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Advanced UNIX

Advanced UNIX. 240-491 Special Topics in Comp. Eng. 2 Semester 2, 2000-2001. Objectives examine a few system data files (and their C interfaces) which record user and system information. 19. User and System Information. Overview. 1. /etc/passwd 2. /etc/group 3. Other Data Files

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Advanced UNIX

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  1. Advanced UNIX 240-491 Special Topics in Comp. Eng. 2Semester 2, 2000-2001 • Objectives • examine a few system data files (and their C interfaces) which record user and system information 19. User and SystemInformation

  2. Overview 1. /etc/passwd 2. /etc/group 3. Other Data Files 4. /etc/hosts 5. /etc/protocols continued

  3. 6. /etc/services 7. Login Accounting 8. The System Log: syslog 9. Process Information 10. System Information

  4. 1. /etc/passwd • The password file: root:jheVopR58x9Fx:0:1:The superuser:/:/bin/shnobody:*:65534:65534::/:stevens:3hKVD8R58r9Fx:224:100: Richard Stevens:/home/stevens:/bin/ksh : : • More details on the passwd file format: • $ man 5 passwd

  5. Notes • root has the user ID 0 • The password is encrypted using crypt() • one-way: there is no known way to decrypt (decode) a password • nobody cannot login, but programs can run as nobody • can only access world readable or writable files

  6. Finger • finger accesses the GECOS field: stevens:3hKVD8R58r9Fx:224:100:Richard &, B232, 555-1111, 555-2222: /home/stevens:/bin/ksh • different fields are separated by commas: • user name, office, work and home phone numbers • & is replaced by the capitalised user name

  7. struct passwd Located in pwd.h • struct passwd{ char *pw_name; /* user name */ char *pw_passwd; /* encrypted passwd */ uid_t pw_uid; /* user ID */ uid_t pw_gid; /* group ID */ char *pw_gecos; /* comment field */ char *pw_dir; /* initial working dir */ char *pw_shell; /* initial shell */}

  8. Get a User’s Details • #include <sys/types.h>#include <pwd.h>struct passwd *getpwuid(uid_t uid);struct passwd *getpwnam(char *name); • Return a pointer to the password structure for the specified user, or NULL on error. • For long term use, the struct should be copied, since it will be over-written on the next call.

  9. Search the passwd file • #include <sys/types.h>#include <pwd.h>struct passwd *getpwent(void);void setpwent(void); // openvoid endpwent(void); // close • getpwent() returns a pointer if ok, NULL on error or end of file.

  10. Search for Stupid Passwords #include <stdio.h>#include <string.h>#include <pwd.h>#include <unistd.h> /* for crypt() */int main(){ struct passwd *pw; char *cry;setpwent(); while ((pw = getpwent()) != NULL) { printf(“Trying %s\n”, pw->pw_name); : continued

  11. if (pw->pw_passwd[0] == ‘\0’) printf(“**%s has no password\n”, pw->pw_name); else { cry = crypt(pw->pw_name, pw->pw_passwd); if (strcmp(cry, pw->passwd) == 0) printf(“##%s used as own passwd\n”, pw->pw_name); } }endpwent(); return 0;}

  12. crypt() • #include <unistd.h>char *crypt(char *text, char *salt); • Returns a pointer to the encrypted version of the text or NULL on error. • salt is a string. crypt() takes the first two chars and treats them as a 12-bit number between 0 and 4095 to slightly modify things.

  13. The salt is stored at the start of the encrypted password: e.g. “mi” in “miqkFWCm1fNJI” • When the encrypted password is first created, /bin/passwd uses the time of day as salt. • Salt means that the same (original) password will be encrypted in different ways on different machines.

  14. The Shadow Password File • /etc/shadow stores encrypted password strings • only readable by root • /etc/passwd contains only ‘x’s in its password fields • This prevents password cracking by copying /etc/passwd and then using ‘guess and test’ • some crypt()’s can generate 50,000 encrypted strings/second • many passwords are very simple!

  15. 2. /etc/group $ man 5 group • Lists every group on the system, an optional password, its group ID, and the users who are members: wheel:*:0:root, racheluucp:*:10:uucpvision:AweHG67Ket4Ds:101:keith, arlinusers:*:100:

  16. Joining Groups • /etc/group lists group users in addition to the ones who are members because of their /etc/passwd group ID. • e.g. stevens is in users because he has group ID 100 • A user can change group with newgrp • usually must be a member of that group • some groups have passwords (e.g. vision)

  17. Accessing /etc/group • Use struct group and its operations in <grp.h>: • struct group { char *gr_name; /* group name */ char *gr_passwd; /* encrypted passwd */ int gr_gid; /* group id */ char **gr_mem; /* array of names */} • gr_mem is terminated by NULL.

  18. Fetch Group Details • #include <sys/types.h>#include <grp.h>struct group *getgrgid(gid_t gid);struct group *getgrnam(char *name); • Return a pointer to a group structure, NULL on error. • For long term use, the structure should be copied since it is over-written on the next call.

  19. Search the group file • #include <sys/types.h>#include <grp.h>struct group *getgrent(void);void setgrent(void); // openvoid endgrent(void); // close • getgrent() returns a pointer if ok, NULL on error.

  20. Supplementary Group IDs • In earlier UNIXs, each user belonged to one group at a time. • change was possible with newgrp • Some UNIXs now have supplementary group IDs: • a user can belong to up to 16 additional groups • no longer need to use newgrp (so much)

  21. Supplementary Group Functions • #include <unistd.h>int getgroups(int gidsetsize, gid_t grouplist[]);int setgroups(int ngroups, gid_t grouplist[]);int initgroups(char *username, gid_t basegid);

  22. getgroups() returns the number of supplementary group IDs if ok, -1 on error. • Both setgroups() and initgroups() return 0 if ok, -1 on error. • initgroups() is called at login, and makes use of setgroups() to initialise a user’s supplementary group IDs by examining /etc/group.

  23. 3. Other Data Files $ man 5 <data> • Most UNIX data files have similar interfaces to those used to access/change /etc/passwd and /etc/group. • At least three ‘search’ functions: • set??() Opens the file and rewinds it. • get??() Reads next record. Returns a pointer to a struct which will be over-written on the next call. • end??() Close the file.

  24. “Lookup a record” functions: • uses keys to identify the record of interest • e.g. user name, service ID • The next three sections illustrate these patterns used with the files: • /etc/hosts • /etc/protocols • /etc/services

  25. 4. /etc/hosts • Keeps track of the network addresses for every host on the local network. • Often incomplete since the system can also ask address servers on other machines. • Typical /etc/hosts: 127.0.0.1 localhost192.100.77.3 ratree.psu.ac.th ratree loghost192.100.77.5 ns.psu.ac.th ns203.154.130.11 ratree2.psu.ac.th ratree2

  26. Accessing /etc/hosts • Header: <netdb.h> • C structure: hostent • Keyed lookup functions: • gethostbyname() • gethostbyaddr()

  27. 5. /etc/protocols • Stores details about network protocols supported by the system. • Fragment of /etc/protocols: tcp 6 TCP # transmission control protocol :udp 17 UDP # user datagram protocol :

  28. Accessing /etc/protocols • Header: netdb.h • C structure: protoent • Keyed lookup functions: • getprotobyname() • getprotobynumber()

  29. 6. /etc/services • Stores details on the network services supported by the system • built on top of network protocols • Fragment of /etc/services ftp 21/tcpsmtp 25/tcp mail :irc 194/tcp # internet relay chatirc 194/udp :

  30. Accessing /etc/services • Header: netdb.h • C structure: servent • Keyed lookup functions: • getservbyname() • getservbyport()

  31. 7. Login Accounting • /var/run/utmp • records which users are currently logged in • used by who, users, finger, ps • may be located in /var/adm/ • /var/log/wtmp • records all logins, logouts, shutdowns, reboots • used by last • may be located in /var/adm/

  32. File Format • Both files are binary files (unlike all the previous examples). • Each record has the basic form: struct utmp { char ut_line[8]; /* ttty line: ttyp0, etc. */ char ut_name[8]; /* login name */ long ut_time; /* secs since 1st Jan 1970 */}

  33. At login: • create a utmp struct, add to utmp and wtmp files • At logout: • entry in utmp is wiped (filled with 0s) • new entry added to wtmp, withut_name filled with ‘\0’ characters • At shutdown, reboot, time change: • special entries added to wtmp

  34. Linux utmp & wtmp (non-standard) $ man 5 utmp • struct utmp { short ut_type; /* login type */ pid_t ut_pid; /* process pid */ char ut_line[UT_LINESIZE]; /* device name */ char ut_id[2]; /* abbrev ttyname */ time_t ut_time; /* login time */ char ut_user[UT_NAMESIZE]; /* uname */ char ut_host[UT_HOSTSIZE]; /* host nm */ long ut_addr; /* host address */ :}

  35. String fields may end with ‘\0’ if there is enough space! • Some login types: • UT_UNKNOWN unknown • BOOT_TIME started at system boot • INIT_PROCESS started at system init • LOGIN_PROCESS login process • USER_PROCESS user-created process • DEAD_PROCESS dead (zombie)

  36. Accessing utmp/wtmp Entries • #include <utmp.h>void utmpname(char *file);void setutent(void);void endutent(void);struct utmp *getutent(void);struct utmp *getutid(struct utmp *ut);struct utmp *getutline(struct utmp *ut);void pututline(struct utmp *ut);

  37. utmpname() can be supplied with the default pathnames stored in _PATH_UTMP and _PATH_WTMP in <paths.h>. • getutline() is restricted to entries with login type LOGIN_PROCESS and USER_PROCESS. • Updates can only be done by root.

  38. Example: sw.c, a simple who #include <stdio.h>#include <string.h>#include <utmp.h>#include <pwd.h>#include <time.h> /* for ctime() */int main(){ struct utmp *ut; struct passwd *pw; char name[UT_NAMESIZE+1];utmpname(“/var/run/utmp”);setutent(); :

  39. while ((ut = getutent()) != NULL) { if (ut->ut_user[0] != ‘\0’) { strncpy(name,ut->ut_user,UT_NAMESIZE); name[UT_NAMESIZE] = ‘\0’; if ((pw = getpwnam(name)) == NULL) printf(“%s has no passwd!\n”,name); else printf(“%s %s %s %s”, name, ut->ut_line, pw->pw_gecos, ctime(&(ut->ut_time)) ); } }endutent(); return 0;}

  40. Usage • $ swreboot has no password!runlevel has no password!LOGIN has no password!LOGIN has no password!LOGIN has no password!LOGIN has no password!LOGIN has no password!LOGIN has no password!s4210075 pts/0 ????,,, Thu Feb 15 15:56:33 2001ad pts/5 Dr.Andrew DAVISON,,, Thu Feb 15 16:00:17 2001s4010041 pts/6 MR. Kemarat CHAIYO,,, Thu Feb 15 15:32:36 2001s4010237 pts/7 MR. Paween CHOKENUKUL,,, Thu Feb 15 15:58:57 2001s4010041 pts/8 MR. Kemarat CHAIYO,,, Thu Feb 15 15:34:00 2001$

  41. Notes • “Simple who” returns similar information to who, but also includes details about: • system processes • dead user processes

  42. last • Displays wtmp in an understandable form. • Lists all logins, logouts, etc. since file creation. • $ lastrich ttypb mit.usa Tue Aug 19 13:19 still logged inzonk ttyp3 129.10.1.22 Tue Aug 19 13:12 - 13:14 (00:02)rich ttypa lisa.ac.th Tue Aug 19 13:11 still logged inzonk ttyp3 lenny Tue Aug 19 12:06 - 12:21 (00:14) : continued

  43. $ last richrich ttypb mit.usa Tue Aug 19 13:19 still logged inrich ttypa foo.lisa.ac.th Tue Aug 19 13:11 still logged inrich ttyp0 goo.lisa.ac.th Mon Aug 18 11:01 - 11:45 (00:44)rich ftp mit.usa Sat Aug 16 00:03 - 00:04 (00:01) : • $ last | grep bootreboot System boot Fri Aug 15 22:15reboot System boot Fri Aug 15 15:21reboot System boot Fri Aug 4 17:24reboot System boot Fri Aug 4 15:41

  44. $ man 8 syslogd 8. The System Log: syslog files, consoleor e-mail syslogd userprocess syslog() UDP port 514 /dev/log /dev/klog Unix domaindatagram socket Internet domaindatagram socket log() kernelroutines Kernel TCP/IP network

  45. Logging Messages • Any program can generate log messages. • A log message should include: • the program name, a facility, a priority, and the message text • Example: login: Root LOGIN REFUSED on ttya • sent by an authorization facility (login); it is critical

  46. Some syslog Facilities • Name Facilitykern The kernel.user Regular user processes.mail The mail system.lpr The printer system. :auth The authorization system, or programs that ask for user names and passwords (e.g. login, su, getty, ftp).

  47. Some Syslog Priorities (levels) • Priority Meaningemerg Emergency (e.g. crash).alert Fix immediately (e.g. bad db).crit Critical (e.g. hardware error).err Ordinary error. :notice Not an error, but important. :debug Debug messages.

  48. Configuring syslog • At system start-up, it reads the /etc/syslog.conf configuration file. • syslog.conf specifies what messages to log, and where to log them • see $ man 5 syslog.conf

  49. General format of a syslog.conf line: facility.priority action • facility and priority can be one of the labels listed in the ealier slides (or * to mean all) • action can be: • log to a file / device / program • send message to a user • send message to all users (*) • send message to another machine

  50. Typical syslog.conf file $ man 5 syslog.conf • kern.debug /dev/console*.err /dev/consoleauth.notice /usr/adm/messageslpr.* /usr/adm/lpd-errsauth.* root,adauth.* @catsix.coe.psu.ac.thauth.* /dev/console*.emerg *

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