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Information has been obtained by Osborne/McGraw-Hill from sources believed to be reliable However, because of the possibility of human or mechanical error by our sources, Osborne/McGraw-Hill, or others, Osborne/McGraw-Hill does not guarantee the accuracy, adequacy, or completeness of any information and is not responsible for any errors or omissions or the results obtained from use of such information
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Hacking Linux Exposed: Linux Security Secrets & Solutions
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assword security is one of the most important security measures to implement for your Linux system Without strong password security, your system will never be safe A hacker who manages to compromise a firewall (see 13) can attempt to log in as a user and gain access to machines on the network However, if all your users have strong passwords, you stand a good chance of foiling the hacker s illegal attempts to break into your network This chapter describes how passwords work, what hackers try to do to crack them, and what measures you can take to protect yourself
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HOW PASSWORDS WORK IN LINUX
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Linux passwords are stored on the machine in encrypted form Encryption involves converting a text string, based on a repeatable algorithm, into a form that is very different from the original string The algorithm must be repeatable so that when you log in, Linux can take your password and reproduce the encrypted form that it stores For instance, if your password is
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HelloWorld
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the value stored on the Linux machine might resemble
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HelloWorld is a very bad password! For information on what makes a password good or bad, see Password Protection, later in the chapter Linux uses a one-way encryption algorithm You can encrypt a password, but you cannot generate a password from an encrypted value You can only try to guess passwords based on a dictionary attack or a brute force attack, which we discuss later in the chapter
/etc/passwd
Most early versions of Linux stored passwords in an encrypted form in the file /etc/ passwd During the login process, a user is asked for a username and password The operating system takes the username and looks up that user s record in /etc/passwd to obtain his encrypted password Then, the username and password are passed into an encryption algorithm function named crypt() to produce the encrypted password If the result matches the encrypted password stored in /etc/passwd, the user is allowed access Here is an example of /etc/passwd:
[jdoe@machine1 jdoe]$ cat /etc/passwd root:a1eGVpwjgvHGg:0:0:root:/root:/bin/bash
9:
Password Cracking
bin:*:1:1:bin:/bin: daemon:*:2:2:daemon:/sbin: adm:*:3:4:adm:/var/adm: lp:*:4:7:lp:/var/spool/lpd: sync:*:5:0:sync:/sbin:/bin/sync mail:*:8:12:mail:/var/spool/mail: news:*:9:13:news:/var/spool/news: uucp:*:10:14:uucp:/var/spool/uucp: gopher:*:13:30:gopher:/usr/lib/gopher-data: ftp:*:14:50:FTP User:/home/ftp: nobody:*:99:99:Nobody:/: xfs:*:100:101:X Font Server:/etc/X11/fs:/bin/false jdoe:2bTlcMw8zeSdw:500:500:John Doe:/home/jdoe:/bin/bash student:9d9WE322:501:100::/home/student:/bin/bash
Each line in /etc/passwd is a colon-separated record The fields in /etc/passwd represent w s s s s s v The username The encrypted password The user ID number The group ID number A comment about the user (often the user s name) The home directory The default shell
Notice that the encrypted password is in view in the second field in the record:
jdoe:2bTlcMw8zeSdw:500:500:John Doe:/home/jdoe:/bin/bash
This file is readable by all users:
[jdoe@machine1 jdoe]$ ls -l /etc/passwd -rw-r--r-1 root root 842 Sep 12 16:24 /etc/passwd
The fact that the encrypted passwords are viewable by everyone leaves the system vulnerable to a password attack The term password attack is a broad term, but it generally means any attempt to crack, decrypt, or delete passwords A deleted password is one that is blank; this is as good as a decrypted password since the password is simply the ENTER key Recall that Linux uses a one-way encryption algorithm: given an encrypted version of a password, the password cannot be derived However, if someone has an encrypted version of a password, an attempt can be made to guess the password
Hacking Linux Exposed: Linux Security Secrets & Solutions
Linux Encryption Algorithms
An encryption algorithm is a repeatable formula to convert a string into a form that is unrecognizable and very different from the original There exist many different encryption algorithms, from very simple and easy to decrypt to very complicated and virtually impossible to decrypt As an example, let s look at one of the simplest encryption algorithms rot13 Rot13, or rotate 13, is an algorithm that takes a string and rotates the uppercase and lowercase alphabetic characters 13 character positions: a n b o m z n a o b z m Given the string
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