I have a task migrate Elasticsearch from v7 to v8. I'd like new cluster add old cluster as a remote cluster. A cluster Elasticsearch v8 require enable https ssl/tls, however a cluster Elasticsearch v7 in prod environment disabled ssl/tls. I research and read the document just connect between trust 2 CA cert each other, the problem is remote cluster is not able to have any certs.
What should I do in this case, thank you everyone
I don't think you have any other option besides enabling SSL/TLS in your v7 cluster.
The documentation is pretty clear about it.
To use cross-cluster replication or cross-cluster search safely with remote clusters, enable security on all connected clusters and configure Transport Layer Security (TLS) on every node.
Thank you for your advance, but I can not enable that because this cluster is running on production, the action will cause down the entire of application services 
@Huy_Nguyen Then I would suggest using Snapshot and Restore between the 2 clusters. CRR requires a secure connection i.e. TLS / SSL.
The requirement to use TLS for cross cluster connections has not changed between v7 and v8.
If you have security enabled on a v7 cluster you would need to enable TLS to support CCS/CCR.
So, as mentioned you could use snapshots, but this also may have issues, first you need a snapshot that would be accessible by both cluster, ideally you would use a cloud storage snapshot on AWS, GCP or Azure, if you do not have a snapshot configured you would need to install the snapshot repository plugin and do a rolling restart of your nodes.
Another option is to use logstash to transfer the data, you would use an elasticsearch input consuming from your v7 cluster and an elasticsearch output sending to your v8 cluster.
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