Networking

DNS issues are deceptively simple. “It’s always DNS” is a meme because it’s true. Here’s how to actually debug it. The Essential Commands dig: Your Primary Tool 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Basic lookup dig example.com # Short answer only dig +short example.com # Specific record type dig example.com MX dig example.com TXT dig example.com CNAME # Query specific nameserver dig @8.8.8.8 example.com # Trace the full resolution path dig +trace example.com Understanding dig Output ; ; ; ; e ; ; ; ; ; e ; x ; ; ; ; x a Q a A m Q S W M U m N p u E H S E p S l e R E G S l W e r V N D T e E . y E : S i I . R c R I G O c o t : S Z N o S m i a E 9 m E . m 1 t . S . C e 9 1 E T : 2 F r 8 C I . e c . T O 2 1 b v 1 I N 3 6 d O : 8 2 : N m . 8 : s 1 5 e . 2 6 c 1 0 # : e 5 3 x 3 3 0 a 6 ( : m 0 1 0 p 0 9 0 l 2 e . E . 1 S c 6 T o I I 8 m N N . 2 1 0 . 2 1 6 ) A A 9 3 . 1 8 4 . 2 1 6 . 3 4 Key fields: ...

SSH tunnels are one of those tools that seem magical until you understand the three basic patterns. Once you do, you’ll use them constantly. The Three Types Local forwarding (-L): Access a remote service as if it were local Remote forwarding (-R): Expose a local service to a remote network Dynamic forwarding (-D): Create a SOCKS proxy through the SSH connection Let’s break down each one. Local Forwarding: Reach Remote Services Locally Scenario: You need to access a database that’s only available from a server you can SSH into. ...

netcat (nc) does one thing: move bytes over a network. That simplicity makes it incredibly versatile — port scanning, file transfers, chat servers, proxies, and network debugging all become one-liners. Basic Usage 1 2 3 4 5 6 7 8 # Connect to host:port nc hostname 80 # Listen on port nc -l 8080 # Listen (keep listening after disconnect) nc -lk 8080 Test if Port is Open 1 2 3 4 5 6 7 8 9 # Quick port check nc -zv hostname 22 # Connection to hostname 22 port [tcp/ssh] succeeded! # Scan port range nc -zv hostname 20-25 # With timeout nc -zv -w 3 hostname 443 Simple Client-Server Communication Server (listener) 1 nc -l 9000 Client 1 nc localhost 9000 Now type in either terminal — text flows both ways. Ctrl+C to exit. ...

“It’s always DNS” is a meme because it’s usually true. When something network-related breaks, DNS is the first suspect. Here’s how to investigate. The Essential Tools dig - The DNS Swiss Army Knife 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Basic lookup dig example.com # Specific record type dig example.com MX dig example.com TXT dig example.com CNAME # Short output (just the answer) dig +short example.com # Query a specific nameserver dig @8.8.8.8 example.com # Trace the full resolution path dig +trace example.com nslookup - Quick and Simple 1 2 3 4 5 6 7 8 9 10 # Basic lookup nslookup example.com # Query specific server nslookup example.com 8.8.8.8 # Interactive mode nslookup > set type=MX > example.com host - Even Simpler 1 2 3 4 5 # Basic lookup host example.com # Specific record type host -t MX example.com Common DNS Problems Problem: Domain Not Resolving Symptoms: NXDOMAIN or SERVFAIL errors ...

DNS is invisible until it breaks. Then everything breaks. Understanding how DNS works helps you debug issues faster and configure services correctly. The Basics DNS translates human-readable names to IP addresses: e x a m p l e . c o m → 9 3 . 1 8 4 . 2 1 6 . 3 4 The resolution process: 1 2 3 4 5 . . . . . B O Q R A r S u e n o e s s w c r o w s h y l e e e v r r c g e k o r c c s e a h s q c e i u h c t t e e k s o r d s i c c e a l a o s t o c n c h f r e a e i o a l g o c u t h c r a e s l c d e e h r v e r v e e e l s r o s b l a v → s e e r T d L ( D o e n . s g e T . r T , v L e 8 r . s 8 . → 8 . a 8 u ) t h o r i t a t i v e s e r v e r s Record Types A Record (IPv4 Address) e x a m p l e . c o m . 3 0 0 I N A 9 3 . 1 8 4 . 2 1 6 . 3 4 Maps a name to an IPv4 address. ...

You’ve scaled horizontally — multiple servers ready to handle requests. Now you need something to decide which server handles each request. That’s load balancing, and the strategy you choose affects latency, reliability, and resource utilization. Round Robin: The Default Each server gets requests in rotation: Server 1, Server 2, Server 3, Server 1, Server 2… 1 2 3 4 5 upstream backend { server app1:8080; server app2:8080; server app3:8080; } Pros: Simple to understand and implement Even distribution over time No state to maintain Cons: ...