Dns

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. ...

Hardcoded IPs are a maintenance nightmare. Here’s how to let services find each other dynamically. The Problem 1 2 3 4 5 6 7 # Bad: Hardcoded api_url = "http://192.168.1.50:8080" # What happens when: # - IP changes? # - Service moves to new host? # - You add a second instance? Service discovery solves this: services register themselves, and clients look them up by name. DNS-Based Discovery The simplest approach: use DNS. ...

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: ...

“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. ...

DNS is the foundation everything else depends on. A misconfigured record can take down your entire infrastructure. Yet DNS is often managed through web consoles with no version control, no review process, and no automation. Let’s fix that. Terraform for DNS Route53 Basics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 # dns.tf resource "aws_route53_zone" "main" { name = "example.com" tags = { Environment = "production" } } # A record resource "aws_route53_record" "www" { zone_id = aws_route53_zone.main.zone_id name = "www.example.com" type = "A" ttl = 300 records = ["203.0.113.10"] } # CNAME record resource "aws_route53_record" "app" { zone_id = aws_route53_zone.main.zone_id name = "app.example.com" type = "CNAME" ttl = 300 records = ["app-lb-123456.us-east-1.elb.amazonaws.com"] } # Alias to ALB (no TTL, resolved at edge) resource "aws_route53_record" "api" { zone_id = aws_route53_zone.main.zone_id name = "api.example.com" type = "A" alias { name = aws_lb.api.dns_name zone_id = aws_lb.api.zone_id evaluate_target_health = true } } # MX records resource "aws_route53_record" "mx" { zone_id = aws_route53_zone.main.zone_id name = "example.com" type = "MX" ttl = 3600 records = [ "10 mail1.example.com", "20 mail2.example.com" ] } # TXT for SPF resource "aws_route53_record" "spf" { zone_id = aws_route53_zone.main.zone_id name = "example.com" type = "TXT" ttl = 3600 records = ["v=spf1 include:_spf.google.com ~all"] } Dynamic Records from Infrastructure 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 # Generate records from other resources locals { services = { "api" = aws_lb.api.dns_name "admin" = aws_lb.admin.dns_name "docs" = aws_cloudfront_distribution.docs.domain_name } } resource "aws_route53_record" "services" { for_each = local.services zone_id = aws_route53_zone.main.zone_id name = "${each.key}.example.com" type = "CNAME" ttl = 300 records = [each.value] } # From Kubernetes ingresses data "kubernetes_ingress_v1" "all" { for_each = toset(["api", "web", "admin"]) metadata { name = each.key namespace = "production" } } resource "aws_route53_record" "k8s_services" { for_each = data.kubernetes_ingress_v1.all zone_id = aws_route53_zone.main.zone_id name = "${each.key}.example.com" type = "CNAME" ttl = 300 records = [each.value.status[0].load_balancer[0].ingress[0].hostname] } DNS Failover Health Check Based Routing 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 # Health check for primary resource "aws_route53_health_check" "primary" { fqdn = "primary-api.example.com" port = 443 type = "HTTPS" resource_path = "/health" failure_threshold = 3 request_interval = 30 tags = { Name = "primary-api-health" } } # Health check for secondary resource "aws_route53_health_check" "secondary" { fqdn = "secondary-api.example.com" port = 443 type = "HTTPS" resource_path = "/health" failure_threshold = 3 request_interval = 30 tags = { Name = "secondary-api-health" } } # Primary record with failover resource "aws_route53_record" "api_primary" { zone_id = aws_route53_zone.main.zone_id name = "api.example.com" type = "A" ttl = 60 records = ["203.0.113.10"] set_identifier = "primary" health_check_id = aws_route53_health_check.primary.id failover_routing_policy { type = "PRIMARY" } } # Secondary record (used when primary fails) resource "aws_route53_record" "api_secondary" { zone_id = aws_route53_zone.main.zone_id name = "api.example.com" type = "A" ttl = 60 records = ["203.0.113.20"] set_identifier = "secondary" health_check_id = aws_route53_health_check.secondary.id failover_routing_policy { type = "SECONDARY" } } Weighted Routing for Gradual Migration 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 # 90% to current, 10% to new resource "aws_route53_record" "api_current" { zone_id = aws_route53_zone.main.zone_id name = "api.example.com" type = "A" ttl = 60 records = ["203.0.113.10"] set_identifier = "current" weighted_routing_policy { weight = 90 } } resource "aws_route53_record" "api_new" { zone_id = aws_route53_zone.main.zone_id name = "api.example.com" type = "A" ttl = 60 records = ["203.0.113.20"] set_identifier = "new" weighted_routing_policy { weight = 10 } } External DNS for Kubernetes Automatically create DNS records from Kubernetes resources. ...