How HTTP Requests Work
Published on 05/19/2020, 1541 words, 6 minutes to read
Reading this webpage is possible because of millions of hours of effort with tens of thousands of actors across thousands of companies. At some level it's a minor miracle that this all works at all. Here's a preview into the madness that goes into hitting enter on xeiaso.net and this website being loaded.
The user types in
https://xeiaso.net into the address bar and hits
enter on the keyboard. This sends a signal over USB to the computer and the
kernel polls the USB controller for a new message. It's recognized as from the
keyboard. The input is then sent to the browser through an input driver talking
to a windowing server talking to the browser program.
The browser selects the memory region normally reserved for the address bar. The browser then parses this string as an RFC 3986 URI and scrapes out the protocol (https), hostname (xeiaso.net) and path (/). The browser then uses this information to create an abstract HTTP request object with the Host header set to xeiaso.net, HTTP method (GET), and path set to the path. This request object then passes through various layers of credential storage and middleware to add the appropriate cookies and other headers in order to tell my website what language it should localize the response to, what compression methods the browser understands, and what browser is being used to make the request.
The browser then checks if it has a connection to xeiaso.net open already. If it does not, then it creates a new one. It creates a new connection by figuring out what the IP address of xeiaso.net is using DNS. A DNS request is made over UDP on port 53 to the DNS server configured in the operating system (such as 184.108.40.206, 220.127.116.11 or 18.104.22.168). The UDP connection is created using operating system-dependent system calls and a DNS request is sent.
The packet that was created then is destined for the DNS server and added to the operating system's output queue. The operating system then looks in its routing table to see where the packet should go. If the packet matches a route, it is queued for output to the relevant network card. The network card layer then checks the ARP table to see what mac address the ethernet frame should be sent to. If the ARP table doesn't have a match, then an arp probe is broadcasted to every node on the local network. Then the driver waits for an arp response to be sent to it with the correct IP -> MAC address mapping. The driver then uses this information to send out the ethernet frame to the node that matches the IP address in the routing table. From there the packet is validated on the router it was sent to. It then unwraps the packet to the IP layer to figure out the destination network interface to use. If this router also does NAT termination, it creates an entry in the NAT table for future use for a site-configured amount of time (for UDP at least). It then passes the packet on to the correct node and this process is repeated until it gets to the remote DNS server.
The DNS server then unwraps the ethernet frame into an IP packet and then as a UDP packet and a DNS request. It checks its database for a match and if one is not found, it attempts to discover the correct name server to contact by using a NS record query to its upstreams or the authoritative name server for the WEBSITE namespace. This then creates another process of ethernet frames and UDP packets until it reaches the upstream DNS server which hopefully should reply with the correct address. Once the DNS server gets the information that is needed, it sends this back the results to the client as a wire-format DNS response.
UDP is unreliable by design, so this packet may or may not survive the entire round trip. It may take one or more retries for the DNS information to get to the remote server and back, but it usually works the first time. The response to this request is cached based on the time-to-live specified in the DNS response. The response also contains the IP address of xeiaso.net.
The protocol used in the URL determines which TCP port the browser connects to. If it is http, it uses port 80. If it is https, it uses port 443. The user specified HTTPS, so port 443 on whatever IP address DNS returned is dialed using the operating system's network stack system calls. The TCP three-way handshake is started with that target IP address and port. The client sends a SYN packet, the server replies with a SYN ACK packet and the client replies with an ACK packet. This indicates that the entire TCP session is active and data can be transferred and read through it.
However, this data is UNENCRYPTED by default. Transport Layer Security is used to encrypt this data so prying eyes can't look into it. TLS has its own handshake too. The session is established by sending a TLS ClientHello packet with the domain name (xeiaso.net), the list of ciphers the client supports, any application layer protocols the client supports (like HTTP/2) and the list of TLS versions that the client supports. This information is sent over the wire to the remote server using that entire long and complicated process that I spelled out for how DNS works, except a TCP session requires the other side to acknowledge when data is successfully received. The server on the other end replies with a ClientHelloResponse that contains a HTTPS certificate and the list of protocols and ciphers the server supports. Then they do an encryption session setup rain dance that I don't completely understand and the resulting channel is encrypted with cipher (or encrypted) text written and read from the wire and a session layer translates that cipher text to clear text for the other parts of the browser stack.
The browser then uses the information in the ClientHelloResponse to decide how to proceed from here.
If the browser notices the server supports HTTP/2 it sets up a HTTP/2 session (with a handshake that involves a few roundtrips like what I described for DNS) and creates a new stream for this request. The browser then formats the request as HTTP/2 wire format bytes (binary format) and writes it to the HTTP/2 stream, which writes it to the HTTP/2 framing layer, which writes it to the encryption layer, which writes it to the network socket and sends it over the internet.
If the browser notices the server DOES NOT support HTTP/2, it formats the request as HTTP/1.1 wire formatted bytes and writes it to the encryption layer, which writes it to the network socket and sends it over the internet using that complicated process I spelled out for DNS.
This then hits the remote load balancer which parses the client HTTP request and uses site-local configuration to select the best application server to handle the response. It then forwards the client's HTTP request to the correct server by creating a TCP session to that backend, writing the HTTP request and waiting for a response over that TCP session. Depending on site-local configuration there may be layers of encryption involved.
Now, the request finally gets to the application server. This TCP session is accepted by the application server and the headers are read into memory. The path is read by the application server and the correct handler is chosen. The HTML for the front page of xeiaso.net is rendered and written to the TCP session and travels to the load balancer, gets encrypted with TLS, the encrypted HTML gets sent back over the internet to your browser and then your browser decrypts it and starts to parse and display the website. The browser will run into places where it needs more resources (such as stylesheets or images), so it will make additional HTTP requests to the load balancer to grab those too.
The end result is that the user sees the website in all its glory. Given all these moving parts it's astounding that this works as reliably as it does. Each of the TCP, ARP and DNS requests also happen at each level of the stack. There are layers upon layers upon layers of interacting protocols and implementations.
This is why it is hard to reliably put a website on the internet. If there is a god, they are surely the one holding all these potentially unreliable systems together to make everything appear like it is working.
Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.
Tags: http, ohgod, philosophy