A few things I've learned about computer networking

Somebody asked a few months ago “hey, what’s the best way to understand computer networking?”. I don’t really know how to answer this question – I’ve learned a lot of the things I know at work, and I think picking up new things when I need them has been fine.

But I thought it could maybe be useful to list a bunch of concrete skills and concepts I’ve learned along the way. Like anything else, “computer networking” involves a large number of different concepts and skills and tools and I’ve learned them all one at a time. I picked most of these things up over the last 4 years.

• How to set up an Apache web server by copying and pasting things from the internet. (pre-2010)
• What a http request looks like (GET, POST, etc). How to use curl to send GET and POST requests. (2010?)
• How to send a http request by hand with netcat (2013)
• how to do ARP spoofing (and what ARP is)
• What a MAC address is and how packets are addressed to a MAC address on a local network
• How traceroute works (which involves learning the basics of how the the IP protocol works and what a TTL is)
• What a network packet is, how to look at a networking packet with Wireshark
• The basics of how TCP works (for example by looking at an http request with wireshark, and by building a tcp stack in Python). Key things: what’s a SYN packet?
• how DNS works (like, what’s an A record, what’s a CNAME record, what does a DNS query look like – wireshark is good here too).
• More HTTP (like cache headers and how they interact with CDNs). More about what CDNs are for
• MTU exists and can cause networking issues
• Having badly tuned TCP connection settings (like TCP_NODELAY) can cause noticeable networking performanace issues (why you should understand (a little) about TCP) (2015)
• HTTP security headers like CORS
• What “SNI” means
• how to use tcpdump to debug firewall issues (2016)
• how to capture packets with tcpdump in somewhat weird ways (for instance “only this very specific kind of DNS response”)
• “can reliably use tcpdump without reading the man page”
• SSL/TLS: what’s a SSL cert? how do I get one issued? how is a SSL cert put together? (tools: openssl x509). here’s a blog post about TLS
• more advanced HTTP+SSL stuff, like the Strict-Transport-Security header
• very basic understanding of what BGP is and how packets get routed on the internet
• slightly more advanced DNS (what’s an authoritative dns server, what’s a recursive dns server)
• a vague understanding of how the linux networking stack handles packets, like – do packets get sent to tcpdump before or after routing? (after!)
• how to slow down my internet on purpose with tc
• how to set up NAT rules with iptables
• how to inspect a route table with iproute2
• container/docker networking (network namespaces, route tables) (2017)

tools I’ve found useful

Per this tweet:

• ping (are these computer connected??)
• whois (is this domain registered)
• ssh
• curl (for making HTTP requests)
• tcpdump (record packets! check for traffic on a port!)
• dig/nslookup (debugging DNS issues)
• netstat/ss (is that port being used?)
• ifconfig (what’s my IP address?)
• iproute2 (that is, the ip command. replacement for ifconfig. very useful.)
• wireshark (look at packets with a GUI)
• ngrep (grep for your network)
• iptables
• socat (connect a unix domain socket to a tcp socket)
• nsenter for debugging container networking problems

learning takes a lot of time

I spend a fair amount of time trying to learn new computer things. I’ve found it really useful to take it one step at a time – my learning process with a lot of this stuff is basically

• identify something small I don’t know (how to, from the command line, check the expiration date on https://google.com's TLS certificate)
• figure it out (sometimes with help from my great coworkers)
• repeat

That’s all! It’s really fun to see how learning a bunch of tiny things adds up over time. Like today I feel like I can handle most things about computer networking that I run into in my job, and I don’t feel like there are that many Big New Ideas about networking I don’t know about. (though, well, wifi is still a mystery to me :) )

Read the full article here by Julia Evans