Type "Google.com" into the Browser and Hit Enter: What Happens Next?

Type “google.com” into the browser and hit enter

What happens next?1 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

How the Internet worksGraeme Mathieson

Email me: [email protected] me: @mathie

2 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

google.com ⏎3 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

Is it a URL?· Yep. OK, cool, my work here is done.

· Kinda. Well, let’s turn it into a well formed URL.· Nope. OK, I’m gonna assume you meant to search for something. Let’s turn it into a well

formed URL for a web search.


HTTP Strict Transport Security

Does this site prefer HTTPS?· Strict-Transport-Security header from

a previous request?· In the browser’s list of HSTS preloads?


HTTP Strict Transport Security

Does this site prefer HTTPS?· Yep OK, set the URL scheme to https.

· Nope Fine then. If you don’t care for security…


https://google.com/7 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

Browser cacheIs the URL in the browser cache?

· Yep Let’s check it’s still valid.· Nope Well, we’re going to have to fetch it.


Browser cacheIs the cached content still valid?

ExpiresCache-Control: max-age

· Yep Awesome. We might skip a network request!· Nope OK, let’s check in with the server.


Browser cacheShould the cached content be revalidated?Cache-Control: must-revalidate

· Yep OK, let’s check in with the server.· Nope Awesome. Skip to rendering!


Parse the URL· Scheme: “https”

· Authority: “google.com”· Path: “/“


DNS Lookup: Browser cacheIs the hostname in the browser’s cache?· Yep Awesome, let’s use that IP address.

· Nope OK, we’re going to have to do this the hard way.


DNS Lookup: OS resolver

Is the hostname in the operating system’s cache?· Yep Job done. We’ll use that IP address.

· Nope OK, we’re really going to have to look it up.


Name Service Switch· Check /etc/hosts

· Try multicast DNS· Perform a DNS lookup


DNS LookupGet the IP address of a name server

· From DHCP· Statically configured


DNS Record Types· A and AAAA are address records: mappings from

name to IP address.· PTR is a reverse mapping from IP address to

name· NS is a pointer to a name server.

· Other record types: SOA, CNAME, MX, TXT.


Send the DNS requestNew is Apple iOS 9 & El Capitan

· Send out an AAAA request; and· Send out an A request, in parallel.


Recursive DNS requestIs the record in the name server’s cache?

· Yep Is it still valid? (TTL) If so, return the record. Job done.

· Nope OK, we’ll need to look it up.


Upstream DNS serverIs our local DNS server configured to have one or

more upstream servers?· Yep OK, let’s pass the request off to an upstream

and let it figure out the answer.· Nope Damn. We’re going to have to do the hard

work ourselves.


Root DNS Servers· 13 well-known IP addresses of root servers.

· Really, they’re hundreds of machines distributed globally.

· Authoritative for the root zone.


DNS AuthorityRoot servers are authoritative for the root zone.Know the canonical answer for who serves each

TLD: “.com”, “.net”, “.uk”, etc.


What’s the A record for “google.com”?


Root serversWhat’s the A record for “google.com”?

· No idea, but here’s the list of name servers for “.com”.

· Oh, and have the IP addresses of those name servers, too.


Authoritative servers for “.com”What’s the A record for “google.com”?

· No idea, but here’s a list of name servers for “google.com”.

· Oh, and have the IP addresses of those name servers, too.


Authoritative servers for “google.com”What’s the A record for “google.com”?

· Hey, I know this! Here’s a list of IP addresses!


Figuring out the TCP portWhat TCP port should we connect to?

· Figure out from the URL scheme· Ask the operating system: getservbyname()

· Name Service Switch· grep '^https.*tcp' /etc/services


Making a TCP connectionWe know the IP address and port. Now we can

connect!


TCP: Three way handshakeOpen connection, and agree initial sequence

numbers.· -> SYN

· <- SYN+ACK· -> ACK


Transmission Control Protocol (TCP)· Ordered data transfer· Reliable data transfer

· Flow control· Congestion control


Transport Layer SecurityCreate a secure connection between the client

and server.· Authentication of the server (and, optionally,

the client).· Negotiate a session key.

· Encrypt data between client and server.30 — © 2015 Graeme Mathieson. CC BY-SA 4.0.

HTTP: GET

GET / HTTP/1.1Host: google.com


FINFIN -> ACK -> FIN -> ACK

TCP/IP Illustrated by W. Richard Stevens<https://woss.name/>
