#!/bin/bash
################################################################################
# Script Name: add-fail2ban-head-crawler.sh
# Version: 1.0
# Description: Adds a Fail2ban jail to block HEAD-only crawlers — bots that
#              systematically send HEAD requests with no referer to probe or
#              index your site while spoofing real browser user agents.
#
# Author:  Phil Connor
# Contact: contact@mylinux.work
# Website: https://mylinux.work
# License: MIT
#
# Usage:
#   sudo ./add-fail2ban-head-crawler.sh
#   sudo ./add-fail2ban-head-crawler.sh --logpath /var/log/nginx/access.log
#   sudo ./add-fail2ban-head-crawler.sh --maxretry 10
#   sudo ./add-fail2ban-head-crawler.sh --dry-run
#
################################################################################

set -euo pipefail

# ============================================================================
# DEFAULTS
# ============================================================================

readonly VERSION="1.0"
readonly SCRIPT_NAME="${0##*/}"

LOGPATH="auto"
BANTIME="86400"
MAXRETRY="5"
FINDTIME="300"
DRY_RUN=false

# Colors
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
CYAN='\033[0;36m'
NC='\033[0m'

# ============================================================================
# HELPER FUNCTIONS
# ============================================================================

log_info()  { echo -e "${GREEN}[INFO]${NC} $*"; }
log_warn()  { echo -e "${YELLOW}[WARN]${NC} $*"; }
log_error() { echo -e "${RED}[ERROR]${NC} $*" >&2; }
log_step()  { echo -e "${CYAN}[STEP]${NC} $*"; }

show_usage() {
    cat <<EOF
Usage: sudo $SCRIPT_NAME [OPTIONS]

Adds a HEAD-crawler blocking jail to an existing Fail2ban installation.
Catches bots that systematically send HEAD requests with no referer —
a common pattern for scrapers and SEO tools that spoof real browser
user agents to avoid detection.

OPTIONS:
    --logpath PATH      Access log path (default: auto-detect)
    --bantime SECS      Ban duration in seconds (default: 86400 / 24 hours)
    --maxretry NUM      HEAD requests before ban (default: 5)
    --findtime SECS     Window for counting requests (default: 300 / 5 min)
    --dry-run           Show what would be done without making changes
    --remove            Remove the filter and jail
    -h, --help          Show this help message

WHAT IT CATCHES:
    - HEAD requests with no referer ("-")
    - Bots rotating through AWS/cloud IPs with spoofed user agents
    - SEO crawlers, uptime probes, and content scrapers
    - Any automated tool doing HEAD-only reconnaissance

WHAT IT IGNORES:
    - Normal GET/POST traffic (even with no referer)
    - HEAD requests with a valid referer (browser prefetch)
    - Single HEAD requests (below maxretry threshold)

EXAMPLES:
    # Install with defaults (ban after 5 HEAD requests in 5 min)
    sudo $SCRIPT_NAME

    # Stricter: ban after 3 requests in 2 minutes
    sudo $SCRIPT_NAME --maxretry 3 --findtime 120

    # More lenient: ban after 15 requests in 10 minutes
    sudo $SCRIPT_NAME --maxretry 15 --findtime 600

    # Preview without changes
    sudo $SCRIPT_NAME --dry-run

    # Remove the jail
    sudo $SCRIPT_NAME --remove

EOF
    exit 0
}

# ============================================================================
# ARGUMENT PARSING
# ============================================================================

REMOVE=false

parse_args() {
    while [[ $# -gt 0 ]]; do
        case "$1" in
            --logpath)   LOGPATH="$2"; shift 2 ;;
            --bantime)   BANTIME="$2"; shift 2 ;;
            --maxretry)  MAXRETRY="$2"; shift 2 ;;
            --findtime)  FINDTIME="$2"; shift 2 ;;
            --dry-run)   DRY_RUN=true; shift ;;
            --remove)    REMOVE=true; shift ;;
            -h|--help)   show_usage ;;
            *)           log_error "Unknown option: $1"; show_usage ;;
        esac
    done
}

# ============================================================================
# CHECKS
# ============================================================================

check_root() {
    if [[ $EUID -ne 0 ]]; then
        log_error "This script must be run as root (sudo)"
        exit 1
    fi
}

check_fail2ban() {
    if ! command -v fail2ban-client &>/dev/null; then
        log_error "Fail2ban is not installed"
        log_error "Install it first: https://mylinux.work/guides/fail2ban-setup/"
        exit 1
    fi

    if ! systemctl is-active --quiet fail2ban; then
        log_error "Fail2ban is not running"
        exit 1
    fi

    log_info "Fail2ban is installed and running"
}

detect_logpath() {
    if [[ "$LOGPATH" != "auto" ]]; then
        # shellcheck disable=SC2086
        local matches=( $LOGPATH )
        if [[ ${#matches[@]} -eq 0 || ! -f "${matches[0]}" ]]; then
            log_error "Log file not found: $LOGPATH"
            exit 1
        fi
        log_info "Using specified log path: $LOGPATH"
        return
    fi

    log_step "Auto-detecting web server access log..."

    # HestiaCP — apache domains
    local hestia_apache=( /var/log/apache2/domains/*.log )
    if [[ -f "${hestia_apache[0]:-}" ]]; then
        LOGPATH="/var/log/apache2/domains/*.log"
        log_info "Detected HestiaCP apache: $LOGPATH"
        return
    fi

    # HestiaCP — nginx domains
    local hestia_nginx=( /var/log/nginx/domains/*.log )
    if [[ -f "${hestia_nginx[0]:-}" ]]; then
        LOGPATH="/var/log/nginx/domains/*.log"
        log_info "Detected HestiaCP nginx: $LOGPATH"
        return
    fi

    # Nginx (standard)
    if [[ -f /var/log/nginx/access.log ]]; then
        LOGPATH="/var/log/nginx/access.log"
        log_info "Detected nginx: $LOGPATH"
        return
    fi

    # Apache (Debian/Ubuntu)
    if [[ -f /var/log/apache2/access.log ]]; then
        LOGPATH="/var/log/apache2/access.log"
        log_info "Detected apache2: $LOGPATH"
        return
    fi

    # Apache (RHEL/Rocky)
    if [[ -f /var/log/httpd/access_log ]]; then
        LOGPATH="/var/log/httpd/access_log"
        log_info "Detected httpd: $LOGPATH"
        return
    fi

    log_error "Could not auto-detect access log. Use --logpath to specify."
    exit 1
}

# ============================================================================
# REMOVE
# ============================================================================

do_remove() {
    local filter_file="/etc/fail2ban/filter.d/head-crawler.conf"
    local jail_file="/etc/fail2ban/jail.d/head-crawler.conf"

    log_step "Removing HEAD crawler jail..."

    if $DRY_RUN; then
        log_info "[DRY RUN] Would remove $filter_file"
        log_info "[DRY RUN] Would remove $jail_file"
        log_info "[DRY RUN] Would reload fail2ban"
        return
    fi

    if [[ -f "$jail_file" ]]; then
        rm -f "$jail_file"
        log_info "Removed: $jail_file"
    else
        log_warn "Jail config not found: $jail_file"
    fi

    if [[ -f "$filter_file" ]]; then
        rm -f "$filter_file"
        log_info "Removed: $filter_file"
    else
        log_warn "Filter not found: $filter_file"
    fi

    fail2ban-client reload
    sleep 2
    log_info "Fail2ban reloaded — head-crawler jail removed"
    exit 0
}

# ============================================================================
# INSTALL FILTER
# ============================================================================

install_filter() {
    local filter_file="/etc/fail2ban/filter.d/head-crawler.conf"

    log_step "Installing filter: $filter_file"

    if $DRY_RUN; then
        log_info "[DRY RUN] Would create $filter_file"
        echo ""
        generate_filter
        echo ""
        return
    fi

    if [[ -f "$filter_file" ]]; then
        log_warn "Filter already exists — backing up to ${filter_file}.bak"
        cp "$filter_file" "${filter_file}.bak"
    fi

    generate_filter > "$filter_file"
    log_info "Filter installed: $filter_file"
}

generate_filter() {
    cat <<'EOF'
# Fail2ban filter to block HEAD-only crawlers
# https://mylinux.work
#
# Catches bots that send HEAD requests with no referer. These are typically
# scrapers, SEO tools, or reconnaissance bots that spoof real browser user
# agents and rotate through cloud IPs to avoid detection.
#
# The filter matches:
#   - HTTP HEAD method
#   - No referer (logged as "-")
#   - Any user agent (spoofed or otherwise)
#
# Combined with a low maxretry (default: 5 in 5 min), this catches
# systematic crawlers while ignoring occasional legitimate HEAD requests
# (browser prefetch, monitoring probes).

[Definition]

# HEAD request with no referer — combined log format
# Format: IP - - [date] "HEAD /path HTTP/x.x" status size "-" "user agent"
failregex = ^<HOST> \S+ \S+ \[.*\] "HEAD \S+ \S+" \d+ \d+ "-" ".*"

ignoreregex =

# Author: Phil Connor — https://mylinux.work
EOF
}

# ============================================================================
# INSTALL JAIL
# ============================================================================

install_jail() {
    local jail_file="/etc/fail2ban/jail.d/head-crawler.conf"

    log_step "Installing jail: $jail_file"

    if $DRY_RUN; then
        log_info "[DRY RUN] Would create $jail_file"
        echo ""
        generate_jail
        echo ""
        return
    fi

    if [[ -f "$jail_file" ]]; then
        log_warn "Jail config already exists — backing up to ${jail_file}.bak"
        cp "$jail_file" "${jail_file}.bak"
    fi

    generate_jail > "$jail_file"
    log_info "Jail config installed: $jail_file"
}

generate_jail() {
    cat <<EOF
# Fail2ban jail to block HEAD-only crawlers
# https://mylinux.work
#
# Bans IPs that send multiple HEAD requests with no referer.
# Default: 5 HEAD requests in 5 minutes = 24 hour ban.
#
# This catches scrapers and SEO bots that spoof real browser
# user agents and rotate through cloud provider IPs.

[head-crawler]
enabled  = true
port     = http,https
filter   = head-crawler
logpath  = $LOGPATH
maxretry = $MAXRETRY
findtime = $FINDTIME
bantime  = $BANTIME

# Author: Phil Connor — https://mylinux.work
EOF
}

# ============================================================================
# RELOAD AND VERIFY
# ============================================================================

reload_fail2ban() {
    log_step "Reloading Fail2ban..."

    if $DRY_RUN; then
        log_info "[DRY RUN] Would reload fail2ban"
        return
    fi

    if ! fail2ban-client --test 2>/dev/null; then
        log_warn "Config test not available — reloading directly"
    fi

    fail2ban-client reload
    sleep 2

    if systemctl is-active --quiet fail2ban; then
        log_info "Fail2ban reloaded successfully"
    else
        log_error "Fail2ban failed to restart — check: journalctl -u fail2ban"
        exit 1
    fi
}

verify_jail() {
    log_step "Verifying head-crawler jail..."

    if $DRY_RUN; then
        log_info "[DRY RUN] Would verify jail status"
        return
    fi

    echo ""
    if fail2ban-client status head-crawler 2>/dev/null; then
        echo ""
        log_info "HEAD crawler jail is active and monitoring $LOGPATH"
    else
        log_error "Jail 'head-crawler' is not running — check: fail2ban-client status"
        log_error "Debug with: fail2ban-regex $LOGPATH /etc/fail2ban/filter.d/head-crawler.conf"
        exit 1
    fi
}

test_against_logs() {
    if $DRY_RUN; then
        # shellcheck disable=SC2086
        local matches=( $LOGPATH )
        if [[ -f "${matches[0]}" ]]; then
            log_step "Testing filter against existing logs..."
            echo ""
            fail2ban-regex "${matches[0]}" /dev/stdin <<'FILTER' 2>&1 | tail -5
[Definition]
failregex = ^<HOST> \S+ \S+ \[.*\] "HEAD \S+ \S+" \d+ \d+ "-" ".*"
ignoreregex =
FILTER
            echo ""
        fi
    fi
}

# ============================================================================
# MAIN
# ============================================================================

main() {
    parse_args "$@"

    echo ""
    echo "============================================"
    echo "  Fail2ban HEAD Crawler Blocker v${VERSION}"
    echo "  https://mylinux.work"
    echo "============================================"
    echo ""

    check_root
    check_fail2ban

    if $REMOVE; then
        do_remove
    fi

    detect_logpath
    test_against_logs
    install_filter
    install_jail
    reload_fail2ban
    verify_jail

    echo ""
    echo "============================================"
    echo "  Setup Complete"
    echo "============================================"
    echo ""
    echo "  Jail:      head-crawler"
    echo "  Log:       $LOGPATH"
    echo "  Ban time:  ${BANTIME}s ($(( BANTIME / 3600 ))h)"
    echo "  Max retry: $MAXRETRY (HEAD requests before ban)"
    echo "  Find time: ${FINDTIME}s ($(( FINDTIME / 60 ))m window)"
    echo ""
    echo "  Useful commands:"
    echo "    fail2ban-client status head-crawler"
    echo "    fail2ban-client set head-crawler unbanip <IP>"
    echo "    fail2ban-regex $LOGPATH /etc/fail2ban/filter.d/head-crawler.conf"
    echo ""
}

main "$@"
