24-08-2012, 12:05 PM
IP SPOOFING
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What is IP spoofing
IP spoofing is the creation of IP packets using somebody else’s IP source addresses.
This technique is used for obvious reasons and is employed in several of the attacks
discussed later. Examining the IP header, we can see that the first 12 bytes contain
various information about the packet. The next 8 bytes contains the source and
destination IP addresses. Using one of several tools, an attacker can easily modify these
addresses – specifically the “source address” field. A common misconception is that IP
spoofing can be used to hide our IP address while surfing the Internet, chatting online,
sending e-mail, and so on. This is generally not true. Forging the source IP address
causes the responses to be misdirected, meaning you cannot create a normal network
connection.
Normal network traffic
Valid source IP address, illustrates a typical interaction between a workstation with a
valid source IP address requesting web pages and the web server executing the
requests. When the workstation requests a page from the web server the request
contains both the workstation’s IP address (i.e. source IP address 192.168.0.5) and the
address of the web server executing the request (i.e. destination IP address 10.0.0.23).
The web server returns the web page using the source IP address specified in the
request as the destination IP address (192.168.0.59) and its own IP address as the
source IP address (10.0.0.23).
Network traffic with spoofed IP address
Spoofed source IP address illustrates the interaction between a workstation requesting
web pages using a spoofed source IP address and the web server executing the
requests. If a spoofed source IP address (i.e. 172.16.0.6) is used by the workstation, the
web server executing the web page request will attempt to execute the request by
sending information to the IP address of what it believes to be the originating system
(i.e. the workstation at 172.16.0.6). The system at the spoofed IP address will receive
unsolicited connection attempts from the web server that it will simply discard.
IP ROUTING MECHANISM AND PROBLEMS
The IP routing mechanism is hop by hop. Every IP packet is routed separately. The
route of a IP packet is decided by all the routers the packet goes through. IP address
spoofing is possible because routers only require inspection of the destination IP
address in the packet to make routing decisions. The source IP address is not required
by routers and an invalid source IP address will not affect the delivery of packets. That
address is only used by the destination machine when it responds back to the source.
IP ADDRESS SPOOFING
ASYMMETRIC ROUTING (SPLITTING ROUTING)
Asymmetric routing means traffic goes over different interfaces for directions in and out.
In other words, asymmetric routing is when the response to a packet follows a different
path from one host to another than the original packet did. The more correct and more
general answer is, for any source IP address ‚A’ and destination ‚B’, the path followed by
any packet (request or response) from ‚A’ to ‚B’ is different than the path taken by a
packet from ‚B’ to ‚A’.
IMPLEMENTATION OF ASYMMETRIC ROUTING
Modern operating systems allows us to receive packets from an input interface, different
from the output interface.
In Linux, we can implement asymmetric routing using iptables (linux 2.4):
iptables –A POSTROUTING –t nat –j SNAT –to 192.168.0.5 –o eth0
This means, for all the packets going out via eth0, their source IP address will be
changed to 192.168.0.5. We also have to „disable“ reverse path filtering:
echo “0” > /proc/sys/net/ipv4/conf/all/rp_filter
IP ADDRESS SPOOFING ATTACKS
BLIND IP SPOOFING
Usually the attacker does not have access to the reply, abuse trust relationship between
hosts. For example: Host C sends an IP packet with the address of some other host
(Host A) as the source address to Host B. Attacked host (B) replies to the legitimate host
(A).
Progress of IP spoofing
MAN-IN-THE-MIDDLE ATTACKS
If an attacker controls a gateway that is in the delivery route, he can
• sniff the traffic
• intercept / block / delay traffic
• modify traffic
Progress of a man-in-the-mittle attack
This is not easy in the Internet because of hop-by-hop routing, unless you control one of
the backbone hosts or source routing is used. This can also be done combined with IP
source routing option. IP source routing is used to specify the route in the delivery of a
packet, which is independent of the normal delivery mechanisms. If the traffic can be
forced through specific routes (=specific hosts), and if the reverse route is used to reply
traffic, a host on the route can easily impersonate another host.
ATTACKS CONCERNING THE ROUTING PROTOCOLS
A host can send spoofed RIP packets in order to “inject” routes into a host. This is easy
to implement, it only requires IP/UDP spoofing. On a LAN with RIPv2 passwords have to
be used for updating routes, but plaintext passwords are used. The plaintext passwords
can be sniffed.
IP address spoofing attack with ICMP
ICMP is short for Internet Control Message Protocol, an extension to the Internet
Protocol (IP) defined by RFC 792. ICMP supportspackets containing error, control, and
informational messages. The PING command, for example, uses ICMP to test an
internet connection.
ICMP ECHO ATTACKS
Map the hosts of a network: The attack sends ICMP echo datagram to all the hosts in a
subnet, then he collects the replies and determines which hosts are alive.
Denial of service attack (SMURF attack): The attack sends spoofed (with victim‘s IP
address) ICMP Echo Requests to subnets, the victim will get ICMP Echo Replies from
every machine.
ICMP REDIRECT ATTACKS
ICMP redirect messages can be used to re-route traffic on specific routes or to a specific
host that is not a router at all.
The ICMP redirect attack is very simple: just send a spoofed ICMP redirect message
that appears to come from the host‘s default gateway.
For example: Host A sends a forged ICMP packet to host B, saying the route through A
is a better way to internet. The source IP address of this forged ICMP packet is the
gateway’s IP address C. Then all the traffic from B to internet will go through A.
ICMP DESTINATION UNREACHABLE ATTACKS
ICMP destination unreachable message is used by gateways to state that the datagram
cannot be delivered. It can be used to cut out nodes from the network. It is a denial of
service attack (DOS)
Example: An attacker injects many forged destination unreachable messages stating
that 100.100.100.100 is unreachable) into a subnet (e.g. 128.100.100.*). If someone
from the 128.100.100.* net tries to contact 100.100.100.100, he will immediately get an
ICMP Time Exceeded from the attacker‘s host. For 128.100.100.* this means that there
is no way to contact 100.100.100.100, and therefore communication fails.
UDP attacks
UDP is an unreliable transport layer protocol. It relies on IP, it is connectionless, and its
checksum is optional. Therefore, the delivery, integrity, non-duplication and ordering are
not guaranteed. It is easy to send a forged packet to the target. Compared with this,
TCP is connection oriented and the TCP connection setup sequence number is hard to
predicated, so it is hard to insert forged packet into the TCP connection. Therefore UDP
traffic is more vulnerable for IP spoofing than TCP.