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Zone Transfers

Configure and optimize DNS zone transfers between primary and secondary instances.

Overview

Zone transfers replicate DNS zone data from primary to secondary servers using AXFR (full transfer) or IXFR (incremental transfer).

Configuring Zone Transfers

Primary Instance Setup

Allow zone transfers to secondary servers:

apiVersion: bindy.firestoned.io/v1alpha1
kind: Bind9Instance
metadata:
  name: primary-dns
spec:
  config:
    allowTransfer:
      - "10.0.0.0/8"        # Secondary network
      - "192.168.100.0/24"  # Specific secondary subnet

Secondary Instance Setup

Configure secondary zones to transfer from primary:

apiVersion: bindy.firestoned.io/v1alpha1
kind: DNSZone
metadata:
  name: example-com-secondary
spec:
  zoneName: example.com
  type: secondary
  instanceSelector:
    matchLabels:
      dns-role: secondary
  secondaryConfig:
    primaryServers:
      - "10.0.1.10"  # Primary DNS server IP
      - "10.0.1.11"  # Backup primary IP

Transfer Types

Full Transfer (AXFR)

Transfers entire zone:

  • Used for initial zone load
  • Triggered manually or when IXFR unavailable
  • More bandwidth intensive

Incremental Transfer (IXFR)

Transfers only changes since last serial:

  • More efficient for large zones
  • Requires serial number tracking
  • Automatically used when available

Transfer Triggers

NOTIFY Messages

Primary sends NOTIFY when zone changes:

graph TB
    primary["Primary Updates Zone"]
    sec1["Secondary 1"]
    sec2["Secondary 2"]
    sec3["Secondary 3"]
    transfer["Secondaries initiate IXFR/AXFR"]

    primary -->|NOTIFY| sec1
    primary -->|NOTIFY| sec2
    primary -->|NOTIFY| sec3
    sec1 --> transfer
    sec2 --> transfer
    sec3 --> transfer

    style primary fill:#e8f5e9,stroke:#1b5e20,stroke-width:2px
    style sec1 fill:#e1f5ff,stroke:#01579b,stroke-width:2px
    style sec2 fill:#e1f5ff,stroke:#01579b,stroke-width:2px
    style sec3 fill:#e1f5ff,stroke:#01579b,stroke-width:2px
    style transfer fill:#fff9c4,stroke:#f57f17,stroke-width:2px

Refresh Timer

Secondary checks for updates periodically:

soaRecord:
  refresh: 3600  # Check every hour
  retry: 600     # Retry after 10 minutes if failed

Manual Trigger

Force zone transfer:

# On secondary pod
kubectl exec -n dns-system deployment/secondary-dns -- \
  rndc retransfer example.com

Monitoring Zone Transfers

Check Transfer Status

# View transfer logs
kubectl logs -n dns-system -l dns-role=secondary | grep "transfer of"

# Successful transfer
# transfer of 'example.com/IN' from 10.0.1.10#53: Transfer completed: 1 messages, 42 records

# Check zone status
kubectl exec -n dns-system deployment/secondary-dns -- \
  rndc zonestatus example.com

Verify Serial Numbers

# Primary serial
kubectl exec -n dns-system deployment/primary-dns -- \
  dig @localhost example.com SOA +short | awk '{print $3}'

# Secondary serial  
kubectl exec -n dns-system deployment/secondary-dns -- \
  dig @localhost example.com SOA +short | awk '{print $3}'

# Should match when in sync

Transfer Performance

Optimize Transfer Speed

  1. Use IXFR - Only transfer changes
  2. Increase Bandwidth - Adequate network resources
  3. Compress Transfers - Enable BIND9 compression
  4. Parallel Transfers - Multiple zones transfer concurrently

Transfer Limits

Configure maximum concurrent transfers:

# In BIND9 config (future enhancement)
options {
  transfers-in 10;   # Max incoming transfers
  transfers-out 10;  # Max outgoing transfers
};

Security

Access Control

Restrict transfers by IP:

spec:
  config:
    allowTransfer:
      - "10.0.0.0/8"  # Only this network

TSIG Authentication

Use TSIG keys for authenticated transfers:

# 1. Create a Kubernetes Secret with RNDC/TSIG credentials
apiVersion: v1
kind: Secret
metadata:
  name: transfer-key-secret
  namespace: dns-system
type: Opaque
stringData:
  key-name: transfer-key
  secret: K2xkajflkajsdf09asdfjlaksjdf==  # base64-encoded HMAC key

---
# 2. Reference the secret in Bind9Cluster
apiVersion: bindy.firestoned.io/v1alpha1
kind: Bind9Cluster
metadata:
  name: production-dns
  namespace: dns-system
spec:
  rndcSecretRefs:
    - name: transfer-key-secret
      algorithm: hmac-sha256  # Algorithm for this key

The secret will be used for authenticated zone transfers between primary and secondary servers.

Troubleshooting

Transfer Failures

Check network connectivity:

kubectl exec -n dns-system deployment/secondary-dns -- \
  nc -zv primary-dns-service 53

Test manual transfer:

kubectl exec -n dns-system deployment/secondary-dns -- \
  dig @primary-dns-service example.com AXFR

Check ACLs:

kubectl get bind9instance primary-dns -o jsonpath='{.spec.config.allowTransfer}'

Slow Transfers

Check zone size:

kubectl exec -n dns-system deployment/primary-dns -- \
  wc -l /var/lib/bind/zones/example.com.zone

Monitor transfer time:

kubectl logs -n dns-system -l dns-role=secondary | \
  grep "transfer of" | grep "msecs"

Transfer Lag

Check refresh interval:

kubectl get dnszone example-com -o jsonpath='{.spec.soaRecord.refresh}'

Force immediate transfer:

kubectl exec -n dns-system deployment/secondary-dns -- \
  rndc retransfer example.com

Best Practices

  1. Use IXFR - More efficient than full transfers
  2. Set Appropriate Refresh - Balance freshness vs load
  3. Monitor Serial Numbers - Detect sync issues
  4. Secure Transfers - Use ACLs and TSIG
  5. Test Failover - Verify secondaries work when primary fails
  6. Log Transfers - Monitor for failures
  7. Geographic Distribution - Secondaries in different regions

Example: Complete Setup

# Primary Instance
apiVersion: bindy.firestoned.io/v1alpha1
kind: Bind9Instance
metadata:
  name: primary-dns
  labels:
    dns-role: primary
spec:
  replicas: 2
  config:
    allowTransfer:
      - "10.0.0.0/8"
---
# Primary Zone
apiVersion: bindy.firestoned.io/v1alpha1
kind: DNSZone
metadata:
  name: example-com-primary
spec:
  zoneName: example.com
  type: primary
  instanceSelector:
    matchLabels:
      dns-role: primary
  soaRecord:
    primaryNs: ns1.example.com.
    adminEmail: admin@example.com
    serial: 2024010101
    refresh: 3600
    retry: 600
    expire: 604800
    negativeTtl: 86400
---
# Secondary Instance  
apiVersion: bindy.firestoned.io/v1alpha1
kind: Bind9Instance
metadata:
  name: secondary-dns
  labels:
    dns-role: secondary
spec:
  replicas: 2
---
# Secondary Zone
apiVersion: bindy.firestoned.io/v1alpha1
kind: DNSZone
metadata:
  name: example-com-secondary
spec:
  zoneName: example.com
  type: secondary
  instanceSelector:
    matchLabels:
      dns-role: secondary
  secondaryConfig:
    primaryServers:
      - "primary-dns-service.dns-system.svc.cluster.local"

Next Steps