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Understanding file /etc/passwd

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User management in Linux OS is obviusly simple, all of user in Linux system saved in a file named “/etc/passwd”. This file format is like this

root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin

Each field is separated by “:” (colon), where:
– field 1: login name
– field 2: password value (encrypted) or shadowed
– field 3: user id
– field 4: group id
– field 5: real name
– field 6: home directory
– field 7: shell command for this user

Please note that:
1. To convert from shadowed mode to unshadowed password you can use “pwunconv”
so on /etc/passwd you can see something like this:

    root:$1$WZL4Ar01$eqxo7DFNztslTojbhABlV1:0:0:root:/root:/bin/bash
    bin:*:1:1:bin:/bin:/sbin/nologin

To rollback again to shadowed mode you can use “pwconv”. This mode off course is more
secure, because user system will not able read the encrypted password. The encrypted password will be put in /etc/shadow which only readable by root

# ls -l /etc/shadow
-r--------  1 root root 2202 Nov  7 03:24 /etc/shadow

And the content of shadow password file is like this

root:$1$WZL4Ar01$eqxo7DFNztslToaKMOlV1:13823:0:99999:7:::
bin:*:13823:0:99999:7:::

Format of /etc/shadow is define like this

       struct spwd {
             char          *sp_namp; /* user login name */
             char          *sp_pwdp; /* encrypted password */
             long int      sp_lstchg; /* last password change */
             long int      sp_min; /* days until change allowed. */
             long int      sp_max; /* days before change required */
             long int      sp_warn; /* days warning for expiration */
             long int      sp_inact; /* days before account inactive */
             long int      sp_expire; /* date when account expires */
             unsigned long int  sp_flag; /* reserved for future use */
       }

Written by adywicaksono

November 6, 2007 at 6:02 pm

Posted in Linux

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