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Private Endpoint Cutover and DNS Negative Caching

Status: Published

Experiment completed with real data collected on 2026-04-12 from Azure App Service P1v3 (koreacentral). Container Apps VNet integration encountered subnet configuration issues; App Service results only.

1. Question

When enabling a private endpoint for an Azure service and updating DNS to resolve to the private IP, how does DNS negative caching affect the transition period, and can it cause extended outages beyond the expected DNS TTL?

2. Why this matters

Private endpoint migrations are high-risk operations. The expected flow is: create private endpoint → update DNS → traffic flows to private IP. But DNS negative caching (caching of NXDOMAIN or failed lookups) can cause problems:

  • If the private DNS zone is created but the link to the VNet is delayed, clients may cache the negative result
  • If the DNS query returns NXDOMAIN during the transition window, that negative response gets cached
  • The negative cache TTL may be longer than the positive TTL, extending the outage

3. Customer symptom

  • "We created the private endpoint and DNS zone, but resolution still shows the public IP after 30 minutes."
  • "Some instances resolve the private IP, others still resolve the public IP."
  • "After restart, everything works — but without restart, the old resolution persists for hours."

4. Hypothesis

  1. DNS negative caching (NXDOMAIN caching) in the VNet resolver causes extended resolution failures during private endpoint cutover.
  2. The negative cache TTL (SOA minimum) can be 5-60 minutes, independent of the A record TTL.
  3. If the private DNS zone link is created after the first DNS query, the negative cache prevents the client from seeing the new private IP until the negative cache expires.
  4. Different Azure compute services (App Service, Functions, Container Apps) handle DNS cache differently, leading to inconsistent behavior across services.

5. Environment

Parameter Value
Service App Service (VNet-integrated)
SKU / Plan P1v3
Region Korea Central
Runtime Python 3.11 (Flask)
OS Linux
Storage Account Standard LRS, public access disabled
Private Endpoint IP 10.90.4.4
Date tested 2026-04-12

6. Variables

Experiment type: Hybrid (Config: does resolution switch? Performance: how long does it take?)

Controlled:

  • Private endpoint target: Storage Account, SQL Database
  • DNS configuration sequence: zone first vs link first vs simultaneous
  • VNet DNS: Azure Default (168.63.129.16) vs custom DNS forwarder
  • Client compute: App Service, Functions, Container Apps

Observed:

  • DNS resolution result (public IP vs private IP) over time per client
  • Negative cache duration
  • Time from DNS change to consistent private resolution across all instances
  • Connectivity success/failure to private endpoint during transition

Independent run definition: Clean VNet with no prior DNS cache, fresh deployment, create private endpoint and DNS zone with specified sequencing

Planned runs per configuration: 3 (minimum; this is expensive to set up)

Warm-up exclusion rule: None — the transition IS the measurement

Primary metric: Time from DNS zone link to consistent private IP resolution; meaningful effect threshold: >5 minutes

Comparison method: Descriptive statistics per sequencing strategy

7. Instrumentation

  • Application code: DNS resolution polling every 30 seconds with resolved IP logging
  • nslookup/dig from Kudu/console: direct DNS queries with TTL inspection
  • Application Insights: dependency calls with resolved endpoint IP
  • Azure Monitor: Private DNS Zone query logs

8. Procedure

8.1 Infrastructure Setup

export SUBSCRIPTION_ID="<subscription-id>"
export RG="rg-pe-dns-negative-cache-lab"
export LOCATION="koreacentral"
export VNET_NAME="vnet-pe-dns-negative-cache"
export APP_SUBNET_NAME="snet-appsvc"
export FUNC_SUBNET_NAME="snet-functions"
export ACA_SUBNET_NAME="snet-containerapps"
export PE_SUBNET_NAME="snet-private-endpoint"
export STORAGE_NAME="stpednscache$RANDOM"
export APP_PLAN="plan-pe-dns-negative-cache"
export WEBAPP_NAME="app-pe-dns-cache-$RANDOM"
export FUNC_NAME="func-pe-dns-cache-$RANDOM"
export ACA_ENV_NAME="cae-pe-dns-cache"
export ACA_NAME="ca-pe-dns-cache"
export PRIVATE_DNS_ZONE="privatelink.blob.core.windows.net"

az account set --subscription "$SUBSCRIPTION_ID"
az group create --name "$RG" --location "$LOCATION"

az network vnet create \
  --resource-group "$RG" \
  --name "$VNET_NAME" \
  --location "$LOCATION" \
  --address-prefixes "10.90.0.0/16" \
  --subnet-name "$APP_SUBNET_NAME" \
  --subnet-prefixes "10.90.0.0/24"

az network vnet subnet create --resource-group "$RG" --vnet-name "$VNET_NAME" --name "$FUNC_SUBNET_NAME" --address-prefixes "10.90.1.0/24"
az network vnet subnet create --resource-group "$RG" --vnet-name "$VNET_NAME" --name "$ACA_SUBNET_NAME" --address-prefixes "10.90.2.0/23"
az network vnet subnet create --resource-group "$RG" --vnet-name "$VNET_NAME" --name "$PE_SUBNET_NAME" --address-prefixes "10.90.4.0/24"

az network vnet subnet update \
  --resource-group "$RG" \
  --vnet-name "$VNET_NAME" \
  --name "$ACA_SUBNET_NAME" \
  --delegations "Microsoft.App/environments"

az storage account create \
  --resource-group "$RG" \
  --name "$STORAGE_NAME" \
  --location "$LOCATION" \
  --sku Standard_LRS \
  --kind StorageV2 \
  --allow-shared-key-access false \
  --allow-blob-public-access false

8.2 Application Code

import json
import os
import socket
from datetime import datetime, timezone

from flask import Flask

app = Flask(__name__)
TARGET = os.getenv("TARGET_FQDN")


@app.get("/dns-probe")
def dns_probe():
    ips = []
    error = None
    try:
        ips = sorted({item[4][0] for item in socket.getaddrinfo(TARGET, 443)})
    except Exception as ex:
        error = str(ex)
    return {
        "timestamp_utc": datetime.now(timezone.utc).isoformat(),
        "target": TARGET,
        "resolved_ips": ips,
        "error": error,
    }

env:
  TARGET_FQDN: <storage-name>.blob.core.windows.net
probes:
  intervalSeconds: 30

8.3 Deploy

az appservice plan create \
  --resource-group "$RG" \
  --name "$APP_PLAN" \
  --location "$LOCATION" \
  --sku P1v3 \
  --is-linux

az webapp create \
  --resource-group "$RG" \
  --name "$WEBAPP_NAME" \
  --plan "$APP_PLAN" \
  --runtime "PYTHON|3.11"

az functionapp create \
  --resource-group "$RG" \
  --name "$FUNC_NAME" \
  --storage-account "$STORAGE_NAME" \
  --runtime python \
  --runtime-version 3.11 \
  --functions-version 4 \
  --flexconsumption-location "$LOCATION"

az containerapp env create \
  --resource-group "$RG" \
  --name "$ACA_ENV_NAME" \
  --location "$LOCATION" \
  --infrastructure-subnet-resource-id "/subscriptions/<subscription-id>/resourceGroups/$RG/providers/Microsoft.Network/virtualNetworks/$VNET_NAME/subnets/$ACA_SUBNET_NAME"

az containerapp create \
  --resource-group "$RG" \
  --name "$ACA_NAME" \
  --environment "$ACA_ENV_NAME" \
  --image "mcr.microsoft.com/azuredocs/containerapps-helloworld:latest" \
  --ingress external \
  --target-port 80

az webapp vnet-integration add \
  --resource-group "$RG" \
  --name "$WEBAPP_NAME" \
  --vnet "$VNET_NAME" \
  --subnet "$APP_SUBNET_NAME"

az functionapp vnet-integration add \
  --resource-group "$RG" \
  --name "$FUNC_NAME" \
  --vnet "$VNET_NAME" \
  --subnet "$FUNC_SUBNET_NAME"

8.4 Test Execution

export TARGET_FQDN="$STORAGE_NAME.blob.core.windows.net"

# 1) Baseline: public endpoint resolution before private endpoint creation
for i in $(seq 1 10); do
  nslookup "$TARGET_FQDN"
  sleep 10
done

# 2) Create private endpoint first, but delay private DNS zone link (induce NXDOMAIN window)
az network private-endpoint create \
  --resource-group "$RG" \
  --name "pe-storage" \
  --location "$LOCATION" \
  --subnet "$PE_SUBNET_NAME" \
  --vnet-name "$VNET_NAME" \
  --private-connection-resource-id "$(az storage account show --resource-group "$RG" --name "$STORAGE_NAME" --query id --output tsv)" \
  --group-id blob \
  --connection-name "pe-storage-conn"

az network private-dns zone create \
  --resource-group "$RG" \
  --name "$PRIVATE_DNS_ZONE"

# 3) Query during unlinked-zone window to trigger potential negative caching
for i in $(seq 1 20); do
  date -u +"%Y-%m-%dT%H:%M:%SZ"
  nslookup "$TARGET_FQDN" || true
  sleep 15
done

# 4) Link zone to VNet and create zone group
az network private-dns link vnet create \
  --resource-group "$RG" \
  --zone-name "$PRIVATE_DNS_ZONE" \
  --name "link-pe-dns-cache" \
  --virtual-network "$VNET_NAME" \
  --registration-enabled false

az network private-endpoint dns-zone-group create \
  --resource-group "$RG" \
  --endpoint-name "pe-storage" \
  --name "default" \
  --private-dns-zone "$PRIVATE_DNS_ZONE" \
  --zone-name "zonegroup-storage"

# 5) Poll every 30 seconds from each client compute and track first private IP response
for i in $(seq 1 60); do
  nslookup "$TARGET_FQDN" || true
  curl --silent "https://$WEBAPP_NAME.azurewebsites.net/dns-probe" || true
  curl --silent "https://$FUNC_NAME.azurewebsites.net/api/dns-probe" || true
  sleep 30
done

# 6) Restart clients and compare cache-cleared behavior
az webapp restart --resource-group "$RG" --name "$WEBAPP_NAME"
az functionapp restart --resource-group "$RG" --name "$FUNC_NAME"
az containerapp revision restart \
  --resource-group "$RG" \
  --name "$ACA_NAME" \
  --revision "$(az containerapp revision list --resource-group "$RG" --name "$ACA_NAME" --query "[?properties.active].name | [0]" --output tsv)"

8.5 Data Collection

az network private-endpoint show \
  --resource-group "$RG" \
  --name "pe-storage" \
  --query "customDnsConfigs" \
  --output table

az network private-dns record-set a list \
  --resource-group "$RG" \
  --zone-name "$PRIVATE_DNS_ZONE" \
  --output table

az monitor metrics list \
  --resource "/subscriptions/<subscription-id>/resourceGroups/$RG/providers/Microsoft.Web/sites/$WEBAPP_NAME" \
  --metric "Http5xx" "Requests" \
  --interval PT1M \
  --aggregation Total \
  --output table

az monitor metrics list \
  --resource "/subscriptions/<subscription-id>/resourceGroups/$RG/providers/Microsoft.Web/sites/$FUNC_NAME" \
  --metric "FunctionExecutionCount" "FunctionExecutionUnits" \
  --interval PT1M \
  --aggregation Total \
  --output table

az monitor log-analytics query \
  --workspace "$(az monitor log-analytics workspace list --resource-group "$RG" --query "[0].customerId" --output tsv)" \
  --analytics-query "AppTraces | where TimeGenerated > ago(6h) | where Message has_any ('NXDOMAIN','Temporary failure','resolved_ips') | project TimeGenerated, AppRoleName, Message | order by TimeGenerated asc" \
  --output table

8.6 Cleanup

az group delete --name "$RG" --yes --no-wait

9. Expected signal

  • Zone-first deployment: brief NXDOMAIN period until A record is created → negative cache delays resolution by 5-30 minutes
  • Simultaneous deployment: minimal negative cache impact, switch happens within positive TTL
  • Custom DNS forwarder: additional caching layer adds 0-15 minutes delay
  • Restart of compute service clears DNS cache and immediately resolves new IP

10. Results

10.1 Baseline: Public IP Resolution

Before Private Endpoint creation, DNS resolved to public IP:

Timestamp (UTC) Resolved IP Private Resolution Time
2026-04-12T07:32:17 20.150.4.36 No 32.25 ms
2026-04-12T07:32:19 20.150.4.36 No 9.06 ms
2026-04-12T07:32:21 20.150.4.36 No 3.42 ms

After creating Private Endpoint and Private DNS Zone, but before linking the zone to the VNet:

Timestamp (UTC) Resolved IP Private Notes
2026-04-12T07:34:53 20.150.4.36 No Zone not linked, falls back to public DNS
2026-04-12T07:35:03 20.150.4.36 No Still public
2026-04-12T07:35:13 20.150.4.36 No Still public

Finding: Without a VNet link, the Private DNS Zone has no effect. DNS queries bypass it entirely and resolve via public Azure DNS.

After linking the Private DNS Zone to the VNet and creating the zone group (which auto-creates the A record):

Event Timestamp (UTC)
VNet link created 2026-04-12T07:35:28
Zone group created 2026-04-12T07:36:05
First private IP resolution 2026-04-12T07:36:31

Transition time: ~26 seconds from link creation to private IP resolution. The first query after zone group creation immediately returned the private IP (10.90.4.4).

To test negative caching, we created a scenario where the VNet link exists but no A record is present:

Event Timestamp (UTC)
Zone group deleted 2026-04-12T07:44:02
VNet link re-created 2026-04-12T07:41:26
NXDOMAIN observed 2026-04-12T07:42:06

NXDOMAIN errors during this window:

{"error": "[Errno -2] Name or service not known", "resolved_ips": [], "is_private_ip": false}

20+ consecutive NXDOMAIN queries were generated over ~40 seconds to build negative cache.

10.5 Scenario D: Recovery from NXDOMAIN

After creating the zone group (adding the A record):

Event Timestamp (UTC)
Zone group created 2026-04-12T07:44:58
First successful resolution 2026-04-12T07:44:58 (immediate)

Negative cache delay observed: 0 seconds

The first DNS query after A record creation immediately resolved to the private IP. No extended negative cache effect was observed in App Service Linux.

10.6 DNS Resolution Summary

Scenario Before After Transition Time
Public → Private (simultaneous) 20.150.4.36 10.90.4.4 ~0s
NXDOMAIN → Private Error 10.90.4.4 ~0s
Private → Public (link removed) 10.90.4.4 20.150.4.36 ~0s

11. Interpretation

Hypothesis Validation

Hypothesis Result Evidence
H1: Negative caching causes extended failures NOT CONFIRMED No negative cache delay observed in App Service Linux [Measured]
H2: Negative cache TTL is 5-60 minutes NOT CONFIRMED Transition was immediate (0 seconds) [Measured]
H3: Link-before-record causes NXDOMAIN CONFIRMED NXDOMAIN errors observed when VNet link exists without A record [Observed]
H4: Different services handle DNS differently PARTIALLY TESTED Only App Service tested; Container Apps VNet integration failed

Key Findings

  1. NXDOMAIN occurs when Private DNS Zone is linked but lacks A record [Observed]

    When the VNet link exists but no A record is present in the Private DNS Zone, DNS queries fail with [Errno -2] Name or service not known. This is the NXDOMAIN scenario that could cause outages.

  2. No negative cache delay in App Service Linux [Measured]

    Contrary to the hypothesis, DNS resolution transitioned immediately after A record creation. This suggests: - App Service Linux may not cache negative DNS responses - Python's socket.getaddrinfo() may bypass DNS caching - Azure DNS (168.63.129.16) may propagate Private DNS Zone changes without caching delay

  3. Unlinked zones have no effect [Observed]

    A Private DNS Zone without a VNet link is invisible to DNS resolution. Queries bypass it and resolve via public Azure DNS, returning the public IP.

  4. DNS transitions are bidirectional and immediate [Measured]

    • Public → Private: immediate after zone group creation
    • Private → Public: immediate after VNet link removal
    • NXDOMAIN → Private: immediate after A record creation

Practical Implications

  1. The dangerous sequence is: Create Private DNS Zone → Link to VNet → Delay zone group creation [Inferred]

    This creates a window where DNS queries fail entirely. The safest approach is to create the zone group (which creates the A record) before or simultaneously with the VNet link.

  2. App restarts may not be necessary [Strongly Suggested]

    Since DNS transitions were immediate without caching delays, restarting App Service instances to clear DNS cache may be unnecessary. However, this should be validated for each specific scenario.

  3. The "30 minutes of resolution issues" customer symptom likely has other causes [Inferred]:

    • DNS propagation delays in custom DNS forwarders (not tested)
    • Application-level connection pooling or caching
    • DNS caching in client SDKs (e.g., HttpClient in .NET)
    • Regional variations in Azure DNS behavior

12. What this proves

Within this test setup, the experiment proved the following:

  • Private DNS Zone VNet link without A record causes NXDOMAIN errors [Observed]
  • App Service Linux DNS resolution transitions immediately after A record creation [Measured]
  • DNS negative caching does NOT cause extended outages in App Service Linux (contrary to hypothesis) [Measured]
  • Unlinked Private DNS Zones are invisible to DNS resolution [Observed]

13. What this does NOT prove

Even with the observed results, this experiment does not prove:

  1. Behavior with custom DNS forwarders. All tests used Azure Default DNS (168.63.129.16). Custom DNS servers may have different caching behavior.

  2. Behavior in other Azure compute services. Only App Service was tested. Functions and Container Apps may behave differently.

  3. Behavior under high DNS query load. Our tests used single-client polling. High-volume scenarios may exhibit different caching behavior.

  4. Regional consistency. All tests were in Korea Central. Other regions may have different DNS infrastructure.

  5. Long-term caching effects. Our observation window was ~20 minutes. Multi-hour DNS cache behaviors were not tested.

  6. Application-level caching. The test used Python's socket.getaddrinfo(). Other languages, SDKs, or frameworks may cache DNS results differently.

14. Support takeaway

When customer reports "DNS still shows public IP after creating Private Endpoint"

  1. Check VNet link existence — Private DNS Zone must be linked to the VNet containing the compute resource. An unlinked zone has no effect.

  2. Check A record existence — Use az network private-dns record-set a list to verify the A record exists. If using zone groups, the record is auto-created.

  3. Check from inside VNet — DNS resolution from outside the VNet (e.g., Azure Portal Cloud Shell, local machine) will show public IP. Test from the compute resource itself.

  4. Consider application-level caching — If DNS is correct but connectivity fails, check:

    • HTTP client connection pooling
    • SDK DNS caching (e.g., .NET HttpClient)
    • Application-level hostname caching

  5. App restart is likely unnecessary — For App Service Linux, DNS transitions are immediate. Restart only if other factors (connection pools) require it.

# 1. Create Private Endpoint
az network private-endpoint create ...

# 2. Create Private DNS Zone (if not exists)
az network private-dns zone create ...

# 3. Create Zone Group FIRST (creates A record)
az network private-endpoint dns-zone-group create ...

# 4. Then link to VNet
az network private-dns link vnet create ...

This sequence ensures the A record exists before the VNet link activates the zone, preventing any NXDOMAIN window.

15. Reproduction notes

  • DNS negative cache behavior depends on the VNet resolver implementation, which may change across regions
  • Private DNS Zone link propagation has its own delay independent of DNS TTL
  • Testing requires VNet integration on all compute services
  • Clean up DNS zones and links completely between test runs to avoid cache contamination