techblog 

Spurred by a recent Slashdot posting, I’ve produced some graphs showing the relationships between the RFCs which define the DNS protocol.

The graphs (which are in SVG format) split the DNS-related RFCs into three groups (although some RFCs end up in more than one group):

• Core Protocol RFCs
• Resource Record Definitions
• DNS Operational Guidelines

The point of these graphs is not to show which RFCs refer to other RFCs, but to show which RFCs update or obsolete other RFCs. Hence the graphs give an “at a glance” overview of which RFCs define the DNS protocol as it is now.

Boxes in grey indicate obsoleted RFCs, and square boxes indicate Informational or Best Current Practice documents.  Hovering over a box should tell you the title of the RFC, and clicking on a box will take you to the RFC itself.

The picture below is just a low resolution sample - click on the picture or on the links above to access the scalable SVG versions.

Please let me know if you believe I’ve missed anything, or miscategorised any document.

I’ve recently written and released source code for “evldns”.

evldns is a software mashup - it takes libevent’s fast event processing code and combines it with ldns’s DNS packet handling.  It’s derived from the server-side half of libevent’s “evdns” component.

The resulting framework is particularly intended for writing servers which generate custom responses. Examples included are:

• an AS112 server which has been benchmarked at over 60,000 queries per second on an HP DL385 server.
• a server which responds with the IP address of the client which sent the query - this can be useful for network discovery

The framework could also be used to write a “fuzzing” DNS server - one that deliberately returns malformed responses so as to trigger and test for bugs in DNS clients.

Here’s an extract from the package’s README:

evldns works using callback functions. A list of packet matching patterns
may be registered, along with a pointer to the function that will be
invoked when each pattern is matched.

The packet match works on the usual DNS triple of (QNAME, QCLASS, QTYPE)
where QNAME may be an exact match or a wildcard, and QCLASS or QTYPE may
be “ANY”.

The callback function is passed two parameters:

void callback(struct evldns_server_request *req, void *data)

The “req” parameter contains the complete received DNS request as an
“ldns_pkt”. The callback should create a response packet and populate
“req” with that response, which may either be in raw wire format
(req->wire_response and req->wire_len) or in ldns format (req->response).

If the callback function fails to populate either of the response fields
then the evldns system will pass the received packet onto the next
matching callback.

Should no callback match then evldns will automatically generate and
return a packet with RCODE = 5 (Refused).

The “data” parameter is used to pass an additional parameter supplied when
the callback function was registered. See “mod_txtrec.c” for an example
of how “data” may be used to pass expected response data into a callback.

A complete evldns application requires just a few lines of code:

event_init(); /* initialise libevent */
evldns_init(); /* initialise evldns */

/* create an evldns server context */
struct evldns_server *server = evldns_add_server();

/* register a UDP socket with evldns */
evldns_add_server_port(server, bind_to_udp4_port(53));

/* register callbacks here */
evldns_add_callback(server, qname, qclass, qtype, callback, data);
...

/* and set libevent running */
event_dispatch();

Please see the project home page for more information. There is also a Google hosted discussion group.

Ray Bellis, Advanced Projects Team

The day following the death of Michael Jackson, Google published a graph showing that their system were heavily hit by queries related to this news. Details can be found on the Google Official Blog.

Our experiments suggest that Nominet systems experienced an analogous, although orders of magnitude smaller, phenomenon. The following figures show the number of new registrations per hour of domain names that contain the name of Michael Jackson (or part of it) and the number of WHOIS queries that Nominet systems received in the same period.

The two graphs are highly correlated because it is common practice for domain name owners to make WHOIS lookups around the period of time they register new domains. The peak around the 27 of June in the second graph is probably related to news stories concerning suspicions about Michael’s death.  Apparently, it did not lead to an immediate rise in the number of domain name registrations.

We have conducted an informal analysis of the domain names that were registered in the last week. The majority of them belong to three categories: parking pages, commercial pages and commemorative sites such as blogs and forums. At the moment, we have no evidence of domain names used for scam or phishing.

In general, this episode confirms (again) that the dynamics of the Domain Name System follow those of the “real world”. A question that is still partially unanswered is at which degree these dynamics are followed by Internet users, i.e. how much their navigation behaviour depends on news stories. In the following months we plan to study the correlation between DNS data and other public events. Google has done something similar in the past, by correlating Google searches for flu-related terms with the spread of flu in North America. The results are very interesting and definitely merit extension to other data sources such as DNS traffic.

I’m pleased to announce the release of enumdroid.

This application adds ENUM (E.164 Number Mapping) support to your Android phone.

Each time you dial a full international number (i.e. starting with a ‘+’) your phone will check the DNS for additional routing information and offer you a list of alternate contact methods.

The application is open source (under the Apache License) and the code is available for download from Google Code.  The application can be downloaded from the Google Market under Applications -> Communication

Here are some screenshots, which show in turn:

1. Nominet’s switchboard number being dialled
2. ENUM results being returned
3. A call being placed over the PSTN to a tel: URI
4. The ENUM application’s settings page

I got bitten by a silly bug in my dnsruby code recently - I thought I’d share it here in case anyone else starts pulling their hair out over this in future!

DNSSEC RRSIG records contain the signatures required to prove that a DNS zone has been correctly signed by an entity which possesses the correct keys for the zone. DNS clients can obtain the correct keys for the zone, then use the RRSIG records to prove that the zone (or record of interest) is correct. Of course, these records shouldn’t last forever - the data records are periodically re-signed (possibly with different keys), and the RRSIGs updated. The RRSIG includes an inception time, and an expiration time, to show the period over which it is valid. To verify a set of records, the DNS client must first produce an array of bytes, the digest of which is taken and used as the signature for the records - the salient data of of the RRSIG record (including the inception and expiration times) is included in this set of bytes.

The code I had written to do this was working fine - I had coded in the examples from the RFCs, and done a lot of work with actual signed zones (reading the data from the authoritative servers, and proving that it was correct). However, when I started to try to do this with real-world zone files, I started noticing that the signatures weren’t verifying. At least, they *sometimes* weren’t verifying. Very odd. Of course, with this kind of work, all you get is a Pass/Fail - no clue as to what is going wrong. I could see that the records were being translated to and from text format correctly - all the data in the RRs was showing as fine. However, when I compared the byte sequences prepared by my DNS client and Net::DNS, I noticed that four bytes were different. It turned out that, of the four byte sequences written to for the RRSIG inception and expiration time, there was a difference of 0xE100 - this works out to 16 hours worth of 3600 seconds. At last, I was onto something!

There are several ways to express time in the presentation format of the RRSIG record - “1234567890″ (seconds since 01/01/1970), or “YYYYMMDDHHMMSS” (e.g. “20090608123435″). Dnsruby worked fine with the first, and even translated the second from presentation format and back to presentation format correctly. However, when read using the following line :

return Time.mktime(year, mon, day, hour, min, sec).to_i

I got a 16 hour offset from the correct time! [When converting this time to/from the text format, the translation worked perfectly - it was only when inspecting the internal epoch time that the difference could be noticed]

I changed this to :

return Time.gm(year, mon, day, hour, min, sec).to_i

And suddenly everything worked just fine.

I’m sure that seasoned Rubyists will sneer at me for my stupidity - it did take me some time to track this one down! So, if you start noticing strange 16 hours offsets in your Ruby code, it’s worth checking your usage of the Time class…

In this post, I’d like to look at how to use Dnsruby with DNSSEC. As before, I’ll run these examples in irb, and assume that you’ve included Dnsruby there.

Dnsruby has DNSSEC support switched on by default. This means it will attempt to validate any DNS responses against its trust anchors. However, by default, no trust anchors are configured - to get dnsruby to validate responses, you must first configure a trust anchor (or DLV repository).

Trust Anchors

DNSSEC works by following a chain of trust from parent zone to child zone. This chain of trust must start somewhere - the “trust anchor”. In a world with a signed root, the root would be the anchor. Delegations to children zones would be signed, all the way down to the domain that is being queried. The querier can then be sure that the signed response is genuine.

Unfortunately, the root is not yet signed - we have many “islands of security”. Each island is signed, but has no chain to it from the root. It is possible to configure dnsruby with the keys for these zones using Dnsruby::Dnssec#add_trust_anchor() - it’s also possible to define an expiration time for each anchor. Dnsruby will then follow the chain of trust from the anchor down to the queried domain in the signed zone.

Managing these trust anchors quickly becomes a headache. You need to have secure means of obtaining and verifying them, and rolling over to new keys as time goes on. Fortunately, there are two mechanisms to help with this : IANA’s TAR and ISC’s DLV repository.

IANA (who manage the root zone) have created a Trust Anchor Repository (ITAR) which can be used until the root is signed. This holds delegation records for the DNSSEC-signed TLDs. It is possible to download this repository and configure dnsruby with the anchors. A method to do this is defined in Dnsruby::Dnssec#load_itar, but it is not currently secure. If you need to use the ITAR securely, you are currently advised to add the trust anchors from the ITAR directly into dnsruby. A secure method will be provided in future releases.

DLV

Even if the root was signed, there will still be some domains in unsigned zones, which wish to benefit from DNSSEC security. For example, signed-zone.unsigned-zone.example.org - there can be no chain of trust from the root to signed-zone. A solution exists for signed-zone : DNSSEC Lookaside Validation (DLV). Here, a DLV repository holds secure delegation records for zones like signed-zone. Instead of following the chain of trust from the root, a validator follows the chain of trust from the closest parent zone known to the DLV repository. Of course, this method involves more validation queries for each application query.

As an example, considering querying for random.example.com - first, the query itself must be made. Then, if unsuccessful, a DLV query for random.example.com.dlv.isc.org must be made, followed by a query for example.com.dlv.isc.org, followed by a query for com.dlv.isc.org. If none of these succeed, then the message cannot be validated. Imagine that a response was received for the com.dlv.isc.org zone : then, the chain of trust could be followed through example.com down to random.example.com. Keys discovered from the DLV repository are cached.

Configuring Trust Anchors

To configure a trust anchor (in this case for the uk-dnssec.nic.uk DNSSEC test zone) :

    trusted_key = Dnsruby::RR.create({:name => "uk-dnssec.nic.uk.",
        :type => Dnsruby::Types.DNSKEY,
        :flags => 257,
        :protocol => 3,
        :algorithm => 5,
        :key=> "AQPJO6LjrCHhzSF9PIVV7YoQ8iE31FXvghx+14E+jsv4uWJR9jLrxMYm sFOGAKWhiis832ISbPTYtF8sxbNVEotgf9eePruAFPIg6ZixG4yMO9XG LXmcKTQ/cVudqkU00V7M0cUzsYrhc4gPH/NKfQJBC5dbBkbIXJkksPLv Fe8lReKYqocYP6Bng1eBTtkA+N+6mSXzCwSApbNysFnm6yfQwtKlr75p m+pd0/Um+uBkR4nJQGYNt0mPuw4QVBu1TfF5mQYIFoDYASLiDQpvNRN3 US0U5DEG9mARulKSSw448urHvOBwT9Gx5qF2NE4H9ySjOdftjpj62kjb Lmc8/v+z"
      })
    Dnsruby::Dnssec.add_trust_anchor(trusted_key)

Dnsruby will now attempt to validate any responses from the uk-dnssec.nic.uk zone (or its children).

To configure dnsruby to use ISC’s DLV repository, you must first obtain the key (from here). You can then configure dnsruby :

    dlv_key = RR.create("DLV_KEY_STRING_FROM_ISC")
    Dnssec.add_dlv_key(dlv_key)

This method queries the DLV registry to get the ZSK (zone signing key) from the above KSK (key signing key). Dnsruby will now attempt to validate all responses against the DLV repository, if it can’t validate from any trust anchors.

Configuring Validation Policy

It is possible to configure the validation policy to vary the precedence of search order - from the root only, or local anchors only, or either first. Separate key caches are maintained by each validator, making it possible to configure them dynamically. DLV validation is only performed once the DLV key has been added. Here is an example of changing the validation policy :

    Dnsruby::Dnssec.validation_policy = Dnsruby::Dnssec::ValidationPolicy::ROOT_THEN_LOCAL_ANCHORS

It is possible to clear all trusted keys (which will also stop DLV validation) by calling :

    Dnsruby::Dnssec.clear_trusted_keys()

You can remove just the trust anchors (leaving DLV keys and validation from the root, and all keys generated from them), and the keys generated from them, by calling :

    Dnsruby::Dnssec.clear_trust_anchors()

Configuring Validation Resolver

When a response is validated, it may be necessary to make several more queries in order to follow the chain of trust. As more queries are made, more chains are followed. Trusted keys are cached as they are discovered (for the length of time they are indicated to be good for) - this means that future queries for domains in those zones will not require so many validation queries to be performed.

It’s possible to configure dnsruby to use different methods for performing the validation queries. They can either be directed to recursive nameservers (which can be the system defaults, or a client-supplied set of addresses), or they can be performed recursively. I have found that many resolvers do not yet speak a perfect dialect of DNSSEC - performing validation queries recursively ensures that the correct DNSSEC-signed responses are received. The default is to perform validation recursively. Of course, while the caches are being built up when dnsruby starts, more queries will be performed than if the queries were directed to recursive nameservers.

To ask dnsruby to use query a recursive nameserver, call :

    Dnsruby::Dnssec.do_validation_with_recursor = false

Dnsruby will now use the system default configured nameservers for validation.

To use a specific set of servers to perform validation :

    res = Dnsruby::Resolver.new({:nameserver => ['192.168.1.1', '192.168.2.1']})
    Dnsruby::Dnssec.default_resolver = res

Validating Responses

Once dnsruby has been configured with a trust anchor, it will attempt to validate any responses for domains within that zone (or its subzones). If it detects that validation is necessary, then it will fire up a new thread to handle that validation. Since many queries may need to be performed in order to validate the reponse, this can take some time longer than the original query would have done alone. This means that the query timing settings in the Resolver class apply only to each query - *not* to the whole validation process.

For example, a query may have a Resolver#query_timeout of 5 seconds. As long as the answer for that query is returned in 5 seconds, then no timeout will occur - even if it then takes another 6 seconds to validate that response. Future versions of dnsruby will include the ability for client applications to receive events detailing the progress of each asynchronous query (e.g. RECEIVED, VALIDATED).

It is possible to disable validation on a Message basis. Simply set :

  msg.do_validation = false

before sending the Message - dnsruby will not validate the response to that query.

Message Security Levels

Messages can have one of four security levels (defined in Dnsruby::Message::SecurityLevel) : BOGUS, UNCHECKED, INSECURE and SECURE. Dnsruby will only raise an error if it detects that a response is BOGUS - this means that the message does not contain the correct set of signatures. INSECURE means that the response has been verified to have come from a non-secured zone. SECURE means that the chain of trust has been correctly followed from a configured trust anchor to the response, and that all signatures check OK. Note that an NXDOMAIN response can still be SECURE - this means that the NSEC(3) records have been verified to prove non-existence.

To check the security level of a Message, use Message#security_level :

  if (msg.security_level == Dnsruby::Message::SecurityLevel::SECURE)
      print "Response was validated OKn"
  end

Examples of Use

DNSSEC examples can be found in the EXAMPLES file in the dnsruby distribution.

Limitations

Dnsruby does not yet perform NSEC3 validation (although NSEC3/NSEC3PARAM records can be read from the wire, or presentation format). This will be added to a future release.

In this post, I’d like to look at how to use dnsruby to accomplish some common tasks.

Getting started

To follow these examples, you’ll need to install dnsruby :

$ gem install dnsruby

I’ll run these examples in Ruby’s interactive shell :

$ irb

First, I need to include Dnsruby :

>> require 'rubygems'
>> require 'dnsruby'
>> include Dnsruby

Now I’ll load the system’s default resolvers :

>> res = Resolver.new

And display them :

>> res.single_resolvers.each {|s| print "Server address : #{s.server}n"}
Server address : 192.168.1.1
Server address : 192.168.2.2

Now I’ll use them to run a couple of queries :

>> ret = res.query("example.com") # Defaults to A record
>> print ret.answer
example.com.	172789	IN	A	208.77.188.166=> nil
>> res.query("example.com", "MX") # Query the MX record

This time, I’ll use some defined nameservers :

>> res = Resolver.new({:nameserver => ["ns1.nic.uk",
        "ns1.nic.uk"]})

Asynchronous Queries

To run an asynchronous query, I’ll define a Queue to hold the results, and then prepare the query. This time, I’ll construct a Message to hold the query data, and set the RD (recursion desired) bit on the header to 0 :

>> queue = Queue.new
>> m = Message.new("co.uk", Types.NS)
>> m.header.rd = false
>> message_id = res.send_async(m, queue, 1)

Now my code can get on with other tasks, until I’m ready to get the response. Queue#pop is a blocking call, but you can check if it is empty using Queue#empty?.

>> id, reply, error = queue.pop # id == message_id

The [id, reply, error] tuple is popped off the queue. The id identifies which query the response is for (it should match the id returned by the send_async call), reply holds the best response that was received, and any errors will be held in error (which should be nil in this example).

Message Options

Now I’ll ask for a Message to be sent without checking (or the response being stored in) the cache. I’ll also make sure that no DNSSEC validation is performed on the response :

>> m.do_caching = false
>> m.do_validation = false
>> res.send_message(m)

I can ask for a Message to be sent without any pre- or post-processing. No EDNS headers are applied, the header flags are not adjusted, and no caching or validation is performed. This method is most useful for tools authors :

>> res.send_plain_message(Message.new("nic.uk"))

TSIG and Dynamic Updates

I can also use TSIG signatures to communicate securely with a resolver. In this example, I’ll use TSIG to sign a dynamic update. First, I’ll have to define the server to use, and the TSIG key to speak to it with :

>> res = Dnsruby::Resolver.new("ns0.validation-test-servers.nominet.org.uk")
>> res.dnssec = false
>> tsig = Dnsruby::RR.create({
        :name        => "rubytsig",
        :type        => "TSIG",
         :key         => "8n6gugn4aJ7MazyNlMccGKH1WxD2B3UvN/O/RA6iBupO2/03u9CTa3Ewz3gBWTSBCH3crY4Kk+tigNdeJBAvrw==",
      })

Now I’ll create the dynamic update packet :

>> update = Dnsruby::Update.new("validation-test-servers.nominet.org.uk")
>> # ... add stuff to the update
>> update.absent("notthere.update.validation-test-servers.nominet.org.uk", 'TXT')

And apply the TSIG signature and send the message :

>> tsig.apply(update)
>> response = res.send_message(update)
>> print "TSIG response was verified? : #{response.verified?}n"

I could also have configured the Resolver to sign *all* packets with TSIG :

>> res.tsig=tsig.name, tsig.key

Recursive Queries

In addition to defining nameservers to do recursive queries on my behalf, I can also get Dnsruby to query recursively from the root. A static cache is built up, so the more client queries that are run, the less packets need be sent per client query.

>> rec = Recursor.new
>> ret = rec.query("uk-dnssec.nic.uk", "NS")

In my next article, I’ll look at how use Dnsruby with DNSSEC.

I’m very pleased to announce the release of dnsruby version 1.30 (RDoc available here). This version constitutes a considerable advance of functionality on previous dnsruby releases. Although it is still possible to use dnsruby in just the same way (which proves useful for tools authors), it is now also capable of full DNSSEC validation, using either ISC’s DLV registry, IANA’s Trust Anchor Repository, or any trust anchors it is configured with.

Clients can configure the validation policy (e.g. whether to try local trust anchors first, followed by ITAR, then DLV, or some other order), whether to use recursion or a local resolver in the validation process, and manage the store of trusted keys themselves. Dnsruby still offers the verify method to verify a Message or RRSet against a set of keys, but also includes the validate method (called automatically by default) which kicks off a separate validation thread (if validation is necessary). This thread follows the chain of trust from the closest trusted key, and records the result in the Dnsruby::Message#security_level. Exceptions are only raised if the SecurityLevel is BOGUS.

Dnsruby::Recursor now includes a static authority cache. This means that dnsruby can now be used to perform entirely recursive querying - especially useful if you don’t trust your local resolvers, or they don’t yet speak a correct dialect of DNSSEC.

Other improvements include dynamic management of configured nameservers - more responsive ones will be preferred over slower, and servers which don’t respond at all, or give broken responses, will be pushed to the back of the line. A cache has been added for received (and validated) Messages. It is possible to avoid use of this cache on a Message basis.

Support for EventMachine has been removed. I’ve had to rework dnsruby’s event system considerably in order to include validation support, and it was no longer feasible to maintain two IO loops. The native Ruby event loop in dnsruby has no known issues, and that is the recommended (and only) option. It should still be possible to work dnsruby into EventMachine projects.

I plan to write another article soon, with more detail on how to achieve common DNS(SEC) tasks with dnsruby. In the meantime, please let me know if you have any issues using the latest version!

In what could be seen as further proof of the imperfection of multi-threaded programming, I have just released another version of dnsjnio. This version includes fixes from Allan O’Driscoll that deal with very obscure synchronisation issues.

It also includes changes to the test system which cope with the fact that Java DatagramSockets have slight differences of behaviour on different platforms. Regardless of the operating system you use, and the power of your computer, you should now have no problems with the dnsjnio test code. Previously, some versions of Linux (and Solaris), and slower machines, could have some issues.

If you’re using dnsjnio in a very high volume environment, I’d recommend upgrading to the latest version.

When Ruby version 1.9 was released at Christmas, I was awfully excited. I had a couple of days to make a release of dnsruby before I headed off to New Zealand, and I was very keen for the release to be compatible with Ruby 1.9.

I spent a few very stressful days, and released dnsruby with no support for 1.9. Everything just seemed broken, and I still had a long way to go! Happily for me, it turned out I was not alone. As Dave Thomas notes, 1.9.1 was “less than stable”.

So I was pleased to be able to spend some time with Ruby 1.9.1 recently. Again, nothing worked at first, but there wasn’t too much to getting dnsruby working with Ruby 1.9.1. The main issues I had were to do with Strings and binary data (presumably owing to the internationalisation work which has gone into Ruby 1.9). At least, that’s they turned out to be once I’d debugged some very odd symptoms!

I’m now happy to be able to announce that the latest version of dnsruby (1.23) is now fully compatible with Ruby versions 1.8.7 and 1.9.1. There are a few more platform checks than I’d have liked (dnsruby now checks whether it is running on windows or java, and what version of Ruby it is running). I can only see these proliferating if Dave Thomas’ recent advice to fork Ruby is followed.

Other improvements to the library include the DLV record, SHA-2 support in DS record processing, and various security features and fixes.

As always, I’m keen to get any feedback on the release.