IP fragmentation attacks

In computer networking the process of breaking up a single IP datagram into two or more IP datagrams of smaller size is called IP fragmentation. This comes from the fact that every datalink medium has a limit on the size of the transmitting frame called the MTU (Maximum Transmission Unit).

Part of the TCP/IP suite is the Internet Protocol (IP), which resides at the Network layer of the OSI model. IP is responsible for the transmission of packets between network end points. IP includes some features which provide basic measures of fault-tolerance (time to live, checksum), traffic prioritization (type of service) and support for the fragmentation of larger packets into multiple smaller packets (ID filed, fragment offset). The support for fragmentation of larger packets provides a protocol allowing routers to fragment a packet into smaller packets when the original packet is too large for the supporting datalink frames. IP fragmentation exploits (attacks) use the fragmentation protocol within IP as an attack vector.

Fragmentation process

Since IP datagrams are encapsulated in datalink frames, MTU affects larger IP datagrams and forces them to be split into pieces each of which is smaller than the MTU size.

To solve this issue we have 3 choices of which only one is applicable:

* To keep IP datagram size smaller than the MTU. This is not applicable since no one knows what datalink mediums a packet will traverse until the end of its journey. You may send a packet through a 802.3 datalink medium with MTU equal to 1500 and the packet may then be routed through different mediums such as Token ring on the way. Each datalink medium has their own specific MTU. (However, in practice, an MTU of 576 is honored across almost all Internet systems and considerable amounts of code exist which assume this and will fail if the MTU is smaller since they assume messages shorter than 576 bytes will not fragment and so only do one read to receive that message, rather than looping on read until the correct number of bytes have been read.)Fact|date=January 2008

* To set the IP datagram size equal or smaller than the directed attached medium (In our case 802.3) and hand away all further fragmentation of datagrams to routers. Meaning that routers decide if the current datagram should be re-fragmented or not. This one is a really ugly solution. The problem is, this offloads a lot of work on to routers, and in the worst case, can also result In packets being segmented by several IP routers one after another, resulting in very peculiar fragmentation.

* To preview all datalink medium up to destination and choose the smallest MTU belongs to this route. This way we make sure that the fragmentation can be done by the ending hosts and there is no re-fragmentation anymore. This solution, called Path MTU Discovery, allows a sender to fragment/segment a long internet packet, Rather than relying on routers to perform IP-level fragmentation. This is more efficient and more scalable. It is therefore the recommended method in the current Internet. (The problem with this approach is that each packet is routed independently; they may well typically follow the same route, but they may not, and so a probe packet to determine fragmentation may follow a path different from paths taken by later packets).

Three fields in the IP header are used to implement fragmentation and reassembly. The Identification, "Flags" and "Fragment Offset" fields.

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version| IHL | Differentiative Services | Total Length
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Identification | Flags | Fragment Offset |<-- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Time to Live | Protocol | Header Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Destination Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Options | Padding
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Flags:

: A 3 bit field which says if the datagram is a part of a fragmented data frame or not.

: Bit 0: reserved, must be zero: Bit 1: (AF) 0 = May Fragment, 1 = Don't Fragment.: Bit 2: (AF) 0 = Last Fragment, 1 = More Fragments.

0 1 2 13 bits +---+---+---+ +-----------------------------+
| D | M | | Fragment Offset
0 | F | F | +-----------------------------+ +---+---+---+

Fragment Offset specifies the fragment's position within the original Datagram, measured in 8-byte units.

Accordingly, every fragment except the last must contain a multiple of 8 bytes of data. It is obvious that Fragment Offset can hold 8192 (2^13) units but the datagram can't have 8192 * 8 = 65536 bytes of data because "Total Length" field of IP header records the total size including the header and data. An IP header is at-least 20 bytes long so the maximum value for "Fragment Offset" is restricted to 8189 which leaves room for 3 bytes in the last fragment.

Because an IP internet can be connectionless, fragments from one datagram may be interleaved with those from another at the destination. The "Identification field" uniquely identifies the fragments of a particular datagram.

The source system sets "Identification" in each datagram to a unique value. For all datagrams using the same source IP address, destination IP address, and "Protocol" values for the lifetime of the datagram on the internet. This way the destination can distinguish which incoming fragments belong to a unique datagram and buffer all of them until the last fragment received. The last fragment sets the "More Fragment" bit to 0 and this tells the receiving station to start reassembling the data if all fragments have been received.

The following is a real-life fragmentation example:

The following was obtained using the Ethereal protocol analyzer to capture ICMP echo request packets. To simulate this open up a terminal and type ping ip_dest -n 1 -l 65000.

The results are as follows:

No. Time Source Destination Protocol Info 1 0.000000 87.247.163.96 66.94.234.13 ICMP Echo (ping) request 2 0.000000 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=1480) 3 0.002929 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=2960) 4 6.111328 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=4440) 5 6.123046 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=5920) 6 6.130859 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=7400) 7 6.170898 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=8880) 8 6.214843 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=10360) 9 6.239257 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=11840) 10 6.287109 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=13320) 11 6.302734 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=14800) 12 6.327148 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=16280) 13 6.371093 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=17760) 14 6.395507 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=19240) 15 6.434570 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=20720) 16 6.455078 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=22200) 17 6.531250 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=23680) 18 6.550781 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=25160) 19 6.575195 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=26640) 20 6.615234 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=28120) 21 6.634765 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=29600) 22 6.659179 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=31080) 23 6.682617 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=32560) 24 6.699218 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=34040) 25 6.743164 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=35520) 26 6.766601 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=37000) 27 6.783203 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=38480) 28 6.806640 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=39960) 29 6.831054 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=41440) 30 6.850586 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=42920) 31 6.899414 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=44400) 32 6.915039 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=45880) 33 6.939453 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=47360) 34 6.958984 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=48840) 35 6.983398 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=50320) 36 7.023437 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=51800) 37 7.046875 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=53280) 38 7.067382 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=54760) 39 7.090820 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=56240) 40 7.130859 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=57720) 41 7.151367 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=59200) 42 7.174804 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=60680) 43 7.199218 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=62160) 44 7.214843 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=63640) 45 7.258789 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=65120)

The first packet details: No.Time Source Destination Protocol Info 1 0.000000 87.247.163.96 66.94.234.13 ICMP Echo (ping) request

Frame 1 (1514 bytes on wire, 1514 bytes captured)Ethernet II, Src: OmronTat_00:00:00 (00:00:0a:00:00:00), Dst: 40:0f:20:00:0c:00 (40:0f:20:00:0c:00)Internet Protocol, Src: 87.247.163.96 (87.247.163.96), Dst: 66.94.234.13 (66.94.234.13)Internet Control Message Protocol Type: 8 (Echo (ping) request) Code: 0 Checksum: 0x6b7d Identifier: 0x0600 Sequence number: 0x0200 Data (1472 bytes)

The second packet details: No. Time Source Destination Protocol Info 2 0.000000 87.247.163.96 66.94.234.13 IP Fragmented IP protocol (proto=ICMP 0x01, off=1480)

Frame 2 (1514 bytes on wire, 1514 bytes captured)Ethernet II, Src: OmronTat_00:00:00 (00:00:0a:00:00:00), Dst: 40:0f:20:00:0c:00 (40:0f:20:00:0c:00)Internet Protocol, Src: 87.247.163.96 (87.247.163.96), Dst: 66.94.234.13 (66.94.234.13)Data (1480 bytes)

Note that Only the first fragment contains the ICMP header and all remaining fragments generated without ICMP header.

Two important points here:

# In some datalink protocols such as Ethernet, Only the first fragment contains the full upper layer header. Meaning that other fragments look like beheaded datagrams.

# Additional overhead imposed over network because all fragments contains their own IP header. Additional overhead = (number_of_fragments - 1) * (ip_header_len);

IP fragmentation exploits

IP fragment overlapped

The IP fragment overlapped exploit occurs when two fragments contained within the same IP datagram have offsets that indicate that they overlap each other in positioning within the datagram. This could mean that either fragment A is being completely overwritten by fragment B, or that fragment A is partially being overwritten by fragment B. Some operating systems do not properly handle fragments that overlap in this manner and may throw exceptions or behave in other undesirable ways upon receipt of overlapping fragments. This is the basis for the teardrop Denial of service attacks.

IP Fragmentation Buffer Full

The IP Fragmentation Buffer Full exploit occurs when there is an excessive amount of incomplete fragmented traffic detected on the protected network. This could be due to an excessive number of incomplete fragmented datagrams, a large number of fragments for individual datagrams or a combination of quantity of incomplete datagrams and size/number of fragments in each datagram. This type of traffic is most likely an attempt to bypass security measures or Intrusion Detection Systems by intentional fragmentation of attack activity.

IP Fragment Overrun

The IP Fragment Overrun exploit is when a reassembled fragmented datagram exceeds the declared IP data length or the maximum datagram length. By definition, no IP datagram should be larger than 65,535 bytes. Systems that try to process these large datagrams can crash, and can be indicative of a denial of service attempt.

IP Fragment Overwrite

Overlapping fragments may be used in an attempt to bypass Intrusion Detection Systems. In this exploit, part of an attack is sent in fragments along with additional random data; future fragments may overwrite the random data with the remainder of the attack. If the completed datagram is not properly reassembled at the IDS, the attack will go undetected.

IP Fragment Too Many Datagrams

The Too Many Datagrams exploit is identified by an excessive number of incomplete fragmented datagrams detected on the network. This is usually either a denial of service attack or an attempt to bypass security measures.

IP Fragment Incomplete Datagram

This exploit occurs when a datagram can not be fully reassembled due to missing data. This can indicate a denial of service attack or an attempt to defeat packet filter security policies.

IP Fragment Too Small

An IP Fragment Too Small exploit is when any fragment other than the final fragment is less than 400 bytes, indicating that the fragment is likely intentionally crafted. Small fragments may be used in denial of service attacks or in an attempt to bypass security measures or detection.

References

#
# [http://www.ietf.org/rfc/rfc1858.txt RFC 1858]

External links

[http://www.packetforum.org network programming and network security research forum]