System Data and Information

1
System Data and Information

• systems files include password, group, hosts,
  services, etc.
• traditionally, systems files are in ASCII text
  format
• portable
• human readable
• contemporary usage calls for cleanly structured,
  possibly binary files for optimum performance,
  security

2
passwd File

• structure defined in pwd.h
• traditionally contains seven elements
• username
• encrypted password
• user ID
• group ID
• comment (GECOS)
• home directory
• default shell

3
passwd File Entries

• contains root (UID 0)
• one-way encrypted password
• can be reverse engineered if visible
• fields may be empty
• some systems support structured comment field

4
passwd functions

• five basic functions
• getpwuid() - fetch entry from uid
• getpwnam() - fetch entry from login name
• getpwent() - get next entry
• setpwent() - rewinds stream(s)
• endpwent() - closes stream(s)

5
passwd function details

• getpwuid, getpwnam, and getpwent all return
  pointers to passwd entry structures
• passwd entry structures are usually static within
  the function, and are overwritten by subsequent
  calls
• getpwent opens files automatically, but endpwent
  must be called to close.

6
Shadow Passwords

• encrypted passwords can be exploited for access
  to systems
• scan dictionary for common matches
• encrypt and compare
• shadow password files hide encrypted passwords
  from world
• only limited number of system programs need
  access to encrypted passwords in shadow file

7
Group File

• group file structures contain four elements
• group name
• encrypted password (almost never used)
• group ID
• array of ptrs to user names

8
group file functions

• like passwd, five basic file functions
• getgrgid() - fetch entry from gid
• getgrnam() - fetch entry from login name
• getgrent() - get next entry
• setgrent() - rewind stream(s)
• endgrent() - close streams(s)

9
Supplementary Group IDs

• originally, all users had one (login) group
• newgrp command used to change effective GID
• today, users can belong to as many as 16
  supplementary groups
• file permissions are checked against both login
  group, and supplementary groups

10
Supplementary GID functions

• there are three primary supplementary GID
  functions
• getgroups() - get supplementary GIDs for user
• setgroups() - set supplementary GIDs for user
  (superuser only)
• initgroups() - read group file and set groups
  including login group (superuser only)

11
Accessing Other System Data Files

• other files use same access function types
• two keyed search functions (if supported)
• three file walking functions
• get next entry
• set to first entry (rewind)
• end (close all files)

12
Other Data Files

• some data files common to almost all
  installations
• hosts - list of hostnames matched to network
  addresses
• networks - list of networks supported
• protocols - list of protocols supported
• services - list of services supported

13
uname() details

• uname returns five element structure
• sysname - name of OS
• nodename - name of node
• release - current release of OS
• version - current version of release
• machine - hardware type

14
Login Accounting

• two primary login accounting files
• utmp - currently logged in users
• wtmp - all logins and logouts
• structures vary from machine to machine
• not all machines have low level functions for
  access
• those that do have functions that generally
  mirror the other file access functions, a get,
  a set, and an end

15
System Identification

• two primary functions for access to system ID
  information
• uname() - returns structure with host information
• gethostbyname() - returns network hostname

16
Time and Date

• basic time service returns number of seconds
  since epoch (1/1/70).
• time_t times are referred to as calendar times
• break down functions convert time_t to tm
  structure
• formatting functions produce user friendly
  output

17
Time tm structure functions

• three main functions
• break down functions
• gmtime() - universal time
• localtime() - local time
• build up function
• mktime() - convert tm to time_t

18
time formatting functions

• three main formatting functions
• asctime() - default time string from tm
• ctime() - default time string from time_t
• strftime() - programmer definable format from tm