There are differences in DNS errors returned by Ubuntu 18.04 and 20.04 . Namely, when running the following test:

TestClientSideEncryptionProse/7._custom_endpoint 

See here for an example: https://parsley.mongodb.com/test/mongo_go_driver_tests_42_plus_zlib_zstd_support__version~latest_os_ssl_40~ubuntu2004_64_go_1_20_test_replicaset_auth_nossl_patch_3efbd23a061a4069af96e56b894f4f72bcd51999_64cd9b4cd6d80a3c8e5e519e_23_08_05_00_44_12/0/08d34aefd7aaf227df0beac7993d28c6?bookmarks=0,16745,23928&shareLine=16551

The "no such hosts" error returned by 18.04 is expected , but when upgrading to 20.04 you get the following:

expected error dial tcp: lookup doesnotexist.invalid on 127.0.0.53:53: server misbehaving to contain substring 'no such host' 

This still comes from the Go DNS Lookup, here. 

Definition of done: Identify the reason why a different DNS results with different Ubuntu versions. Determine if the "misbehaving" error is masking a bug in the Go Driver.

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Additionally, the following test results in different timeout errors betwen RHEL 8.7 and all other OS:

TestClientSideEncryptionProse/18._Azure_IMDS_Credentials/Case_6:_Slow_Response 

Here is an example:

https://parsley.mongodb.com/test/mongo_go_driver_tests_latest_zlib_zstd_support__version~latest_os_ssl_40~rhel87_64_go_1_20_test_replicaset_auth_nossl_patch_7fa4f6d1f2fb3725118a8084ee19c7c77fa9bc13_65b972f732f417bd50811a62_24_01_30_22_07_01/0/5ff6b7bd9bfb2d35c7e9dd2998509463?shareLine=15031

Both come from the "send" net/http method. The Go version also seems to matter here, versions older than 1.20 will result in the deadline exceeded error.