This repository is an attempt to answer the age old interview question "What happens when you type google.com into your browser and press enter?"
Except instead of the usual story, we're going to try to answer this question in as much detail as possible. No skipping out on anything.
This is a collaborative process, so dig in and try to help out! There's tons of details missing, just waiting for you to add them! So send us a pull request, please!
This is all licensed under the terms of the Creative Commons Zero license.
To pick a zero point, lets choose the enter key on the keyboard hitting the bottom of its range. At this point, an electrical circuit specific to the enter key is closed (either directly or capacitively). This allows a small amount of current to flow into the logic circuitry of the keyboard, which scans the state of each key switch, debounces the electrical noise of the rapid intermittent closure of the switch, and converts it to a keycode integer, in this case 13. The keyboard controller then encodes the keycode for transport to the computer. This is now almost universally over a Universal Serial Bus (USB) connection, but historically has been over PS/2 or ADB connections.
The USB circuitry of the keyboard is powered by the 5V supply provided over pin 1 from the computer's USB host controller. 17.78 mA of this current is returned on either the D+ or D- pin (the middle 2) of the keyboard's USB connector. Which pin carries the current is rapidly toggled between the two creating a high speed bitstream (the rate depending on USB 1, 2, or 3) serially encoding the digital value of the enter key. This serial signal is then decoded at the computer's host USB controller, and interpreted by the computer's Human Interface Device (HID) universal keyboard device driver. The value of the key is then passed into the operating system's hardware abstraction layer.
- Browser checks if the domain is in it's cache.
- If not found, calls
gethostbynamelibrary function (varies by OS) to do the lookup. - If
gethostbynamedoes not have it cached then a request is made to the known DNS server that was given to the network stack. This is typical the local router or the ISP's caching DNS server - If the local/ISP DNS server does not have it, then a recursive search is requested and that flows up the list of DNS servers until the SOA is reached, and if found an answer is returned.
Once the browser receives the IP address of the destination server it takes
that and the given port number from the URL and makes a call to the system
library function names socket and requests a TCP socket stream -
AF_INET and SOCK_STREAM.
This request is passed to the Transport Layer where the extra love that TCP/IP requires for ensuring packet delivery and ordering is added and then a UDP datagram is fashioned. The UDP datagram is handed off to the physical network layer which inspects the target IP address, looks up the subnet in it's route tables and wraps the datagram in an envelope with the proper gateway address as the recipient.
This address lookup and wrapping of datagrams continues until one of two things happen, the time-to-live value for a datagram reaches zero at which point the packet is dropped or it reaches the destination.
This send and receive happens multiple times following the TCP connection flow:
- SYN request is sent to the target
- target receives SYN and if it's in an agreeable mood, replies with SYN/ACK
- source sends ACK to acknowledge socket is established
- data is transferred until FIN/ACK is sent and acknowledged