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Supporting Desktop Applications : Repair a Corrupted Operating System (part 4)

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3/20/2011 11:16:31 AM

System File Checker (SFC)

Catastrophic failure isn’t the only risk to a system. You have to have a plan for recovery if you ever become concerned that your computer has become compromised by bad guys. Bad guys often gain access to computers through vulnerabilities in the programs that are running on a computer. It is estimated that approximately 1% of all software code written, even today, has vulnerabilities. With Windows Vista having in the ballpark of 100,000,000 lines of code, that says there are approximately 1,000,000 vulnerabilities in the operating system alone. These are the doorways that bad guys use to break into your computer.

Bad guys can also gain access to a computer by having you run malware, like when your browser connects to a website, or when you download and run that free copy of Whack-a-mole from the Internet.

After a bad guy breaks into a computer, one of the first things he tries to accomplish is to strengthen his hold by having your computer download his collection of malware. When the bad guy successfully implants his cocktail of Trojaned software that runs at startup, your computer is said to be “rooted.”

The bad guy’s software that gets installed on your computer is usually composed of several of the operating systems files that he has “Trojaned” by including additional software to them. This additional software allows him to connect to your computer without going through the standard access controls, like logging in. These access control bypass mechanisms are called backdoors.

The bad guy uses the operating system files because the operating system needs these programs running and launches them automatically at system bootup. They are always available for the bad guy to use.

That’s the bad news.

The good news is that Microsoft includes a command-line tool to help detect and repair operating system files on your computer when they appear to be different from the files that Microsoft releases. This tool is called the System File Checker (SFC); the executable is SFC.exe. It compares the following attributes of the system files on your computer to those that should be on your system:

  • Location

  • Source

  • Cyclical Redundancy Checksum (CRC)

  • Date and time stamp

  • Size

  • Version

Alert

SFC has several switches that it can use. Type SFC /? at a command prompt to view them.

SFC /scannow— Scans protected system files and repairs files that appear damaged.

SFC /verifyonly— Scans protected system files and reports on files that appear damaged. No repair is performed.

SFC /scanfile— Scans specified protected system files and repairs files that appear damaged.

SFC /verifyfile— Scans specified protected system files and reports on files that appear damaged. No repair is performed.

SFC /offbootdir— Performs offline scan of the boot directory protected system files and repairs files that appear damaged.

SFC /offwindir— Performs offline scan of Windows directory protected system files and repairs files that appear damaged.


One Last “Oops...” to Deal With—Convert x: /FS:NTFS

Windows Vista (still) supports the FAT and NTFS file systems. Table 1 shows which Microsoft operating systems support which file systems.

Table 1. Supported File Systems in Windows Operating Systems
OSFile Systems Supported
Windows NT 4.xFAT16, NTFS
Windows 9xFAT16, FAT32
Windows MEFAT16, FAT32
Windows 2000FAT16, FAT32, NTFS
Windows XPFAT16, FAT32, NTFS
Windows Server 2003FAT16, FAT32, NTFS
Windows VistaFAT16, FAT32, NTFS
Windows Server 2008FAT16, FAT32, NTFS

FAT actually is the acronym for its File Allocation Table, which is how FAT file systems keep track of the files, their location on the hard drive, and their basic attributes. The basic attributes are

  • File name

  • File size

  • Read Only

  • Archive

  • System

  • Hidden

  • Directory (versus file object)

  • Time stamp (of last save)

  • Date stamp (of last save)

The FAT table is a linked list table that is read sequentially when a file has been requested. Floppy disks use FAT12. Hard disks can use FAT16 or FAT32. FAT16 partitions are limited to 4GB, whereas FAT32 partitions are limited to 2TB typically; however, Vista limits FAT32 partitions to 32GB.

Of course, NTFS is substantially superior to FAT in many ways. NTFS is faster for large volumes and file counts. NTFS is managed by something called the Master File Table (MFT), which is similar in structure to a relational database that holds all information about a file. NTFS volumes are limited to 16TB and can contain over 4 billion individual entries.

NTFS is the default file system for all Windows Vista editions. It is the suggested file system for all newly created volumes due to all the support features added to NTFS.

NTFS volumes support all the basic attributes, plus additional extended attributes that include

  • Security (permissions)

  • Auditing

  • Ownership

  • Compression

  • Encryption (using the Encrypting File System, or EFS)

It is possible to convert a FAT partition into NTFS to be able to take advantage of all the great bells and whistles that NTFS has to offer.

The command to accomplish this is

Convert x: /FS:NTFS

where x: is the drive letter of the FAT partition you want to convert to NTFS. This is supposedly risk free, with no data loss. Practicality and common sense both scream “Back up your system and data before converting the file system!”

Note

How Do You Move Back to FAT? It is not possible to convert or, more correctly, revert from NTFS back to FAT. This is a destructive process. If you need to do this, you must back up all data on the NTFS volume, delete the NTFS volume, create a new partition, format the new partition with FAT, and then restore your data from the backup onto the new FAT partition.


If the OS can dismount the partition that you’re converting from FAT to NTFS, the conversion could happen during your live session on the Windows Vista computer. Typically, though, you receive a message that indicates the OS cannot dismount the partition, and the conversion will occur at the next reboot. When this happens, a flag gets set in the Registry to trigger the conversion process when the OS is starting up.

If you execute the Convert x: /FS:NTFS command on a computer and then change your mind for any reason, you must edit the Registry to remove the flag that signals the file system conversion.

To do this, launch the Registry Editor utility, Regedit.exe. Change the Registry setting from

HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Control\Session
Manager\BootExecute\autoconv\DosDevices\x: /FS:NTFS

to

HKEY_LOCAL_MACHINE \SYSTEM\CurrentControlSet\Control\Session
Manager\BootExecute\autocheck autochk *

as shown in Figure 13.

Figure 13. Using the Undo the Convert command.

Other -----------------
- Maintain Desktop Applications (part 2) - Using Group Policy to Manage Application Compatibility
- Maintain Desktop Applications (part 1) - New Program Compatibility Wizard
- Supporting Desktop Applications : Troubleshoot Software Restrictions
- Support Deployed Applications
- Configure Network Security (part 2 ) - Windows Firewall
- Configure Network Security (part1 ) - Secure Files and Printer Shares with Access Control Lists (ACLs)
- Configure and Troubleshoot Remote Access (part 2) - Troubleshooting Windows Vista Remote Access Connections
- Configure and Troubleshoot Remote Access (part 1) - Remote Client Access Connections
- Configure and Troubleshoot Wireless Networking (part 3) - Troubleshooting Wireless Connections
- Configure and Troubleshoot Wireless Networking (part 2) - Wireless Security
 
 
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