scratch/dns-server/lookup.go (view raw)
| 1 | package main |
| 2 | |
| 3 | import ( |
| 4 | "bytes" |
| 5 | "fmt" |
| 6 | "net" |
| 7 | ) |
| 8 | |
| 9 | func Lookup(qname string, qtype QueryType, server string) (Packet, error) { |
| 10 | conn, _ := net.Dial("udp", server) |
| 11 | defer conn.Close() |
| 12 | |
| 13 | p := Packet{} |
| 14 | p.Header.ID = 6666 |
| 15 | p.Header.RecursionDesired = true |
| 16 | p.Questions = append(p.Questions, Question{ |
| 17 | Name: qname, |
| 18 | Type: qtype, |
| 19 | Class: 1, // IN |
| 20 | }) |
| 21 | |
| 22 | buf := &bytes.Buffer{} |
| 23 | if err := p.Write(buf); err != nil { |
| 24 | return Packet{}, fmt.Errorf("failed to write packet: %v", err) |
| 25 | } |
| 26 | |
| 27 | if _, err := conn.Write(buf.Bytes()); err != nil { |
| 28 | fmt.Printf("failed to write to connection: %v", err) |
| 29 | } |
| 30 | |
| 31 | res := make([]byte, 512) |
| 32 | n, err := conn.Read(res) |
| 33 | if err != nil { |
| 34 | fmt.Printf("failed to read from connection: %v", err) |
| 35 | } |
| 36 | |
| 37 | return ParsePacket(res[:n]) |
| 38 | } |
| 39 | |
| 40 | func RecursiveLookup(qname string, qtype QueryType) (Packet, error) { |
| 41 | ns := "198.41.0.4" // For now we're always starting with *a.root-servers.net*. |
| 42 | for { |
| 43 | fmt.Printf("attempting lookup of %v %s with ns %s\n", qtype, qname, ns) |
| 44 | |
| 45 | response, err := Lookup(qname, qtype, ns+":53") |
| 46 | if err != nil { |
| 47 | return Packet{}, err |
| 48 | } |
| 49 | |
| 50 | if len(response.Answers) > 0 && response.Header.Rescode == NOERROR { |
| 51 | return response, nil |
| 52 | } |
| 53 | |
| 54 | if response.Header.Rescode == NXDOMAIN { |
| 55 | return response, nil |
| 56 | } |
| 57 | |
| 58 | if newNS, ok := response.GetResolvedNS(qname); ok { |
| 59 | ns = newNS.String() |
| 60 | continue |
| 61 | } |
| 62 | |
| 63 | newNSName, ok := response.GetUnresolvedNS(qname) |
| 64 | if !ok { |
| 65 | return response, nil |
| 66 | } |
| 67 | |
| 68 | recursive, err := RecursiveLookup(newNSName, AType) |
| 69 | if err != nil { |
| 70 | return response, nil |
| 71 | } |
| 72 | |
| 73 | if ip, ok := recursive.GetRandomA(); ok { |
| 74 | ns = ip.String() |
| 75 | } else { |
| 76 | return response, nil |
| 77 | } |
| 78 | } |
| 79 | } |