System Programming
Chapter 6
1
Administration Misc.
• The mid-term will be held on April 26, 2006 in class.
It will cover Chapters 1 to 6.
• MP2 is due on April 28, 2006 (Friday).
2
Fun Stuff
•
•
•
•
SuperCilia Skin (e.g., glass blowing in the wind, tactile interface)
Fuzzmail (http://www.fuzzmail.org/)
Pendland’s social network
You are in control?
3
Simple Chapter!
• What is system data files?
–
–
–
–
Commonly used “system files” by programs
What are they?
Where are they?
How to access them (data structure)?
• Password file / Shadow passwords file
• Group file (Primacy group vs. supplementary groups)
4
What/where are they?
• User identification
– Password & shadow passwords
– Group file (Primacy group vs. supplementary groups)
• System identification
– Networks, services, protocols, OS, etc.
• Time information
5
User Identification
Password file
• Where?
– /etc/passwd
– /etc/group
• /etc/passwd – local machine
or NIS DB
– User database in POSIX.1
<pwd.h>
– Login-name, encrypted
passwd, numeric user-ID,
numeric group ID, comment,
home dir, shell program
linux1:~> cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
6
User Identification
Password file
• /etc/passwd (continue)
– Superuser root – UID =
0
– Command finger
linux1:~> finger hchu
Login: hchu
Name:
Directory: /home/professor/hchu
Shell: /bin/tcsh
On since Sun Apr 16 15:21 (CST) on
pts/25 from 61-229-10275.dynamic.hinet.net
Mail last read Sun Dec 11 17:38 2005
(CST)
No Plan.
7
User Identification
access password file
#include <sys/types.h>
#include <pwd.h>
struct passwd *getpwuid(uid_t uid);
get password by uid
struct passwd *getpwnam(const char
*name);
– getpwuid() used by Command ls
– getpwnam() used by the login
program
– Both use a static variable for
returning
– POSIX.1
struct passwd *getpwent(void);
void setpwent(void);
void endpwent(void);
– No order in the returned pwd
entries.
– setpwent()/endpwent
rewind/close these files.
– Non-POSIX.1 but XSI extension
8
User Identification
access password file?
struct passwd {
char *pw_name;
/* user name */
char *pw_passwd; /* encrypted password */
uid_t pw_uid;
/* user uid */
gid_t pw_gid;
/* user gid */
time_t pw_change;
/* password change time */
char *pw_class;
/* user access class */
char *pw_gecos;
/* Honeywell login info */
char *pw_dir;
/* home directory */
char *pw_shell;
/* default shell */
time_t pw_expire;
/* account expiration */
int pw_fields;
/* internal: fields filled in */
};
9
Figure 6.2
#include <pwd.h>
#include <stddef.h>
#include <string.h>
struct passwd *
getpwnam(const char *name)
{
struct passwd *ptr;
setpwent();
while ((ptr = getpwent()) != NULL)
if (strcmp(name, ptr->pw_name) == 0)
break;
/* found a match */
endpwent();
return(ptr);
/* ptr is NULL if no match found */
}
10
User Identification
Shadow Passwords
• /etc/shadow – shadow passwd file
– /etc/passwd
• root:x:0:1:Super-User:/root:/bin/tcsh
• with “x” indicated for passwd
– Store encrypted password in the shadow file
• Username, passwd, passwd aging
• Not readable by the world
• Readable by set-user-ID login/passwd programs
– Why? avoid a brute force approach in trying to guess passwds
11
Cryptography 101 (MD5)
• Basically a hash function producing 128 bits
value
• User enters a plaintext password K (after
login: prompt)
• System applies a hash function MD5 to
generate crypotext C
– E.g., MD5(“quickbrownfox") =
9e107d9d372bb6826bd81d3542a419d6
• System matches C to encrypted password
on /etc/shadow
• Very computationally expensive (infeasible)
to get K from MD5 & C
12
User Identification
access shadow file
#include <shadow.h>
struct spwd *getspnam(const char *name);
struct spwd *getspent(void);
void setspent(void);
void endspent(void);
– No order in the returned pwd entries.
– setspent()/endspent rewind/close these files.
– Non-POSIX.1 but XSI extension
13
User Identification
access shadow passwords
struct spwd {
char *sp_namp;
char *sp_pwdp;
long sp_lstchg;
long sp_min;
long sp_max;
long sp_warn;
/* Login name */
/* Encrypted password */
/* Date of last change */
/* Min #days between changes */
/* Max #days between changes */
/* #days before pwd expires
to warn user to change it */
long sp_inact;
/* #days after pwd expires
until account is disabled */
long sp_expire;
/* #days since 1970-01-01
until account is disabled */
unsigned long sp_flag; /* Reserved */
};
14
User Identification
group file
• /etc/group – the group database
– nuucp::9:root,nuucp
– Figure 6.2 – Page 149
• gr_passwd not in POSIX.1 (in
SVR4 & 4.3+BSD)
#include <sys/types.h>
#include <grp.h>
struct group *getgrent(void);
void setgrent(void);
void endgrent(void);
– setgrent() open (if not) and
rewind the group file.
– endgrent() close the group file.
struct group *getgrgid(gid_t gid);
struct group *getgrnam(const char
*name);
– A static variable for returned
values.
15
Supplementary Group IDs
• Introduction of supplementary group ID’s – 4.2BSD
– newgrp is the way to change gid since Version 7
– They all can be used to check for file access permissions
– Optional in POSIX.1, NGROUP_MAX (16 in common)
16
Supplementary Group IDs
#include <sys/types.h>
#include <unistd.h>
int getgroups(int gidsetsize, gid_t grouplist[]);
– Up to gidsetsize elements stored in grouplist[]
– Special case: gidsetsize = 0  only number is returned.
int setgroups(int ngroups, const gid_t grouplist[]);
int initgroups(const char *usrname, gid_t basegid);
– Only superusers can call setgroups() and initgroups()
• Called by the login program
17
Other System Data Files and Info
• BSD Networking Software
–
–
–
–
/etc/services – getservbyname, getservbyport
/etc/protocols – getprotobyname, getprotobynumber
/etc/networks – getnetbyname, getnetbyaddr
/etc/hosts – gethostbyname, gethostbyaddr
• General Principle to the Interfaces
–
–
–
–
–
A get function to read the next record
A set function to rewind the file
An end function to close the file
Keyed lookup functions if needed.
Figure 6.3 – Page 153
• Routines for System File Access
18
linux1:~> cat /etc/hosts
127.0.0.1
localhost.localdomain localhost
140.112.30.32 linux1.csie.ntu.edu.tw linux1
linux1:~> more /etc/networks
localnet 140.112.28.0
linux1:~> more /etc/protocols
ip
0
IP
# internet protocol, pseudo protocol number
#hopopt 0
HOPOPT
# IPv6 Hop-by-Hop Option [RFC1883]
icmp 1
ICMP
# internet control message protocol
igmp 2
IGMP
# Internet Group Management
tcp 6
TCP
# transmission control protocol
pup 12
PUP
# PARC universal packet protocol
udp 17
UDP
# user datagram protocol
linux1:~> more /etc/services
tcpmux
1/tcp
echo
7/tcp
echo
7/udp
systat
11/tcp
users
daytime
13/tcp
daytime
13/udp
# TCP port service multiplexer
19
Login Accounting
utmp
• Track all current logins
• /etc/utmp:
– /var/adm/utmp in SVR4
– /var/run/utmp in 4.3+BSD and Linux
• Updated by the login program, erased by init
• tty is a Unix command that prints to standard output the name of the file
connected to standard input.
struct utmp {
char ut_line[8]; // tty line
char ut_name[8]; // login name
long ut_time; // seconds since epoch
}
20
who cmd reads from utmp
linux1:~> who
b93043 pts/1
r89033 pts/4
b89013 pts/10
b89013 pts/11
b89013 pts/5
2006-04-07 13:39 (council:S.0)
2006-04-15 02:31 (bsd5.csie.ntu.edu.tw)
2006-04-07 14:51 (218-174-143-212:S.0)
2006-04-07 14:51 (218-174-143-212:S.1)
2006-04-07 16:13 (218-174-143-212:S.2)
21
Login Accounting
wtmp
• Track all logins and logouts
• /etc/wtmp:
– var/adm/wtmp in SVR4
– /var/log/wtmp in 4.3+BSD and Linux
• Updated by the login and init programs, reboot
linux1:~> last | grep hchu
hchu pts/43
61-229-102-75.dy Sun Apr 16 17:43 still logged in
hchu pts/25
61-229-102-75.dy Sun Apr 16 15:21 - 17:43 (02:21)
hchu pts/32
140.112.29.47 Sat Apr 15 20:57 - 21:15 (00:17)
hchu pts/28
140.112.29.47 Tue Apr 11 20:17 - 04:02 (07:45)
hchu pts/28
140.112.29.47 Tue Apr 11 20:11 - 20:16 (00:04)
22
System Identification
#include <sys/utsname.h>
int uname(struct utsname *name);
struct utsname {
char sysname[9] /* name of OS */
char nodename[9]; /* name of the node */
char release[9]; /* current release of the OS */
char version[9]; /* current ver of the release */
char machine[9]; /* name of the HW type */
}
– sysconf()
linux1:~> uname -a
Linux linux1 2.6.16-1-686-smp #1 SMP Mon Apr 3 13:02:49 UTC 2006 i686
GNU/Linux
23
#include <sys/utsname.h>
int gethostname(char *name, int namelen);
– Name of the host on a TCP/IP network – BSD systems
– MAXHOSTNAMELEN in <sys/param.h>
linux1:~> hostname
linux1
linux1:~> hostname -d
csie.ntu.edu.tw
linux1:~> hostname -i
140.112.30.32
24
System Identification
• Commands: hostname, sethostname (/etc/rc),
uname
• $ uname –a
SunOS ntucsa 5.6 Generic_105181-26 sun4u sparc sun4u
– Nodename is not good to reference on a network.
– Most versions of System V has this info compiled
into the kernel when the kernel is built.
25
Time and Date Routines
• Time Values
– Calendar time
• In seconds since the Epoch (00:00:00 January 1, 1970, Coordinated
Universal Time, i.e., UTC)
• type time_t
– Remark: Times in Unix
• Keeping time in UTC
• Automatic handling of conversions, such as daylight saving time
• Keeping of time and date as a single quantity.
26
Time and Date Routines
• Time Values (continued)
– Process time
• In clock ticks (divided by CLK_TCK -> secs)
• type clock_t
• Clock time, user/system CPU time
> time grep _POSIX_SOURCE */*.h > /dev/null
0.25u 0.25s 0:03.51 14.2%
27
Calendar Time
#include <time.h>
time_t time(time_t *calptr);
– As a func in BSD, call gettimeofday (1us)
– Time initialization: settimeofday (BSD, 1us), stime (SVR4)
(broken-down time)
string
struct tm
strftime
time_t
(calendar time)
time
kernel
formatted string
mktime
localtime
gmtime
ctime
asctime
28
Affected by env var TZ
Calendar Time to GMT/Local
#include <time.h>
struct tm *gmtime(const time_t *calptr);
struct tm *localtime(const time_t *calptr);
struct tm { /* borken-down time */
int tm_sec; /* [0, 61], >= 59 for leap seconds*/
int tm_min; /* [0, 59] */
int tm_hour; /* [0, 23] */
int tm_mday; /* [1, 31] */
int tm_mon; /* [0, 11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday: [0, 6] */
int tm_yday; /* days since January 1: [0, 365] */
int tm_isdst; /* daylight saving time flag: > 0, 0, < 0 (not available) */
};
– localtime()  local time, gmtime()  UTC time
29
More calendar time conversion
functions
time_t mktime(struct tm*tmptr) // convert tm to time_t
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 *tmptr);
$ date
Sun Apr 16 18:39:21 2006 // char format in asctime & ctime
• strftime produces formatted string, like printf (see conversion
specifiers in Figure 6.9 )
30
Process time
• Higher resolution (microseconds)
#include <sys/time.h>
int gettimeofday(struct timeval * restrict tp, void *restrict tzp);
struct timeval {
time_t tv_sec;
long tv_usec;
}
31