istio.io/content/en/about/faq/security/verify-mtls-encryption.md

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How can I verify that traffic is using mutual TLS encryption?	25

If you installed Istio with values.global.proxy.privileged=true, you can use tcpdump to determine encryption status. Also in Kubernetes 1.23 and later, as an alternative to installing Istio as privileged, you can use kubectl debug to run tcpdump in an ephemeral container. See Istio mutual TLS migration for instructions.