**A warm welcome to DNS** Note: this page is part of the '[hello-dns](https://powerdns.org/hello-dns/)' documentation effort. # tauth: the teaching authoritative server `tauth` is based on the [`tdns`](README.md.html) teachable DNS library. `tauth` is a relatively full-featured authoritative server. # Current status All 'basic DNS' items are implemented: * UDP & TCP * AXFR (incoming and outgoing) * Wildcards * Delegations * Glue records * Truncation * Compression As a bonus: * EDNS (buffer size, no options) * Serving of DNSSEC signed zones Known broken: * TCP/IP does not follow recommended timeouts The code is not quite in a teachable state yet and still contains ugly bits. But well worth [a read](https://github.com/ahupowerdns/hello-dns/tree/master/tdns). # Layout Key to a good DNS implementation is having a faithful DNS storage model, with the correct kind of objects in them. Over the decades, many many nameservers have started out with an incorrect storage model, leading to pain later on with empty non-terminals, case sensitivity, setting the 'AA' bit on glue (or not) and eventually DNSSEC ordering problems. When storing DNS as a tree, as described in RFC 1034, a lot of things go right "automatically". When DNS Names are a fundamental type composed out of DNS Labels with the correct case-insensitive equivalence and identity rules, lots of problems can never happen. The core or `tauth` therefore is the tree of nodes as intended in 1034, containing DNS native objects like DNS Labels and DNS Names. # The DNS Tree The DNS Tree is of fundamental importance, and is used a number of times within `tauth`. When storing the contents of the `org` zone, it may look like this: ************************************************************************************************* * * * .---. * * 1 +---------+ +--------+ * * / '-+-' \ * * / | \ * * .-+-. .-+-. .-+-. * * 2 + ietf+ | ietg+ | ... + * * '-+-' '-+-' '---' * * / \ | * * / \ | * * .--+. +---. .-+-. * * 3 + ord | | fra + | ... + * * '-+-' '-+-' '---' * * | | * * .-+-. .-+-. * * 4 + ns1 | | ns2 + * * '-+-' '---' * * * ************************************************************************************************* This tree has a depth of four. The top node has an empty name, and is relative to the name of the zone, in this case `org`. On layer 4, we find the names `ns1.ord.ietf.org` and `ns2.fra.ietf.org`. Key to looking up anything in DNS is to follow the tree downwards and to observe what nodes are passed. For example, a lookup for `www.ietf.org` starts as a lookup for `www.ietf` in the `org` zone (if loaded, of course). Layer 1 is where we start (and find the Start of Authority record), and we look if there is a child node called `ietf`. And there is. As we look at that node, we could see NS records attached to it (`ietf.org NS ns1.ord.ietf.org`) for example. This means our lookup is done: we've found a zonecut. The authoritative server should now respond with a delegation by returning those NS records in the Nameserver section. To complete the packet, we need to look up the IPv4 and IPv6 addresses of `ns1.ord.ietf.org` and `ns2.fra.ietf.org`. To do this, we traverse the tree downward again, starting at the apex with `ns1.ord.ietf` and going to the `ietf`, `ord` and finally `ns1` labels. There we find attached the IP(v6) addresses. ## Objects `tdns` uses a DNS tree in three places: 1) to quickly find the right zone for a query 2) within that zone, to traverse the names 3) DNS name compression. The DNS tree within `tdns` consists of `DNSNode` objects, each of which can have: * Child nodes * Pointer to a zone * Attached RRSets, keyed on type The child nodes are always used in the DNS tree. The pointer to a zone is only used when consulting the 'tree of zones'. The attached RRsets meanwhile are only consulted when the right zone is found, to provide actual DNS answers. ## Manipulating the tree To add nodes to the DNS tree, or to add things to existing nodes, use the `add` method like this: ``` newzone->add({"www"})->addRRs(CNAMEGen::make({"server1","powerdns","org"})); newzone->add({"www"})->rrsets[DNSType::CNAME].ttl = 1200; ``` The first line creates the `www` node, and provisions a CNAME there. The second line updates the new node to set the ttl. Note that `addRRs` accepts multiple 'generator' parameters, more about which later. `add` accepts `DNSName`s as parameter, so to populate www.fra.ietf.org, use `newzone->add({"www", "fra", "ietf", "org"})`. Within `tdns`, the sample `powerdns.org` zone is populated within [contents.cc](https://github.com/ahupowerdns/hello-dns/blob/master/tdns/contents.cc). Finding nodes in the tree uses a slightly more complicated method called `find`. Unlike `add` it will not modify the tree, even though it has in common that it will return a pointer to a node. `find` however also returns some additional things: which parts of the `DNSName` did not match a node, if a DNS zonecut was encountered while traversing the tree, and what name it had. The syntax: ``` DNSName searchname({"www", "ietf", "org"}), lastname, zonecutname; DNSNode* passedZonecut; DNSNode* node = bestzone->find(searchname, lastname, &passedZonecut, &zonecutname); ``` When this operates on the `org` zone tree displayed above, after the call to `find`, `searchname` will be `www`, while `lastname` is `{"ietf", "org"}`. What this means was that the `www` label could not be matched in the tree, since it isn't there. `passedZonecut` is set to the node that describes `ietf.org`, where NS records live that describe the delegation. `zonecutname` is therefore set to `ietf.org`. To clarify this further, a lookup for `ns1.ord.ietf.org` would end up with: * `searchname` empty: all labels of `ns1.ord.ietf.org` were matched * `lastname` is then `ns1.ord.ietf.org` * `passedZonecut` again points to the `{"ietf", "org"}` node, which has the NS RRSet that describes the delegation * `zonecutname` is set to `{"ietf", "org"}`. The DNS Tree is aware of `*` semantics, and when traversing nodes and not finding a match, it will look for a `*` node. The tree does not do any special processing for CNAMEs though. Based on the `find` method, implementing the RFC 1034 DNS algorithm is very straightforward. ## Record generators As noted above, `RRSet`s contain things like `CNAMEGen::make`. These are generators that are stored in a `DNSNode` and that know how to put their content into a `DNSMessageWriter`. Each implemented `DNSType` has at least one associated generator. A more complete example of populating a zone looks like this: ``` newzone->addRRs(SOAGen::make({"ns1", "powerdns", "org"}, {"admin", "powerdns", "org"}, 1), NSGen::make({"ns1", "powerdns", "org"}), NSGen::make({"ns2", "powerdns", "org"}), MXGen::make(25, {"server1", "powerdns", "org"}) ); newzone->add({"server1"})->addRRs(AGen::make("213.244.168.210"), AAAAGen::make("::1")); ``` This attaches SOA, NS and MX records to the apex of a zone, and defines a `server1` node that is also referenced in the MX record. This code can be found in [record-types.cc](https://github.com/ahupowerdns/hello-dns/blob/master/tdns/record-types.cc) and [record-types.hh](https://github.com/ahupowerdns/hello-dns/blob/master/tdns/record-types.cc). Since there are many record types, it is imperative that adding a new one needs to happen in only one place. Within `tauth`, it actually requires two places: the `DNSType` enum needs to be updated with the numerical value of the type, and a 'XGen` struct needs to be written. Luckily this is simple enough. Here is the entire MX record implementation: ``` 1 struct MXGen : RRGen 2 { 3 MXGen(uint16_t prio, const DNSName& name) : d_prio(prio), d_name(name) {} 4 static std::unique_ptr< RRGen > make(uint16_t prio, const DNSName& name) 5 { 6 return std::make_unique< MXGen >(prio, name); 7 } 8 void toMessage(DNSMessageWriter& dpw) override; 9 DNSType getType() const override { return DNSType::MX; } 10 uint16_t d_prio; 11 DNSName d_name; 12 }; ... 13 void MXGen::toMessage(DNSMessageWriter& dmw) 14 { 15 dmw.putUInt16(d_prio); 16 dmw.putName(d_name); 17 } ``` Line 3 stores the priority and server name of this MX record (as defined in lines 10 and 11). Lines 4-7 are mechanics so we can make a smart pointer for an MXGen type using a call to `make`. This smart pointer is sort of reference counted in that its reference count is always 1. This means there is no overhead. Line 8 defines the call that transposes this record into a `DNSMessageWriter`. Line 9 announces to anyone who wants to know what the `DNSType` of this generator is. This is used by `addRRs` as shown above to put the generator in the right RRSet place. 13 to 17 show the construction of the actual DNS resource record in a packet: the 16 bit priority, followed by the name. ## A bit of fun: dynamic record contents Although names can not easily be dynamic within the DNS tree (either they exist or they don't), contents can be changed at will. `tdns` defines a `time.tdns.powerdns.org` node which has a `ClockTXTGen`: ``` newzone->add({"time"})->addRRs(ClockTXTGen::make("The time is %a, %d %b %Y %T %z")); ``` The code behind this generator: ``` void ClockTXTGen::toMessage(DNSMessageWriter& dmw) { struct tm tm; time_t now = time(0); localtime_r(&now, &tm); std::string txt("overflow"); char buffer[160]; if(strftime(buffer, sizeof(buffer), d_format.c_str(), &tm)) txt=buffer; TXTGen gen(txt); gen.toMessage(dmw); } ``` Note that this generator uses the existing TXT code to encode itself. # The RFC 1034 algorithm As noted in the [basic DNS](../basic.md.html) and [authoritative](../auth.md.html) pages, the RFC 1034 algorithm can be simplified for a pure authoritative server. ## Finding the right zone and node In [tdns.cc](https://github.com/ahupowerdns/hello-dns/blob/master/tdns/tdns.cc) , processing starts like this: ``` 1 DNSName zonename; 2 auto fnd = zones.find(qname, zonename); 3 ... 4 response.dh.aa = 1; 5 6 auto bestzone = fnd->zone; 7 DNSName searchname(qname), lastnode, zonecutname; 8 const DNSNode* passedZonecut=0; 9 auto node = bestzone->find(searchname, lastnode, &passedZonecut, &zonecutname); ``` In line 1 we declare the DNSName where we will store the name of the matching zone. On line 2 we look up the query name, and get the node containing the zone, plus its name. Line 3 elides error response if no zone was found. In line 4 we declare we have authority. Line 6 saves some typing later on. Lines 7 and 8 declare what we are looking for, and reserves names for where we store what we found. Line 9 finally calls `find` to find the best node within our zone. As noted above, `find` not only finds the best node, but also lets us know if we passed any NS records along the way. ## If we passed a zone cut ``` 1 if(passedZonecut) { 2 response.dh.aa = false; 3 cout<<"This is a delegation, zonecutname: '" << zonecutname << "'" << endl; 4 auto iter = passedZonecut->rrsets.find(DNSType::NS); 5 if(iter != passedZonecut->rrsets.end()) { 6 const auto& rrset = iter->second; 7 vector< DNSName > toresolve; 8 for(const auto& rr : rrset.contents) { 9 response.putRR(DNSSection::Authority, zonecutname+zonename, DNSType::NS, rrset.ttl, rr); 10 toresolve.push_back(dynamic_cast< NSGen* >(rr.get())->d_name); 11 } 12 addAdditional(bestzone, zonename, toresolve, response); 13 } 14 } ``` This is the first thing we check: did we pass a zone cut? If so, on line 2 we drop the aa bit, since we clearly are not providing an authoritative answer. Lines 4 and 5 lookup and verify if there is actually an NS record at the zone cut. This should always be true. In line 7 we store room for the NS server names we will need to look up glue for. In line 8 we iterate over the NS records, which we put in the `DNSMessageWriter` on line 9. On line 10 we store glue record names. Finally on line 12, we call `addAdditional` which will look up the glue names for us. This completes the response in case of a delegation. Note that contrary to RFC 1034, `addAdditional` **only** looks for glue within the `bestzone` itself. ## NXDOMAIN ``` 1 else if(!searchname.empty()) { 2 if(!CNAMELoopCount) // RFC 1034, 4.3.2, step 3.c 3 response.dh.rcode = (int)RCode::Nxdomain; 4 const auto& rrset = bestzone->rrsets[DNSType::SOA]; 5 6 response.putRR(DNSSection::Authority, zonename, DNSType::SOA, rrset.ttl, rrset.contents[0]); 7 } ``` If `find` returned with a non-empty `searchname`, it meant there were parts of the query name that could not be matched to a node. We checked for a zonecut earlier (in the previous section), there was none. So this name really does not exist. In line 3 we set the response status to NXDOMAIN, unless we've looped through a CNAME already. In line 4 we look up the SOA record of our `bestzone` and in line 6 we put it in the message. ## Node exists At this stage we know a node exists for this name, although it may actually be a wildcard node. We do not actually care if it is. Here is what we have to do first though. ### Check for a CNAME ``` 1 auto iter = node->rrsets.cbegin(); 2 if(iter = node->rrsets.find(DNSType::CNAME), iter != node->rrsets.end()) { 5 const auto& rrset = iter->second; 6 response.putRR(DNSSection::Answer, lastnode+zonename, DNSType::CNAME, rrset.ttl, rrset.contents[0]); 7 DNSName target=dynamic_cast(rrset.contents[0].get())->d_name; 8 if(target.makeRelative(zonename)) { 9 searchname = target; 10 if(CNAMELoopCount++ < 10) { 11 lastnode.clear(); 12 zonecutname.clear(); 13 goto loopCNAME; 14 } 15 } 16 else 17 cout<<" CNAME points to record " << target << " in other zone, good luck" << endl; 18 } ``` Line 1 defines an iterator for our subsequent lookup in line 2: is there a CNAME at this node? If so, in line 6 we put it in the DNSMessage. In line 7 we extract the target of the CNAME. In line 8 we again violate the RFC 1034 algorithm by checking if the CNAME points to somewhere within our own zone. If it points to another zone, we are not going to chase this CNAME. On line 9 we redirect ourselves if within the same zone. We also check if we haven't looped 'too much' already. It appears everyone has picked the number 10 for this. We do some cleanup on lines 11 and 12 and finally on line 13 we restart our algorithm. With a goto. ### Name exists, no CNAME, matching types ``` 1 if(iter = node->rrsets.find(qtype), iter != node->rrsets.end() || (!node->rrsets.empty() && qtype==DNSType::ANY)) { 2 auto range = make_pair(iter, iter); 3 if(qtype == DNSType::ANY) 4 range = make_pair(node->rrsets.begin(), node->rrsets.end()); 5 else 6 ++range.second; 7 for(auto i2 = range.first; i2 != range.second; ++i2) { 8 const auto& rrset = i2->second; 9 for(const auto& rr : rrset.contents) { 10 response.putRR(DNSSection::Answer, lastnode+zonename, i2->first, rrset.ttl, rr); 11 if(i2->first == DNSType::MX) 12 additional.push_back(dynamic_cast< MXGen* >(rr.get())->d_name); 13 } 14 } 15 } ``` On line 1 is a somewhat tricky lookup that tries to find the query type in the RRSET, and if it could not be found, if the query maybe was for ANY and there are records that could be matched. On lines 2 to 6 we either pick the matching RRSet to put in the DNSMessage, or we set it up so we iterate over all types, which we then do on lines 8 to 14. Note that again we gather up the server name of the MX record for additional processing. If we supported SRV records, we would do the same for them. ### The name exists, but no types or no types match Finally one of the most vexing parts of DNS: a name that exists, but there are no types or at least no matching types. This could be an 'empty non-terminal', created out of thin air by 'some.long.name.powerdns.org'. This DNS Name populates nodes all along its length, even if no RRSets are attached to 'long.name.powerdns.org' for example. In many servers this is tricky, but since we followed a DNS tree based design with nodes, our code is trivial: ``` 1 else { 2 const auto& rrset = bestzone->rrsets[DNSType::SOA]; 3 response.putRR(DNSSection::Authority, zonename, DNSType::SOA, rrset.ttl, rrset.contents[0]); 4 } ``` All we have to do is 'else' off the previous case, and add the SOA record. # AXFR AXFR over TCP/IP consists of a series of DNS messages, each prefixed by a 16 bit length field. The first and last RRSet contained within these DNS message(s) must be the SOA record of a zone. Code: ``` 1 DNSMessageWriter response(std::numeric_limits< uint16_t >::max()-sizeof(dnsheader)); 2 DNSName zone; 3 auto fnd = zones->find(name, zone); 4 if(!fnd || !fnd->zone || !name.empty() || !fnd->zone->rrsets.count(DNSType::SOA)) { 5 cout<< " This was not a zone, or zone had no SOA" << endl; 6 return; 7 } 8 response.dh = dm.dh; 9 response.dh.ad = response.dh.ra = response.dh.aa = 0; 10 response.dh.qr = 1; 11 response.setQuestion(zone, type); 12 13 auto node = fnd->zone; 14 15 // send SOA 16 response.putRR(DNSSection::Answer, zone, DNSType::SOA, node->rrsets[DNSType::SOA].ttl, node->rrsets[DNSType::SOA].contents[0]); 17 18 writeTCPResponse(sock, response); ``` In line 1 we allocate a `DNSMessageWriter` of maximum size. Lines 2-7 find the best zone, as in the RFC 1034 algorithm. Of specific note is that 'empty non-terminal zones' could be found by this tree walking function, so we check for this. The response is then prepared, copying in the original dnsheader (with the transaction id), and setting the flags, qname and qtype correctly. Line 13 is again a convenience to save some typing. Line 16 adds the initial SOA record, and the response gets sent out on line 18. Note that it is possible to use this first DNSMessage for the initial SOA record and subsequent records too. To keep things simple, we don't do this here. Next up is the loop to pass the rest of the zone contents: ``` 1 response.setQuestion(zone, type); 2 3 node->visit([&response,&sock,&name,&type,&zone](const DNSName& nname, const DNSNode* n) { 4 for(const auto& p : n->rrsets) { 5 if(p.first == DNSType::SOA) 6 continue; 7 for(const auto& rr : p.second.contents) { 8 retry: 9 try { 10 response.putRR(DNSSection::Answer, nname, p.first, p.second.ttl, rr); 11 } 12 catch(std::out_of_range& e) { // exceeded packet size 13 writeTCPResponse(sock, response); 14 response.setQuestion(zone, type); 15 goto retry; 16 } 17 } 18 } 19 }, zone); 20 21 writeTCPResponse(sock, response); ``` In line 1, the DNS message is emptied of RRSets. Line 3 launches a visitor that walks the DNS Tree and calls putRR on all RRSets it finds, except the SOA record, which was sent already., so we skip it on line 5. Lines 9 to 11 attempt to put this resource record in the message. If the record does not fit, `putRR` rolls back the addition, and throws an exception which we catch on line 12. There we write out the message to TCP, reset the packet, and try again. Finally in line 21 we write out the last `DNSMessageWriter` we filled. To terminate the AXFR, we now need to resend the SOA record, which we do as follows: ``` response.putRR(DNSSection::Answer, zone, DNSType::SOA, node->rrsets[DNSType::SOA].ttl, node->rrsets[DNSType::SOA].contents[0]); writeTCPResponse(sock, response); ``` Note: this code, in `tcpClientThread` of [tdns.cc](https://github.com/ahupowerdns/hello-dns/blob/master/tdns/record-types.cc) does not yet implement best TCP practices on timeouts and keeping open connections.