scratch/dns-server/record.go (view raw)
| 1 | package main |
| 2 | |
| 3 | import ( |
| 4 | "bytes" |
| 5 | "encoding/binary" |
| 6 | "fmt" |
| 7 | "strings" |
| 8 | ) |
| 9 | |
| 10 | type Record struct { |
| 11 | Name string |
| 12 | Type uint16 |
| 13 | Class uint16 |
| 14 | TTL uint32 |
| 15 | Data string |
| 16 | } |
| 17 | |
| 18 | func ReadRecord(r *bytes.Reader, packet []byte) (Record, error) { |
| 19 | name, err := readName(r, packet) |
| 20 | if err != nil { |
| 21 | return Record{}, err |
| 22 | } |
| 23 | |
| 24 | var rtype, class, rdlen uint16 |
| 25 | var ttl uint32 |
| 26 | _ = binary.Read(r, binary.BigEndian, &rtype) |
| 27 | _ = binary.Read(r, binary.BigEndian, &class) |
| 28 | _ = binary.Read(r, binary.BigEndian, &ttl) |
| 29 | _ = binary.Read(r, binary.BigEndian, &rdlen) |
| 30 | |
| 31 | var data string |
| 32 | switch rtype { |
| 33 | case 1: // A |
| 34 | var ip [4]byte |
| 35 | _, _ = r.Read(ip[:]) |
| 36 | data = fmt.Sprintf("%d.%d.%d.%d", |
| 37 | ip[0], ip[1], ip[2], ip[3]) |
| 38 | |
| 39 | default: |
| 40 | buf := make([]byte, rdlen) |
| 41 | _, _ = r.Read(buf) |
| 42 | data = fmt.Sprintf("%x", buf) |
| 43 | } |
| 44 | |
| 45 | return Record{ |
| 46 | Name: name, |
| 47 | Type: rtype, |
| 48 | Class: class, |
| 49 | TTL: ttl, |
| 50 | Data: data, |
| 51 | }, nil |
| 52 | } |
| 53 | |
| 54 | func readName(r *bytes.Reader, packet []byte) (string, error) { |
| 55 | var labels []string |
| 56 | for { |
| 57 | length, err := r.ReadByte() |
| 58 | if err != nil { |
| 59 | return "", err |
| 60 | } |
| 61 | if length == 0 { |
| 62 | break |
| 63 | } |
| 64 | // pointer: top two bits set (0xC0) |
| 65 | if length&0xC0 == 0xC0 { |
| 66 | low, err := r.ReadByte() |
| 67 | if err != nil { |
| 68 | return "", err |
| 69 | } |
| 70 | offset := int(uint16(length&0x3F)<<8 | uint16(low)) |
| 71 | sub := bytes.NewReader(packet[offset:]) |
| 72 | name, err := readName(sub, packet) |
| 73 | if err != nil { |
| 74 | return "", err |
| 75 | } |
| 76 | labels = append(labels, name) |
| 77 | break // pointer always ends the name |
| 78 | } |
| 79 | buf := make([]byte, length) |
| 80 | if _, err := r.Read(buf); err != nil { |
| 81 | return "", err |
| 82 | } |
| 83 | labels = append(labels, string(buf)) |
| 84 | } |
| 85 | return strings.Join(labels, "."), nil |
| 86 | } |