I realize it's a one-off, but you could potentially mix lines of output with a script like that.

If you gnu parallel (xargs++) installed - turn that second line into a script (and replace $line with $1).

  cat domains.txt | parallel -n 1 -P 50 script.sh

Notice that with parallel, you don't need the -n 1, since that's the default.

Ah, didn't realize that - throwback to my xargs days! Thanks for the tip.

Thanks for the note on parallel, it looks to be an excellently useful tool. I can find a lot of ways to save time with it.

FYI: On OSX, there is a Homebrew formula (brew install parallel).

If you are looking for which domain point to 1.2.3.4:

   cat domains.txt | parallel -P 100 --tag host | grep 1.2.3.4

Somewhat slower runtime than the go-solution, but may be faster to write.

You don't need to spawn N awks,

  <domains.txt while read line; do
    host $line | head -n1 &
  done | awk '{print $1 " -> " $4}'