Users and Groups

• Each user has an associated user id
  • This should be unique
  • It is one of the fields of /etc/passwd
    • username: unique
    • password: encrypted or something invalid.
      • This is ignored if shadow passwords are enabled.
    • User id: 0 through 65,535 on older versions
      • 2^32 on newer versions.
      • Multiple names with the same value allows multiple accounts with the same privileged.
    • Group ID
    • gecos : a comma separated field
      • Name,
      • office,
      • Office phone,
      • Other info
      • (chfn), finger, ...
    • Home directory
    • Login Shell
  • This information is actually stored in a distributed data base for our system.
    • On a NIS server
    • Or it could be LDAP
  • /etc/shadow, as we have discussed.
• Groups
  • /etc/group
    • group name
    • group password
    • group id
    • members
  • You log in to the group indicated in the password file.
    • newgrp allows you to change to another authorized group
    • As long as you have the password for that group
• There are a pair of functions for getting user and group information

  •  
           #include <sys/types.h>
           #include <pwd.h>
    
           struct passwd *getpwnam(const char *name);
           struct passwd *getpwuid(uid_t uid);
           int getpwnam_r(const char *name, struct passwd *pwd,
                       char *buf, size_t buflen, struct passwd **result);
           int getpwuid_r(uid_t uid, struct passwd *pwd,
                       char *buf, size_t buflen, struct passwd **result);
            

  •            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;      /* user information */
                   char   *pw_dir;        /* home directory */
                   char   *pw_shell;      /* shell program */
               };
                 

  • These functions work regardless of password storage system.
  • Programs like ls and who use this call
  • This points to memory area which will be overwritten in subsequent calls.
  • Do not free this either.
  • These are DEFINITELY not thread safe.
• Just for fun

  •  
           #include <unistd.h>
           #include <sys/types.h>
    
           uid_t getuid(void);
           uid_t geteuid(void);
           

  • These functions are always successful
• There are equivalent functions for groups

  •        #include <sys/types.h>
           #include <grp.h>
    
           struct group *getgrnam(const char *name);
    
           struct group *getgrgid(gid_t gid);
    
           struct group {
               char   *gr_name;       /* group name */
               char   *gr_passwd;     /* group password */
               gid_t   gr_gid;        /* group ID */
               char  **gr_mem;        /* group members */
           };