linux下的/etc/shadow文件详解

原址：http://www.cyberciti.biz/faq/understanding-etcshadow-file/

Q. Can you explain /etc/shadow file used under Linux or UNIX?

A. /etc/shadow file stores actual password in encrypted format for user's account with additional properties related to user password i.e. it stores secure user account information. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file Generally, shadow file entry looks as follows (click to enlarge image):

/etc/shadow file fields

(Fig.01: /etc/shadow file fields)

1. User name : It is your login name
2. Password: It your encrypted password. The password should be minimum 6-8 characters long including special characters/digits
3. Last password change (last changed): Days since Jan 1, 1970 that password was last changed
4. Minimum: The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password
5. Maximum: The maximum number of days the password is valid (after that user is forced to change his/her password)
6. Warn : The number of days before password is to expire that user is warned that his/her password must be changed
7. Inactive : The number of days after password expires that account is disabled
8. Expire : days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used

The last 6 fields provides password aging and account lockout features (you need to use chage command to setup password aging). According to man page of shadow - the password field must be filled. The encrypted password consists of 13 to 24 characters from the 64 character alphabet a through z, A through Z, 0 through 9, \. and /. Optionally it can start with a "$" character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with "$1$" it means the MD5-based algorithm was used.

原址：http://en.wikipedia.org/wiki/Passwd

passwd is a tool on most Unix and Unix-like operating systems used to change a user's password. The password entered by the user is run through a key derivation function to create a hashed version of the new password, which is saved. Only the hashed version is stored; the entered password is not saved for security reasons.

When the user logs on, the password entered by the user during the log on process is run through the same key derivation function and the resulting hashed version is compared with the saved version. If the hashes are identical, the entered password are considered to be identical, and so the user is authenticated. In theory, it is possible to occur that two different passwords produce the same hash. However, cryptographic hash functions are designed in such way that finding any password that produces the given hash is very difficult and practically unfeasible, so if the produced hash matches the stored one, the user can be authenticated.

The passwd command may be used to change passwords for local accounts, and on most systems, can also be used to change passwords managed in a distributed authentication mechanism such as NIS, Kerberos, or LDAP.

Password file

The /etc/passwd file is a text-based database of information about users that may login to the system or other operating system user identities that own running processes.

In many operating systems this file is just one of many possible back-ends for the more general passwd name service.

The file's name originates from one of its initial functions as it contained the data used to verify passwords of user accounts. However, on modern Unix systems the security-sensitive password information is instead often stored in a different file using shadow passwords, or other database implementations.

The /etc/passwd file typically has file system permissions that allow it to be readable by all users of the system (world-readable), although it may only be modified by the superuser or by using a few special purpose privileged commands.

The /etc/passwd file is a text file with one record per line, each describing a user account. Each record consists of seven fields separated by colons.^[1] The ordering of the records within the file is generally unimportant.

An example record may be:

jsmith:x:1001:1000:Joe Smith,Room 1007,(234)555-8910,(234)555-0044,email:/home/jsmith:/bin/sh

The fields, in order from left to right, are:^[2]

1. The first field is the user name, i.e. the string a user would type in when logging into the operating system: the logname. Each record in the file must have a unique user name field.
2. The second field stores information used to validate a user's password; however in most modern uses this field is usually set to "x" (or some other indicator) with the actual password information being stored in a separate shadow password file. Setting this field to an asterisk "*" is the typical way to deactivate an account to prevent it being used.
3. The third field is the user identifier, the number that the operating system uses for internal purposes. It does not have to be unique.
4. The fourth field is the group identifier. This number identifies the primary group of the user; all files that are created by this user may initially be accessible to this group.
5. The fifth field, called the Gecos field, is commentary that describes the person or account. Typically, this is a set of comma-separated values including the user's full name and contact details.
6. The sixth field is the path to the user's home directory.
7. The seventh field is the program that is started every time the user logs into the system. For an interactive user, this is usually one of the system's command line interpreters (shells).

Shadow file

/etc/shadow is used to increase the security level of passwords by restricting all but highly privileged users' access to hashed password data. Typically, that data is kept in files owned by and accessible only by the super user.

Systems administrators can reduce the likelihood of brute force attacks by making the list of hashed passwords unreadable by unprivileged users. The obvious way to do this is to make the passwd database itself readable only by the root user. However, this would restrict access to other data in the file such as username-to-userid mappings, which would break many existing utilities and provisions. One solution is a "shadow" password file to hold the password hashes separate from the other data in the world-readable passwd file. For local files, this is usually /etc/shadow on Linux and Unix systems, or /etc/master.passwd on BSD systems; each is readable only by root. (Root access to the data is considered acceptable since on systems with the traditional "all-powerful root" security model, the root user would be able to obtain the information in other ways in any case). Virtually all recent Unix-like operating systems use shadowed passwords.

The shadow password file does not entirely solve the problem of attacker access to hashed passwords, as some network authentication schemes operate by transmitting the hashed password over the network (sometimes in cleartext, e.g., Telnet^[3]), making it vulnerable to interception. Copies of system data, such as system backups written to tape or optical media, can also become a means for illicitly obtaining hashed passwords. In addition, the functions used by legitimate password-checking programs need to be written in such a way that malicious programs cannot make large numbers of authentication checks at high rates of speed.

On a system without shadowed passwords (typically older Unix systems dating from before 1990 or so), the passwd file holds the following user information for each user account:

• Username
• Salt combined with the current hash of the user's password (usually produced from a cryptographic hash function)
• Password expiration information
• User ID (UID)
• Default group ID (GID)
• Full name
• Home directory path
• Login shell

The passwd file is readable by all users so that name service switch can work (e.g., to ensure that user names are shown when the user lists the contents of a folder), but only the root user can write to it. This means that an attacker with unprivileged access to the system can obtain the hashed form of every user's password. Those values can be used to mount a brute force attack offline, testing possible passwords against the hashed passwords relatively quickly without alerting system security arrangements designed to detect an abnormal number of failed login attempts. Users often select passwords vulnerable to such password cracking techniques.^[4]

With a shadowed password scheme in use, the /etc/passwd file typically shows a character such as '*', or 'x' in the password field for each user instead of the hashed password, and /etc/shadow usually contains the following user information:

• User login name
• salt and hashed password OR a status exception value e.g.:
  • "$id$salt$hashed", where "$id" is the algorithm used (On GNU/Linux, "$1$" stands for MD5, "$2a$" is Blowfish, "$5$" is SHA-256 and "$6$" is SHA-512, crypt(3) manpage, other Unix may have different values, like NetBSD).
  • "NP" or "!" or null - No password, the account has no password.
  • "LK" or "*" - the account is Locked, user will be unable to log-in
  • "!!" - the password has expired
• Days since epoch of last password change
• Days until change allowed
• Days before change required
• Days warning for expiration
• Days before account inactive
• Days since Epoch when account expires
• Reserved

The format of the shadow file is simple, and basically identical to that of the password file, to wit, one line per user, ordered fields on each line, and fields separated by colons. Many systems require the order of user lines in the shadow file be identical to the order of the corresponding users in the password file.

History

Password shadowing first appeared in UNIX systems with the development of System V Release 3.2 in 1988 and BSD4.3 Reno in 1990. But, vendors who had performed ports from earlier UNIX releases did not always include the new password shadowing features in their releases, leaving users of those systems exposed to password file attacks.

System administrators may also arrange for the storage of passwords in distributed databases such as NIS and LDAP, rather than in files on each connected system. In the case of NIS, the shadow password mechanism is often still used on the NIS servers; in other distributed mechanisms the problem of access to the various user authentication components is handled by the security mechanisms of the underlying data repository.

In 1987 the author of the original Shadow Password Suite, Julie Haugh, experienced a computer break-in and wrote the initial release of the Shadow Suite containing the login, passwd and su commands. The original release, written for the SCO Xenix operating system, quickly got ported to other platforms. The Shadow Suite was ported to Linux in 1992 one year after the original announcement of the Linux project, and was included in many early distributions, and continues to be included in many current Linux distributions.

In the past, it was necessary to have different commands to change passwords in different authentication schemes. For example, the command to change a NIS password was yppasswd. This required users to be aware of the different methods to change passwords for different systems, and also resulted in wasteful duplication of code in the various programs that performed the same functions with different back ends. In most implementations, there is now a single passwd command, and the control of where the password is actually changed is handled transparently to the user via pluggable authentication modules (PAMs). For example, the type of hash used is dictated by the configuration of the pam_unix.so module. By default, the MD5 hash has been used, while current modules are also capable of stronger hashes such as blowfish, SHA256 and SHA512.

原址：https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Introduction_To_System_Administration/s3-acctsgrps-shadow.html

6.3.2.2. /etc/shadow

Because the /etc/passwd file must be world-readable (the main reason being that this file is used to perform the translation from UID to username), there is a risk involved in storing everyone's password in /etc/passwd. True, the passwords are encrypted. However, it is possible to perform attacks against passwords if the encrypted password is available.

If a copy of /etc/passwd can be obtained by an attacker, an attack that can be carried out in secret becomes possible. Instead of risking detection by having to attempt an actual login with every potential password generated by password-cracker, an attacker can use a password cracker in the following manner:

• A password-cracker generates potential passwords
• Each potential password is then encrypted using the same algorithm as the system
• The encrypted potential password is then compared against the encrypted passwords in /etc/passwd

The most dangerous aspect of this attack is that it can take place on a system far-removed from your organization. Because of this, the attacker can use the highest-performance hardware available, making it possible to go through massive numbers of passwords very quickly.

Therefore, the /etc/shadow file is readable only by the root user and contains password (and optional password aging information) for each user. As in the /etc/passwd file, each user's information is on a separate line. Each of these lines is a colon delimited list including the following information:

• Username — The name the user types when logging into the system. This allows the login application to retrieve the user's password (and related information).
• Encrypted password — The 13 to 24 character password. The password is encrypted using either the crypt(3) library function or the md5 hash algorithm. In this field, values other than a validly-formatted encrypted or hashed password are used to control user logins and to show the password status. For example, if the value is ! or *, the account is locked and the user is not allowed to log in. If the value is !! a password has never been set before (and the user, not having set a password, will not be able to log in).
• Date password last changed — The number of days since January 1, 1970 (also called the epoch) that the password was last changed. This information is used in conjunction with the password aging fields that follow.
• Number of days before password can be changed — The minimum number of days that must pass before the password can be changed.
• Number of days before a password change is required — The number of days that must pass before the password must be changed.
• Number of days warning before password change — The number of days before password expiration during which the user is warned of the impending expiration.
• Number of days before the account is disabled — The number of days after a password expires before the account will be disabled.
• Date since the account has been disabled — The date (stored as the number of days since the epoch) since the user account has been disabled.
• A reserved field — A field that is ignored in Red Hat Enterprise Linux.

Here is an example line from /etc/shadow:

 juan:$1$.QKDPc5E$SWlkjRWexrXYgc98F.:12825:0:90:5:30:13096: 

This line shows the following information for user juan:

• The password was last changed February 11, 2005
• There is no minimum amount of time required before the password can be changed
• The password must be changed every 90 days
• The user will get a warning five days before the password must be changed
• The account will be disabled 30 days after the password expires if no login attempt is made
• The account will expire on November 9,2005

For more information on the /etc/shadow file, see the shadow(5) man page.

还有一个参考网址：http://linux.die.net/man/5/shadow