k8s-ndots-search-domain-nxdomain-flood

name	k8s-ndots-search-domain-nxdomain-flood
description	Fix for massive NxDomain query floods to external DNS servers caused by Kubernetes ndots:5 search domain expansion. Use when: (1) DNS server shows low cache hit rate with 60%+ NxDomain responses, (2) DNS logs show queries like "service.namespace.svc.cluster.local.yourdomain.lan", (3) external DNS receives thousands of junk queries per hour for non-existent names ending in your search domain, (4) DNS cache hit ratio is unexpectedly low despite stable workloads. Applies to any Kubernetes cluster using CoreDNS with a custom DNS search domain.
author	Claude Code
version	1.1.0
date	"2026-02-17T00:00:00.000Z"

Kubernetes ndots:5 Search Domain NxDomain Flood

Problem

Kubernetes pods have ndots:5 and a custom search domain (e.g., viktorbarzin.lan) in their /etc/resolv.conf. When resolving internal service names like redis.redis.svc.cluster.local (4 dots < ndots:5), glibc tries all search domain suffixes before the absolute name. This generates queries like:

1. redis.redis.svc.cluster.local.namespace.svc.cluster.local (CoreDNS handles, NxDomain)
2. redis.redis.svc.cluster.local.svc.cluster.local (CoreDNS handles, NxDomain)
3. redis.redis.svc.cluster.local.cluster.local (CoreDNS handles, NxDomain)
4. redis.redis.svc.cluster.local.yourdomain.lan (CoreDNS forwards to external DNS, NxDomain)
5. redis.redis.svc.cluster.local (finally resolves)

Step 4 is the problem: CoreDNS forwards *.yourdomain.lan queries to the external DNS server, flooding it with junk NxDomain requests. With hundreds of pods making DNS lookups, this generates tens of thousands of useless queries per day.

Context / Trigger Conditions

• DNS server (e.g., Technitium, Pi-hole, BIND) shows high NxDomain percentage (50%+)
• DNS cache hit rate is unexpectedly low
• DNS logs show queries ending in *.svc.cluster.local.yourdomain.lan
• CoreDNS Corefile has a server block forwarding yourdomain.lan to an external DNS
• Node resolv.conf has search yourdomain.lan (set by DHCP)
• Top DNS clients by query volume are Kubernetes node IPs (not pod IPs), because CoreDNS forwards via NodePort and the source IP becomes the node IP

Solution

Step 1: Confirm the problem

Check DNS query logs for the pattern:

# Enable Technitium query logging temporarily
# API: /api/settings/set?token=TOKEN&enableLogging=true&logQueries=true&loggingType=File

# Check for junk queries
kubectl exec -n technitium PODNAME -- grep "cluster.local.yourdomain" /etc/dns/logs/*.log

Step 2: Add generic CoreDNS template regex (RECOMMENDED)

Instead of creating specific catch-all blocks for each junk suffix pattern, add a single template directive with a regex inside the yourdomain.lan server block. This catches ALL multi-label junk queries (e.g., *.cluster.local.yourdomain.lan, *.yourdomain.lan.yourdomain.lan, www.cloudflare.com.yourdomain.lan) in one rule:

yourdomain.lan:53 {
  errors
  template ANY ANY yourdomain.lan {
    match ".*\..*\.yourdomain\.lan\.$"
    rcode NXDOMAIN
    fallthrough
  }
  forward . <your-dns-server-ip>
  cache {
    success 10000 300 6
    denial 10000 300 60
  }
}

How it works: The regex .*\..*\.yourdomain\.lan\.$ matches any query with 2+ labels before .yourdomain.lan — meaning only single-label queries like idrac.yourdomain.lan fall through to the real DNS server. All junk multi-label queries get instant NXDOMAIN.

Important: The fallthrough directive is required so that legitimate single-label queries (which don't match the regex) continue to the forward plugin.

Alternative: Specific catch-all blocks (DEPRECATED)

The older approach used separate server blocks per junk suffix pattern:

cluster.local.yourdomain.lan:53 {
  errors
  template ANY ANY {
    rcode NXDOMAIN
  }
  cache {
    denial 10000 3600
  }
}

This requires adding a new block for each pattern and doesn't catch arbitrary junk queries like www.cloudflare.com.yourdomain.lan. The generic regex approach above is preferred.

Step 3: Apply the CoreDNS ConfigMap

kubectl apply -f coredns-configmap.yaml
# CoreDNS auto-reloads via the `reload` plugin (default 30s)

Step 4: Manage in Terraform (this cluster)

The CoreDNS ConfigMap is managed in modules/kubernetes/technitium/main.tf as kubernetes_config_map.coredns. To import an existing ConfigMap:

terraform import 'module.kubernetes_cluster.module.technitium["technitium"].kubernetes_config_map.coredns' 'kube-system/coredns'

Verification

1. Test that the template returns NXDOMAIN instantly:

kubectl run dns-test --rm -i --restart=Never --image=busybox -- \
  nslookup redis.redis.svc.cluster.local.yourdomain.lan 10.96.0.10
# Should return NXDOMAIN immediately

2. Check DNS logs - no more *.cluster.local.yourdomain.lan queries to external DNS
3. NxDomain percentage on external DNS should drop significantly within an hour

Additional Fix: Enable DNS Cache Persistence

If the DNS server (Technitium) loses its cache on pod restart, enable saveCache:

/api/settings/set?token=TOKEN&saveCache=true

This prevents the cache hit rate from resetting to zero after every restart.

Notes

• The same ndots:5 issue also causes *.yourdomain.lan.yourdomain.lan (double suffix) and *.yourdomain.me.yourdomain.lan patterns — the generic regex catches all of these
• The top DNS client IPs will be the node IPs (not pod IPs) because CoreDNS forwards via NodePort, and the source becomes the node's IP
• ndots:5 is the Kubernetes default and shouldn't be changed cluster-wide as it breaks short-name service resolution
• Individual pods can set dnsConfig.options: [{name: ndots, value: "2"}] to reduce search domain lookups, but this is a per-pod opt-in
• Prometheus scrape targets using .yourdomain.lan hostnames should add a trailing dot (e.g., idrac.yourdomain.lan.:161) to bypass ndots expansion entirely
• ExternalName services don't need trailing dots — the generic template regex handles them

See also

• pfsense-dnsmasq-interface-binding — Related: preserve client IPs for DNS port forwarding
• crowdsec-agent-registration-failure — another common K8s DNS-adjacent issue
• loki-helm-deployment-pitfalls — Loki deployment patterns