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Web Servers

Lecture 6

lemurseven & storce

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What happens when we visit a website?

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Overview

Domain Name System
Web servers
Load balancing

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Domain Name System

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What is DNS?

169.229.226.23

1.2.3.4

DNS

9.9.9.9

Gimme the A record of www.ocf.berkeley.edu

169.229.226.23

TTL = 300s

GET / HTTP/1.1 Host: www.ocf.berkeley.edu

HTTP/1.1 200 OK

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What is actually happening?

169.229.226.23

TTL = 300s

1.2.3.4

DNS

9.9.9.9

Gimme the A record of www.ocf.berkeley.edu

Recursive resolver

root

a.root-servers.net

NS for edu domains

a.edu-servers.net

Gob ears

adns1.berkeley.edu

A, www.o.b.e

edu

a.edu-servers.net

adns1.berkeley.edu

NS for berkeley.edu

ns.ocf.berkeley.edu

OCF

169.229.226.23, TTL = 300s

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Types of DNS records

A: returns an IPv4 address (e.g. 74.125.142.147)
AAAA: returns an IPv6 address (e.g. 2607:f140:0:32::70)
CNAME: returns the canonical domain name (e.g. uptime.ocf.io points to stats.uptimerobot.com)
MX: redirects email to a mail server (e.g. MX ocf.b.e points to aspmx.l.google.com etc.)
NS: stores the authoritative name server for a domain (e.g. ocf.io’s NS record points to ns1.o.b.e)

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More types of DNS records

TXT: contains information about the domain (e.g. site verification, etc.)
SRV: specifies a host and port for specific services
SOA: stores administrative information about a domain (such as the email address of the admin, when the domain was last updated, and how long the server should wait between refreshes)

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The DNS records of OCF

https://github.com/ocf/dns

death IN A 169.229.226.23

death IN AAAA 2607:f140:8801::1:23

www IN CNAME death

fallingrocks IN A 169.229.200.70

fallingrocks IN AAAA 2607:f140:0:32::70

apt IN CNAME fallingrocks

mirrors IN CNAME fallingrocks

lb IN A 169.229.226.79

lb IN AAAA 2607:f140:8801::1:79

auth IN CNAME lb

...

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TTL record

Time to live
Tells a DNS server or the local resolver how long it should keep the record in its cache
Longer TTLs can speed up DNS resolution but causes updates to the zone to take longer to propagate to users

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DNS poisoning

169.229.226.23

1.2.3.4

9.9.9.9

Gimme the A record of www.ocf.berkeley.edu

169.229.226.23

TTL = 300s

GET / HTTP/1.1 Host: www.ocf.berkeley.edu

HTTP/1.1 200 OK

171.67.215.200

TTL = 300s

ZZZ…

DNS

furd

171.67.215.200

Mallory

OCF

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More DNS poisoning

2.4.6.8

DNS

69.69.69.69

Gimme the A record of www.ocf.berkeley.edu

171.67.215.200

TTL = 300s

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More DNS poisoning

1.2.3.4

DNS

9.9.9.9

Gimme the A record of www.ocf.berkeley.edu

169.229.226.23

TTL = 300s

root

a.root-servers.net

NS for edu domains

Recursive resolver

a.edu-servers.net

edu

a.edu-servers.net

adns1.berkeley.edu

Gob ears

adns1.berkeley.edu

A, www.o.b.e

NS for berkeley.edu

169.229.226.23, TTL = 300s

ns.ocf.berkeley.edu

OCF

It can happen at any of these levels!

Example: Great Firewall of China

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Web Servers

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What do web servers do?

https://youtu.be/dQw4w9WgXcQ

5.0.0.1

1.2.3.4

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What do web servers do?

https://…

page assets

video metadata

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How do web servers scale?

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How do web servers scale?

https://youtu.be/dQw4w9WgXcQ

5.0.0.1

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How do web servers scale?

https://youtu.be/dQw4w9WgXcQ

5.0.0.1

vertical scaling

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How do web servers scale?

5.0.0.1

5.0.0.2

https://…

horizontal scaling

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What do load balancers do?

traffic balancing

5.0.0.1

5.0.0.2

5.0.0.3

5.0.1.1

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What do load balancers do?

5.0.1.1

5.0.0.1

5.0.0.2

5.0.0.3

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What do load balancers do?

Static load balancing algorithms (round robin, IP Hash)

1

2

3

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What do load balancers do?

load balancing algorithms (response time, connections, dynamic)

14ms

10ms

21ms

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What do load balancers do?

healthchecks

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Won’t Load Balancers be overwhelmed?

5.0.1.1

5.0.0.1

5.0.0.2

5.0.0.3