There are many possible causes for a user password is lost, which is the system administrator, it may not be a problem, as long as the Root account login, and then Passwd command to change the user password on it. However, if you lose the Root password, and how to do it? Is it nothing, reinstall the system? This will not only time consuming, but also may cause damage and loss of critical data applications. Obviously this is not feasible.
---- In fact, Linux password recovery is not a difficult thing. Linux password recovery has two aspects: First, the user creates a new password, the system enables users to log in again; the second is to find the original user's password, rather than a new password to replace the old password. Under normal circumstances, users only want to be able to log into the system again, and not get the original password, which is the first aspect mentioned here, relatively speaking, this area is much easier to achieve.
---- Linux password recovery is a typical case of system maintenance, some of the methods mentioned in this article from the perspective of system maintenance, not only for Linux password recovery, but also for some reason can not log into the normal maintenance of the system, the management of Linux system maintains a reference.
---- Herein, all recovery Linux passwords mentioned methods are tested in practice, the system is mainly used in Linux RedHat Linux For x86, the actual password recovery, perhaps because Linux publisher, release, processor and different hard disk while slightly different, but the difference is not great. Some of these methods are also applicable to other Unix system password recovery even recover Windows operating system and other passwords.
First, the password recovery route
---- Linux recovery following way passwords for different circumstances, vary the degree of difficulty.
---- 1. Emergency repair mode
---- When you can not boot Linux, it is often necessary magnetic, optical or other methods to start the basic Linux environment, enter the emergency repair mode by Linux. In the emergency repair mode, Linux systems can access files on the hard disk and restore the system to normal, including the restoration of passwords.
---- 2. Single-user mode
---- Linux has a plurality of operating levels, such as single-user mode, multi-user mode without network service, full multi-user mode and X11 graphical multi-user mode run level. Single-user mode means that the system is running at only user --Root user mode, when entering this mode, the system only loads the minimum software and hardware can be configured to run, direct access to Root, no password verification.
---- In single-user mode, you can use the passwd command to change a user's password, can also / etc / passwd and other account passwords directly read and write files, password recovery purposes.
---- 3 Modify the password file
---- Linux password file has two kinds of saving forms: one is the account information and password via encrypted ciphertext are stored in / etc / passwd file, this form is not enough security, the use of Unix in the early; another species is the account information and password ciphertext stored separately, / etc / passwd file is used to save the account information, / etc / shadow file is used to save the password in cipher text. As to what form and save the encryption algorithm, you can use / usr / sbin / authconfig program settings.
For ---- no shadow passwd file, as long as the corresponding account password fields deleted, you can log in directly without password authentication systems, such as the root account a passwd file is as follows:
---- Root: $ 1 $ dPTzzYkE $ Zd3Vs6yCu
---- VH8RC1gwKXX01: 0: 0: root: / root: / bin / bash
---- Be amended as follows:
---- Root :: 0: 0: root: / root: / bin / bash
---- If there is a shadow passwd file, the password field with "x" instead of letters, the password is stored in cipher text Shadow file. Passwd file can be deleted in the "x" letter, or delete shadow password file ciphertext, can make the user login without a password authentication system directly, to achieve the purpose of the password recovery.
---- 4 Restore password
---- In some cases, trying to find the original password is lost, and not log into the system to generate a new password. However, Linux using DES (encryption function type is the Crypt) or MD5 (function type is Md) encryption algorithm, due to the large amount of calculation, they are not likely to be almost the reverse break. DES ciphertext password is a string of 13 ASCII characters, while the MD5 password Kai ciphertext characters always start "$ 1 $."
---- Although difficult to reverse cracks, but to find out through the password ciphertext original password is not impossible, to extract a string from a dictionary or a number of permutations and combinations of characters in the string extracted with the same encryption algorithm encrypted ciphertext ciphertext comparison will produce and password, if it indicates that the string is consistent with the original password, so as to achieve the purpose of reducing password. If the password is complex, the use of such methods is very difficult to find the original password, but there is a general impression of their user password length, used characters, etc., which makes the likelihood of success will be greater.
---- Linux password cracking tools are many, such as John the Ripper, Crack by Alex Muffett and Cracker Jack, etc., in which John the Ripper the most powerful, fastest speed.
---- 5 System attacks break
---- If you can turn off the power to restart the system, then it is easy to recover the password, but a very important service to run on Linux systems sometimes can not turn off the power, otherwise it will cause damage systems and data.
---- Like this to get the system password when the system is running, it is often more difficult, but because the system administrator of the operating system and application versions, configurations, etc. have a comprehensive understanding, coupled with no firewall and protection intrusion detection systems, etc., then the opportunity to discover and exploit system vulnerabilities still a lot of chance of success password recovery is still there.
Content ---- This involves too complicated, we do not introduced here. If you do have needs in this area, it is best to help by professionals, so as not to damage the system and data.