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**A warm welcome to DNS**
This series of documents attempts to provide a correct introduction to the Domain Name System as of 2018. The original RFCs remain the authoritative source of normative text, but this document tries to make this venerable protocol more accessible, while maintaining full alignment with all relevant and useful RFCs.
This effort is developed cooperatively on GitHub, the repository can be found https://github.com/ahupowerdns/hello-dns/ and help is highly welcome!
Although we start from relatively basic principles, the reader is expected to know what IP addresses are, what a (stub) resolver is and what an authoritative server is supposed to do. When in doubt: authoritative servers 'host' DNS data, 'resolvers' look up things over at authoritative servers and clients run 'stub resolvers' to look things up over at resolvers. This document is aimed at developers, but may also be of aid for administrators.
DNS was originally written down in August 1979 in 'IEN 116', part of a parallel series of documents describing the Internet. IEN 116-era DNS is not compatible with today's DNS. In 1983, RFC 882 and 883 were released, describing a version of the DNS very similar but not quite interoperable with the one we have today.
DNS attained its modern form in 1987 when RFC 1034 and 1035 were published. Although much of 1034/1035 remains valid, these standards are not that easy to read because they were written in a very different time. There are 100s of pages of updates that can only be found in later document.
The main goal of this document is not to contradict the DNS RFCs but to provide an easier entrypoint into understanding the Domain Name System.
If you will, the goal is to be a mini "TCP/IP Illustrated" of DNS. For more about the philosophy of these documents, and how to contribute, please read meta.md. Your help & insights are highly welcome!
I want to thank Ólafur Guðmundsson and Job Snijders for their input and enthusiasm for improving the state of DNS.
The content is spread out over several documents:
- The core of DNS (this file)
- Relevant to stub resolvers and applications
- Relevant to authoritative servers
- Relevant to resolvers
- Optional elements: EDNS, TSIG, Dynamic Updates, DNAME, DNS Cookies
- DNSSEC
We start off with a general introduction of DNS basics: what is a resource record, what is an RRSET, what is a zone, what is a zone-cut, how are packets laid out. This part is required reading for anyone ever wanting to query a nameserver or emit a valid response.
We then specialize into what applications can expect when they send questions to a resolver, or what a stub-resolver can expect.
The next part is about what an authoritative server is supposed to do. On top of this, we describe in slightly less detail how a resolver could operate. Finally, there is a section on optional elements like EDNS, TSIG, Dynamic Updates and DNSSEC
RFCs, especially earlier ones, tend to describe servers that perform both authoritative and resolver functions. This turns out to make both code and troubleshooting harder. Therefore, in these documents, the authoritative and caching functions are described separately.
Note that this file, which describes DNS basics, absolutely must be read from beginning to end in order for the rest of the documents (or DNS) to make sense.
In this section we will initially ignore optional extensions that were added to DNS later, specifically EDNS and DNSSEC.
This file corresponds roughly to the fundamental parts of RFCs 1034, 1035, 2181, 2308, 3596, 4343, 5452, 6604.
DNS is mostly used to serve IP addresses and mailserver details, but it can contain arbitrary data. DNS is all about names. Every name can have data of several types. The most well known externally useful types are A for IPv4 addresses, AAAA for IPv6 addresses and MX for mailserver details. DNS also has types that have meaning for its own use, like NS, CNAME and SOA.
When we ask a DNS question we call this a query. We call the reply the response. These queries and responses are contained in DNS messages. When UDP is used, the message is also the packet.
A DNS message has:
- A header
- A query name and query type
- An answer section
- An authority section
- An additional section
In basic DNS, query messages should have no answer, authority or additional sections.
The header has the following fields that are useful for queries and responses:
- ID: a 16 bit identifier used as part of the process of matching queries to responses
- QR: Set to 0 to identify a message as a query, 1 for a response
- OPCODE: 0 for a standard query, other opcodes also exist
- RD: Set to indicate that this question wants recursion
Relevant for responses:
- AA: This response has Authoritative Answers
- RA: Recursive service was available
- TC: Not all the required parts of the response fit in the UDP message
- RCODE: Result code. 0 is ok, 2 is SERVFAIL, 3 is NXDOMAIN.
DNS queries are mostly sent over UDP, and UDP packets can easily be spoofed. To recognize the authentic response to a query it is important that the ID field is random or at least unpredictable. This is however not enough protection, so the source port of a UDP DNS query must also be unpredictable.
DNS messages can also be sent over TCP/IP. Because TCP is not a datagram oriented protocol, each DNS message in TCP/IP is preceded by a 16 bit network endian length field.
DNS servers must listen on both UDP and TCP, port 53.
The header of a question for the IPv6 address of www.ietf.org looks like this:
-
1 1 1 1 1 1 -
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | ID = random 16 bits |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- |QR| Opcode |AA|TC|RD|RA| Z | RCODE |
- |0 | 0 |0 | 0| 0|0 | 0 | 0 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | QDCOUNT = 1 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | ANCOUNT = 0 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | NSCOUNT = 0 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | ARCOUNT = 0 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Note that we did not spend time on field Z, this is because it is defined to be 0 at all times. This packets does not request recursion. QDCOUNT = 1 means there is 1 question. In theory DNS supported several questions in one message, but this has not been implemented. ANCOUNT, NSCOUNT and ARCOUNT are all zero, indicating there as no answers in this question packet.
Here is the actual question:
-
1 1 1 1 1 1 * -
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 * - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 3 | w | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | w | w | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 4 | i | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | e | t | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | f | 3 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | o | r | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | g | 0 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 0 | 28 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 0 | 1 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
This consists of the 'www.ietf.org' encoded in DNS wire format (for which see below), followed by a 16 bit type field. For AAAA, which denotes the IPv6 address, this is 28. This is then followed by the 'class' of the question. It was originally intended that DNS records would exist in different 'classes', but the semantics of this were not specified completely and it was not really implemented. For now, always set class to 1.
Of specific note is the somewhat unusual way the name 'www.ietf.org' is serialized in DNS. 'www.ietf.org' consists of 3 'labels' of lengths 3, 4 and 3 respectively. In DNS messages, this is encoded as the value 3, then www, then the value 4, then ietf, then 3 followed by org. Then there is a trailing 0 which denotes this is the end.
This format is unusual, but has several highly attractive properties. For example, it is binary safe and it needs no escaping. When writing DNS software, it may be tempting to pass DNS names around as "ASCII". This then leads to escaping an unescaping code in lots of places. It is highly recommended to use the native DNS encoding to store DNS names. This will save a lot of pain when processing DNS names with spaces or dots in them.
Finally, DNS queries are case-insensitive. This however is defined rather mechanically. Operators do not need to know that in some ASCII encodings a Ü is equivalent to ü when compared case insensitively. For DNS purposes, the fifth bit (0x20) is ignored when comparing octets within a-Z and A-Z.
Note that individual labels of a name may only be 63 octets long.
Next up, a DNS response. Note that this again is a DNS message, and it looks a lot like the original DNS query. Here is the beginning of a response:
-
1 1 1 1 1 1 * -
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 * - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | ID = same random 16 bits | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- |QR| Opcode |AA|TC|RD|RA| Z | RCODE | *
- |1 | 0 | 1| 0| 0| 0| 0 | 0 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | QDCOUNT = 1 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | ANCOUNT = 1 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | NSCOUNT = 0 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | ARCOUNT = 0 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 3 | w | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | w | w | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 4 | i | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | e | t | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | f | 3 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | o | r | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | g | 0 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 0 | 28 (= 0x1c)| *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
- | 0 | 1 | *
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ *
Note that QR is now set to 1 to denote a response. The 'AA' bit was set because this answer came from a from a server authoritative for this name.
In addition, ANCOUNT is now set to '1', indicating a single answer is to be found in the message, immediately after the original question, which has been repeated from the query message.
To recognize the right response, check that the ID field is the same as the query, make sure the answer arrives on the right source port and that the query name and type match up with the original query. In addition, make sure not to send out more than one equivalent query when still waiting for the response, as doing so opens a security hole.
After the header and the original question we find the answer:
-
1 1 1 1 1 1 -
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 - +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 0xc0 | 0x0c |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 28 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 01 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | TTL = 3600 |
- | |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | RDLENGTH = 16 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--|
- | 24 | 00 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | cb | 00 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 20 | 48 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 01 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 00 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 00 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 68 | 14 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
- | 00 | 55 |
- +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
The first two bytes (0xc0 0c0c) look rather mysterious. When DNS was created, 512 octets was considered the maximum size of a UDP datagram and thus the maximum size of a DNS message transported without using the (then slow) TCP protocol.
In order to squeeze as much information as possible into the 512 bytes, DNS names can (and often MUST) be compressed. The details of this compression are arcane and easy to get wrong, leading to infinite loops or buffer overflows. So tread very carefully. If you remember one thing, make sure that a pointer always has to go to a lower position in the packet. Also beware of signed/unsigned arithmetic.
In this case, the DNS name of the answer is encoded is '0xc0 0x0c'. The c0 part has the two most significant bits set, indicating that the following 6+8 bits are a pointer to somewhere earlier in the message. In this case, this points to position 12 (= 0x0c) within the packet, which is immediately after the DNS header. There we find 'www.ietf.org'.
So what this means is that the answer about the DNS name 'www.ietf.org' is also called 'www.ietf.org'.
This is then followed in the packet by '28', which denotes AAAA (IPv6), and the usual 'class' of 1. Then a whole 32 bits are devoted to the Time To Live of this record, followed by a 16 bits length field. Since this is an IPv6 address, the actual answer payload length is 16 bytes (or 128 bits).
This is then followed by the binary representation of the current IPv6 address of www.ietf.org, 2400:cb00:2048:1::6814:55.
If there had been further answers, these would follow this first one, and the ANCOUNT would have been higher than 1. If there had been data in the 'authoritative' and 'additional' sections, that would follow here too, with the corresponding adjustments to 'NSCOUNT' and 'ARCOUNT' fields. More about these sections later.
In the example above, the question for the AAAA record of 'www.ietf.org' had exactly one corresponding resource record. In a human readable 'zone file', this would stored as:
www.ietf.org IN AAAA 3600 2400:cb00:2048:1::6814:55
It is however possible to have multiple AAAA records for the same name. Even if there is only one record, the DNS specifications talk about 'Resource Record Sets', or RRSETs. These operate in unity. So even though the encoding in the DNS packet allows different TTL values within a single RRSET, this should never happen.
Zone files are one way of storing DNS data, but these are not integral to the operation of a nameserver. The zone file format is standardised, but it is highly non-trivial to parse. It is entirely possible to write useful nameserver that does not read or write DNS zone files. When embarking on parsing zonefiles, do not do so lightly. As an example, various fields within a single line can appear in many orders. Most fields are optional, and some will then be copied from the previous line. But not all.
Of specific note, many people have attempted to write a grammar (parser) for zonefiles and it is almost impossible.
The concept of a DNS name is non-trivial and frequently misunderstood. Despite writing 'www.ietf.org' from left to right, within DNS it is fairer to describe it as 'org' below the root node, with below the 'org' node a node called 'ietf'. Finally to the 'ietf' node is attached a node called 'www'.
Or in graphical form:
-
+-----+ -
| | -
+--+--+ -
| -
+--+--+ -
| ORG | -
+--+--+ -
| -
+--+---+ -
| IETF | -
+--+---+ -
| -
+--+--+ -
| WWW | -
+-----+
The 'tree' of nodes as shown above is real and not just another way of visualizing a DNS name. This for example means that if there is a name called 'www.fr.ietf.org' and a query comes in for 'fr.ietf.org', that name exists - even though no records may be assigned to it.
NOTE: This means that any implementation that sees DNS as a simple 'key/value' store, where only records that exist can match, is headed for trouble down the line.
As noted, DNS is more complicated than a simple key/value store. This is not only because of the tree style nature of names but also because the same data can live in multiple places, but always lives in a 'zone'.
Various DNS implementations over time have found out that you can mostly ignore the concept of 'zone' for simple nameservers or load balancers, but not implementing zones correctly will eventually trip you up.
To make life confusing, 'www.ietf.org' can be defined in four different places. It could be in the 'root' zone itself, fully written out:
www.ietf.org IN AAAA 3600 2400:cb00:2048:1::6814:55
Or it could be in the org zone, where it might look like this:
$origin ORG
www.ietf IN AAAA 3600 2400:cb00:2048:1::6814:55
Or, (as is actually the case), this name could live in the 'ietf.org' zone:
$origin ietf.org
www IN AAAA 3600 2400:cb00:2048:1::6814:55
And finally, it is even possible that there is a zone called 'www.ietf.org', where the record lives like this:
$origin www.ietf.org
@ IN AAAA 3600 2400:cb00:2048:1::6814:55
A zone always starts with a SOA or Start Of Authority record. A SOA record is DNS metadata. It stores various things that may be of interest about a zone, like the email address of the maintainer, the name of the most authoritative server. It also has values that describe how or if a zone needs to be replicated. Finally, the SOA record has a number that influences TTL values for names that do not exist.
There is only one SOA that is guaranteed to exist on the internet and that is the one for the root zone (called '.'). As of 2018, it looks like this:
. 86400 IN SOA a.root-servers.net. nstld.verisign-grs.com. 2018032802 1800 900 604800 86400
For details of what all these fields mean, please see the authoritative server document.
The final number however is important here. 86400 denotes that if a response says a name or RRSET does not exist, it will continue to not exist for the next day, and that this knowledge may be cached.
As noted, 'www.ietf.org' can live in four places. If it lives where it currently does, in the 'ietf.org' zone, it passes through two zone cuts: From . to org, from org to ietf.org.
When an authoritative server receives a query for 'www.ietf.org', it consults which zones it knows about and answers from the most specific zone it has available.
For a root-server, which only knows about the root zone, this means consulting the '.' zone. As noted, 'www.ietf.org' is actually a tree, 'org' -> 'ietf' -> 'www'. And as luck will have it, the first node 'org' is present in the root zone.
Attached to that node is an NS RRSET, which has the names of nameservers that host the ORG zone.
If we ask these servers about 'www.ietf.org', they too find the best zone to answer from, which in this case is 'org'. Within the 'org' zone they then find the 'ietf' node, which again contains an NS RRSET.
When we ask the servers named in that RRSET about 'www.ietf.org', they find a node called 'www' with several RRSETs on it, one if which is for AAAA and contains the IPv6 address we were looking for.
Any authoritative server which does not implement 'zones' in this way will eventually run into trouble. It is not enough to consult a list of known names and answer records attached to those names.
These are a mandatory part of a zone, at the 'apex'. The 'apex' is the name of the zone, at which point there is also a SOA record. So a typical zone will start like this:
$ORIGIN ietf.org.
@ IN SOA ns1 admin 2018032802 1800 900 604800 86400
IN NS ns1
IN NS ns2
Note how in this zone file example names not ending on a '.' are interpreted as being part of ietf.org. The '@' is a way to specify the name of the apex. Lines two and three omit a name, so they default to '@' too.
This zone lists ns1.ietf.org and ns2.ietf.org as its nameservers. Being part of the zone, this data is authoritative. Any queries sent to this nameserver for the NS RRSET of 'ietf.org' will receive responses with the AA bit set.
Note however that above we learned that the parent zone, 'org' also needs to list the nameservers for example.org, and it does:
$ORIGIN org.
...
ietf IN NS ns1.ietf
ietf IN NS ns2.ietf
If we ask the 'org' nameservers for the NS RRSET of 'ietf.org', we receive a response with AA=0, indicating that the 'org' servers know they aren't 'authoritative' for ietf.org.
The astute reader will have spotted a chicken and egg problem here. If ns1.ietf.org is the nameserver for ietf.org.. where do we get the IP address of ns1.ietf.org?
To solve this problem, the parent zone can provide a free chicken. In the org zone, we would actually find:
$ORIGIN org.
...
ietf IN NS ns1.ietf
ietf IN NS ns2.ietf
ns1.ietf IN A 192.0.2.1
ns2.ietf IN A 198.51.100.1
These entries are mirrored in the 'ietf.org' zone hosted on ns1.ietf.org and ns2.ietf.org. And as with the NS records, any queries for ns1.ietf.org sent to the org servers receive AA=0 answers, whereas ns1.ietf.org itself answers with AA=1.
Note that for various reasons the AA=0 answer from the parent zone may be different than the AA=1 answer, and resolvers must be aware of the difference.
The description up to this point is correct, but far from functionally complete even for basic DNS. The following sections describe additional aspects of basic DNS:
A CNAME provides the 'Canonical Name' for another DNS name. For example:
www IN CNAME www.ietf.org.cdn.cloudflare.net.
This is frequently used to redirect to a Content Distribution Network. The CNAME is for a name, and not for a type. This means that any query for www.ietf.org is sent to cloudflare. This simultaneously means that what everyone wants is impossible:
$ORIGIN ietf.org
@ IN CNAME this.does.not.work.int.
This collides with the SOA and NS records, which are then also redirected and not found. Frequently doing this 'apex CNAME' appears to work, but it really doesn't.
In hindsight, the CNAME should have been 'typed' to apply only to specific query types.
When a server encounters a CNAME with the name of a name it was looking for, it will 'follow' the chain to where it points. And please be aware that this can loop.
Wildcards allow for the following:
$ORIGIN ietf.org.
* IN A 192.0.2.1
IN AAAA 2001:db8:85a3::8a2e:0370:7334
smtp IN A 192.0.2.222
A query for the A record of 'smtp.ietf.org' will return 192.0.2.222. A query for 'www.ietf.org' however will return 192.0.2.1.
Interestingly, as another example of how DNS really is a tree, a query for the AAAA record of smtp.ietf.org will return.. nothing. This is because the node 'smtp.ietf.org' does exist, and processing ends there. The wildcard match will not proceed to the '*' entry.
Wildcards synthesize new answers. This means that, unless explicitly queried, no '*.ietf.org' record will be served. Instead, a 'www.ietf.org' record is created on the fly.
Without implementing the optional EDNS protocol extension, all UDP responses must fit in 512 bytes of payload. If on writing an answer a server finds itself exceding this limit, it must truncate the packet and set the TC bit to 1.
The originator of the query will then resend the query over TCP.
Sometimes DNS responses contain optional data that could be left out, and this could be done to stay under the 512 byte limit.
It is recommended however to keep it simple and send an empty response packet with TC=1 whenever the byte limit is reached.
DNS queries can fail to match in two ways: the whole node does not exist, or, the requested type is not present at that node.
As an example of the first case, 'doesnotexist.ietf.org' really does not exist, which leads to a response with RCODE NXDOMAIN and no answer records.
As an example of the second case, 'www.ietf.org' does exist, but has no MX record. The RCODE is normal, but there are no answer records.
Empty answers however are hard to cache. To alleviate this situation, in these cases the authoritative server sends a copy of the SOA record in the Authority section of the response. The TTL of that record tells us how long the knowledge of 'no such name' or 'no such data' can be cached.
This is the core of DNS. There are quite some parts that have not been discussed, but based on the explanations above, it is possible to write a compliant authoritative server.
These (1034 & 1035) describe the core of DNS in 1987 language. When reading, disregard mentions of IQUERY and experimental records. They did not survive. Also realize that in this world, authoritative and resolver service were described as a single function. We now know this to be confusing.
From 1997, 2181 performs a decade of cleanup work on 1034/1035. It also talks about an early version of DNSSEC (NXT, SIG, KEY records), these sections should not be read as this is unrelated to current DNSSEC (aka DNSSEC-bis).
Of specific note, 5.4.1 describes very exact ordering rules which data a server is supposed to prefer. This list becomes a lot simpler when split up between pure authoritative and pure resolver functions.
This rfc describes how negative responses are to be cached. The details matter for both authoritative and resolvers. Of specific note are the parts that dwell on CNAME chains which lead to a 'no data' or 'NXDOMAIN' situation.
As with 2181, this RFC speaks about an earlier version of DNSSEC, and these parts should be fully ignored.
This rfc describes the AAAA record, which is core to DNS as it is required to look up addresses of nameservers.
4343 clarifies the somewhat odd case insensitivity of DNS but also writes out the escaping rules when using non-ASCII or whitespace in DNS names. As noted before, try not to have to use these rules except when reading DNS data from text files or showing DNS data meant for human consumption. Use native DNS names as much as possible, and create 4343-compliant comparison and equivalence functions.
This RFC makes source port randomization mandatory for UDP-based DNS messages and also has rules on preventing "birthday attacks".
6604 further describes the meanings of header bits (AA) and RCODEs when following CNAME chains. Also discusses an earlier version of DNAMEs, these parts are best ignored in lieu of (later) reading the newer DNAME specification.
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