DNS, packets, proxies, and the wires that turn a server into something the internet can reach.
sudo iftop -i eth0 shows a live, per-connection bandwidth table: which host pairs are moving traffic and at what rate. The interface flag people forget, the -n and -P switches that make the output readable, the 2s/10s/40s columns, the filter syntax, and when nload or iftop is the right tool.
The nload command draws a live graph of incoming and outgoing bandwidth per interface, right in the terminal. Install it, run nload, switch interfaces with the arrow keys, and read the In/Out, Avg, and Max numbers. The -m multi-interface view, the -u/-U unit flags, the refresh interval, and where iftop and nmap fit instead.
Practical nmap command examples I reach for: host discovery, the default port scan, service and version detection, OS fingerprinting, scoped scans with timing control, and the scripting engine, with notes on when each one needs root.
Switch to TCP BBR congestion control with two sysctl lines and a reboot-safe config file. The throughput it buys you on lossy long-haul links, the fq qdisc it needs, the kernel version that ships it, and when CUBIC is still the right default.
The full reference for blocking AI crawlers: which bots matter, what each one does, robots.txt and server-level rules, Cloudflare's managed approach, WordPress paths, and the bots that ignore robots.txt entirely.
How to block the Internet Archive's Wayback Machine from future crawls with robots.txt, how to request removal of existing snapshots by emailing info@archive.org, what the 'This URL has been excluded' message means, and the DMCA path for copyrighted material.
I blocked every AI bot I could when they first started slamming my servers. A few months later I unblocked most of them. This is the decision framework I wish I'd had at the start.
A practical DNS health check covers nameservers, A and AAAA records, MX, SPF, DKIM, DMARC, and CAA. Here is the full checklist, what each record actually tells you, and how to verify all of them in one pass.