Users and Groups

• This information is from chapter 8.
• Each user has a unique number, or user id (UID)
  • This is perhaps the most important user credential.
• They may have one or more group ids
• Password information is stored in /etc/passwd and /etc/shadow
  • Colon delimited
  • username
  • encrypted password
    • If blank, no password
    • If not the right size, can't log in
    • Might not be used (/etc/shadow)
  • UID
    • 0 is root
    • Other positive integers.
    • Duplicates mean same account, different passwords
  • Primary group id
  • gecos field
  • Home directory
  • Login shell
• This might not be in use due to distributed password systems
  • Lightweight Directory Access Protocol (LDAP)
  • Network Information Systems (NIS)
• /etc/shadow
  • Encrypted password, username, other information.
• /etc/group
  • Group name
  • group password
  • Group id
  • Comma seperated list of members.
• /etc/gshadow
• getpwnam
  • struct passwd *getpwnam(const char *name);
  • struct passwd *getpwuid(uid_t uid);
  • #include <pwd.h>
  • Both have reentrant versions. getpwnam_r, getpwuid_r

  •        The passwd structure is defined in  as follows:
    
               struct passwd {
                   char   *pw_name;       /* username */
                   char   *pw_passwd;     /* user password */
                   uid_t   pw_uid;        /* user ID */
                   gid_t   pw_gid;        /* group ID */
                   char   *pw_gecos;      /* real name */
                   char   *pw_dir;        /* home directory */
                   char   *pw_shell;      /* shell program */
               };
           

  • NULL or a valid entry on return.
    • NULL means either not found or an error
• getgrnam
  • struct group *getgrnam(const char *name);
  • struct group *getgrgid(gid_t gid);
  • #include <grp.h>
  • Reentrent versions as well

  •            struct group {
                   char   *gr_name;       /* group name */
                   char   *gr_passwd;     /* group password */
                   gid_t   gr_gid;        /* group ID */
                   char  **gr_mem;        /* group members */
               };
           

  • gr_mem is null terminated
• See ~dbennett/310/user/userid.C