Chapter 6: System Data Files and InformationIntroductionPassword FileDetailsAccessing the password fileSearching the password fileShadow PasswordsGroup FileAccessing Group FileSupplementary Group IDsOther data filesLogin AccountingSystem identificationTime and Date RoutinesChapter 6: System Data Files and InformationCMPS 105: Systems ProgrammingProf. Scott BrandtT Th 2-3:45Soc Sci 2, Rm. 167Introduction Lots of system parameters, configuration information, and status information is stored in files Usually ASCII text files Why? Why are some things compiled in and others in config files?Password File User name: char *pw_name; Encrypted password: char *pw_passwd; Numerical user ID: uid_t pw_uid; Numerical group ID: gid_t pw_gid; Comment field: char *pw_gecos; Initial working directory: char *pw_dir; Initial shell: char *pw_shell;Details Usually an entry with username root One-way password encryption 13 characters (from 64 character set) Fields can be empty Some unixes support other fields root:jheVopR58x9Fx:The superuser:/:/bin/shAccessing the password file #include <sys/types.h> #include <pwd.h> struct passwd *getpwuid(uid_t uid); maps uid (from file i-node) to password entry struct passwd *getpwnam(const char *name); maps username (from login) to password entrySearching the password file #include <sys/types.h> #include <pwd.h> struct passwd *getpwent(void); Return the next entry in the password file void setpwent(void); Rewind password file to the beginning void endpwent(void); Close the password file  Note no corresponding open: getpwent() does that for usShadow Passwords Hackers can guess lots of passwords, encrypt them, and compare them to password file entries If there is a match, they know the password Note: do NOT try this at home Some systems avoid this by storing the encrypted passwords in a shadow file Sometimes also employ password aging Other password questions?Group File Group name: char *gr_name; Encrypted password: char *gr_passwd; Not part of POSIX Numerical group ID: gr_gid; Array of pointers to individual user names: char **gr_mem;Accessing Group File #include <sys/types.h> #include <grp.h> struct group *getgrgid(gid_t gid); struct group *getgrnam(const char *name); struct group *getgrent(void); void setgrent(void); void endgrent(void); Parallel the passwd functionsSupplementary Group IDs Used to have to use newgrp() to change groups Now, all group IDs are checked upon any access #include <sys/types.h> #include <unistd.h> int getgroups(int gidsetsize, gid_t grouplist[]); Gets list of groups for current user int setgroups(int ngroups, const gid_t grouplist[]); Sets list of groups for current user int initgroups(const char *username, gid_t basegid); Reads group file and then calls setgroups() for a user Used by loginOther data files Some systems support similar files for other purposes /etc/hosts /etc/services /etc/protocols /etc/networks All support get, set, end, similar to passwd and group filesLogin Accounting Two data files: utmp and wtmp utmp tracks all users currently logged in wtmp keeps track of all logins and logouts Let’s check them outSystem identification #include <sys/utsname.h> int uname(struct utsname *name); struct utsname { char sysname[9];// OS char nodename[9]; // Host char release[9]; // OS Release char version[9]; // OS Version char machine[9]; // Machine (hw) }; Some systems: int gethostname(char *name, intnamelen);Time and Date Routines Basic time services counts seconds since the Epoch #include <time.h> See figure on page 156 time_t time(time_t *calptr); struct tm *gmtime(const time_t *calptr); struct tm *localtime(const time_t *calptr); time_t mktime(struct tm *tmptr); char *asctime(const struct tm *tmptr); char *ctime(const time_t *calptr); size_t strftime(char *buf, size_t maxsize, const char *format, const struct tm