC CHECKDB, and fixing any errors, please make sure to check the database for logical consistency as well. Because a new log has been built, the transactions in the old log are lost, hence you must also verify the logical consistency of the data as well.

bash shortcuts

from http://www.howtogeek.com/howto/ubuntu/keyboard-shortcuts-for-bash-command-shell-for-ubuntu-debian-suse-redhat-linux-etc/

Ctrl + A : Go to the beginning of the line you are currently typing on
Ctrl + E : Go to the end of the line you are currently typing on
Ctrl + L : Clears the Screen, similar to the clear command
Ctrl + U : Clears the line before the cursor position. If you are at the end of the line, clears the entire line.
Ctrl + H : Same as backspace
Ctrl + R : Let’s you search through previously used commands
Ctrl + C : Kill whatever you are running
Ctrl + D : Exit the current shell
Ctrl + Z : Puts whatever you are running into a suspended background process. fg restores it.
Ctrl + W : Delete the word before the cursor
Ctrl + K : Clear the line after the cursor
Ctrl + T : Swap the last two characters before the cursor
Esc + T : Swap the last two words before the cursor
Alt + F : Move cursor forward one word on the current line
Alt + B : Move cursor backward one word on the current line
Tab : Auto-complete files and folder names

SCSI Tape status codes in Linux

Status Bit   Description
BOT          The tape is positioned at the beginning of the first
            file.
EOT          A tape operation has reached the physical End Of
            Tape.
EOF          The tape is positioned just after a filemark.
WR_PROT      The tape (or drive) is write-protected. For some
            drives this can also mean that the drive does not
            support writing on the current medium  type.
ONLINE       The drive has a tape in place and ready for
            operation.
DR_OPEN      Door is open. Depending on the type of drive, this
            usually means that the drive does not have a tape in
            place.
IM_REP_EN    Immediate report mode. This bit is set if there are
            no guarantees that the data has been physically
            written to the tape when the write call  returns. It
            is set to zero only when the driver does not buffer
            data and the drive is set not to buffer data.
SM           The tape is currently positioned at a setmark. DDS
            specific.
EOD          The tape is positioned at the end of recorded data.
            DDS specific.
D_6250
D_1600
D_800        This "generic" status information reports the current
            density setting for 9-track 1/2 inch tape drives only.

Linux kernel debugging first steps on i386

(from the SGI XFS mailing list)

If you have compiled in kdb and you get an oops (at least in 2.4 series kernels):

Next time it hangs, enter these kdb commands and
capture the output on the serial console.

set LINES 10000
set BTAPROMPT 0
-md1c320 system_utsname
-id %eip-0x40
-cpu
-ps
-dmesg 200
-bt
-bta

Hide OS and Apache version number

By default (at least on RHEL 4, CentOS 4.x, and related distros), Apache web server will give the version number of Apache, as well as your OS. This will make it easier for an attacker to guess what vulnerabilities your web server (or host OS) may have. This information can be hidden by changing these two lines in /etc/httpd/conf/httpd.conf:

ServerTokens ProductOnly
ServerSignature Off

Note that this won't actually fix any vulnerabilities, but it will make an attacker have to either guess or scan to determine what vulnerabilities you may have.

An even better idea is to front your apache with modsecurity (perhaps even on a separate (virtual) server), and have modsecurity tell everyone that your apache web server is really an IIS server.

To hide the Apache PHP version number, add/change this line in /etc/php.ini:
expose_php = Off