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The Problem & The Question

I'm trying to create an AWS ElastiCache Redis-flavored cluster and connect to it from an instance in the same VPC. When I create the cluster initially, I can connect to it just fine via redis-cli. If I wait a day or two and then try to connect to it again via redis-cli, I get the following error:

$ redis-cli -h <aws-elasticache-cluster-primary-endpoint>
Could not connect to Redis at <aws-elasticache-cluster-primary-endpoint>:6379: Name or service not known
not connected>

I should be able to connect again with no issue. Why would I be getting DNS errors the next day when it worked initially? Nothing has changed about the cluster since I created it. How could I fix this?

Steps to Reproduce

  1. ElastiCache Dashboard -> Redis -> Create.

  2. Set the following options:

    Option Value
    Cluster Engine Redis
    Location Amazon Cloud
    Engine version compatibility 6.x
    Port 6379
    Parameter group default.redis6.x
    Node type cache.t3.micro
    Number of replicas 0
    Multi-AZ false
    Subnet group default
    Security groups default
    Encryption at-rest false
    Encryption in-transit false

  3. Create.

  4. After creating, verify you can connect to the cluster with:

    redis-cli -h <aws-elasticache-cluster-primary-endpoint>

  5. Disconnect.

  6. Try reconnecting with the same redis-cli command a few days later. You should see the errors shown in the section above.

Additional Information

nping

$ sudo nping --tcp -p 6379 <aws-elasticache-cluster-primary-endpoint>
Failed to resolve given hostname/IP: <aws-elasticache-cluster-primary-endpoint>.  Note that you can't use '/mask' AND '1-4,7,100-' style IP ranges
Cannot find a valid target. Please make sure the specified hosts are either IP addresses in standard notation or hostnames that can be resolved with DNS

nslookup

$ sudo nslookup <aws-elasticache-cluster-primary-endpoint>
Server:         75.75.75.75
Address:        75.75.75.75#53

** server can't find <aws-elasticache-cluster-primary-endpoint>: REFUSED

Reachability Analyzer

I followed AWS' instructions for testing connectivity from my EC2 instance to the ElastiCache cluster. The results I got back for the connectivity were:

Reachability Status State
Reachable Succeeded

Cluster Metrics

Metric Value
CPU Utilization ~1.000%
Engine CPU Utilization ~0.283%
Database Memory Usage Percentage ~1.093%

Local Wireguard configuration

[Interface]
PrivateKey = <value>
ListenPort = 21841
Address = 10.0.0.2/32
DNS = 9.9.9.9 # 75.75.75.75 75.75.76.76

[Peer]
PublicKey = <value>
AllowedIPs = 0.0.0.0/0, ::/0
Endpoint = <public-IP>:51820
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1 Answer 1

Reset to default
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It could be due to a "VPC DNS throttling", you can find more information on the official documentation.

https://aws.amazon.com/premiumsupport/knowledge-center/vpc-find-cause-of-failed-dns-queries/

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  • It can also be an issue if the OP is using some form of third-party DNS resolver, such as Microsoft AD Domain Controller (i.e. if the environment relies on AD or LDAP). Generally, public DNS services such as Google or Cloudflare are going to be good, but a misconfigured localdomain DNS host can cause tons of problems.
    – maplebird
    Apr 4, 2021 at 5:50

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