#!/usr/bin/env bash
#
# JbTecWiz Support Centre -- generated fix script
#
# Fault : sshd will not start, or refuses connections after a configuration change
# Fix   : Fix the configuration sshd is rejecting
# Source: https://jbtecwiz.com/support/lnx-net-sshd-config
#
# Run as     : Console or recovery access
# Expect     : 20 minutes
# Risk       : medium
# Reversible : yes
#
# WHEN THIS IS THE RIGHT FIX
#   sshd will not start at all.
#
# HOW TO UNDO IT
#   Keep a copy: sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak
#   before editing.
#
# 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

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 $n of 5"
  prose "$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 "  sshd will not start, or refuses connections after a configuration change"
echo "  Fix the configuration sshd is rejecting"
echo
echo "  Risk: medium   Reversible   20 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 'Read the exact complaint.' '' cmd 'sudo sshd -t' 'sudo journalctl -u sshd -n 40 --no-pager'; then
  sudo sshd -t
  sudo journalctl -u sshd -n 40 --no-pager
  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 'Check for host keys -- a rebuilt or cloned machine may have none, and sshd will not start without them.' 'ssh-keygen -A generates any missing host key types with the correct permissions. Note that new host keys will make every client warn about a changed key, which is expected here and not an attack.' cmd 'ls -l /etc/ssh/ssh_host_*' 'sudo ssh-keygen -A'; then
  ls -l /etc/ssh/ssh_host_*
  sudo ssh-keygen -A
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 2 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 3 'Check permissions, which sshd enforces strictly and refuses to start without.' '' cmd 'sudo chmod 600 /etc/ssh/ssh_host_*_key' 'sudo chmod 644 /etc/ssh/ssh_host_*_key.pub'; then
  sudo chmod 600 /etc/ssh/ssh_host_*_key
  sudo chmod 644 /etc/ssh/ssh_host_*_key.pub
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 3 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 4 'Watch for directives placed after a Match block. Everything following a Match applies only to that match until the next one or the end of the file.' 'This catches the most confusing sshd fault there is: a setting that is present in the file, spelled correctly, and simply not applying because of where it sits.' cmd 'sudo grep -n -E '\''^\s*(Match|PermitRootLogin|PasswordAuthentication|AllowUsers)'\'' /etc/ssh/sshd_config'; then
  sudo grep -n -E '^\s*(Match|PermitRootLogin|PasswordAuthentication|AllowUsers)' /etc/ssh/sshd_config
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 4 failed. The rest of the fix may depend on it." >&2
  fi
fi

if step 5 'Also check the drop-in directory, which overrides the main file on modern distributions.' '' cmd 'ls -l /etc/ssh/sshd_config.d/' 'sudo sshd -T | grep -E '\''permitrootlogin|passwordauthentication|pubkeyauthentication'\'''; then
  ls -l /etc/ssh/sshd_config.d/
  sudo sshd -T | grep -E 'permitrootlogin|passwordauthentication|pubkeyauthentication'
  if [ $? -ne 0 ]; then
    failed=$((failed+1))
    echo "  Step 5 failed. The rest of the fix may depend on it." >&2
  fi
fi

rule "  Confirm it worked"
prose 'sshd -t is silent and sshd -T shows the effective settings you intended.'
if [ "$DRYRUN" = "0" ]; then
  sudo sshd -t && sudo sshd -T | grep -E 'port|permitrootlogin|passwordauthentication'
fi

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-net-sshd-config"
else
  echo "  Finished."
fi
echo
prose 'To undo: Keep a copy: sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak before editing.'
rule