Much has been said and written in recent times about the use of the DNS as a means of looking at the behaviour of end systems and inferring user behaviours. Almost every transaction starts with a DNS query, and if one were to assemble the complete set of DNS queries generated by an Internet user it would be possible to assemble a relatively complete picture of their online activity. For many years this aspect of the DNS as a means of observation into the activities of others received little attention from the mainstream, but the more recent sensitivities over state and private digital surveillance has brought significant attention to the overall topic of DNS privacy. Another reason for all this attention is that in terms of privacy DNS resolution protocol has been sadly lacking in some basic privacy protections. The DNS name resolution protocol was not designed with privacy as the foremost consideration. The queries and responses are unencrypted, which makes them prone to hostile man-in-the-middle manipulation and they leak superfluous information to third party onlookers.
There are two major approaches to try and mediate the DNS privacy issues. The first is to make it harder to eavesdrop on DNS queries by using encryption for DNS transactions. The issues around encryption and the efforts with DNS-over-TLS (DoT) and DNS-over-HTTP (DoH) are a current topic of very high interest in the DNS world. The second approach is to reduce the information leak by reducing the amount of information in each DNS query. The IETF proposed an approach to achieve this using a technique called “Query Name Minimisation” (Qname minimisation), described in an Experimental RFC document (RFC 7816, “DNS Query Name Minimisation to Improve Privacy” by Stephan Bortzmeyer, March 2016).
In this article we will look at Qname minimisation in a little more detail and present some results of our measurement of the current level of use of this resolver query technique in today’s iInternet.
Query Name Minimisation
The technique described in RFC 7816 is query management approach based on a principle described in RFC 6973, “Privacy Considerations for Internet Protocols” (July 2013), which could be summarized as: the less data you emit the fewer privacy issues you are likely to encounter.
The DNS has conventionally optimised its behaviour for simplicity and performance. The underlying factor in the DNS name resolution protocol is that a DNS recursive resolver does not necessarily know in advance which servers are authoritative for a given zone, so it has to discover this information. Also, in a name that has a number of labels the resolver does not necessarily know where the zone cuts occur between labels. In the absence of this meta-information about the structure of the namespace it uses the full query name in all queries as it descends the name hierarchy looking for the lowest level authoritative name server, as this iterative technique will expose the zone cuts and the name servers for each zone.
To expand on this a little, DNS resolution occurs in a top down manner, and when an authoritative server for a zone receives a query for a name that is only resolvable in a delegated zone, it returns a NOERROR code and no Answer section in its response (a “NODATA” response). The response includes the name of the delegated zone and the name servers, as enumerated in the delegation record (the point of the zone cut), in the Authority section of the response and the IP addresses of these name servers in the Additional section of the response, assuming that these addresses are known to the authoritative server.
The DNS is a strict hierarchal namespace, so each server is only aware of immediately delegated zones. The name resolution process will iterate down through the hierarchy to either reach the server that can provide an authoritative response for this query name or obtain a response indicating that the name does not exist in the DNS. This process is illustrated in Figure 1.
Of course, an efficient recursive resolver will use cached information whenever it can, so the process is typically nowhere near as exhaustive and slow as it may appear from this informal description.
This description is not exactly the case in all situations. A server may be an authoritative server for both a parent zone and some or all of its delegated child zone or zones. The query does not specify which zone is the intended subject of its query, allowing the server to answer the query using data from the most specific served zone in the name hierarchy that partially matches the query name.
For example, if a server is authoritative for both example.com and c.example.com, then when the server receives a query for a.b.c.example.com, the server will be able to respond with the delegation information contained in the zone c.example.com. The response to such a query from this server will therefore list the name servers of the zone b.c.example.com.
How does Query Name Minimisation alter this behaviour? To quote from RFC 7816:
Instead of sending the full QNAME and the original QTYPE upstream, a resolver that implements QNAME minimisation and does not already have the answer in its cache sends a request to the name server authoritative for the closest known ancestor of the original QNAME. The request is done with:
- the QTYPE NS
- the QNAME that is the original QNAME, stripped to just one label more than the zone for which the server is authoritative
A resolver using Qname minimisation implicitly assumes that each label in the query name corresponds to a zone cut. The resolver queries a parent zone server, using an abbreviated query name that is truncated after the name of the immediate child label, and uses a query type of NS. This altered resolution process is illustrated in Figure 2.
Let’s look at the query sequence in two cases to illustrate the difference between full name queries and minimised name queries. In the case of a full name query for the name myspecialname.me.example.com the query name has been exposed to a root server, a .com server, an example.com server and a me.example.com server. If the query logs from any of these servers were to be inspected my “interest” in the name myspecialname.me.example.com would be evident. In the minimised case the information ‘leak’ has been trimmed considerably. The root server only sees a query for the .com zone, the com servers only see a query for example.com, and so on.
In terms of an improvement to DNS privacy, this technique sounds like a great step forward. Nothing changes for authoritative servers and it’s only the recursive resolvers that change their behaviour to trim the query name and alter the query type to a ‘neutral’ query for the NS record rather than expose the intended query type to these servers. Only at the target zone is the full query name used with the original query type. This approach essentially removes superfluous information from the DNS query stream. The approach can be deployed incrementally, and the benefits are immediately available to those recursive resolvers, and their user population, that use this Qname minimisation technique.
In short, it seems like an ideal technology change, where current incumbent service providers need to do nothing to allow those who want to adopt this approach to proceed. The benefit for those who do this is that they cease to broadcast their actions and intent to a larger circle of potential DNS onlookers.
Query Name Minimisation Considerations
Why hasn’t this technique been deployed in all resolvers already? Why isn’t this the default mode of operation of the DNS? Assuming that the concerns relating to DNS privacy aren’t just the products of the fevered imagination of a few activists in the IETF, but a reflection of a larger set of very real user community concerns over obsessive levels of DNS surveillance, then surely there would be a clear consumer preference for services that use such techniques to improve DNS privacy. Why have vendors not identified this consumer preference and deployed product to meet this incipient demand? If all this is so simple and easy, and is deployable in a piecemeal and uncoordinated manner then what’s stopping us for doing this?
The Qname minimisation picture is nowhere as simple as you might think at this juncture. There are a number of DNS structures that need to be considered, and three such cases are considered here.
Empty Non-Terminal (ENT) zones
What if the query name does not exist?
The simple response is that whenever the Qname minimising resolver receives an NXDOMAIN response then it should stop and return NXDOMAIN as the response to the querier. NXDOMAIN is a very particular form of response indicating that this name does not exist in any form in the DNS, not even as a delegation point. NXDOMAIN means that this name, and any name that shares this name as a common suffix, does not exist.
Only in theory do theory and practice coincide. In practice they don’t.
The practice of the DNS is filled with odd behaviours and stupid DNS tricks that tend to assume a particular mode of resolver behaviour. As Shumon Huque has pointed out in a presentation on Qname minimisation some very common Content Data Networks (CDNs) host content by using CNAME records to map a client’s name into their CDN name space and then assume that subsequent queries into the CDN zone’s name space always contain the full query name. Rather than assuming that every name needs to be “discoverable” as a top-down hierarchical search, they assume that their part of the DNS is an exact match lookup.
A common CDN hosting technique is to map a hosted content name into the content provider’s managed name space through a CNAME DNS alias record.
For example, if the CDN provider uses the common DNS suffix such as hosted-service.cdn then the service name www.example.com would be mapped into the hosted service by placing a CNAME record for www.example.com, aliasing this name to www.example.com.hosted-service.cdn.
The strict definition of a CNAME record is that the recursive resolver follows the CNAME record and re-commences name resolution for this alias name.
In this example, recursive resolver would then use the query name www.example.com.hosted-service.cdn to query the DNS. When the server for hosted-service.cdn is queried for this name it will then return the provider’s hosting point for the client www.example.com.
The service provider is not hosting example.com, nor .com, so rather than synthesizing a delegation hierarchy that includes empty non terminal zones for com.hosted-service.cdn and example.com.hosted-service.cdn the service provider often uses a zone structure that emulates a flattened enumerated name space. In other words, the hosted-service.cdn zone server behaves in a manner that is consistent with have a zone file that has an entry for www.example.com.hosted-service.cdn. In this light it is not entirely inconsistent for the server to respond with NXDOMAIN for all name queries in hosted-service.cdn apart from precisely those names that are mapped to hosted content.
If a partial form of these mapped names is passed to the CDN’s authoritative server, then an NXDOMAIN may be generated by the server.
These are instances of so-called empty non-terminal (ENT) zones, where the zone exists in the DNS hierarchy, but aside from a delegation record it has no other record. The expected response when an ENT is queried is NODATA (response code 0 (NOERROR) and an empty answer section). The NXDOMAIN is an overclaim in this case as NXDOMAIN is intended to be interpreted as “this name does not exist and there are no delegated names in the name hierarchy below this name.”
As long as the recursive resolver used the full query name this anomalous use of NXDOMAIN does not have any visible impact. Qname minimisation exposes this anomaly as it expects queries for all shortened name forms of a defined query name to return the names for the servers of the delegated zone.
NS vs A query types
RFC 7816 points out some issues that have been encountered with DNS load distributors, where the response to a NS query is the somewhat unhelpful response code “REFUSED”. The specification suggests that a possible workaround is to use a A query with the minimised query name.
Don’t forget that a Qname minimising resolver asks the parent zone server about the child zone name, so this A query type is analogous to asking for the NS record, and the anticipated response to the A query type is a NODATA response with the details of the name servers of the child zone in the Authority serction. This is the same information to that provided if the NS query type was correctly handled. Don’t forget that the parent zone is not authoritative for the child zone, so the NS query to the parent can only generate a NODATA response, rather than an authoritative answer.
If the only reasons to use NS queries is to mask the intended query type for intermediate queries, then it can be argued that an A query type is so common that in itself it gives out even less information than the NS query type. Our measurement show that this is the conclusion reached by resolver vendors and the predominate query type in Qname minimising resolvers is for an A record.
DNS Zone server Misconfiguration
As has been said many times the DNS is nowhere near as simple as it looks. Configuring authoritative servers for zones can be prone to all kinds of subtle errors. A server for a delegated zone does not necessarily know that it is a ‘properly delegated server.
For example, a DNS server can be set up to serve the zone b.c.example.com, but it is not explicitly aware whether or not the server for c.example.com has listed this server as a delegated nameserver for the zone. The server will still answer all queries for names in b.c.example.com if it is asked. If the zone was DNSSEC-signed, then DNSSEC validation would expose any attempt pass off false data in this manner, but for unsigned domain names or non-validating resolvers, this can have unintended consequences.
Most of the time it’s not a problem, as it is difficult for the DNS to discover this rogue server. A top-down conventional name server discovery process will use the parent zone delegation details to find the child zone’s name servers, and so on. As the parent zone’s delegation records do not point to the rogue server, the server will not be discovered in the normal course of events.
However, consider the case where a server is a duly delegated server for both the parent zone and is also an undelegated server for a child zone.
Continuing our example, if our server (an undelegated server for b.c.example.com) was also a duly delegated server for c.example.com, and this zone contained a delegation record for b.c.example.com that pointed to an entirely different server. When a recursive resolver passes a query to this server for the name a.b.c.example.com it does so because it has been told that this is an authoritative server for the zone c.example.com. However, the query does not contain any such information about intention, and the server will use the most specific served zone, in this case the undelegated b.c.example.com zone, to answer the query.
Qname minimisation imposes a stricter regime on this situation. A Qname minimising resolver will use the query name b.c.example.com when querying this server and will follow the zone delegation directions to the duly delegated server for this zone.
An illustration of the the difference between these two cases is shown in Figure 3.
This form of DNS configuration, where a single server is configured to serve both zones and direct or indirect ancestors of these zones is not uncommon in the DNS. As long as all servers of a zone are kept in sync with each other and serve the same information then this DNS server situation will be largely unnoticed. However, two tools will explicitly follow the full delegation path and will not ‘short cut’ across zone cuts, namely DNSSEC and Qname minimisation.
This behaviour has been used to set up a simple resolver test for the existence of Qname minimisation: the name a.b.qnamemin-test.internet.nl has been configured in a manner similar to that shown in Figure 3, and a query for the TXT record of this name will provide a different answer depending on whether the resolver is performing Qname minimisation or not.
Status of Recursive Resolvers and Qname Minimisation
In Bind 9, Qname minimisation is on by default since version 9.14.0. The configuration option is called qname-minimization and it can be set to off, relaxed and strict. The off setting disables qname minimisation completely, strict proceeds with qname minimisation as described by RFC 7816, and relaxed first tries Qname minimisation, but falls back to regular resolution if it fails (presumably through the ENT issues described previously. The default setting is relaxed, although that may change in future releases of Bind.
In Unbound Qname minimisation has been included since release 1.7.2. This setting is on by default. There are two directives: qname-minimisation: which is either yes or no, and qname-minimisation-strict: which determines fallback behaviour if the name fails to resolve. Strict mode yes turns off this fallback behaviour. The default in Unbound is not to use strict mode.
In Knot Qname minimisation is enabled by default. In the struct kr_qflags the member NO_MINIMIZE can be turned on to disable this behaviour.
In terms of the larger open DNS resolvers deployed in the Internet, Google’s public DNS server does not appear to support Qname minimisation, nor does the Quad9 DNS service nor Yandex’s service. Cloudflare’s 188.8.131.52 service, and the OpenDNS service resolve their queries using Qname minimisation.
It is unclear to me whether Qname minimisation in a very heavily used public DNS resolver provides any substantive beneficial privacy outcome to the users of this service or not. In many ways each user is “hiding in a crowd” and their individual queries are lost in the volume of queries being made by such recursive resolvers in the first place. It would also be expected that the open resolver’s caches would be heavily populated so the full query name would be unlikely to be passed to the servers at the higher levels of the DNS name hierarchy. Yes, the recursive resolver is privy to each user’s DNS activity, but that is part of the direct consequences of using such a service in the first place, and is unrelated to the Qname minimisation aspect of the resolver’s behaviour.
The story changes completely when using a small volume DNS resolver, such as a resolver in a home network. The small client pool means that the resolver can be linked to end users, particularly if the resolver’s clients share an IP address subnet with the resolver. A small volume recursive resolver may not have a continually refreshed local cache, so the full query names are more likely to be passed across to DNS servers at all levels in the DNS hierarchy.
Measuring Qname Minimisation
Let’s turn to the measurement results. We want to understand the extent of deployment of Qname minimisation in the DNS today, both as a count of the number of visible resolvers that ask authoritative servers and as a count of the proportion of users who send their queries to Qname minimising resolvers.
As usual, when attempting to measure the DNS we need to take into consideration the conventional caching behaviour of resolvers, so in order to expose the queries being made by resolvers we use a pair of unique dynamically generated labels in the test scenario. The labels were served by DNS servers that are operated as part of the measurement experiment and the query logs were analysed to determine the extent to which resolvers were performing Qname minimisation.
We ran this test from the 6th February 2019 until the 24th July 2019. In that period the we saw 644,406 “visible” resolvers (recursive resolvers that query authoritative servers). Of this set of visible resolvers some 69,869 resolvers queried for the intermediate name form, indicating that they were performing some form of Qname minimisation.
|11%||2%||9%||0%||% of all resolvers|
|21%||79%||0%||% of Qmin resolvers|
This figure of 11% of all visible resolvers does not show to what extent Qname minimisation is being used in today’s DNS. For that we need to count relative use, and one way of doing this is to count the query load.
|98%||2%||0%||1%||0%||% of all queries|
|23%||77%||0%||% of Qmin queries|
Some 2% of all queries were using QMin, and of these queries some three quarters of these Qname minimised queries used the A query type, not the NS type.
We can break this down a little further, looking at the query patterns for each individual experiment.
|3%||1%||2%||0%||% of all experiments|
|25%||75%||0%||% of Qmin experiments|
The number of users that we observed using Qmin resolvers is quite small: some 3% of users send their queries through QMin resolvers.
Where are these users? Table 4 lists those economies where we collected more than 20,000 sample points over the duration of the measurement period, and where 10% or more of the users in these economies used a recursive resolver that performed Query name minimisation.
What a curious collection of economies! It is unclear whether service providers in these economies have enabled Qname minimisation deliberately, or whether this is an outcome of using a recursive resolver such as the recent version of the Bind 9 resolver or the Knot resolver, where this functionality has been enabled by default.
Two economies of interest are not listed in Table 4: China, where some 4% of users pass queries through Qname minimising resolvers, and the United States where the number is just 0.8% of users.
Our measurements indicate that some 3% of users pass their queries through resolvers that actively work to minimize the extent of leakage of superfluous information in DNS queries.
In many ways this is a very disappointing number, in that it indicates a somewhat lacklustre attitude on the part of administrators of DNS resolver services to take every possible step to minimise information exposure to third parties. Maybe we like to talk about privacy and security far more than we actually are prepared to do something tangible about it!
It’s likely that we will return to this measurement of the use of Qname minimisation in a year or so to see if anything has changed from the picture today.
Making the DNS More Private with QNAME Minimisation, Wouter de Vries, Ripe Labs Blog, April 2019.
Qname Minimisation and your Privacy, Vicky Risk, ISC.