Securing the Domain Name System

The Domain Name System provides a critical service in the Internet infrastructure since it maps the domain names that are used by end users onto IP addresses. Since end users rely on names to identify the servers that they connect to, any incorrect information distributed by the DNS would direct end users’ connections to invalid destinations. Unfortunately, several attacks of this kind occurred in the past. A detailed analysis of the security threats against the DNS appeared in RFC 3833. We consider three of these threats in this section and leave the others to RFC 3833.

The first type of attack is eavesdropping. An attacker who can capture packets sent to a DNS resolver or a DNS server can gain valuable information about the DNS names that are used by a given end user. If the attacker can capture all the packets sent to a DNS resolver, he/she can collect a lot of meta data about the domain names used by the end user. Preventing this type of attack has not been an objective of the initial design of the DNS. There are currently discussions with the IETF to carry DNS messages over TLS sessions to protect against such attacks. However, these solutions are not yet widely deployed.

The second type of attack is the man-in-the-middle attack. Consider that Alice is sending DNS requests to her DNS resolver. Unfortunately, Mallory sits in front of this resolver and can capture and modify all the packets sent by Alice to her resolver. In this case, Mallory can easily modify the DNS responses sent by the resolver to redirect Alice’s packets to a different IP address controlled by Mallory. This enables Mallory to observe (and possibly modify) all the packets sent and received by Alice. In practice, executing this attack is not simple since DNS resolvers are usually installed in protected datacenters. However, if Mallory controls the WiFi access point that Alice uses to access the Internet, he could easily modify the packets on this access point and some software packages automate this type of attacks.

If Mallory cannot control a router on the path between Alice and her resolver, she could still launch a different attack. To understand this attack, it is important to correctly understand how the DNS protocol operates and the roles of the different fields of the DNS header which is reproduced in the figure below.

../_images/dnsheader.png

DNS header

The first field of the header is the Identification field. When Alice sends a DNS request, she places a 16-bits integer in this field and remembers it. When she receives a response, she uses this Identification field to locate the initial DNS request that she sent. The response is only used if its Identification matches a pending DNS request (containing the same question).

Mallory has studied the DNS protocol and understands how it works. If he can predict a popular domain for which Alice will regularly send DNS requests, then he can prepare a set of DNS responses that map the name requested by Alice to an IP address controlled by Mallory instead of the legitimate DNS response. Each DNS response has a different Identification. Since there are only 65,536 values for the Identification field, it is possible for Mallory to send them to Alice hoping that one of them will be received while Alice is waiting for a DNS response with the same identifier. In the past, it was difficult to send 65,536 DNS responses quickly enough. However, with the high speed links that are available today, this is not an issue anymore. A second concern for Mallory is that he must be able to send the DNS responses as if they were coming directly from the DNS resolver. This implies that Mallory must be able to send IP packets that appear to originate from a different address. Although networks should be configured to prevent this type of attack, this is not always the case and there are networks where it is possible for a host to send packets with a different source IP address 1. If the attack targets a single end user, e.g. Alice, this is annoying for this user. However, if the attacker can target a DNS resolver that serves an entire company or an entire ISP, the impact of the attack can be much larger in particular if the injected DNS response carries a long TTL and thus resides in the resolver’s cache for a long period of time.

Fortunately, designers of DNS servers and resolvers have found solutions to mitigate this type of attack. The easiest approach would have been to update the format of the DNS requests and responses to include a larger Identifier field. Unfortunately, this elegant solution was not possible with the DNS because the DNS messages do not include any version number that would have enabled such a change. Since the DNS messages are exchanged inside UDP segments, the DNS developers found an alternate solution to counter this attack. There are two ways for the DNS library used by Alice to send her DNS requests. A first solution is to bind one UDP source port and always send the DNS requests from this source port (the destination port is always port 53). The advantage of this solution is that Alice’s DNS library can easily receive the DNS responses by listening to her chosen port. Unfortunately, once the attacker has found the source port used by Alice, he only needs to send 65,536 DNS responses to inject an invalid response. Fortunately, Alice can send her DNS requests in a different way. Instead of using the same source port for all DNS requests, she can use a different source port for each request. In practice, each DNS request will be sent from a different source port. From an implementation viewpoint, this implies that Alice’s DNS library will need to listen to one different port number for each pending DNS request. This increases the complexity of her implementation. From a security viewpoint there is a clear benefit since the attacker needs to guess both the 16 bits Identifier and the 16 bits UDP source port to inject a fake DNS response. To generate all possible DNS responses, the attacker would need to generate almost \(2^32\) different messages, which is excessive in today’s networks. Most DNS implementations use this second approach to prevent these cache poisoning attacks.

These attacks affect the DNS messages that are exchanged between a client and its resolver or between a resolver and name servers. Another type of attack exploits the possibility of providing several resource records inside one DNS response. A frequent optimization used by DNS servers and resolvers is to include several related resource records in each response. For example, if a client sends a DNS query for an NS record, it usually receives in the response both the queried record, i.e. the name of the DNS server that serves the queried domain, and the IP addresses of this server. Some DNS servers return several NS records and the associated IP addresses. The cache poisoning attack exploits this DNS optimization.

Let us illustrate it on an example. Assume that Alice frequently uses the example.net domain and in particular the web server whose name is www.example.net. Mallory would like to redirect the TCP connections established by Alice towards www.example.net to one IP address that he controls. Assume that Mallory controls the mallory.net domain. Mallory can tune the DNS server of his domain and add special DNS records to the responses that it sends. An attack could go roughly as follows. Mallory forces Alice to visit the www.mallory.net web site. He can achieve this by sending a spam message to Alice or buying advertisements on a web site visited by Alice and redirect one of these advertisements to www.mallory.net. When visiting the advertisement, Alice’s DNS resolver will send a DNS request for www.mallory.net. Since Mallory control the DNS server, he can easily add in the response a AAAA record that associates www.example.net to the IP address controlled by Mallory. If Alice’s DNS library does not check the returned response, the cache entry for www.example.net will be replaced by the AAAA record sent by Mallory.

To cope with these security threats and improve the security of the DNS, the IETF has defined several extensions that are known as DNSSEC. DNSSEC exploits public-key cryptography to authenticate the content of the DNS records that are sent by DNS servers and resolvers. DNSEC is defined in three main documents RFC 4033, RFC 4034, RFC 4035. With DNSSEC, each DNS zone uses one public-private key pair. This key pair is only used to sign and authenticate DNS records. The DNS records are not encrypted and DNSSEC does not provide any confidentiality. Other DNS extensions are being developed to ensure the confidentiality of the information exchanged between a client and its resolvers RFC 7626. Some of these extensions exchange DNS records over a TLS session which provides the required confidentiality, but they are not yet deployed and outside the scope of this chapter.

DNSSEC defines four new types of DNS records that are used together to authenticate the information distributed by the DNS.

  • the DNSKEY record allows to store the public key associated with a zone. This record is encoded as a TLV and includes a Base64 representation of the key and the identification of the public key algorithm. This allows the DNSKEY record to support different public key algorithms.

  • the RRSIG record is used to encode the signature of a DNS record. This record contains several sub-fields. The most important ones are the algorithm used to generate the signature, the identifier of the public key used to sign the record, the original TTL of the signed record and the validity period for the signature.

  • the DS record contains a hash of a public key. It is used by a parent zone to certify the public key used by one of its child zones.

  • the NSEC record is used when non-existent domain names are queried. Its usage will be explained later

The simplest way to understand the operation of DNSSEC is to rely on a simple example. Let us consider the example.org domain and assume that Alice wants to retrieve the AAAA record for www.example.org using DNSSEC.

The security of DNSSEC relies on anchored keys. An anchored key is a public key that is considered as trusted by a resolver. In our example, we assume that Alice’s resolver has obtained the public key of the servers that manage the root zone in a secure way. This key has been distributed outside of the DNS, e.g. it has been published in a newspaper or has been received in a sealed letter.

To obtain an authenticated record for www.example.org, Alice’s resolver first needs to retrieve the NS which is responsible for the .org Top-Level Domain (TLD). This record is served by the DNS root server and Alice’s resolver can retrieve the signature (RRSIG record) for this NS record. Since Alice knows the DNSKEY of the root, she can verify the validity of this signature.

The next step is to contact ns.org, the NS responsible for the .org TLD to retrieve the NS record for the example.org domain. This record is accompanied by a RRSIG record that authenticates it. This RRSIG record is signed with the key of the .org domain. Alice’s resolver can retrieve this public key as the DNSKEY record for the .org, but how can it trust this key since it is distributed by using the DNS and could have been modified by attackers ? DNSSEC solves this problem by using the DS record that is stored in the parent zone (in this case, the root zone). This record contains a hash of a public key that is signed with a RRSIG signature. Since Alice’s resolver’s trusts the root key, it can validate the signature of the DS record for the .org domain. It can then retrieve the DNSKEY record for this domain from the DNS and compare the hash of this key with the DS record. If they match, the public key of the .org domain can be trusted. The same technique is used to obtain and validate the key of the example.org domain. Once this key is trusted, Alice’s resolver can request the AAAA record for www.example.org and validate its signature.

Thanks to the DS record, a resolver can validate the public keys of client zones as long as their is a chain of DS -> DNSKEY records from an anchored key. If the resolver trusts the public key of the root zone, it can validate all DNS replies for which this chain exists.

There are several details of the operation of DNSSEC that are worth being discussed. First, a server that supports DNSSEC must have a public-private key pair. The public key is distributed with the DNSKEY record. The private key is never distributed and it does not even need to be stored on the server that uses the public key. DNSSEC does not require the DNSSEC servers to perform any operation that requires a private key in real time. All the RRSIG records can be computed offline, possibly on a different server than the server that returns the DNSSEC replies. The initial motivation for this design choice was the CPU complexity of computing the RRSIG signatures for zones that contain millions of records. In the early days of DNSSEC, this was an operational constraint. Today, this is less an issue, but avoiding costly signature operations in real time has two important benefits. First, this reduces the risk of denial of service attacks since an attacker cannot force a DNSSEC server to perform computationally intensive signing operations. Second, the private key can be stored offline, which means that even if an attacker gains access to the DNSSEC server, it cannot retrieve its private key. Using offline signatures for the RRSIG records has some practical implications that are reflected in the content of this record. First, each RRSIG record contains the original TTL of the signed record. When DNS resolvers cache records, they change the value of the TTL of these cached records and then return the modified records to their clients. When a resolver receives a signed DNS record, it must replace the received TTL of the record with the original TTL (and check that the received TTL is smaller than the original one) before checking the signature. Second, the RRSIG records contain a validity period, i.e. a starting time and an ending time for the validity of the signature. This period is specified as two timestamps. This period is only the validity of the signature. It does not affect the TTL of the signed record and is independent from the TTL. In practice, the validity period is important to allow DNS server operators to update their public/private keys. When such a key is changed, e.g. because the private could have been compromised, there is some period of time during which records signed with the two keys coexist in the network. The validity period allows to ensure that old signatures do not remain in DNS caches for ever.

The last record introduced by DNSSEC is the NSEC record. It is used to authenticate a negative response returned by a DNS server. If a resolver requests a domain name that is not defined in the zone, the server replies with an error message. The designers of the original version of the DNS thought that these errors would not be very frequent and resolvers were not required to cache those negative responses. However, operational experience showed that queries for invalid domain names are more frequent than initially expected and a large fraction of the load on some servers is caused by repeated queries for invalid names. Typical examples include queries for invalid TLDs to the root DNS servers or queries caused by configuration errors [WF2003]. Current DNS deployments allow resolvers to cache those negative answers to reduce the load on the entire DNS RFC 2308.

The simplest way to allow a DNSSEC server to return signed negative responses would be for the serve to return a signed response that contains the received query and some information indicating the error. The client could then easily check the validity of the negative response. Unfortunately, this would force the DNSSEC server to generate signatures in real time. This implies that the private key must be stored in the server memory, which leads to risks if an attacker can take control of the server. Furthermore, those signatures are computationally complex and a simple denial of service attack would be to send invalid queries to a DNSSEC server.

Given the above security risks, DNSSEC opted for a different approach that allows the negative replies to be authenticated by using offline signatures. The NSEC record exploits the lexicographical ordering of all the domain names. To understand its usage, consider a simple domain that contains three names (the associated AAAA and other records that are not shown) :

alpha.example.org
beta.example.org
gamma.example.org

In this domain, the DNSSEC server adds three NSEC records. A RRSIG signature is also computed for each of these records.

alpha.example.org
alpha.example.org NSEC beta.example.org

beta.example.org
beta.example.org NSEC gamma.example.org

gamma.example.org
gamma.example.org NSEC alpha.example.org

If a resolver queries delta.example.org, the server will parse its zone. If this name were present, it would have been placed, in lexicographical order, between the beta.example.org and the gamma.example.org names. To confirm that the delta.example.org name does not exist, the server returns the NSEC record for beta.example.org that indicates that the next valid name after beta.example.org is gamma.example.org. If the server receives a query for pi.example.org, this is the NSEC record for gamma.example.org that will be returned. Since this record contains a name that is before pi.example.org in lexicographical order, this indicates that pi.example.org does not exist.

Footnotes

1

See http://spoofer.caida.org/summary.php for an ongoing measurement study that analyses the networks where an attacker could send packets with any source IP address.