1. Lecture 4: Unix Security Basics
Prof. Guntis Barzdins
Asist. Girts Folkmanis
Lekt. Leo Trukšāns
University of Latvia

2. Top UNIX Vulnerabilities
U1 BIND Domain Name System
U2 Remote Procedure Calls (RPC)
U3 Apache Web Server
U4 General UNIX Authentication Accounts with No Passwords or Weak Passwords
U5 Clear Text Services
U6 Sendmail
U7 Simple Network Management Protocol (SNMP)
U8 Secure Shell (SSH)
U9 Misconfiguration of Enterprise Services NIS/NFS
U10 Open Secure Sockets Layer (SSL)
Source:

3. Favourite TCP Ports 20 FTP (data) 21 FTP (control) 23 Telnet
7-19 echo, discard, daytime, chargen, netstat
22 SSH
42 wins
53 dns
111 sun rpc
113 identd
123 ntp
135 loc-srv/epmap – used to attack wintel
netbios
161 snmp
rexec, rlogin, rsh, talk, syslog, who
635 mountd – Linux
2049 nfs
6670 Deepthroat
31337 BackOrifice
20 FTP (data)
21 FTP (control)
23 Telnet
25 SMTP (mail)
70 Gopher
79 Finger
80 HTTP also 8000 or 8001 or 8080
110 Pop3
119 NNTP (news)
143 Imap

4. No system is perfectly secure, but still we need security
A number of toolkits exist that allow total amateurs to become holy terrors.
The good news is that if you can beat the popular intrusion toolkits, 90 percent of the bad guys will go bother somebody else who's less secure.

6. Protection
Operating system consists of a collection of objects, hardware or software
Each object has a unique name and can be accessed through a well-defined set of operations.
Protection problem - ensure that each object is accessed through correct set of operations and only by those processes that are allowed to do so.

7. UNIX Security Basics Permissions UID GID Superuser SUID, SGID
Sticky bit
Umask
Filesystem restrictions
Advanced: Systrace, Veriexec, iptables, etc.

8. Domain Implementation in UNIX
Two domain groups
User
Superuser (can do everything, UID=0)
User domain group
Domain = user-id (UID)
Domain switch accomplished via file system.
Each file has associated with it a domain bit (setuid bit = SUID bit).
When file is executed and setuid = on, then effective user-id is set to owner of the file being executed. When execution completes user-id is reset (exit() for child process ).

9. Subjects and Objects
Each subject (process) and object (file, socket, etc) has a 16-bit UID.
Each object also has a 16-bit GID and each subject has one or more GIDs.
Objects have access control lists that specify read, write, and execute permissions for user, group, and world.
Super-users (uid=0 root) can do anything.

10. Subjects and Objects UID GID Others GID-main+ GID-list
Objects = files (regular and devices /dev)
UID
GID
Others
User permissions
GID-main+
GID-list
Group permissions
Others permissions
Subjects = processes (effective UID, GID counts)

11. inodes inodes contain a lot of information about a file
mode and type of file
number of links to the file
owner's UID
owners GID
number of bytes in file
times (last accessed, modified, inode changed)
physical disk addresses (direct and indirect blocks)
number of blocks
access information

12. Unix File System (UFS) Structure

13. Directory Under UNIX directories are special (OS writable only) files.
The directory file is an unsorted linked list of filenames to file-inode (attributes and location of file on hard disk)
Directory size will always increase to be large enough to hold all the file entries. If the number of files latter shrinks the directory size WILL NOT! 5
apples 4
oranges
aboli 2 . 7 ..

14. ls -l > ls -l foo -rw-rw---- 1 hollingd grads 13 Jan 10 23:05 foo
size
permissions
name
owner
group
time

15. File Time Attributes Time Attributes:
when the file was last changed ls -l
when the file was created* ls -lc
when the file was last read (accessed) ls -ul
*actually it’s the time the file status in the directory last changed (e.g. file renamed).

16. File Types In Unix All Files Text: Readable characters
Binary: Uses all characters
Documents, etc.
Directories
Source: Readable Programs
Programming Language: Interpreted or Compiled
Compiler
Machine Code: Directly executed
Shell scripts: Interpreted by shell
Executable Files

17. Types of Files Regular Files binary text
GIF, JPEG, Executable etc.
text
scripts, program source code, documentation
Supports sequential and random access

18. Types of Files (cont.) Directory Device File
Can contain ANY kind of files
. (Dot) The special name for the current directory.
.. (Dot) (Dot) The special name for the directory above the current directory.
Device File
Allows programs to communicate with hardware.
Kernel modules handle device management.

19. Types of Files (cont.) Device Files (cont.) Character Device
Accepts a stream of characters, without regard to any block structure.
It is not addressable, therefore no seek operation
Block Device
Information stored in fixed-sized block
It is addressable, therefore seek operation is possible.

20. Types of Files (cont.) UNIX Domain Sockets (BSD) Named Pipe
sockets that are local to a particular host and are referenced through a file system object rather than a network port.
X windows
Named Pipe
Allow processes to communicate with each other.

21. Types of Files (cont.) Hard links Soft links
Linking files by reference
System maintains a count of the number of links
Does not work across file systems.
Soft links
Linking files by name
No counter is maintained
Work across file system

22. From “man ln”
There are two concepts of `link' in Unix, usually called hard link and soft link
A hard link is just a name for a file. (And a file can have several names. It is deleted from disk only when the last name is removed. The number of names is given by ls(1). There is no such thing as an `original' name: all names have the same status.
A soft link (or symbolic link, or symlink) is an entirely different animal: it is a small special file that contains a pathname.

23. % ln -s /home/faculty/ostic/prof myprof
Creating a Link
Create a link directory by typing the following command from your home directory:
% ln -s /home/faculty/ostic/prof myprof
You only need to create this link once. It will appear as a subdirectory in your home directory structure every time you log on to the system.
soft link

24. Disk vs. Filesystem
The entire hierarchy can actually include many disk drives.
some directories can be on other computers /
bin
etc
users
tmp
usr
hollid2
scully

25. Disk mount options Override individual file permissions
A major security tool in Unix
fdisk -l
mount /dev/hdb1 /media/new_disk -t ext3 –o ro,nosuid
unmount /media/new_disk

26. -rwxr--r-- File permissions File type Access granted to others
- : plain file
d : directory
c : character device (tty, printer)
b : block device (disk, CD-ROM)
l : symbolic link
s : socket
=, p : FIFO
Access granted to
others
-rwxr--r--
Access granted to
group member
Access granted to owner
r : read / w : write / x : execute

27. Permissions for Files
If you have read permission for a file, you can view its contents.
If you have write permission for a file, you can alter its contents.
If you have execute permission for a file, you can run the file as a program.

28. Permissions for Directories
If you have read permission for a directory, you can list the contents of the directory.
If you have write permission for a directory, you can create or remove files or directories inside that directory.
If you have execute permission for a directory, you can change to this directory using the cd command, or use it as part of a pathname.

29. SUID/SGID/sticky bits
SUID (set uid)
Processes are granted access to system resources based on user who owns the file.
SGID (set gid)
(For file) Same with SUID except group is affected.
(For directory) Files created in that directory will have their group set to the directory's group.
sticky bit
If set on a directory, then a user may only delete files that he owns or for which he has explicit write permission granted, even when he has write access to the directory. (e.g. /tmp )

33. File Permissions File Permissions (ex: rw-r--r--)
owner: rw-, group: r--, others: r--
r: read, w: write, x: execute
When a process executes, it has four values related to file permission
a real user ID, an effective user ID
a real group ID, an effective group ID
When you login, your login shell process’ values are your user ID and group ID

34. Effective User and Group ID
A process’ effective user ID
depends on who executes the process, not who owns the executable
E.g., if you run passwd (owned by root), the effective user ID is your ID, not root; then how can it update /etc/passwd file owned by root ?
Two special file permissions
“set user ID” (SUID) and “set group ID” (GUID)
When an executable with set user ID permission is executed, the process’ effective user ID becomes that of executable; the real user ID is unaffected
File permission of /bin/passwd is r-sr-sr-x

35. Real uids
The uid of the user who started the program is used as its real uid.
The real uid affects what the program can do (e.g. create, delete files).
For example, the uid of /usr/bin/vi is root:
$ ls -alt /usr/bin/vi lrwxrwxrwx 1 root root 20 Apr 13...
But when I use vi, its real uid is dkl (not root), so I can only edit my files.

36. Effective uids Programs can change to use the effective uid
the uid of the program owner
e.g. the passwd program changes to use its effective uid (root) so that it can edit the /etc/passwd file
SUID bit enables this functionality

37. Real and Effective Group-ids
There are also real and effective group-ids.
Usually a program uses the real group-id (i.e. the group-id of the user).
Sometimes useful to use effective group-id (i.e. group-id of program owner):
e.g. software shared across teams
SGID bit enables this functionality

38. Sample SETUID Scenario
/dev/lp is owned by root with protection rw
This is used to access the printer
/bin/lp is owned by root with rwsr-xr-x (with SETUID=1)
User A issues a print command
Shell (running with A’s UID and GID) interprets the command and forks off a child process, say, P
Process P has the same UID/GID as user A
Child process P executes exec(“/bin/lp”,…)
Now P’s domain changes to root’s UID
Consequently, /dev/lp can be accessed to print
When /bin/lp terminates so does P
Parent shell never got the access to /dev/lp

39. File system tips Turning off SUID / SGID in mounted file system
use nosuid (and nodev if possible) when mounting remote file system or allowing users to mount floppies or CD-ROMs
Finding SUID and SGID Files
# find / \( -local -o -prune \) \( -perm o -perm \) -type f -print
( xdev can be used in place of local/prune)

41. Unix Accounts and the Filesystem

42. Unix Accounts To access a Unix system you need to have an account.
Unix account includes:
username and password
userid and groupid
home directory
shell

43. Creating user accounts
useradd or adduser scripts
manually
edit /etc/passwd, etc/shadow, etc/group
remember to lock these files while editing - vipw
run “passwd [user]”
create home directory
chown, chgrp, chmod
copy defaults (e.g umod) from
/etc/skel
/etc/profile

44. username
A username is (typically) a sequence of alphanumeric characters of length no more than 8.
username the primary identifying attribute of your account.
username is (usually) used as a part of address
the name of your home directory is usually related to your username.

45. password
a password is a secret string that only the user knows (not even the system knows!)
When you enter your password the system calculates a hash (one-way) function and compares it to a stored string.
passwords are (usually) no less than 8 characters long.
It's a good idea to include numbers and/or special characters (don't use an english word!)

46. userid
a userid is a number (a 16-bit integer) that identifies a Unix account. Each userid is unique.
It's easier (and more efficient) for the system to use a number than a string like the username.
You don't need to know your userid!

47. Unix Groups and groupid
Unix includes the notion of a "group" of users.
A Unix group can share files and active processes.
Each account is assigned a "primary" group.
The groupid is a number that corresponds to this primary group.
A single account can belong to many groups (but has only one primary group).

48. Home Directory
A home directory is a place in the file system where the account files are stored.
A directory is like a Windows folder (more on this later).
Many unix commands and applications make use of the account home directory (as a place to look for customization files).

49. Additional Password Security
Later versions of Unix have improved the security for password encryption as follows:
Passwords no longer restricted to 8 characters
Use MD5 instead of DES; gives 128-bit output
Use “salt”
Furthermore, the encrypted (hashed) password is removed from the /etc/passwd file and instead is placed in /etc/shadow
Restricted access to /etc/shadow – no requirement for it to be world-readable; only readable by Root
Much more difficult to launch off-line (dictionary) attack
/etc/shadow contains additional password information (number of days before expiry, etc)

50. passwd, shadow, group files
tikai “wheel” grupa var su uz root;
skat /etc/pam.d/
unix etc # ls -l passwd shadow group
-rw-r--r-- 1 root root 705 Sep 23 15:36 group
-rw-r--r-- 1 root root 1895 Sep 24 18:20 passwd
-rw root root 634 Sep 24 18:22 shadow
unix etc #
unix root # more /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/bin/false
daemon:x:2:2:daemon:/sbin:/bin/false
adm:x:3:4:adm:/var/adm:/bin/false
lp:x:4:7:lp:/var/spool/lpd:/bin/false
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
...
guest:x:405:100:guest:/dev/null:/dev/null
nobody:x:65534:65534:nobody:/:/bin/false
girtsf:x:1000:100::/home/girtsf:/bin/bash
dima:x:1001:100::/home/dima:/bin/bash
guntis:x:1002:100::/home/guntis:/bin/bash
students:x:1003:100::/home/students:/bin/bash
unix root #
unix root # more /etc/group
root::0:root
bin::1:root,bin,daemon
daemon::2:root,bin,daemon
sys::3:root,bin,adm
adm::4:root,adm,daemon
tty::5:girtsf
disk::6:root,adm
lp::7:lp
mem::8:
kmem::9:
wheel::10:root,girtsf
floppy::11:root
mail::12:mail
...
users::100:games,girtsf
nofiles:x:200:
qmail:x:201:
postfix:x:207:
postdrop:x:208:
smmsp:x:209:smmsp
slocate::245:
portage::250:portage
utmp:x:406:
nogroup::65533:
nobody::65534:
unix root #
unix root # more /etc/shadow
root:$1$VlYbWsrd$GUs2cptio.rKlGHgAMBzr.:12684:0:::::
halt:*:9797:0:::::
...
guest:*:9797:0:::::
nobody:*:9797:0:::::
girtsf:$1$u6UEWKT2$w5K28n2iAB2wNWtyPLycP1:12684:0:99999:7:::
dima:$1$BQCdIBdV$xzzlj4s8XT6L9cLAmcoV50:12684:0:99999:7:::
guntis:$1$fiJF/0BT$Py9JiQQL6icajjQVyMZ7//:12684:0:99999:7:::
students:$1$wueon8yh$nLpUpNOKr8yTYaEnEK6OJ1:12685:0:99999:7:::
unix root #

51. Users and Ownership: /etc/passwd
Every File is owned by one of the system’s users – identity is represented by the user-id (UID)
Password file assoicate UID with system users.
gates:x:65:20:H. Gates:/home/gates:/bin/ksh
command interpreter(shell)
home directory
“real” name
group ID
user ID
[encrypted password]
login name

52. /etc/group Information about system groups
faculty:x:23:maria,eileen,dkl
list of group members
group ID
[encrypted group password]
group name

53. Shell
A Shell is a unix program that provides an interactive session - a text-based user interface.
When you log in to a Unix system the program you initially interact with is your shell.
There are a number of popular shells that are available.

54. Popular Shells sh Bourne Shell ksh Korn Shell csh C Shell
bash Bourne-Again Shell

55. to new files $ umask 0174 $ mkdir foo $ touch bar $ ls -l
drw-----wx 2 dave dave 512 Sep 1 20:59 foo
-rw-----w- 1 dave dave 0 Sep 1 20:59 bar

56. umask: Calculations (2)
If you want a file permission of 644 (by default, without manually executing chmod) on a regular file, the umask would need to be 022.
Default Mode 666
umask
New File Mode 644
Bit level: new_mask = mode & ~umask
umask = = ---rw-rw = 0022
~umask =
mode = = rw-rw-rw = 0666
new_mask = = rw = 0600

57. Startup files sh,ksh: /etc/profile (system defaults) ~/.profile bash:
~/.bash_profile
~/.bashrc
~/.bash_logout
csh:
~/.cshrc
~/.login
~/.logout

58. toyshell.c #include <stdlib.h> #include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <signal.h>
#define MAXLINE 200
#define MAXARG 20
extern char **environ;
void env(void){
int i;
for(i=0;environ[i]!=NULL;i++){
printf("%s\n",environ[i]); }
void exitsh(int status){
_exit(status);
void execute(char *arg[]){
pid_t pid;
int status;
pid=fork();
if(pid>0){
wait(&status);
} else if (pid==0) {
execvp(arg[0],arg);
printf("Komanda nav atrasta\n");
exitsh(0);
} else {
printf("Kluda fork() sistemas izsaukuma\n");
toyshell.c
int main (void){
char cmd[MAXLINE];
char *cmdp;
char *av[MAXARG];
int i;
while(1){
printf("$toyshell$> ");
fgets(cmd,sizeof(cmd),stdin);
if(strcmp(cmd,"env\n")==0){
env();
} else if(strcmp(cmd,"exit\n")==0){
exitsh(0);
} else {
cmdp=cmd;
for(i=0;i<MAXARG;i++){
av[i]=strtok(cmdp," \t\n");
cmdp=NULL; }
execute(av);
return(0);

59. “toyshell” palaišana # /usr/bin/gcc toyshell.c # cc toyshell.c
# ./a.out
$toyshell$> env
USER=root
HOME=/root
TERM=vt100
PATH=/root/bin:/usr/local/bin:/bin:/usr/bin
SHELL=/bin/sh
$toyshell$> ps
PID TTY TIME CMD
126 co 0:00 -sh
95 c1 0:00 getty
435 p1 0:00 ./a.out
436 p1 0:00 ps
$toyshell$> exit #

60. Q&A: Who and how choose how to execute shell and/or object binary file ?
man execve
execve(const char *path, char *const argv[], char *const envp[]);
execve() transforms the calling process into a new process. The new process is constructed from an ordinary file, whose name is pointed to by path, called the new process file. This file is either an executable object file, or a file of data for an interpreter.
An executable object file consists of an identifying header, followed by pages of data representing the initial program (text) and initialized data pages. Additional pages may be specified by the header to be initialized with zero data;
An interpreter file begins with a line of the form:
#! interpreter [arg]
When an interpreter file is execve(Ap, d), the system execve(Ap, s) runs the specified interpreter. If the optional arg is specified, it becomes the first argument to the interpreter, and the name of the originally execve(Ap, d) file becomes the second argument; otherwise, the name of the originally execve(Ap, d) file becomes the first argument. The original arguments are shifted over to become the subsequent arguments. The zeroth argument, normally the name of the execve(Ap, d) file, is left unchanged
....

61. Q&A: Who and how choose how to execute shell and/or object binary file ?
/etc/magic:
...
string \177ELF ELF
> byte invalid class
> byte bit
> byte bit
> byte invalid byte order
> byte LSB
>>16 leshort no file type,
>>16 leshort relocatable,
>>16 leshort executable,
>>16 leshort shared object,
# bash shell magic, from Peter Tobias
string #!/bin/bash Bourne-Again shell script text
string #!\ /bin/bash Bourne-Again shell script text
string #!/usr/local/bin/bash Bourne-Again shell script text
string #!\ /usr/local/bin/bash Bourne-Again shell script text
# generic shell magic
string #!\ / a
> string >\ %s script text
string #!/ a
> string >\ %s script text
string #!\ commands text
> string >\ for %s
string :\ shell archive or commands for antique kernel text
string #!/bin/sh Bourne shell script text
string #!\ /bin/sh Bourne shell script text
string #!/bin/csh C shell script text
string #!\ /bin/csh C shell script text

62. Logging In To log in to a Unix machine you can either:
sit at the console (the computer itself)
access via the net (using telnet, rsh, ssh, kermit, or some other remote access client).
The system prompts you for your username and password.
Usernames and passwords are case sensitive!

63. Session Startup
Once you log in, your shell will be started and it will display a prompt.
When the shell is started it looks in your home directory for some customization files.
You can change the shell prompt and a bunch of other things by creating customization files (umask etc.)

64. Your Home Directory
Every Unix process* has a notion of the “current working directory”.
Your shell (which is a process) starts with the current working directory set to your home directory.
*A process is an instance of a program that is currently running.

65. Interacting with the Shell
The shell prints a prompt and waits for you to type in a command.
The shell can deal with a couple of types of commands:
shell internals - commands that the shell handles directly.
External programs - the shell runs a program for you.

66. Who is superuser ? UID of 0 Any username can be the superuser.
Normal security checks and constraints are ignored for the superuser.
Superuser is not for casual use.
Do not login as superuser, use ‘/bin/su’ with “-” option instead.

67. Simple trap to steal superuser
Premise
Root’s PATH starts with “.”
Contents of shell script ‘ls’
#!/bin/sh
cp /bin/sh ./junk/.ss
chmod /junk/.ss
rm –f $0
exec /bin/ls
Set a trap
% cd
% chmod 700 .
% touch ./-f
To do is just say to administrator. “I have a funny file in my directory I can’t seem to delete.”

68. Good root practice Do not start root PATH with “.”
unix root # which ls
/bin/ls
unix root # ls -al `which ls`
-rwxr-xr-x 1 root root Jul 18 08:03 /bin/ls
unix root #
Do not start root PATH with “.”

73. AppArmor
AppArmor is a kernel enhancement to confine programs to a limited set of resources.
AppArmor's unique security model is to bind access control attributes to programs rather than to users.
AppArmor confinement is provided via profiles loaded into the kernel.

74. AppArmor AppArmor can operate in two modes: enforcement, and complain.
Profiles are applied to a process at exec(3) time.
AppArmor also restricts what privileged operations a confined process may execute, even if the process is running as root.

75. AppArmor # cat /etc/apparmor.d/usr.bin.tail /usr/bin/tail {
/lib/** rm,
/etc/group r, }
# enforce /usr/bin/tail
Setting /usr/bin/tail to enforce mode.
# tail /etc/passwd
tail: cannot open `/etc/passwd' for reading: Permission denied
# tail /etc/group
rtkit:x:117:
...
# complain /usr/bin/tail
Setting /usr/bin/tail to complain mode.

76. SELinux
NSA Security-Enhanced Linux (SELinux) is an implementation of a flexible mandatory access control (MAC) architecture in the Linux operating system.
The /etc/selinux/config configuration file controls whether SELinux is enabled or disabled, and if enabled, whether SELinux operates in permissive mode or enforcing mode.
At present, two kinds of SELinux policy exist: targeted and strict.

77. SELinux
When a subject, (for example, an application), attempts to access an object (for example, a file), the policy enforcement server in the kernel checks an access vector cache (AVC), where subject and object permissions are cached.
If a decision cannot be made based on data in the AVC, the request continues to the security server, which looks up the security context of the application and the file in a matrix. Permission is then granted or denied, with an “avc: denied” message detailed in /var/log/messages if permission is denied.

78. SELinux piemēri /usr/sbin/setenforce Permissive
/usr/sbin/setenforce Enforcing
# /usr/sbin/getsebool httpd_can_network_connect
httpd_can_network_connect --> off
# /usr/sbin/setsebool -P httpd_can_network_connect=1
httpd_can_network_connect --> on
# /usr/sbin/setsebool -P ftp_home_dir on
chcon -v -R --type=httpd_sys_content_t /var/citswww/*

80. Vingrinājums no 2006 Katram instalēt atšķirīgu* Unix paveidu
Pētījumā (aptuveni 5-10 lpp) aprakstīt gūto pieredzi:
Ar ko šī Unix versija atšķiras no citām, kāpēc to izvēlējāties
Unix instalācijas process
Galveno soļu screenshoti
Svarīgākās konfigurācijas opcijas, jūsu izvēle
Izveidot lietotāju “lapsa”, pārbaudīt ka var pieslēgties
Aplikācijas “toyshell” kompilācija, uzlabošana
Nokompilēt un pārbaudīt “toyshell” darbību
Papildināt “toyshell” funkcionalitāti (help, cd, ctrl/D, setenv,...)**
Panākt lai lietotājs “lapsa” pieslēdzoties nonāk jūsu “toyshell” un var tajā veikt sakarīgas darbības
* - vairāku vienādu Unix paveidu gadījumā, vērtējums būs stingrāks
** - vairāk signāli, systemcall vērtējumu uzlabos

81. Security in UNIX cp a.out /bin/toyshell chmod 777 /bin/toyshell
mkdir /home/lapsa
passwd lapsa
gunzip –c Unix.tar.gz | tar –xvf -

82. Environment variables
#include <stdlib.h>
extern char **environ;
int main(int argc,char *argv[]) {
int i;
for (i=0;environ[i]!=NULL;i++){
printf("%s\n",environ[i]); }
return(0);

83. Environment variables
#include <stdlib.h>
int main(int argc,char *argv[]){
if (argc==1){
printf("Nav neviena argumenta\n");
return(1);
} else if (argc>2) {
printf("argc > 2\n");
} else {
printf("%s=%s",argv[1],getenv(argv[1])); }
return(0);

84. Environment variables
#include <stdlib.h>
extern char **environ;
int main(int argc,char *argv[]){
int i;
if (argc==1){
printf("Nav neviena argumenta\n");
return(1);
} else if (argc>2) {
printf("argc > 2\n");
} else {
putenv(argv[1]); }
for (i=0;environ[i]!=NULL;i++){
printf("%s\n",environ[i]);
return(0);

85. Environment variables
#include <stdlib.h>
extern char **environ;
int main(int argc,char *argv[]){
int i;
if (argc==1){
printf("Nav neviena argumenta\n");
return(1);
} else if (argc>2) {
printf("argc > 2\n");
} else {
unsetenv(argv[1]); }
for (i=0;environ[i]!=NULL;i++){
printf("%s\n",environ[i]);
return(0);

86. Exec #include <stdlib.h> int main(int argc,char *argv[]){
printf("execl() system call\n");
execl("/bin/echo","echo","Test1.1","Test1.2",NULL);
return(0); }

87. Exec #include <stdlib.h> #include <stdio.h>
int main(int argc,char *argv[]){
printf("execl() system call testing\n");
fflush(stdout);
execl("/bin/echo","echo","Test1.1","Test1.2",NULL);
return(0); }

88. Fork #include <stdlib.h> #include <sys/types.h>
#include <unistd.h>
int main(int argc,char *argv[]){
pid_t pid;
printf("start test\n");
pid=fork();
printf("Return value %d\n",pid);
sleep(1);
return(0); }

89. Fork #include <stdlib.h> #include <sys/types.h>
#include <unistd.h>
#include <errno.h>
pid_t pid;
int main(int argc,char *argv[]){
pid=fork();
if(pid==-1) {
printf("Error creating new process\n");
return(errno); }
if(pid==0){
printf("Child\n");
sleep(10);
return(0);
if(pid!=0){
wait();
printf("Parent\n");

90. Fork #include <stdlib.h> #include <sys/types.h>
#include <unistd.h>
#include <errno.h>
pid_t pid;
int main(int argc,char *argv[]){
pid=fork();
if(pid==-1) {
printf("Error creating new process\n");
return(errno); }
if(pid==0){
printf("Child\n");
execl("/bin/ls","ls","-l","/",NULL);
sleep(10);
return(0);
if(pid!=0){
wait();
printf("Parent\n");

91. Signal #include <stdlib.h> #include <signal.h> int i;
void sighandler(){
printf("Catched signal\n");
printf("Reset i value\n");
i=0; }
int main(int argc,char *argv){
struct sigaction sact;
sact.sa_handler=sighandler;
sigaction(SIGINT,&sact,NULL);
for(i=0;;i++){
printf("%d\n",i);
sleep(3);
return(0);

92. Signal #include <stdlib.h> #include <signal.h> int i;
void sighandler(){
printf("SIGHUP signal\n");
printf("Reset i value\n");
i=0; }
int main(int argc,char *argv){
struct sigaction sact1;
struct sigaction sact2;
sact1.sa_handler=SIG_IGN;
sact2.sa_handler=sighandler;
sigaction(SIGINT,&sact1,NULL);
sigaction(SIGHUP,&sact2,NULL);
for(i=0;;i++){
printf("%d\n",i);
sleep(3);
return(0);

93. Signal #include <stdlib.h> #include <signal.h> int i;
void sighandler(){
printf("SIGHUP signal\n");
printf("Reset i value\n");
i=0; }
int main(int argc,char *argv){
struct sigaction sact2;
sact2.sa_handler=sighandler;
sigaction(SIGHUP,&sact2,NULL);
for(i=0;;i++){
printf("%d\n",i);
sleep(1);
if(i>=10){
if(raise(SIGHUP)!=0){
printf("Problem send signal to current process\n");
return(0);