Today I Learned: 08/12/2022 - /dev/tty errors using scp in Docker

Publish date: 8 Dec 2022

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I hit an unexpected error today while using scp in a script executing in a Docker container - an inability to use a tty. This was correct in that I wasn’t running Docker with the interactive or terminal flags (commonly used as -it). The relevant -vv output from scp was:

debug1: read_passphrase: can’t open /dev/tty: No such device or address
Host key verification failed.
lost connection

What confused me is that I was explicitly providing a key file without a passphrase.

Some headscratching later, I found the error was a little misleading - scp did need a tty, but not to ask for a passphrase. It wanted permission to override the existing host key details of my remote server in the ~/.ssh/known-hosts file. Since my Docker container was intended to be removed after each run, this was most easily fixed by telling scp not to check the key, by adding

-o StrictHostKeyChecking=no

to the arguments.