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

SITCON 2017

yenWu(吳彥寬)

● NCKU CSIE (Master, Now)● NCKU Mathematics(Bachelor)● C● Robotic● RTOS● Microkernel

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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.

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Outline

1. Story Time2. Introduction3. Prerequisites4. Aims5. Cautions

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Story Time

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Story Time

Project “Build your own DNS Server in FreeBSD”

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Story Time

Can I build one from scratch to be my final project?

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Story Time

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

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Story Time

OK. That’s better than building one.

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Story Time

OK. That’s better than building one.

I took three days long to fix the lowercase bug when building AMP.

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

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An Episode!

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

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Build TCP/IP Stack from Scratch in C (Part 1)

SITCON 2017

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?

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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?

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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.

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Introduction

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Great VideoDNS Explained

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Internet

● The Internet maintains two principal namespaces

○ Domain name hierarchy

○ Internet Protocol (IP) address spaces

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What is DNS server?

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What is DNS server?

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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

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Hierarchy of DNS servers

“www.yahoo.com”

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Hierarchy of DNS servers

“www.yahoo.com.”

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Hierarchy of DNS servers

“www.yahoo.com.”

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Hierarchy of DNS servers

“www.yahoo.com.”

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Let’s take it deeplyThat’s all I knew before I started this.

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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

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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

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Glance

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Resolver

A Resolver maps a name to an address and vice versa.

Query

Response

Resolver Name Server

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Iterative Resolution

client

edu

2

root

3

com

4

NS: google.com

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iterative request“What is the IP address of www.google.com?”

ncku

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iterative response“The IP address of www.google.com is 216.239.37.99.”

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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.”

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Iterative Query

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Recursive Resolution

client

edu

2

root

3

com

4

google

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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

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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.”

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Recursive Query

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● Why hierarchy?

Questions

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

Questions

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

● Does it always waste a lot of time on process of lookup?

Questions

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● 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

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● 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

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● 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

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● 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

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● 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

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● 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

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● 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

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Process flow

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Client

DHCP Server

Primary DNS ServerZone File

IP Address?

IP Address Update

Dynamic DNS

Fine ! We know the concept of DNS.

Can we start it ?

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Prerequisites

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IPheader

UDPheader DNS message

IPheader

TCPheader DNS message

2-byteDNS 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)

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Transport

DNS Protocol

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Message Format● Header Section● Question Section● RR Section

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Header Section

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

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Flag● QR● Opcode● RCODE

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Flag

Query 0

Respond 1

Query 0000

OpCode Retired

0001

Status 0010

reserved 0011

Notify 0100

Update 0101

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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 101057

Message Formate● Header Section● Question Section● RR Section

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Message Formate● Header Section● Question Section● RR Section

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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)

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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

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RR Section

IN Internet 0x0001

CH Chaos 0x0003

HS Hesiod 0x0004

QCLASS Only

Any 0x00FF

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Aims of this Project

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Aims1. Efficiency2. Uniform Protocol Access Interface3. Consistency with RFC standard4. Pluggable Database and DNS Complement5. Benchmark6. Education

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Efficiency

Is it good enough ?

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EfficiencyIn order to send message rapidly, we should maintain continous memory buffer!

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Uniform

Protocol

Access

Interface

● MACRO● Hard to debug

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Uniform Protocol Access Interface

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Consistency with RFC standard

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Pluggable Database and DNS Complement

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Pluggable Database and DNS Complement

Object-Orien Programming in C !?

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Education

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

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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

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Phonebook

● clear● gnuplot

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Great ! It time to work!

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Cautions

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Pitfalls!!!

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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

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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

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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

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

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Que

stio

nSe

ctio

nA

nsw

erSe

ctio

n

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

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Que

stio

nSe

ctio

nA

nsw

erSe

ctio

n

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

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Que

stio

nSe

ctio

nA

nsw

erSe

ctio

n

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

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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

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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

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struct in C

● make sure continuous memory space

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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

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Demo - Resolver

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

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

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Q & A

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