Networking from Scratch

Build a fully functional HTTP/1.1 server from raw TCP up. You'll parse requests, build responses, route paths, serve static files, handle keep-alive connections, manage worker concurrency, and produce real Common Log Format access logs. By the end you'll deeply understand what nginx, Apache, Caddy, and net/http actually do.

Build a WebSocket server from scratch: HTTP upgrade handshake, frame format with masking, fragmented messages, ping/pong keep-alive, close handshake, and scaling considerations. The protocol behind Slack, Discord, Figma, and every real-time web app.

Build a DNS resolver and authoritative server from scratch. You'll parse the DNS wire format, encode names with compression, handle all major record types, do CNAME chasing, perform iterative resolution from the root, and implement TTL-based caching. By the end you'll deeply understand how BIND, Unbound, dnsmasq, and AWS Route 53 actually work.

Build an FTP server: control + data connection model, USER/PASS/CWD/RETR/STOR commands, active vs passive modes (PORT/PASV/EPSV), ASCII vs binary transfers, MLSD listings, FTPS (TLS) and how it compares to modern alternatives like SFTP and HTTPS upload.

Build an IRC server like ircd/Ergo: client registration, channels with modes, PRIVMSG routing, PING/PONG keepalive, server-to-server federation, IRCv3 capability negotiation + message tags + SASL, and the chat protocol that ran open-source communities for 35+ years.

Build a complete BitTorrent client from scratch. You'll implement bencoding for the wire format, parse .torrent metainfo and magnet URIs, talk to HTTP and UDP trackers, perform the peer handshake, frame typed peer messages, verify pieces with SHA-1, and implement rarest-first piece selection. By the end you'll deeply understand what qBittorrent, Transmission, and libtorrent actually do.

Build an email server like Postfix/Exim: SMTP conversation, EHLO/MAIL/RCPT/DATA, MIME multipart parsing, STARTTLS, SASL authentication, mail queue with retries, SPF/DKIM/DMARC anti-spam authentication, and the protocol behind every email message ever sent.

Build a userspace TCP/IP stack on top of a TUN device: parse IP and UDP headers, implement TCP's three-way handshake, sliding window, retransmission, congestion control, and connection state machine. Same architecture as gVisor, lwIP, slirp4netns.