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Multiple DNS Implementation DoS
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SUMMARY
The Domain Name System (DNS) protocol "is an Internet service that
translates domain names into Internet Protocol (IP) addresses. Because
domain names are alphabetic, they're easier to remember, however the
Internet is really based on IP addresses; hence every time a domain name
is requested, a DNS service must translate the name into the corresponding
IP address".
A vulnerability was found with the recursion process used by some DNS
implementations to decompress compressed DNS messages. Under certain
circumstances, it is possible to cause the DNS server to terminate
abnormally.
DETAILS
The text portions of DNS messages are specified by first giving the
character count, followed by the characters themselves. For example to
specify 'test.test.com', the message would look like
'0x04test0x04test0x03com0x00' using 16-bit numbers. From RFC1035, Section
4.1.4 "Message Compression" specifies a way to create smaller messages so
that they can easily fit into a DNS UDP packet. Hence if the top two bits
of the label length byte are 1, the remaining 14 bits specify an offset
from the beginning of the text on where the remaining characters can be
found. This way, redundant information can be removed and hence create a
smaller message.
Given this type of DNS message, the most obvious method to decode it is by
using recursion. However consider a message that contains a code that
instructs the DNS process to go to an illegal address once the end of the
string is reached; if recursion is used to decode such a message, some DNS
implementation may enter into a loop and eventually exhaust the stack. If
this happens, then it would be possible for the DNS service to terminate
and hence cause a denial-of-service condition.
CVE Information:
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-0036>
CAN-2005-0036
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-0037>
CAN-2005-0037
<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2005-0038>
CAN-2005-0038
ADDITIONAL INFORMATION
The information has been provided by <mailto:g0tcha@webmail.co.za>
g0tcha.
The original article can be found at:
<http://www.niscc.gov.uk/niscc/docs/al-20050524-00433.html>
http://www.niscc.gov.uk/niscc/docs/al-20050524-00433.html
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