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Table 6-1. rpm -V Command Syntax
From time to time, it's necessary to make sure that everything on your
system is "OK". Are you sure the packages you've installed are still
configured properly? Have there been any changes made that you don't
know about? Did you mistakenly start a recursive delete in
/usr and now have to assess the damage?
RPM can help. It can alert you to changes made to any of the files installed by RPM. Also, if a package requires capabilities provided by another package, it can make sure the other package is installed, too.
The command rpm -V (The options -y and --verify are equivalent) verifies an installed package. Before we see how this is done, let's take a step back and look at the big picture.
Every time a package is installed, upgraded, or erased, the changes are logged in RPM's database. It's necessary for RPM to keep track of this information; otherwise it wouldn't be able to perform these operations correctly. You can think of the RPM database (and the disk space it consumes) as being the "price of admission" for the easy package management that RPM provides. 
The RPM database reflects the configuration of the system on which it resides. When RPM accesses the database to see how files should be manipulated during an install, upgrade, or erase, it is using the database as a mirror of the system's configuration.
However, we can also use the system configuration as a mirror of the RPM database. What does this "backward" view give us? What purpose would be served?
The purpose would be to see if the system configuration accurately reflects the contents of the RPM database. If the system configuration doesn't match the database, then we can reach one of two conclusions:
The RPM database has become corrupt. The system configuration is unchanged.
The RPM database is intact. The system configuration has changed.
While it would be foolish to state that an RPM database has never become corrupt, it is a sufficiently rare occurrence that the second conclusion is much more likely. So RPM gives us a powerful verification tool, essentially for free.
It would be handy if RPM did nothing more than verify that every file installed by a package actually exists on your system. In reality, RPM does much more. It makes sure that if a package depends on other packages to provide certain capabilities, the necessary packages are, in fact, installed. If the package builder created one, RPM will also run a special verification script that can verify aspects of the package's installation that RPM cannot.
Finally, every file installed by RPM is examined. No less than nine different attributes of each file can be checked. Here is the list of attributes:
Symbolic Link String
Let's take a look at each of these attributes and why they are good things to check:
Most operating systems today keep track of each file's creator. This is done primarily for resource accounting. Linux and UNIX also use file ownership to help determine access rights to the file. In addition, some files, when executed by a user, can temporarily change the user's ID, normally to a more privileged ID. Therefore, any change of file ownership may have far reaching effects on data security and system availability.
In a similar manner to file ownership, a "group" specification is attached to each file. Primarily used for determining access rights, a file's group specification can also become a user's group ID, should that user execute the file's contents. Therefore, any changes in a file's group specification are important, and should be monitored.
Encompassing the file's "permissions", the mode is a set of bits
that specifies permitted access for the file's owner, group members,
and everyone else. Even more important are two additional bits that
determine whether a user's group or user ID should be changed if
they execute the program contained in the file. Since these little
bombshells can let any user become
root for the
duration of the program, it pays to be extra careful with a file's
The MD5 checksum of a file is simply a 128-bit number that is mathematically derived from the contents of the file. The MD5 algorithm was designed by Ron Rivest, the "R" in the popular RSA public-key encryption algorithm. The "MD" in "MD5" stands for Message Digest, which is a pretty accurate description of what it does.
Unlike literary digests, an MD5 checksum conveys no information about the contents of the original file. However, it possesses one unique trait:
Any change to the file, no matter how small, results in a change to the MD5 checksum. 
RPM creates MD5 checksums of all files it manipulates, and stores them in its database. For all intents and purposes, if one of these files is changed, the MD5 checksum will change, and RPM will detect it.
As if the use of MD5 isn't enough, RPM also keeps track of file sizes. A difference of even one byte more or less will not go unnoticed.
Device character and block files possess a major number. The major number is used to communicate information to the device driver associated with the special file. For instance, under Linux the special files for SCSI disk drives should have a major number of 8, while the major number for an IDE disk drive's special file would be 3. As you can imagine, any change to a file's major number can have disastrous effects, and is tracked by RPM.
A file's minor number is similar in concept to the major number, but conveys different information to the device driver. In the case of disk drives, this information can consist of a unit identifier. Should the minor number change, RPM will detect it.
If the file in question is really a symbolic link, the text string containing the name of the linked-to file is checked.
Most operating systems keep track of the date and time that a file was last modified. RPM uses this to its advantage by keeping modification times in its database.
Actually, the price is fairly low. For a completely RPM-based Linux distribution, it would be unusual to have a database over 5MB in size.
From a strictly theoretical standpoint, this is not entirely true. Using the lingo of cryptologists, it is believed to be "computationally infeasible" to find two messages that produce the same MD5 checksum.