#!/usr/bin/env bash
#
# JbTecWiz Support Centre -- generated fix script
#
# Fault : "Host key verification failed" / REMOTE HOST IDENTIFICATION HAS CHANGED
# Fix   : Verify the fingerprint before trusting it
# Source: https://jbtecwiz.com/support/lnx-ssh-hostkey
#
# Run as     : Shell
# Expect     : 15 minutes
# Risk       : low
# Reversible : yes
#
# WHEN THIS IS THE RIGHT FIX
#   You cannot account for the change. Do this before removing anything --
#   the warning is doing its job.
#
# HOW TO UNDO IT
#   None -- this is a verification step.
#
# Walks the fix one step at a time and asks before each. Steps with no
# command are yours to do -- it prints those and waits. DRYRUN=1 prints
# without executing; UNATTENDED=1 does not ask.
#
# --------------------------------------------------------------------
# NO WARRANTY - USE AT YOUR OWN RISK
#
# This script is provided by JbTecWiz as-is and with no warranty of any
# kind, express or implied. You run it entirely at your own risk.
#
# JbTecWiz accepts no liability for any loss or damage arising from its
# use, including but not limited to data loss, downtime, or configuration
# changes that turn out to be wrong for your system.
#
# You are responsible for reading this script before running it, for
# satisfying yourself that it suits the machine in front of you, and for
# having a working backup first. Some steps cannot be undone.
# --------------------------------------------------------------------

set -uo pipefail
DRYRUN="${DRYRUN:-0}"
UNATTENDED="${UNATTENDED:-0}"
failed=0

# - THIS SCRIPT WILL NOT RUN UNTIL YOU EDIT IT -
cat >&2 <<'EOF'

  This script needs editing before it can run.
  Replace each of these with a real value:
    example.com   (a placeholder domain)

  Then delete this block near the top of the script.

EOF
exit 2

rule() { printf "\n%s\n" "$(printf '-%.0s' $(seq 1 70))"; if [ $# -gt 0 ]; then echo "$1"; fi; }
prose() { echo "$1" | fold -s -w 74 | sed "s/^/  /"; }

# Returns 0 when the caller should run the command, 1 when it should not.
# A manual step always returns 1 -- there is nothing for the caller to run.
step() {  # step <n> <do> <why> <manual|cmd> [command lines...]
  local n="$1" dotext="$2" why="$3" mode="$4"; shift 4
  rule "  Step $n of 4"
  prose "$dotext"
  if [ -n "$why" ]; then echo; prose "$why"; fi
  if [ "$mode" = "manual" ]; then
    echo; echo "  -> Do this yourself, then press Enter to carry on."
    if [ "$UNATTENDED" = "0" ] && [ "$DRYRUN" = "0" ]; then read -r _; fi
    return 1
  fi
  echo; printf "    %s\n" "$@"; echo
  if [ "$DRYRUN" = "1" ]; then echo "  (dry run -- not executed)"; return 1; fi
  if [ "$UNATTENDED" = "0" ]; then
    read -r -p "  Run this step? [Y]es / [S]kip / [Q]uit " a
    case "$a" in
      [Qq]*) echo "  Stopped at your request."; exit 0 ;;
      [Ss]*) echo "  Skipped."; return 1 ;;
    esac
  fi
  return 0
}

rule
echo "  "Host key verification failed" / REMOTE HOST IDENTIFICATION HAS CHANGED"
echo "  Verify the fingerprint before trusting it"
echo
echo "  Risk: low   Reversible   15 minutes"
echo
prose 'No warranty. Use at your own risk - JbTecWiz accepts no liability. Read it before you run it, and have a backup.'
rule
echo
if [ "$UNATTENDED" = "0" ] && [ "$DRYRUN" = "0" ]; then
  read -r -p "  Ready? [y/N] " go
  case "$go" in [Yy]*) ;; *) echo "  Nothing was changed."; exit 0;; esac
fi

if step 1 'Get the fingerprint the server is actually presenting.' '' cmd 'ssh-keyscan -t ed25519 server.example.com 2>/dev/null | ssh-keygen -lf -'; then
  ssh-keyscan -t ed25519 server.example.com 2>/dev/null | ssh-keygen -lf -
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 1 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 2 'Get the real fingerprint from the server itself, through a channel that is not SSH -- a console, a cloud provider'\''s serial output, or a colleague standing at it.' 'Comparing the presented fingerprint against one you fetched over the same suspect connection proves nothing. It has to come from somewhere else.' cmd 'sudo ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub'; then
  sudo ssh-keygen -lf /etc/ssh/ssh_host_ed25519_key.pub
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 2 failed. The rest of the fix may depend on it." >&2
  fi
fi

step 3 'If they match, the change is legitimate -- proceed to remove the old entry.' '' manual || true

step 4 'If they do not match, stop. Do not connect, do not type a password, and investigate the network path.' '' manual || true

rule "  Confirm it worked"
prose 'The two fingerprints, obtained independently, are identical.'

rule
if [ "$failed" -gt 0 ]; then
  echo "  Finished with $failed failed step(s)."
  echo "  Read the full write-up at https://jbtecwiz.com/support/lnx-ssh-hostkey"
else
  echo "  Finished."
fi
echo
prose 'To undo: None -- this is a verification step.'
rule