===================================                    
=INTRODUCTION TO DENIAL OF SERVICE=
===================================

Hans Husman
t95hhu@student.tdb.uu.se
Last updated: Mon Oct 28 14:56:31 MET 1996

.0. FOREWORD

.A. INTRODUCTION
	.A.1. WHAT IS A DENIAL OF SERVICE ATTACK?
	.A.2. WHY WOULD SOMEONE CRASH A SYSTEM?
		.A.2.1. INTRODUCTION
		.A.2.2. SUB-CULTURAL STATUS
		.A.2.3. TO GAIN ACCESS
		.A.2.4. REVENGE
		.A.2.5. POLITICAL REASONS
		.A.2.6. ECONOMICAL REASONS
		.A.2.7. NASTINESS
	.A.3. ARE SOME OPERATING SYSTEMS MORE SECURE?

.B. SOME BASIC TARGETS FOR AN ATTACK
	.B.1. SWAP SPACE
	.B.2. BANDWIDTH
	.B.3. KERNEL TABLES
	.B.4. RAM
	.B.5. DISKS
	.B.6. CACHES
	.B.7. INETD

.C. ATTACKING FROM THE OUTSIDE
	.C.1. TAKING ADVANTAGE OF FINGER
	.C.2. UDP AND SUNOS 4.1.3.
	.C.3. FREEZING UP X-WINDOWS
	.C.4. MALICIOUS USE OF UDP SERVICES
    	.C.5. ATTACKING WITH LYNX CLIENTS
	.C.6. MALICIOUS USE OF telnet
	.C.7. MALICIOUS USE OF telnet UNDER SOLARIS 2.4
	.C.8. HOW TO DISABLE ACCOUNTS
	.C.9. LINUX AND TCP TIME, DAYTIME
	.C.10. HOW TO DISABLE SERVICES
	.C.11. PARAGON OS BETA R1.4
	.C.12. NOVELLS NETWARE FTP
	.C.13. ICMP REDIRECT ATTACKS
	.C.14. BROADCAST STORMS
	.C.15. EMAIL BOMBING AND SPAMMING
	.C.16. TIME AND KERBEROS
	.C.17. THE DOT DOT BUG
	.C.18. SUNOS KERNEL PANIC
	.C.19. HOSTILE APPLETS
	.C.20. VIRUS
	.C.21. ANONYMOUS FTP ABUSE
	.C.22. SYN FLOODING
	.C.23. PING FLOODING
	.C.24. CRASHING SYSTEMS WITH PING FROM WINDOWS 95 MACHINES
	.C.25. MALICIOUS USE OF SUBNET MASK REPLY MESSAGE
	.C.26. FLEXlm
	.C.27. BOOTING WITH TRIVIAL FTP

.D. ATTACKING FROM THE INSIDE
	.D.1. KERNEL PANIC UNDER SOLARIS 2.3
	.D.2. CRASHING THE X-SERVER
	.D.3. FILLING UP THE HARD DISK
	.D.4. MALICIOUS USE OF eval
	.D.5. MALICIOUS USE OF fork()
	.D.6. CREATING FILES THAT IS HARD TO REMOVE
	.D.7. DIRECTORY NAME LOOKUPCACHE
	.D.8. CSH ATTACK
	.D.9. CREATING FILES IN /tmp
	.D.10. USING RESOLV_HOST_CONF
	.D.11. SUN 4.X AND BACKGROUND JOBS	
	.D.12. CRASHING DG/UX WITH ULIMIT 
	.D.13. NETTUNE AND HP-UX
	.D.14. SOLARIS 2.X AND NFS
	.D.15. SYSTEM STABILITY COMPROMISE VIA MOUNT_UNION
	.D.16. trap_mon CAUSES KERNEL PANIC UNDER SUNOS 4.1.X

.E. DUMPING CORE
	.E.1. SHORT COMMENT
	.E.2. MALICIOUS USE OF NETSCAPE
	.E.3. CORE DUMPED UNDER WUFTPD
	.E.4. ld UNDER SOLARIS/X86

.F. HOW DO I PROTECT A SYSTEM AGAINST DENIAL OF SERVICE ATTACKS?
	.F.1. BASIC SECURITY PROTECTION
		.F.1.1. INTRODUCTION
		.F.1.2. PORT SCANNING
		.F.1.3. CHECK THE OUTSIDE ATTACKS DESCRIBED IN THIS PAPER
		.F.1.4. CHECK THE INSIDE ATTACKS DESCRIBED IN THIS PAPER
		.F.1.5. EXTRA SECURITY SYSTEMS
		.F.1.6. MONITORING SECURITY
		.F.1.7. KEEPING UP TO DATE
		.F.1.8. READ SOMETHING BETTER
	.F.2. MONITORING PERFORMANCE
		.F.2.1. INTRODUCTION
		.F.2.2. COMMANDS AND SERVICES                      
		.F.2.3. PROGRAMS
		.F.2.4. ACCOUNTING

.G. SUGGESTED READING
	.G.1. INFORMATION FOR DEEPER KNOWLEDGE
	.G.2. KEEPING UP TO DATE INFORMATION
	.G.3. BASIC INFORMATION

.H. COPYRIGHT

.I. DISCLAIMER

.0. FOREWORD
------------

In this paper I have tried to answer the following questions:

	- What is a denial of service attack?
	- Why would someone crash a system?
	- How can someone crash a system.
	- How do I protect a system against denial of service attacks?
	
I also have a section called SUGGESTED READING were you can find
information about good free information that can give you a deeper
understanding about something.

Note that I have a very limited experience with Macintosh, OS/2 and
Windows and most of the material are therefore for Unix use. 

You can always find the latest version at the following address:
http://www.student.tdb.uu.se/~t95hhu/secure/denial/DENIAL.TXT

Feel free to send comments, tips and so on to address:
t95hhu@student.tdb.uu.se

.A. INTRODUCTION
~~~~~~~~~~~~~~~~

.A.1. WHAT IS A DENIAL OF SERVICE ATTACK?
-----------------------------------------

Denial of service is about without permission knocking off
services, for example through crashing the whole system. This
kind of attacks are easy to launch and it is hard to protect
a system against them. The basic problem is that Unix
assumes that users on the system or on other systems will be
well behaved. 

.A.2. WHY WOULD SOMEONE CRASH A SYSTEM?
---------------------------------------
 
.A.2.1. INTRODUCTION
--------------------

Why would someone crash a system? I can think of several reasons
that I have presentated more precisely in a section for each reason,
but for short:

	.1. Sub-cultural status.
	.2. To gain access.
	.3. Revenge.
	.4. Political reasons.
	.5. Economical reasons.
	.6. Nastiness.

I think that number one and six are the more common today, but that
number four and five will be the more common ones in the future.

.A.2.2. SUB-CULTURAL STATUS
---------------------------

After all information about syn flooding a bunch of such attacks
were launched around Sweden. The very most of these attacks were
not a part of a IP-spoof attack, it was "only" a denial of service
attack. Why? 

I think that hackers attack systems as a sub-cultural pseudo career
and I think that many denial of service attacks, and here in the
example syn flooding, were performed for these reasons. I also think
that many hackers begin their carrer with denial of service attacks.

.A.2.3. TO GAIN ACCESS
----------------------

Sometimes could a denial of service attack be a part of an attack to
gain access at a system. At the moment I can think of these reasons
and specific holes:

	.1. Some older X-lock versions could be crashed with a 
	method from the denial of service family leaving the system
	open. Physical access was needed to use the work space after.

	.2. Syn flooding could be a part of a IP-spoof attack method.

	.3. Some program systems could have holes under the startup, 
	that could be used to gain root, for example SSH (secure shell).

	.4. Under an attack it could be usable to crash other machines
	in the network or to deny certain persons the ability to access 
	the system.  	

	.5. Also could a system being booted sometimes be subverted,
	especially rarp-boots. If we know which port the machine listen
	to (69 could be a good guess) under the boot we can send false
	packets to it and almost totally control the boot.

.A.2.4. REVENGE
---------------

A denial of service attack could be a part of a revenge against a user
or an administrator.

.A.2.5. POLITICAL REASONS
-------------------------

Sooner or later will new or old organizations understand the potential
of destroying computer systems and find tools to do it.

For example imaginate the Bank A loaning company B money to build a
factory threating the environment. The organization C therefor crash A:s
computer system, maybe with help from an employee. The attack could cost
A a great deal of money if the timing is right.

.A.2.6. ECONOMICAL REASONS
--------------------------

Imaginate the small company A moving into a business totally dominated by
company B. A and B customers make the orders by computers and depends
heavily on that the order is done in a specific time (A and B could be
stock trading companies). If A and B can't perform the order the customers
lose money and change company.

As a part of a business strategy A pays a computer expert a sum of money to
get him to crash B:s computer systems a number of times. A year later A
is the dominating company.

.A.2.7. NASTINESS
-----------------

I know a person that found a workstation where the user had forgotten to
logout. He sat down and wrote a program that made a kill -9 -1 at a
random time at least 30 minutes after the login time and placed a call to
the program from the profile file. That is nastiness.

.A.3. ARE SOME OPERATING SYSTEMS MORE SECURE?
---------------------------------------------

This is a hard question to answer and I don't think that it will
give anything to compare different Unix platforms. You can't say that
one Unix is more secure against denial of service, it is all up to the
administrator.

A comparison between Windows 95 and NT on one side and Unix on the
other could however be interesting.

Unix systems are much more complex and have hundreds of built in programs,
services... This always open up many ways to crash the system from
the inside.

In the normal Windows NT and 95 network were is few ways to crash
the system. Although were is methods that always will work.

That gives us that no big different between Microsoft and Unix can
be seen regardning the inside attacks. But there is a couple of
points left:

	- Unix have much more tools and programs to discover an
	attack and monitoring the users. To watch what another user
	is up to under windows is very hard.

	- The average Unix administrator probably also have much more
	experience than the average Microsoft administrator.

The two last points gives that Unix is more secure against inside
denial of service attacks.

A comparison between Microsoft and Unix regarding outside attacks
are much more difficult. However I would like to say that the average
Microsoft system on the Internet are more secure against outside
attacks, because they normally have much less services.

.B. SOME BASIC TARGETS FOR AN ATTACK
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.B.1. SWAP SPACE
----------------

Most systems have several hundred Mbytes of swap space to 
service client requests. The swap space is typical used
for forked child processes which have a short life time.
The swap space will therefore almost never in a normal
cause be used heavily. A denial of service could be based 
on a method that tries to fill up the swap space.

.B.2. BANDWIDTH
---------------

If the bandwidth is to high the network will be useless. Most
denial of service attack influence the bandwidth in some way.

.B.3. KERNEL TABLES
-------------------

It is trivial to overflow the kernel tables which will cause
serious problems on the system. Systems with write through
caches and small write buffers is especially sensitive.

Kernel memory allocation is also a target that is sensitive.
The kernel have a kernelmap limit, if the system reach this
limit it can not allocate more kernel memory and must be rebooted.
The kernel memory is not only used for RAM, CPU:s, screens and so
on, it it also used for ordinaries processes. Meaning that any system
can be crashed and with a mean (or in some sense good) algorithm pretty
fast.

For Solaris 2.X it is measured and reported with the sar command
how much kernel memory the system is using, but for SunOS 4.X there
is no such command. Meaning that under SunOS 4.X you don't even can
get a warning. If you do use Solaris you should write sar -k 1 to
get the information. netstat -k can also be used and shows how much
memory the kernel have allocated in the subpaging.
 
.B.4. RAM
---------

A denial of service attack that allocates a large amount of RAM
can make a great deal of problems. NFS and mail servers are 
actually extremely sensitive because they do not need much
RAM and therefore often don't have much RAM. An attack at 
a NFS server is trivial. The normal NFS client will do a 
great deal of caching, but a NFS client can be anything 
including the program you wrote yourself...

.B.5. DISKS
-----------

A classic attack is to fill up the hard disk, but an attack at
the disks can be so much more. For example can an overloaded disk
be misused in many ways.

.B.6. CACHES
-------------

A denial of service attack involving caches can be based on a method
to block the cache or to avoid the cache.

These caches are found on Solaris 2.X:

Directory name lookup cache: Associates the name of a file with a vnode.

Inode cache: Cache information read from disk in case it is needed
again.

Rnode cache: Holds information about the NFS filesystem.

Buffer cache: Cache inode indirect blocks and cylinders to realed disk
I/O.