This is my final project. The purpose is that I don't get any course about Network, so I try to implement DNS server and learn from practicing. In this slide, it would cover brief introduction of ddns server, prerequisite for building protocol, aims of my project and all of pitfalls I have met. Demo resolver. I would finish this project and provide the part2 of slide.

1. Build Dynamic DNS Server
from Scratch in C (Part 1)
SITCON 2017

2. yenWu(吳彥寬)
● NCKU CSIE (Master, Now)
● NCKU Mathematics(Bachelor)
● C
● Robotic
● RTOS
● Microkernel
2

3. DDNS
● Project Link: https://github.com/yenWu/DDNS
● DDNS = Dynamic DNS
● In order to figure out what's DNS and how it work, I would Implement
Dynamic DNS server in User-level.
3

4. Outline
1. Story Time
2. Introduction
3. Prerequisites
4. Aims
5. Cautions
4

5. Story Time
5

6. Story Time
Project “Build your own DNS Server in FreeBSD”
6

7. Story Time
Can I build one from scratch to be my final project?
7

8. Story Time
Sure. But you should try more difficult one,
“Dynamic DNS Server”.
8

9. Story Time
OK. That’s better than building one.
9

10. Story Time
OK. That’s better than building one.
I took three days long to fix
the lowercase bug when
building AMP.
10

11. Lots of documents ...
● rfc1033
● rfc1034
● rfc1035
● rfc2136
11

12. An Episode!
12

13. An Episode!
I said that I want to build a TCP/IP Stack at first time ...
13

14. Build TCP/IP Stack from
Scratch in C (Part 1)
SITCON 2017

15. Most frequently asked questions and comments
● Why do you try to build from scratch? I already have BIND(FreeBSD)!
● Your project doesn’t more better than BIND. Why do you still do that?
15

16. Most frequently asked questions and comments
● Why do you try to build from scratch? I already have BIND(FreeBSD)!
○ Writing one is the best pratice for me to understand the concept of DNS.
● Your project doesn’t more better than BIND. Why do you still do that?
16

17. Most frequently asked questions and comments
● Why do you try to build from scratch? I already have BIND(FreeBSD)!
○ Writing one is the best pratice for me to understand the concept of DNS.
● Your project doesn’t more better than BIND. Why do you still do that?
○ If you have wrote one, you can have more idea to improve it.
17

18. Introduction
18

19. Great Video
DNS Explained
19

20. Internet
● The Internet maintains two principal namespaces
○ Domain name hierarchy
○ Internet Protocol (IP) address spaces
20

21. What is DNS server?
21

22. What is DNS server?
22

23. What is DNS server?
● Hierarchical decentralized naming system
● Distributed database
● A little similar to Phonebook
● Translate readily memorized domain name to IP address
● DNS client v.s. DNS server
● Two category of the DNS server
○ Recursive DNS server
○ Iterative DNS server
23

24. Hierarchy of DNS servers
“www.yahoo.com”
24

25. Hierarchy of DNS servers
“www.yahoo.com.”
25

26. Hierarchy of DNS servers
“www.yahoo.com.”
26

27. Hierarchy of DNS servers
“www.yahoo.com.”
27

28. Let’s take it deeply
That’s all I knew before I started this.
28

29. What is DNS server?
● DNS client v.s. DNS server
○ DNS Server ⊇ { Resolver, Database, Cache}
○ DNS Client ⊇ { Resolver}
● Unlike a phonebook, DNS can be quickly updated, allowing a service's
location on the network to change without affecting the end users, who
continue to use the same host name.
● Responds with answers to queries against its database.
● Port 53
29

30. What is DNS server?
● The most common types of records
○ Start of Authority (SOA)
○ IP addresses (A and AAAA)
○ SMTP mail exchangers (MX)
○ Name servers (NS)
○ Pointers for reverse DNS lookups (PTR)
○ Domain name aliases (CNAME)
● Different reaction depend on the type of record
30

31. Glance
31

32. Resolver
A Resolver maps a name to an address and vice versa.
Query
Response
Resolver Name Server
32

33. Iterative Resolution
client
edu
2
root
3
com
4
NS: google.com
5
iterative request
“What is the IP address of
www.google.com?”
ncku
1
iterative response
“The IP address of www.google.com is
216.239.37.99.”
6
33

34. Iterative Resolution
client
edu
2
root
3
com
4
NS: google.com
5
iterative request
“What is the IP address of
www.google.com?”
ncku
1
iterative response
“The IP address of www.google.com is
216.239.37.99.”
6
Iterative Query
34

35. Recursive Resolution
client
edu
2
root
3
com
4
google
5
recursive request
“What is the IP address of
www.google.com?”
ncku
1
recursive response
“The IP address of www.google.com is
216.239.37.99.”
6
35

36. Recursive Resolution
client
edu
2
root
3
com
4
google
5
recursive request
“What is the IP address of
www.google.com?”
ncku
1
recursive response
“The IP address of www.google.com is
216.239.37.99.”
6
Recursive Query
36

37. ● Why hierarchy?
Questions
37

38. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
Questions
38

39. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
Questions
39

40. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
Questions
40

41. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
Questions
41

42. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
○ Probably not … because of the cache.
Questions
42

43. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
○ Probably not … because of the cache.
● Is it easy to update the information(RR)?
Questions
43

44. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
○ Probably not … because of the cache.
● Is it easy to update the information(RR)?
○ Maybe not. In RFC 1035, we would load zone and startup file into database at local.
Questions
44

45. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
○ Probably not … because of the cache.
● Is it easy to update the information(RR)?
○ Maybe not. In RFC 1035, we would load zone and startup file into database at local.
● Can we update the information on remote DNS Server?
Questions
45

46. ● Why hierarchy?
○ More efficient than heterarchy.
○ Update could be immediately showed.
● Does it always waste a lot of time on process of lookup?
○ Right! So caching!
● Is it true that react immediately when we update the informaition(RR)?
○ Probably not … because of the cache.
● Is it easy to update the information(RR)?
○ Maybe not. In RFC 1035, we would load zone and startup file into database at local.
● Can we update the information on remote DNS Server?
○ No. We need another mechanism to do that. In RFC 2136, we could name DNS to DDNS with
remote updating mechanism.
● That’s why we need a Dynamic DNS Server.
Questions
46

47. Process flow
47

48. 48
Client
DHCP
Server
Primary DNS Server
Zone File
IP Address?
IP Address Update
Dynamic DNS

49. Fine ! We know the concept of DNS.
Can we start it ?
49

50. Prerequisites
50

51. IP
header
UDP
header
DNS message
IP
header
TCP
header
DNS message
2-byte
DNS msg.
length
● DNS messages are encapsulated in UDP by default.
● If the resolver expects the response to exceed 512 bytes, the
resolver encapsulates the query in TCP instead.
● If a request is sent over UDP and the response is longer than 512
bytes, the server sends the first 512 bytes of the response using
UDP and sets the TC (truncated) flag. The resolver then re-sends
the query using TCP.
max. 512 bytes
no limit (up to max. TCP payload size)
51
Transport

52. DNS Protocol
52

53. Message Format
● Header Section
● Question Section
● RR Section
53

54. Header Section
● ID := identification of DNS transaction
● FLAG
● QDCOUNT := questions record count
● ANCOUNT := answer record count
● NSCOUNT := authority record count
● ARCOUNT := additional record count
54

55. Flag
● QR
● Opcode
● RCODE
55

56. Flag
Query 0
Respond 1
Query 0000
OpCode
Retired
0001
Status 0010
reserved 0011
Notify 0100
Update 0101
56

57. Flag NoError No Error 0000
FormErr Format Error 0001
ServFail Server Failure 0010
NXDomain Non-Existent Domain 0011
NotImp Not Implemented 0100
Refused Query Refused 0101
YXDomain Name Exists when it should not 0110
YXRRSet RR Set Exists when it should not 0111
NXRRSet RR Set that should exist does not 1000
NotAuth Server Not Authoritative for zone 1001
NotZone Name not contained in zone 1010
57

58. Message Formate
● Header Section
● Question Section
● RR Section
58

59. Message Formate
● Header Section
● Question Section
● RR Section
59

60. RR Section
● The most common Type
○ Start of Authority (SOA)
○ IP addresses (A and AAAA)
○ SMTP mail exchangers (MX)
○ Name servers (NS)
○ Pointers for reverse DNS lookups (PTR)
○ Domain name aliases (CNAME)
60

61. RR Section
● The most common TYPE
○ Start of Authority (SOA)
○ IP addresses (A and AAAA)
○ SMTP mail exchangers (MX)
○ Name servers (NS)
○ Pointers for reverse DNS lookups (PTR)
○ Domain name aliases (CNAME)
● RDATA depends on TYPE
61

62. RR Section
IN Internet 0x0001
CH Chaos 0x0003
HS Hesiod 0x0004
QCLASS
Only
Any 0x00FF
62

63. Aims of this Project
63

64. Aims
1. Efficiency
2. Uniform Protocol Access Interface
3. Consistency with RFC standard
4. Pluggable Database and DNS Complement
5. Benchmark
6. Education
64

65. Efficiency
Is it good enough ?
65

66. Efficiency
In order to send message
rapidly, we should maintain
continous memory buffer!
66

67. Uniform
Protocol
Access
Interface
● MACRO
● Hard to debug
67

68. Uniform Protocol Access Interface
68

69. Consistency with RFC standard
69

70. Pluggable Database and DNS Complement
70

71. Pluggable Database and DNS Complement
Object-Orien Programming in C
!?
71

72. Education
● A pioneer have shared all of the pitfalls he met at protocol level.
● Provide uniform protocol access interface.
● Easy to change implementation.
72

73. Benchmark
● Follow the assignment(phonebook) of Embedded System Course in
NCKU
● Mission: You need to optimize the building and searching time on
phonebook.
● Assignment Link: https://hackmd.io/s/rJYD4UPKe
73

74. Phonebook
● clear
● gnuplot
74

75. Great ! It time to work!
75

76. Cautions
76

77. Pitfalls!!!
77

78. Notice !!!
● If you start to build it, maybe you would be trapped in this three pitfalls:
○ Domain Name Format in Message Format
○ Message Compression
○ Endian Transfer
78

79. Domain Name in Message Format
class of network (1 = Internet)
12
5
2
1 Address – IPv4
Name Server (authoritative)
Canonical Name (alias)
Pointer – reverse lookup
15 Mail Exchange
28 Address - IPv6
252 Zone Transfer
PTR
CNAME
NS
A
MX
AAAA
AXFR
sent in query;
repeated in response
w w w . g o o g l e . c o m
Query Name
Query Type Query Class
16 bit 16 bit
Variable Length
79

80. Domain Name in Message Format
class of network (1 = Internet)
12
5
2
1 Address – IPv4
Name Server (authoritative)
Canonical Name (alias)
Pointer – reverse lookup
15 Mail Exchange
28 Address - IPv6
252 Zone Transfer
PTR
CNAME
NS
A
MX
AAAA
AXFR
sent in query;
repeated in response
w w w . g o o g l e . c o m .
Query Name
Query Type Query Class
16 bit 16 bit
Variable Length
80

81. Domain Name in Message Format
class of network (1 = Internet)
12
5
2
1 Address – IPv4
Name Server (authoritative)
Canonical Name (alias)
Pointer – reverse lookup
15 Mail Exchange
28 Address - IPv6
252 Zone Transfer
PTR
CNAME
NS
A
MX
AAAA
AXFR
sent in query;
repeated in response
3 w w w 6 g o o g l e 3 c o m 0
count
Query Name
Query Type Query Class
16 bit 16 bit
Variable Length
81

82. Question
Section
Answer
Section
3 w w w 6 g o o g l e 3 c o m 0
1 1 000 ... 1 1 0 0
2 bit 30 bit
16 bit
Query Name
Query Type Query Class
Header
Domain Name
16 bit 16 bit
Variable Length
Variable Length
12 byte
C0 0C
PointerCompression flag
Compression
82

83. Question
Section
Answer
Section
3 w w w 6 g o o g l e 3 c o m 0
1 1 000 ... 1 1 0 0
2 bit 14 bit
16 bit
Query Name
Query Type Query Class
Header
Domain Name
C0 0C
PointerCompression flag
Compression 11
Reserve 10
Reserve 01
Normal 00
Compression
83

84. Question
Section
Answer
Section
3 w w w 6 g o o g l e 3 c o m 0
1 1 000 ... 1 1 0 0
2 bit 30 bit
16 bit
Query Name
Query Type Query Class
Header
Domain Name
C0 0C
PointerCompression flag
Compression 11
Reserve 10
Reserve 01
Normal 00
Compression
84

85. The difficult one for Endian transfer
0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 0
QR Opcode AA TC RD RA Z AD CD RCODE
85

86. As I've said …………………………………..
0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 0
QR Opcode AA TC RD RA Z AD CD RCODE
1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0
86

87. struct in C
● make sure
continuous memory space
87

88. Reference
● Request For Comments(RFC)
○ rfc1033
○ rfc1034
○ rfc1035
○ rfc2136
● INACON's protocol help
○ DNS Protocol
● University of Delaware
○ https://www.eecis.udel.edu/~amer/856/dns.04f.ppt
● University of Nebraska–Lincoln
○ http://cse.unl.edu/~ylu/csce855/notes/DNS.ppt
● Wikipedia DNS
88

89. Demo - Resolver
89

90. Project would continous when I have free
time (... graduated paper)
90

91. Contact me
● Github: yenWu
● LinkedIn: Yen-Kwan Wu
● NCKU CSIE WIKI: yenWu
● E-Mail: c14006078@gmail.com
91

92. Q & A
92