An awk script that provides the same functionality as ssh-keygen -F <host>

findHostName.sh

#!/usr/bin/env bash

host=$1

awk -v host=$host '
/^|1/ {
  split($1, a, /\|/)
  key=a[3]
  hmac=a[4]

  "echo -n " host " | openssl sha1 -mac HMAC -macopt hexkey:$(echo " key " | base64 -d | xxd -p) -binary | base64" | getline myhmac

  if (hmac == myhmac) {
    print "# Host " host " found: line " NR
    print
  }
}' ~/.ssh/known_hosts

findHostName2.sh

#!/usr/bin/env bash

host=$1

awk -v host=$host '
function pad_key(pad,                                         i, val) {
  for (i = 0; i < 64; i++) {
    val = sprintf("%s%02x", val, xor(strtonum("0x" substr(hexkey, i*2+1, 2)), pad))
  }

  return val
}

/^|1/ {
  split($1, a, /\|/)
  key=a[3]
  hmac=a[4]

  "echo " key " | base64 -d | xxd -p " | getline hexkey
  
  o_key_pad = pad_key(0x5c)
  i_key_pad = pad_key(0x36)
  "bash -c '\''cat <(echo -n " o_key_pad " | xxd -r -p) <(cat <(echo -n " i_key_pad " | xxd -r -p) <(echo -n " host ") | sha1sum | xxd -r -p) | sha1sum | xxd -r -p | base64'\''" | getline myhmac

  if (hmac == myhmac) {
    print "# Host " host " found: line " NR
    print
  }
}' ~/.ssh/known_hosts