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250 lines
9.0 KiB
250 lines
9.0 KiB
/* |
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Package dns implements a full featured interface to the Domain Name System. |
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Server- and client-side programming is supported. |
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The package allows complete control over what is send out to the DNS. The package |
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API follows the less-is-more principle, by presenting a small, clean interface. |
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The package dns supports (asynchronous) querying/replying, incoming/outgoing zone transfers, |
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TSIG, EDNS0, dynamic updates, notifies and DNSSEC validation/signing. |
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Note that domain names MUST be fully qualified, before sending them, unqualified |
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names in a message will result in a packing failure. |
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Resource records are native types. They are not stored in wire format. |
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Basic usage pattern for creating a new resource record: |
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r := new(dns.MX) |
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r.Hdr = dns.RR_Header{Name: "miek.nl.", Rrtype: dns.TypeMX, Class: dns.ClassINET, Ttl: 3600} |
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r.Preference = 10 |
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r.Mx = "mx.miek.nl." |
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Or directly from a string: |
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mx, err := dns.NewRR("miek.nl. 3600 IN MX 10 mx.miek.nl.") |
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Or when the default TTL (3600) and class (IN) suit you: |
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mx, err := dns.NewRR("miek.nl. MX 10 mx.miek.nl.") |
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Or even: |
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mx, err := dns.NewRR("$ORIGIN nl.\nmiek 1H IN MX 10 mx.miek") |
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In the DNS messages are exchanged, these messages contain resource |
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records (sets). Use pattern for creating a message: |
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m := new(dns.Msg) |
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m.SetQuestion("miek.nl.", dns.TypeMX) |
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Or when not certain if the domain name is fully qualified: |
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m.SetQuestion(dns.Fqdn("miek.nl"), dns.TypeMX) |
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The message m is now a message with the question section set to ask |
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the MX records for the miek.nl. zone. |
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The following is slightly more verbose, but more flexible: |
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m1 := new(dns.Msg) |
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m1.Id = dns.Id() |
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m1.RecursionDesired = true |
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m1.Question = make([]dns.Question, 1) |
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m1.Question[0] = dns.Question{"miek.nl.", dns.TypeMX, dns.ClassINET} |
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After creating a message it can be send. |
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Basic use pattern for synchronous querying the DNS at a |
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server configured on 127.0.0.1 and port 53: |
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c := new(dns.Client) |
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in, rtt, err := c.Exchange(m1, "127.0.0.1:53") |
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Suppressing |
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multiple outstanding queries (with the same question, type and class) is as easy as setting: |
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c.SingleInflight = true |
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If these "advanced" features are not needed, a simple UDP query can be send, |
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with: |
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in, err := dns.Exchange(m1, "127.0.0.1:53") |
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When this functions returns you will get dns message. A dns message consists |
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out of four sections. |
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The question section: in.Question, the answer section: in.Answer, |
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the authority section: in.Ns and the additional section: in.Extra. |
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Each of these sections (except the Question section) contain a []RR. Basic |
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use pattern for accessing the rdata of a TXT RR as the first RR in |
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the Answer section: |
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if t, ok := in.Answer[0].(*dns.TXT); ok { |
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// do something with t.Txt |
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} |
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Domain Name and TXT Character String Representations |
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Both domain names and TXT character strings are converted to presentation |
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form both when unpacked and when converted to strings. |
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For TXT character strings, tabs, carriage returns and line feeds will be |
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converted to \t, \r and \n respectively. Back slashes and quotations marks |
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will be escaped. Bytes below 32 and above 127 will be converted to \DDD |
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form. |
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For domain names, in addition to the above rules brackets, periods, |
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spaces, semicolons and the at symbol are escaped. |
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DNSSEC |
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DNSSEC (DNS Security Extension) adds a layer of security to the DNS. It |
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uses public key cryptography to sign resource records. The |
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public keys are stored in DNSKEY records and the signatures in RRSIG records. |
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Requesting DNSSEC information for a zone is done by adding the DO (DNSSEC OK) bit |
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to an request. |
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m := new(dns.Msg) |
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m.SetEdns0(4096, true) |
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Signature generation, signature verification and key generation are all supported. |
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DYNAMIC UPDATES |
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Dynamic updates reuses the DNS message format, but renames three of |
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the sections. Question is Zone, Answer is Prerequisite, Authority is |
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Update, only the Additional is not renamed. See RFC 2136 for the gory details. |
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You can set a rather complex set of rules for the existence of absence of |
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certain resource records or names in a zone to specify if resource records |
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should be added or removed. The table from RFC 2136 supplemented with the Go |
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DNS function shows which functions exist to specify the prerequisites. |
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3.2.4 - Table Of Metavalues Used In Prerequisite Section |
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CLASS TYPE RDATA Meaning Function |
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-------------------------------------------------------------- |
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ANY ANY empty Name is in use dns.NameUsed |
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ANY rrset empty RRset exists (value indep) dns.RRsetUsed |
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NONE ANY empty Name is not in use dns.NameNotUsed |
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NONE rrset empty RRset does not exist dns.RRsetNotUsed |
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zone rrset rr RRset exists (value dep) dns.Used |
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The prerequisite section can also be left empty. |
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If you have decided on the prerequisites you can tell what RRs should |
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be added or deleted. The next table shows the options you have and |
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what functions to call. |
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3.4.2.6 - Table Of Metavalues Used In Update Section |
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CLASS TYPE RDATA Meaning Function |
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--------------------------------------------------------------- |
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ANY ANY empty Delete all RRsets from name dns.RemoveName |
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ANY rrset empty Delete an RRset dns.RemoveRRset |
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NONE rrset rr Delete an RR from RRset dns.Remove |
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zone rrset rr Add to an RRset dns.Insert |
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TRANSACTION SIGNATURE |
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An TSIG or transaction signature adds a HMAC TSIG record to each message sent. |
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The supported algorithms include: HmacMD5, HmacSHA1, HmacSHA256 and HmacSHA512. |
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Basic use pattern when querying with a TSIG name "axfr." (note that these key names |
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must be fully qualified - as they are domain names) and the base64 secret |
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"so6ZGir4GPAqINNh9U5c3A==": |
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c := new(dns.Client) |
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c.TsigSecret = map[string]string{"axfr.": "so6ZGir4GPAqINNh9U5c3A=="} |
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m := new(dns.Msg) |
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m.SetQuestion("miek.nl.", dns.TypeMX) |
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m.SetTsig("axfr.", dns.HmacMD5, 300, time.Now().Unix()) |
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... |
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// When sending the TSIG RR is calculated and filled in before sending |
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When requesting an zone transfer (almost all TSIG usage is when requesting zone transfers), with |
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TSIG, this is the basic use pattern. In this example we request an AXFR for |
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miek.nl. with TSIG key named "axfr." and secret "so6ZGir4GPAqINNh9U5c3A==" |
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and using the server 176.58.119.54: |
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t := new(dns.Transfer) |
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m := new(dns.Msg) |
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t.TsigSecret = map[string]string{"axfr.": "so6ZGir4GPAqINNh9U5c3A=="} |
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m.SetAxfr("miek.nl.") |
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m.SetTsig("axfr.", dns.HmacMD5, 300, time.Now().Unix()) |
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c, err := t.In(m, "176.58.119.54:53") |
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for r := range c { ... } |
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You can now read the records from the transfer as they come in. Each envelope is checked with TSIG. |
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If something is not correct an error is returned. |
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Basic use pattern validating and replying to a message that has TSIG set. |
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server := &dns.Server{Addr: ":53", Net: "udp"} |
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server.TsigSecret = map[string]string{"axfr.": "so6ZGir4GPAqINNh9U5c3A=="} |
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go server.ListenAndServe() |
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dns.HandleFunc(".", handleRequest) |
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func handleRequest(w dns.ResponseWriter, r *dns.Msg) { |
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m := new(Msg) |
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m.SetReply(r) |
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if r.IsTsig() { |
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if w.TsigStatus() == nil { |
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// *Msg r has an TSIG record and it was validated |
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m.SetTsig("axfr.", dns.HmacMD5, 300, time.Now().Unix()) |
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} else { |
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// *Msg r has an TSIG records and it was not valided |
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} |
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} |
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w.WriteMsg(m) |
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} |
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PRIVATE RRS |
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RFC 6895 sets aside a range of type codes for private use. This range |
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is 65,280 - 65,534 (0xFF00 - 0xFFFE). When experimenting with new Resource Records these |
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can be used, before requesting an official type code from IANA. |
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see http://miek.nl/posts/2014/Sep/21/Private%20RRs%20and%20IDN%20in%20Go%20DNS/ for more |
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information. |
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EDNS0 |
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EDNS0 is an extension mechanism for the DNS defined in RFC 2671 and updated |
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by RFC 6891. It defines an new RR type, the OPT RR, which is then completely |
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abused. |
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Basic use pattern for creating an (empty) OPT RR: |
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o := new(dns.OPT) |
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o.Hdr.Name = "." // MUST be the root zone, per definition. |
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o.Hdr.Rrtype = dns.TypeOPT |
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The rdata of an OPT RR consists out of a slice of EDNS0 (RFC 6891) |
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interfaces. Currently only a few have been standardized: EDNS0_NSID |
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(RFC 5001) and EDNS0_SUBNET (draft-vandergaast-edns-client-subnet-02). Note |
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that these options may be combined in an OPT RR. |
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Basic use pattern for a server to check if (and which) options are set: |
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// o is a dns.OPT |
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for _, s := range o.Option { |
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switch e := s.(type) { |
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case *dns.EDNS0_NSID: |
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// do stuff with e.Nsid |
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case *dns.EDNS0_SUBNET: |
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// access e.Family, e.Address, etc. |
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} |
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} |
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SIG(0) |
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From RFC 2931: |
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SIG(0) provides protection for DNS transactions and requests .... |
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... protection for glue records, DNS requests, protection for message headers |
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on requests and responses, and protection of the overall integrity of a response. |
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It works like TSIG, except that SIG(0) uses public key cryptography, instead of the shared |
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secret approach in TSIG. |
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Supported algorithms: DSA, ECDSAP256SHA256, ECDSAP384SHA384, RSASHA1, RSASHA256 and |
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RSASHA512. |
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Signing subsequent messages in multi-message sessions is not implemented. |
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*/ |
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package dns
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