hello-dns/tdns

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                        **A warm welcome to DNS**

Note: this page is part of the 'hello-dns' documentation effort.

teaching DNS: Library, Authoritative, Resolver

Welcome to tdns, a 'from scratch' teaching DNS library. Based on tdns, tauth and tres implement all of basic DNS and large parts of DNSSEC in 2000 3000 3100 lines of code. Code is here. To compile, see the end of this document.

Even though the 'hello-dns' documents describe how basic DNS works, and how servers should function, nothing quite says how to do things like actual running code. tdns is small enough to read in one sitting and shows how DNS packets are parsed and generated. tdns is currently written in C++ 2014, and is MIT licensed. Reimplementations in other languages are highly welcome, as these may be more accessible to other programmers.

Please contact bert.hubert@powerdns.com or @PowerDNS_Bert if you have plans or feedback.

The goals of tdns, tauth & tres are:

• Showing the DNS algorithms 'in code'
• Protocol correctness, except where the protocol needs updating
• Suitable for educational purposes
• Display best practices, both in DNS and security
• Be a living warning for how hard it is to write a nameserver correctly

Non-goals are:

• Performance
• Implementing more features (unless very educational)
• DNSSEC signing, validation

A more narrative explanation of what tdns is and what we hope it will achieve can be found here.

The code for tdns can be found on GitHub and is also documented using Doxygen.

Objects in tdns

These are found in dns-storage.hh and dns-storage.cc.

DNSLabel

The most basic object in tdns is DNSLabel. www.powerdns.com consists of three labels, www, powerdns and com. DNS is fundamentally case insensitive (in its own unique way), and so is DNSLabel. So for example:

	DNSLabel a("www"), b("WWW");
	if(a==b) cout << "The same\n";

Will print 'the same'.

In DNS a label consists of between 1 and 63 characters, and these characters can be any 8 bit value, including 0x0. By making our fundamental data type DNSLabel behave like this, all the rest of tdns automatically gets all of this right.

When DNS labels contain spaces or other non-ascii characters, and a label needs to be converted for screen display or entry, escaping rules apply. The only place in a nameserver where these escaping rules should be enabled is in the parsing or printing of DNS Labels.

The input to a DNSLabel is an unescaped binary string. The escaping example from RFC 4343 thus works like this:

	DNSLabel dl("Donald E. Eastlake 3rd");
	cout << dl << endl; // prints: Donald\032E\.\032Eastlake\0323rd

DNSName

A sequence of DNS Labels makes a DNS name. We store such a sequence as a DNSName. To make this safe, even in the face of embedded dots, spaces and other things, within tdns we make no effort to parse www.powerdns.com in the code. Instead, use this:

	DNSName sample({"www", "powerdns", "com"});
	cout << sample <<"\n"; // prints www.powerdns.com.

	sample.pop_back();
	cout << sample << ", size: " << sample.size() << sample.size() << '\n';
	// prints www.powerdns., size 2

Note: for convenience, when parsing human-generated input, makeDNSName() is available to make a DNSName from a string.

Since a DNSName consists of DNSLabels, it gets the same escaping. To again emphasise how we interpret the input as binary, ponder:

	DNSName test({"powerdns", "com."});
	cout << test << endl; // prints: powerdns.com\..

	const char zero[]="p\x0werdns";
	DNSName test2({std::string(zero, sizeof(zero)-1), "com"});

	cout << test2 << endl; // prints: p\000werdns.com.

DNSType, RCode, DNSSection

These is an enums that contains the names and numerical values of the DNS types and error codes. This means for example that DNSType::A corresponds to 1 and DNSType::SOA to 6.

To make life a little bit easier, an operator has been defined which allows the printing of DNSTypes as symbolic names. Sample:

	DNSType a = DNSType::CNAME;
	cout << a << "\n";    // prints: CNAME

	a = (DNSType) 6;
	cout << a <<" is "<< (int)a << "\n"; // prints: SOA is 6

Similar enums are defined for RCodes (response codes, RCode::Nxdomain for example) and DNS Sections (Question, Answer, Nameserver/Authority, Additional). These too can be printed.

tdig

To discover how tdns works, let's start with the basics: sending DNS queries and parsing responses. For this purpose, the tdig tool is provided, somewhat modelled after the famous dig program created by ISC.

The code:

	int main(int argc, char** argv)
	{
		/* ... */
		DNSName dn = makeDNSName(argv[1]);
		DNSType dt = makeDNSType(argv[2]);
		ComboAddress server(argv[3]);

		DNSMessageWriter dmw(dn, dt);
		dmw.dh.rd = true;
		dmw.setEDNS(4000, false);

This starts out with the basics: it reads a DNSName from the first argument to tdns, a DNSType from the second and finally a server IP address from the third argument.

With this knowledge, in line 8 we create a DNSMessageWriter to make a question for query name dn and query type dt. In addition, we set the 'recursion desired' flag.

Finally on line 10, we indicate our support for up to 4000 byte responses, but we set the 'DNSSEC Ok' flag to false.

Next, mechanics:

1	Socket sock(server.sin4.sin_family, SOCK_DGRAM);
2	SConnect(sock, server);
3	SWrite(sock, dmw.serialize());
4	string resp = SRecvfrom(sock, 65535, server);
5
6	DNSMessageReader dmr(resp);

In line 1 we create a datagram socket appropriate for the protocol of server. This is based on a small set of socket wrappers called simplesockets. On line 2 we connect and on line 3 we serialize our DNSMessageWriter and send the resulting packet. On line 4 we receive a response.

Finally on line 6 we parse that response into a DNSMessageReader.

1	DNSSection rrsection;
2	uint32_t ttl;
3 
4	dmr.getQuestion(dn, dt);
5	
6	cout<<"Received " << resp.size() << " byte response with RCode ";
7	cout << (RCode)dmr.dh.rcode << ", qname " << dn << ", qtype " << dt << endl;
8	std::unique_ptr< RRGen > rr;
9	while(dmr.getRR(rrsection, dn, dt, ttl, rr)) {
10	  cout << dn<< " IN " << dt << " " << ttl << " " << rr->toString() << endl;
11	}

On lines 1 and 2 we declare some variable we'll need later to actually retrieve the resource records. On line 4 we retrieved the name and type we received an answer for, and on line 6 this all is displayed.

Line 8 declares 'rr' ready to receive our Resource Records, which are then retrieved using the getRR method from the DNSMessageReader on line 9.

On line 10 we print what we found. Note that the RRGen object helpfully has a toString() method for human friendly output.

Parsing and generating DNS Messages

This code is in dnsmessages.cc and dnsmessages.hh.

RRGens: dealing with all the record types

DNS knows many record types, so we need a unified interface that can pass all of them. For this purpose, tdns uses RRGen instances. RRGens are classes, one for each record type, all deriving from the RRGen base.

Each RRGen has a method called toString() which emits the record's contents in familiar 'zonefile' format.

RRGens can be created using their specific instance types, for example like this:

	ComboAddress ip("203.0.113.1");
	auto agen = AGen::make(ip);

	cout << agen->toString() << endl; // prints 203.0.113.1

	auto soagen = SOAGen::make({"ns1", "powerdns", "com"}, 
		{"bert.hubert", "powerdns", "com"}, 2018102301);

RRGens also know how to serialize themselves from a DNSMessageReader, or how to write themselves out to a DNSMessageWriter.

When reading DNS Messages (see below), DNSMessageReader::getRR() will return RRGen instances to you, if you want to do more than print their contents, you need to cast them to the specific type, for example:

  ComboAddress ret;
  ret.sin4.sin_family = 0;
  if(auto ptr = dynamic_cast<AGen*>(rr.get()))
    ret=ptr->getIP();
  else if(auto ptr = dynamic_cast<AAAAGen*>(rr.get()))
    ret=ptr->getIP();

This code from tres checks if a record is an IP or IPv6 address and extracts the IP address - all without using ASCII.

DNSMessageReader

This class reads a DNS message, and makes available:

• The query name (qname) and type (qtype)
• The dnsheader containing the flags
• EDNS buffer size and value of DNSSEC Ok flag

Of specific security note, this is one area where we might potentially have to do pointer arithmetic. For security purposes, DNSMessageReader uses bounds checking access methods exclusively.

Somewhat unexpectedly, parsing a packet does not immediately give the user access to the query and type of the query (or response). The reason for this is that there are packets that have no query defined. So to get the query, call getQuery().

Getting resource records from a DNSMessageReader happens via getRR which returns record details and a smart pointer to an RRGen instance (as described above).

A good example of how DNSMessageReader works can be found in tdig.cc.

DNSMessageWriter

This class creates DNS messages, and in its constructor it needs to know the name and type it is creating a message for.

Packets are only written in order. So it is not possible to change the qname after adding a resource record. Resource records must also be added together as RRSets, and in 'section order'.

Internally DNSMessageWriter again only uses bounds checked methods for modifying its state.

A DNSMessageWriter has a maximum length (set via its constructor). If new resource record, as written by putRR, would exceed this maximum length, that record is rolled back and a std::out_of_range() exception is thrown. This allows the caller to either truncate or decide this data was optional anyhow.

Writing actual records to DNSMessageWriter proceeds via putRR() which serializes RRGen instances to the message.

Samples of how to do this can be found in tres.cc and tauth.cc.

Compression

DNS compression is unreasonably difficult to get right. In what happens to be a coincidence, it turns out the DNS Tree can also be used to perform DNS name compression.

For every invocation of putName() in DNSMessageWriter() we check the DNS tree if it has a match on the full name, and if not, we add the name and its components of the name to a DNS tree.

This effectively gets us the desired compression behaviour, except special care has to be taken to not do wildcard processing.

EDNS and truncation

EDNS tells us that a larger buffer size is available. However, even with such a larger buffer size, a packet may exceed the available space. In that case, the standard tells us to truncate the packet, and then still put an EDNS record in the response.

The DNSMessageWriter, in somewhat of a layering violation, takes care of this in serialize().

Internals

tdns uses several small pieces of code not core to dns:

• nenum this is a simple 'named ENUM' construct that enables the printing of DNSName::A
• Simplesocket a small set of convenience functions for making sockets, parsing IP addresses etc.
• Catch2 a unit test framework

Compiling and running tdns

This requires a recent compiler version that supports C++ 2014. If you encounter problems, please let me know (see above for address details).

$ git clone https://github.com/ahupowerdns/hello-dns.git
$ cd hello-dns/tdns
$ git submodule init
$ git submodule update
$ make
$ ./tauth [::1]:5300 &
$ dig -t any time.powerdns.org @::1 -p 5300 +short 
time.powerdns.org.	3600	IN	TXT	"The time is Fri, 13 Apr 2018 12:55:54 +0200"

For more detauls, read on about tauth, tres or the C API.