The output to download this is in binary and that's why you are getting this warning. You can do 2 things:
1. Allow curl to show the output
curl "http://mirrors.estointernet.in/apache/kafka/2.1.1/kafka-2.1.1-src.tgz" --output -
But I would not suggest that because the output will be very messy
2. Send the output to a text file:
curl "https://mirrors.estointernet.in/apache/kafka/2.8.1/kafka-2.8.1-src.tgz" --output kaf.txt
Hope this will help.
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