3.1.2 - IP Behavior I - Fragmentation

5/28/2018 3.1.2 - IP Behavior I - Fragmentation

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Fragmentation - SANS 2000 - 2003 1

Frag mentation

Attackers use fragmentation to mask their probes and exploits. Some intrusion detection systems do

not support packet reassembly and therefore do not detect activity where the signature is split over

multiple datagrams. There are availability, or denial of service attacks such as ssping, that use highly

fragmented traffic to exhaust system resources. Finally, some sophisticated persons of mal-intent use

fragmentation to try to circumvent filtering routers. These are all reasons that you may want to learn

about fragmentation, the topic of the webcast.

By understanding how this facet of IP works, you will be equipped to detect and analyze fragmented

traffic and discover if it is normal fragmentation or fragmentation used for other purposes.


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Objectives

Discuss fragmentation concepts

Examine normal fragmentation

Examine abnormal fragmentation

We will look at fragmentation to see what is happening at the datagram level. We need to be aware of

normal fragmentation before we can identify abnormal; well examine both of these in todays

webcast.


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Normal Fragmentation

Fragmentation can be a very normal and naturally occurring effect of traffic travelling among

variously sized networks. We will consider the theory and composition of normal fragmentation first

to acquaint you with how it should operate.


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Fragmentation theory

Occurs when maximum transmission unit (MTU) smaller

than datagram

Reassembled by destination host

Can be used to bypass routers or intrusion detection

systems

The next slide is titled Fragmentation theory. Fragmentation occurs when an IP datagram

travelling on a network has to traverse a network with a maximum transmission unit (MTU) that is

smaller than the size of the datagram. For instance, for Ethernet, the maximum transmission unit or

maximum size for an IP datagram is 1500 bytes. If a datagram needs to traverse an Ethernet network

and is larger than 1500 bytes, it will have to be fragmented by a router that is directing it to the

Ethernet network. Fragmentation can also occur when a host needs to put a datagram on the network

that exceeds the MTU; in some instances this will be fragmented.

Fragments will continue on to their destination where they will be reassembled by the destination

host. It is even possible for fragments to become further fragmented if they cross an MTU smaller

than the fragment size. While fragmentation is a perfectly normal and naturally occurring event, it is

possible to craft fragments for the purposes of avoiding detection by routers and intrusion detection

systems that dont deal well with fragmentation.


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More fragmentation theory

Reassembled by the receiving host

All fragments:

Must share a common fragment identification number

Must tell what offset in original unfragmented datagram

Must tell length of data payload

Must tell whether another fragment follows this one

Each fragment encapsulated in IP datagram

Continuing with concepts on slideMore fragmentation theory, we examine what kind of

information the fragments must carry for the destination host to reassemble them back to the original

unfragmented state. This information is:

A common fragment identification number. This is cloned from a field in the IP header known as

the IP identification number, also called the fragment ID

Each fragment must say what its place or offset is in the original unfragmented packet

Each fragment must tell the length of the data carried in the fragment

Finally, the fragment must know whether more fragments follow this one

This information will be contained in the IP header. The IP header will be placed in an IP datagram

followed by an encapsulated fragment. All TCP/IP traffic must be wrapped within IP because IP is

the protocol responsible for getting the packet delivered.


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The fragment ID

Each fragment has identifying number - fragment ID

Taken from IP identification field

Value set by a host sending datagram

Value usually increases by 1 for each new datagram sent

Newer TCP/IP stacks randomizing this value

tcpdump output of unfragmented datagram IP

identification value:

ping.com > 192.168.244.2: icmp: echo request (ttl 240, id 202)

Slide The fragment ID examines the origin of the field that identifies fragments. The IP

identification value is a 16 bit field found in the IP header of all datagrams. This uniquely identifies

each datagram sent by the host. Typically, this value is incremented by 1 for each datagram sent by

that host, although we the trend now is to have TCP/IP stacks randomize this value.

When the datagram becomes fragmented, all fragments created from this datagram will contain this

same IP identification number, or the fragment ID. The tcpdump output in this slide shows an IP

identification number of 202 for a datagram that is not fragmented. If this datagram were to be

fragmented on the way to its destination, all fragments created from this datagram would share a

fragment ID of 202.

This tcpdump output was generated using the -vv option. This is a verbose option which says to list

the time to live value and the IP identification values along with normal output.


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Ethernet datagram packaging

Ethernet (MTU = 1500)

20 byte IP

header1480 bytes of embedded data

Turning to the slide Ethernet datagram packaging, we see that a datagram travelling on Ethernet

has a maximum transmission unit of 1500 bytes. Each datagram must have an IP header which is

typically 20 bytes, but can be more if IP options are included.

If you recall, the IP header contains information such as the source and destination IP numbers. It is

considered the network portion of the IP datagram since routers use the information found in the IP

header to direct the datagram towards its destination. Encapsulated after the IP header is some kind

of data. This data can be an IP protocol such as tcp, udp or ICMP. For instance, if this data were tcp,

it would include a tcp header and tcp data.


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Fragmentation using ICMPecho request

ICMP data

IP headerICMP

header

20 8 4000 bytes of ICMP data

4028 total bytes in IP datagram

Ethernet MTU = 1500

1500 bytes 1500 bytes 1068 bytes

(ICMP echo request)

Original 4028 byte fragment broken into 3 fragments of 1500 bytes or less

On slideFragmentation using an ICMP echo request, we have a datagram of 4028 bytes. This

is an ICMP echo request bound for an Ethernet network that has an MTU of 1500. So, the 4028 byte

datagram will have to be divided into fragments of 1500 bytes or less. Each of these 1500 byte

fragmented packets will have a 20 byte IP header so that leaves 1480 bytes maximum for data for

each fragment. Lets examine what each of the individual three fragments looks like.

Normally, you shouldnt encounter a 4,000+ byte echo request. And, if you do, examine it until you

become cross-eyed because something isnt kosher. The reason that this was used for the example

and for instructive purposes is that in the Windows ping, there is a -l switch that allows you to say

how big you want the echo request to be. This allowed the generation and capture by tcpdump of the

packets you see in the upcoming several slides to validate all the information delivered to you is

correct.


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The Breakdown

ICMP data

20 8 4000 bytes of ICMP data4028 total bytes in pre-fragmented IP datagram

1472 ICMP data 1480 ICMP data 1048

1500 1500 1068

1472 + 1480 + 1048 = 4000 bytes of ICMP data= 20 byte IP header

Looking at the slide, The Breakdown, lets see how each fragment is actually formed. Before the

IP datagram is sent on the link that has an MTU of 1500 bytes, we see that is has a total of 4028 bytes

total.

What we have seen is that this IP datagram will be divided into three separate fragments each with a

cloned IP header. The original header is paired with the first fragment and two new headers of 20

bytes each have to be created for the second and third fragments. So, we really need a total of 4068

bytes to send all of this traffic.

The first fragment gets the original IP header, along with the 8 bytes of the ICMP header for a

running total of 28 bytes. With a maximum datagram size of 1500 bytes, 1472 bytes remain for

ICMP data. The second fragment gets a cloned IP header of 20 bytes, and has the remaining 1480

bytes for ICMP data. The final fragment again gets a 20 byte IP header and carries the final 1048

bytes of ICMP data. As a cross check, we see that we have 1500 + 1500 + 1068 bytes of data sent

for a total of 4068 bytes.


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The first fragment

IP header

ICMP header

ICMP echorequest

20 8 1472

Offset = 0Length = 1480

More Fragments = 1

ICMP data

IP Header

1500 total bytes

ICMP dataIP Header

20 8 1472

Looking at the slide The first fragment we turn our concentration to the initial fragment in the

fragment train. The original IP header will be cloned to contain the identical fragment

identification numbers for the first and remaining fragments. Remember, all fragments must be

carried in an IP datagram. An IP datagram requires an IP header to direct it to its destination.

The first fragment is the only one that will carry with it the ICMP message pseudo-header.

As we see, the first fragment has a 0 offset, a length of 1480 bytes, 1472 bytes of data and 8 of ICMP

header, and more fragments follow so that more fragments flag is set.


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Composition of the first fragment

First fragment 1500 total bytes in IP datagram

20 8 1472 ICMP data bytes

Protocol = ICMP

Fragment ID = 21223

More Fragments Flag = 1

Fragment Offset = 0Data Length = 1480

IP Header ICMP pseudo-header

Type = ICMP echo request

SlideComposition of the first fragmentexplains the configuration of the first fragment in the

fragment train. The first 20 bytes of the 1500 bytes are the IP header. The next 8 bytes are occupied

by the ICMP pseudo-header. Recall that this was an ICMP echo request that has an 8 byte pseudo-

header in its original packet. The remaining 1472 bytes are for ICMP data.

In addition to the normal fields carried in the IP header such as source and destination IP and

protocol, in this instance, ICMP, there are fields that are specifically for fragmentation. The

fragment ID with a value of 21223 will be the common link for all the fragments in the fragment

train. There is a field known as the more fragments flag that indicates that another fragment follows

the current one. In this first fragment, the flag will be set to 1 to indicate that more fragments do

follow. Also, the offset of the data contained in this fragment relative to the data of whole datagram

must be stored. For the first record, the offset will be 0. The 1480 bytes of data represents the 8 byte

ICMP pseudo-header followed by the first 1472 bytes of the ICMP data; it does not include the IP

header.


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The second fragment

1480

Offset = 1480

Length = 1480More Fragments = 1

ICMP data

IP Header ICMP data

20 1480

1500 total bytes

Looking at the slide The second fragment we focus on the next fragment in the fragment train.

An IP header will be cloned from the original header with an identical fragment identification

number, and most of the other data in the IP header such as the source and destination numbers will

be replicated for the new header. Embedded after this new IP header will be 1480 ICMP data bytes.

As we see, the second fragment has an offset of 1480, a length of 1480 bytes, and one more fragment

follows so the more fragments flag is set.


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Composition of the secondfragment

Second fragment 1500 total bytes in IP datagram

20 1480 ICMP data bytes

Protocol = ICMP

Fragment ID = 21223



IP Header

Continuing with fragmentation on slideComposition of the second fragment, we examine the IP

datagram carrying the second fragment. As with all fragments, it requires a 20 byte IP header.

Again, the protocol in the header will indicate ICMP. The fragment identification number will

remain 21223. And, the more fragments flag will be turned on because another fragment follows.

The offset is 1480 bytes into the data portion of the original ICMP message data. The previous

fragment occupied the first 1480 bytes. This fragment will be 1480 bytes long as well and it is

composed entirely of ICMP data bytes.

It is worth noting that the ICMP pseudo-header in the first fragment doesnt get cloned along with the

ICMP data. This means if you were to examine this fragment alone, you could not tell what the

ICMP message type is in this case ICMP echo request. This becomes an important issue when we

discuss filtering devices in a while.


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The third fragment

1048

Offset = 2960

Length = 1048More Fragments = 0

IP Header

20 1048

1068 total bytes

ICMP data

ICMP data

Looking at the slide The third fragment we examine the final fragment in the fragment train.

Again, an IP header will be cloned from the original header with an identical fragment

identification number, and other fields will be replicated for the new header. Embedded in this new

IP datagram will be the final 1048 ICMP data bytes.

As we see, the third fragment has an offset of 2960, a length of 1048 bytes, and no more fragments

follow so the more fragments flag is 0.


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Composition of the final

fragmentThird fragment 1068 total bytes in IP Datagram

20 1048 ICMP data bytes

Protocol = ICMP

Fragment ID = 21223



IP Header

SlideComposition of the final fragmentdepicts the last fragment in the fragment train. Again,

20 bytes are reserved for the IP header. The remaining ICMP data bytes are carried in the data

portion of this fragment. The fragment ID is 21223, the more fragments flag is not set because this is

the last fragment. The offset is 2960 (this is the sum of the two 1480 byte previous fragments). There

are only 1048 data bytes carried in this fragment comprised entirely of the remaining ICMP message

bytes.

This fragment, like the second one will have no ICMP pseudo-header and therefore no ICMP

message type to reflect that this is an ICMP echo request.


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tcpdump output of

fragmentationping.com > myhost.com: icmp: echo request (frag 21223:1480@0+)

ping.com> myhost.com: (frag 21223:1480@1480+)

ping.com> myhost.com: (frag 21223:1048@2960)

fragment

ID

Number of data

bytes in currentfragment

Offset into the

data that this

fragment falls

More fragments to

follow

Examining the tcpdump output on slide tcpdump output of fragmentation, we see the three

different records representing the three fragments weve discussed. This means that the host running

tcpdump has collected the ICMP echo request after the fragmentation occurred.

The first line shows ping.com sending an ICMP echo request to myhost.com. The reason that tcpdump

can identify this as an ICMP echo request is because the first fragment contains the 8 byte ICMP

pseudo-header which identifies this as an ICMP echo request. Now, lets look at fragmentation

notation at the right side of the record. tcpdump convention for displaying fragmented output is the

word frag appears followed by the fragment ID, 21223, in our example, followed by a colon. The

length of data in the current fragment follows, 1480, followed by an @ sign, and then you see the offset

into the data, 0, since this is the first fragment. The plus sign indicates that the more fragments flag is

set.

The second record is somewhat different. Notice that there is no ICMP echo request label. This is

because there is no ICMP pseudo-header to tell what kind of ICMP traffic this is. The IP header will

still have the protocol field set to ICMP, yet that is all you can tell looking at this fragment alone. We

see the tcpdump output lists the fragment ID of 21223, the current data length is 1480 and the offset is

1480. The plus sign signifies that the more fragments flag is set.

The last line is very similar to the second one in format. It shows the same fragment ID of 21223, it has

a length of 1048 and a displacement of 2960. But, there is no more fragments flag in the final record,

as we would expect.


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Fragmentation and a packet

filtering device Only first fragment carries protocol header

If packet filter that is not stateful blocks inbound traffic:

May block first fragment only

Subsequent fragments may be allowed in

Fragments reassembled:

By destination host

Not by packet filtering device incapable of keeping state

Slide Fragmentation and a packet filtering device covers fragmentation and how a packet filtering

device such as a router or firewall may deal with it. The problem arises when such a device attempts to

block traffic that is fragmented.

Because only the first fragment of a fragment train will contain any kind of protocol header such as

TCP, UDP, or ICMP, only this fragment will be prevented from entry into the network guarded by a

packet filtering device incapable of examining state of a header field. What we mean by state is that it

appears obvious to us that any fragment sharing the fragment ID of the one that is blocked should also

be blocked. However, some packet filtering devices dont maintain this information. They look at each

fragment as an individual entity and dont connect it with previous or subsequent packets.

If a particular packet doesnt match the blocking criteria, in this instance because of the absence of the

ICMP pseudo-header, it will be allowed into the network. This applies to TCP and UDP as well.

Fragmented TCP or UDP datagrams may contain their respective header information in the first

fragment only. Header information such as destination ports are often used as criteria upon which

blocking is based. So fragmented TCP and UDP are susceptible to the same shortcomings of a

stateless packet filtering device.

One final point to remember is that IP is not a reliable protocol and it is very possible for the firstfragment that contains the protocol header information to be lost. When this occurs, the packet filtering

device has an even more difficult job of allowing or denying traffic. In fact, if one of the fragments does

not arrive at the destination, all must be resent.


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The Dont Fragment Flag

Located in the IP header

If this flag is set, packet will not be fragmented

If fragmentation required, datagram will be discarded

May be set by sending host to determine smallest

MTU on path to destination

Once determined, datagrams will be sent with a size

smaller than MTU

Moving to slide The Dont Fragment flag, we discuss a new flag. In some of the tcpdump output

we looked at in Part I of IP Behavior, you saw the letters DF in parentheses. This means the dont

fragment flag is set. As the name implies, if this flag is set, fragmentation will not be done on the

datagram. If this flag is set and the datagram crosses a network where fragmentation is required, the

router will discover this, discard the datagram and send an ICMP error message back to the sending

host.

The ICMP error message will contain the MTU of the network that required fragmentation. Some

hosts intentionally send an initial datagram across the network with the DF flag set as a way to

discover the MTU for a particular source to destination. If the ICMP error message is returned with

the smaller MTU, the host will then package all datagrams bound for that destination in small enough

units to avoid fragmentation.


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tcpdump Output With DF Flag Set

and Fragmentation Required

11:30:55.270000 router.ru > mail.mysite.com: icmp:

host.ru unreachable - need to frag (mtu 536)(DF)

MTU = 536Send traffic to host.ru, DF set

Datagram too big, cant fragment

mail.mysite.comrouter.ru host.ru

On slide tcpdump output with the DF flag set and fragmentation required, we see tcpdump

output from an ICMP message in which a router discovered that fragmentation was necessary, yet the

dont fragment flag was set. The stimulus for this reply was that mail.mysite.com attempted to send

a datagram larger than 536 bytes to host.ru with the DF flag set. router.ru finds that the datagram

must traverse a smaller network with an MTU of 536 bytes and fragmentation is necessary.

When router.ru examines the record, it finds that the dont fragment flag is set and an ICMP message

is sent back to mail.mysite.com informing it of the problem. Now, mail.mysite.com will either have

to package the datagrams to be smaller than the MTU of 536 so fragmentation doesnt occur or

remove the DF flag so fragmentation can occur and then resend the datagram.


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Malicious Fragmentation

We now examine the topic offragmentation used for purposes other than the intended ones.


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Ping O Death fragmentation attack

Uses fragmented ICMP packets for denial of service

Very large datagram crafted using fragments

When reassembled by victim host, maximum IP

datagram size of 65,535 bytes exceeded

Causes some vulnerable hosts to crash or freeze

The Ping O Death fragmentation attack slide discusses a denial of service attack which uses a

ping system utility to create an IP packet that exceeds the maximum 65,535 bytes of data allowed by

the IP specification. The oversize packet is then sent to a victim host. Systems may crash, hang, or

reboot when they receive such a maliciously crafted packet. This attack is not new, and all OS

vendors should have fixes in place to handle the oversize packets.


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Ping O Death

Hostile Host

Victim Host

Frag 1 Frag 2 Frag3 Frag x

Total byte length of fragments > 65,535

Reassembled length > 65,535

65535

In the pictorial representation of Ping O Death slide, we see a hostile host crafting an oversized

IP datagram from smaller fragments. When the victim host receives these fragments and attempts to

reassemble them, it may experience a denial of service failure when its reassembled datagram length

exceeds 65,535 bytes.


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tcpdump output of Ping O Death

attack

evilping.com > victimhost.com: icmp: echo request (frag 56980:1480@0+)

evilping.com > victimhost.com: (frag 56980:1480@1480+)






evilping.com > victimhost.com: (frag 56980:1480@62160+)evilping.com > victimhost.com: (frag 56980:1480@63640+)

evilping.com > victimhost.com: (frag 56980:1480@65120)

tcpdump output of Ping O Death attack displays how evilping.com fashions the fragments to

send to victimhost.com. Judging from the fragment lengths of 1480 bytes, we conclude that the

MTU is 1500 - 1480 bytes of data and 20 of IP header. This is the MTU of Ethernet.

We see the first fragment is an ICMP echo request because it carries the ICMP pseudo-header

identifying the ICMP message type. The first 6 fragments are displayed and then for space purposes,

we omit the middle fragments. We continue with the final 5 fragments. The fragment of interest is

the final fragment. It begins at an offset of 65120 bytes for a length of 1480 bytes. This brings the

the byte length to 66600. The maximum allowable IP datagram size of 65535 has been exceeded

and victimhost.com may suffer some ill consequences upon reassembly.


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tcpdump output of Teardrop attack

Examine the tcpdump output of the fragmented traffic

below

What is wrong with it?

evilfrag.com.139 > target.net.139: udp 28 (frag 242:36@0+)

evilfrag.com > target.net: (frag 242:4@24)

Examining tcpdump output of Teardrop attack, we see a new type of denial of service using

udp. The Teardrop attack exploits weaknesses in the reassembly process of fragments. The

Teardrop program creates fragments with overlapping offset fields. When these fragments are

reassembled at the destination host, some systems will crash, hang, or reboot. Again, this attack has

been around for several years so patches should be available for vulnerable systems.

The first fragment is not per specification because the length of the fragment is not evenly divisibleby 8. All fragments but the final one should have a payload divisible by 8 or they are considered

non-standard. The reason that fragment sizes need to be a multiple of 8 is because the value found in

the fragment offset field in the IP header is always multiplied by 8. This field is used to determine

the offset of the current fragment into the fragment train. Therefore, if it must be a multiple of 8, all

previous fragments must have payloads evenly divisible by 8.


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Teardrop attack



Byte 0 Byte 24 Byte 27 Byte 35

Fragment 2

Fragment 1

If you look at slide Teardrop attack, youll see tcpdump output from Teardrop fragmentation.

The first fragment delivered is a UDP datagram which has a fragment ID of 242, a length of 36 data

bytes and an offset of zero. We represent this in the diagram in the patterned rectangles. It spans

bytes 0 through 35 inclusive.

Now, the second fragment comes along. It is associated with the first fragment because of fragment

ID of 242, it has a length of 4 and it begins at an offset of 24 bytes into the data portion. It is

depicted in the solid color in the middle. As you can see, it actually overlaps bytes 24 through 27 of

the first fragment. Some unpatched versions of operating systems cannot handle this anomaly and

will reboot or hang when an attempt is made to reassemble received Teardrop fragmentation.


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tcpdump Output of Unknown Activity

Examine the tcpdump output of the fragmented traffic

below

What is wrong with it?



We investigate another instance of malicious activity in tcpdump output of unnamed attack.

While there is no known name for this type of activity, the guess is that this is an attempt at denial of

service by using udp much as the Teardrop attack used. While the teardrop attack used overlapping

fragments, this appears to be exploiting some kind of vulnerability with a gap in fragments.

There are a couple of possibilities for the explanation of the missing fragment. First, it is possiblethat the fragment just got lost on its way to target.net and never reached it. The second, more likely

possibility is that it was some kind of intentional omission. The reason that the second explanation is

favored is because this fragmentation doesnt look like normal fragmentation.

Again, the first fragment payload is not a multiple of 8. Also, you typically dont see fragments that

are smaller than 512 bytes because MTUs are usually not that small.


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Unnamed Activity



Fragment 1 Fragment 2

Byte 0 Byte 17 Byte 48 Byte 163

Missing Data

As we study the Unnamed Attack slide, we see what is going on with offsets. The first fragment

with fragment ID 242 begins at byte zero and has a length of 18 bytes. So, that means it spans bytes

0 through 17 inclusive.

The second fragment with fragment ID 242 begins at offset 48 and has a length of 116 bytes. This

fragment spans bytes 48 through 163 inclusive. As we observe, bytes 18 through 47 appear to be

missing. The suspicion is that this is crafted fragmentation since we are not likely to see MTUs

small enough to cause this naturally. Also, normal fragmentation definitely would not be divided as

we see in this slide.

In the ICMP section of this course, well see where a host that receives an incomplete set of

fragments will discard the fragments after a given amount of time. The time after which the

fragments are expired varies based on the operating system, but can be after half a minute or a

minute, for example. So, the above traffic should not be considered to be an attack. But it is possible

that someone may be attempting to see if target.net is alive by receiving an ICMP error message in

response.


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Fragmentation review

Necessary when datagram crosses a network smaller than the

datagram length

Datagram divided into fragments which will be reassembled by

the receiving host by using:

The fragment ID

The fragment offset

The fragment data length

The more fragments flag

DF flag can be used for MTU discovery to avoid fragmentation

Fragmentation can be used for malicious purposes

As slideFragmentation review discusses, weve seen where fragmentation is a normal

occurrence for a datagram travelling from a larger to a smaller network. If a datagram requires

fragmentation along the path to the destination, a router on the network with the smaller MTU will

fragment the datagram if the dont fragment flag is not set.

Each fragment will be encapsulated in an IP datagram. Every IP header will contain information

such as the fragment ID, the offset, the length of the current fragment and whether other fragments

follow. The destination host will be responsible for reassembling the fragments and will use the

information included in the IP header to do so. The DF flag can be used as a mechanism to discover

the MTU to the destination host and provide more appropriate datagram packaging thus avoiding

fragmentation all together.

We examined several examples of malicious fragmentation. Most of our examples showed attempted

denial of service attacks against a victim host, the last exhibited stealth behavior to sneak past a

router.


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Quiz

1) Fragmented packets are reassembled by:

a) The destination host

b) The source host

c) Every router that examines the packet

d) There is no need to reassemble fragments

2) The first fragment in a normal fragment train will have the following:

a) The dont fragment flag set and a zero offset

b) The more fragments flag set and a zero offset

c) The dont fragment flag set and a non-zero offset

d) The more fragments flag set and a non-zero offset

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Quiz (2)

3) The Ping O Death attack attempts to exceed an IP datagram size of

65535 bytes which represents:

a) The maximum length of an IP datagram header

b) The maximum length of a fragment offset

c) The maximum length of the TCP header

d) The maximum length of an IP datagram

4) Every normal fragment will have:

a) An IP header to assist in routing the datagram to its destination

b) 1480 bytes of data

c) 1500 bytes of data

d) A non-zero offset



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Quiz (3)

5) Normal fragmentation occurs when:

a) The IP header in a datagram exceeds 20 bytes

b) A tcp session is terminated

c) The size of the datagram is greater than the MTU

d) The size of the datagram is less than the MTU

6) One common piece of shared information among all normal fragmentsis:

a) The port numbers

b) The SYN number

c) The ACK number

d) The fragment ID



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Quiz (4)

7) The Ping O Death exploit uses fragmentation to attempt a denial ofservice by:

a) Sending fragments out of order

b) Sending fragments that when reassembled exceed the maximum IPdatagram size

c) Sending fragments that exceed the MTU size

d) Sending very small fragments

8) The fragment ID come from which of the following fields:

a) The time to live

b) The final octet of the source IP

c) The final octet of the destination IPd) The IP identification



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Quiz (5)

9) The dont fragment flag may be used by hosts as a discovery

mechanism to:

a) Fragment datagrams on byte boundaries

b) Let the destination host know fragmentation is occurring

c) Force the router to fragment the datagram

d) Find the MTU from source to destination in an attempt to avoid

fragmentation by packaging datagrams in a size smaller than the MTU

10) The final fragment in a normal fragment train should always have:

a) The dont fragment flag set

b) A zero offset

c) The FIN flag set

d) The more fragments flag set to zero to signify no more fragments

follow



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Quiz (6)

Questions 11-15 pertain to this tcpdump output:

myhost.com > yourhost.com: icmp echo reply (frag 11259:1000@0+)

11) The + at the end of the tcpdump output signifies:

a) Dont fragment this datagram

b) More fragments follow

c) This is the final fragment

d) This is the first fragment

12) The fragment ID for this record is:

a) 0

b) 1500c) 1000

d) 11259



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Quiz (7)


13) The length of this fragment is:

a) 0

b) 1480

c) 11259

d) 1000

14) We know that this is the first fragment because:

a) The offset is 0

b) More fragments followc) It has a fragment ID

d) The length is less than 1500



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Quiz (8)


15) Another reason that we know that this is the first fragment is because

it has an ICMP protocol header embedded in it. We can tell this by:

a) We see a source host

b) We see a destination host

c) We see this is an ICMP echo reply meaning that there is an ICMP

pseudo-header embedded in the datagram

d) We see a non-zero fragment ID



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Quiz (9)

16) (T/F) Fragmentation can be a problem for routers that do not keep

track of state because they are unaware of previous and subsequent

fragments.

17) (T/F) The Teardrop attack attempts a denial of service by sending

overlapping fragments.

18) (T/F) The Dont Fragment flag is used to say more fragments follow.

19) (T/F) One of the possible purposes for sending malicious

fragmentation is for a denial of service.



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Quiz (10)

20) (T/F) Normal fragments can bypass a filtering router because they

may contain only data and may not have protocol header fields upon

which filtering choices are based.

21) (T/F) Normal fragmentation can occur with the tcp protocol only, udp

and ICMP arent big enough to require fragmentation.



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Quiz (11)

Questions 22 - 26 pertain to this tcpdump output:

evilfrag.com.139 > victim.net.139: udp 56 (frag 123:24@0+)

evilfrag.com > victim.net: (frag 123:16@64)

22) (T/F) The two related fragments overlap.

23) (T/F) The first fragment begins at byte 24 and has a length of 0.

24) (T/F) The second fragment begins at byte 64 and has a length of 16.

25) (T/F) The IP identification number for both these fragments is 123.

26) (T/F) No final fragment was sent.



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Quiz (12)

Questions 27 - 30 pertain to this tcpdump output:

evilfrag.com.139 > victim.net.139: udp 56 (frag 456:40@0+)

evilfrag.com > victim.net: (frag 456:8@24)

27) (T/F) These two fragments overlap.

28) (T/F) The first fragment begins at byte 0 and has a length of 40.

29) (T/F) The second fragment begins at byte 8 and has a length of 24.

30) (T/F) Bytes 8 - 23 overlap for these two fragments.

Answers:

1) a 16) T

2) b 17) T

3) d 18) F

4) a 19) T

5) c 20) T

6) d 21) F

7) b 22) F

8) d 23) F

9) d 24) T

10) d 25) T

11) b 26) F

12) d 27) T

13) d 28) T

14) a 29) F15) c 30) F


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Course Revision History

v1.0 - J. Novak Sep 20 00

v1.1 12-15-2000 (JHN) - (per recommendation of S. Northcutt) added slide The Breakdown

v1.2 edited by J. Kolde, minor formatting changes 16 Jan 2001

v1.3 edited by J. Novak, missing fragment clarification 23 Feb 2001

v1.4 edited by J. Novak, new router graphic 22 May 2001

v1.5 edited by J. Novak, minor edit p.27 6 July 2001

v1.6 - edited by J. Novak, intro changed, qualification on fragment offsets 5 July 2002.v.1.7 edited by J. Novak, slide 6 added sentence to slide and notes (end of first paragraph) about

randomizing IPID now. Slide 9 slide color changes for IP header per Stephens request. Slide 11

last paragraph changed one sentence deleted and new last one added. Slide 19 changed MTU of

308 to 536 in all occurrences (slides and notes). Slide 23 added periods in middle of slide to show

break/continuation of records.

Documents

3.1.2 - IP Behavior I - Fragmentation