You try to start your server and get hit with: E r r o r : l i s t e n E A D D R I N U S E : a d d r e s s a l r e a d y i n u s e : : : 3 0 0 0 Or the Python equivalent: ...
You’ve deployed your webhook endpoint. The external service says it’s sending requests. But your logs show nothing. Here’s how to systematically debug incoming webhook issues. The Debugging Checklist Before diving deep, run through these quick checks: 1 2 3 4 5 6 7 8 9 10 11 # 1. Is your endpoint actually reachable? curl -X POST https://your-domain.com/webhook \ -H "Content-Type: application/json" \ -d '{"test": true}' \ -w "\nHTTP Status: %{http_code}\n" # 2. Is DNS resolving correctly? dig your-domain.com +short # 3. Is the port open? nc -zv your-domain.com 443 If your own curl request doesn’t reach the endpoint, the problem is infrastructure. If it does but the external service’s requests don’t arrive, the problem is somewhere in between. ...
Your container builds fine, starts fine, then fails with Could not resolve host or Temporary failure in name resolution. Here’s how to fix it. Quick Diagnosis First, confirm it’s actually DNS and not a network issue: 1 2 3 4 5 6 7 8 # Get a shell in the container docker exec -it <container_name> sh # Test DNS specifically nslookup google.com # or cat /etc/resolv.conf ping 8.8.8.8 # If this works but nslookup fails, it's DNS If ping 8.8.8.8 works but nslookup google.com fails, you have a DNS problem. If both fail, it’s a broader network issue. ...
Kubernetes networking confuses everyone at first. Pods, Services, Ingresses, CNIs — it’s a lot. Here’s how it actually works, layer by layer. The Fundamental Model Kubernetes networking has three simple rules: Every Pod gets its own IP address Pods can communicate with any other Pod without NAT Agents on a node can communicate with all Pods on that node That’s it. Everything else is implementation detail. Layer 1: Pod Networking Each Pod gets an IP from the cluster’s Pod CIDR (e.g., 10.244.0.0/16). This isn’t magic — your CNI plugin handles it. ...
Load balancers are invisible until they break. Then they’re the only thing anyone talks about. Here’s how to get them right. Algorithms That Matter Round Robin Requests go to each server in sequence: A, B, C, A, B, C… 1 2 3 4 5 upstream backend { server 10.0.0.1:8080; server 10.0.0.2:8080; server 10.0.0.3:8080; } Good for: Homogeneous servers, stateless apps Bad for: Servers with different capacities, long-running requests Weighted Round Robin Some servers get more traffic: ...
Still typing ssh -i ~/.ssh/my-key.pem -p 2222 user@server.example.com? There’s a better way. The SSH Config File ~/.ssh/config transforms verbose commands into simple ones. # s # s s s B h A h e f f - t p o i e r r r o e ~ d / . s s h / p r o d - k e y . p e m - p 2 2 2 2 d e p l o y @ p r o d . e x a m p l e . c o m Basic Config # H H H o o o ~ s s s / t t t . H U P I H U I H U s p o s o d s o s d d o s s r s e r e t s e e e s e h o t r t n a t r n v t r / d N t g N t N c a d 2 i i a d i a d o m e 2 t n m e t m e n e p 2 y g e p y e v f l 2 F l F e i p o i s o i 1 l g r y l t y l 9 o o e a e 2 p d g . e . ~ i ~ 1 r e / n / 6 x . g . 8 a s . s . m s e s 1 p h x h . l / a / 1 e p m s 0 . r p t 0 c o l a o d e g m - . i k c n e o g y m - . k p e e y m . p e m Now just: ...
Round robin is the default. It’s also often wrong. Here’s how to choose load balancing strategies that actually match your workload. The Strategies Round Robin Each request goes to the next server in rotation. 1 2 3 4 5 upstream backend { server 10.0.0.1; server 10.0.0.2; server 10.0.0.3; } Good for: Stateless services, similar server capacity Bad for: Long-running connections, mixed server specs, sticky sessions Weighted Round Robin Same rotation, but some servers get more traffic. ...
Port forwarding is a security liability. Opening ports means exposing your network, managing firewall rules, and hoping your ISP doesn’t change your IP. Cloudflare Tunnels solve this elegantly—your services connect outbound to Cloudflare, which handles incoming traffic. How Tunnels Work Traditional setup: I n t e r n e t → Y o u r P u b l i c I P : 4 4 3 → F i r e w a l l → N A T → L o c a l S e r v i c e With Cloudflare Tunnel: ...
DNS is the first thing that breaks and the last thing you check. Understanding it properly saves hours of debugging “it works on my machine.” Record Types You’ll Actually Use A and AAAA Point domain to IP address: e e x x a a m m p p l l e e . . c c o o m m . . A A A A A 9 2 3 6 . 0 1 6 8 : 4 2 . 8 2 0 1 0 6 : . 2 3 2 4 0 : 1 : 2 4 8 : 1 8 9 3 : 2 5 c 8 : 1 9 4 6 CNAME Alias to another domain: ...
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: ...