package delivery

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/http"
	"net/url"
	"time"
)

const (
	// dnsResolutionTimeout is the maximum time to wait for
	// DNS resolution during SSRF validation.
	dnsResolutionTimeout = 5 * time.Second
)

// Sentinel errors for SSRF validation.
var (
	errNoHostname = errors.New("URL has no hostname")
	errNoIPs      = errors.New(
		"hostname resolved to no IP addresses",
	)
	errBlockedIP = errors.New(
		"blocked private/reserved IP range",
	)
	errInvalidScheme = errors.New(
		"only http and https are allowed",
	)
)

// blockedNetworks contains all private/reserved IP ranges
// that should be blocked to prevent SSRF attacks.
//
//nolint:gochecknoglobals // package-level network list is appropriate here
var blockedNetworks []*net.IPNet

//nolint:gochecknoinits // init is the idiomatic way to parse CIDRs once at startup
func init() {
	cidrs := []string{
		"127.0.0.0/8",
		"10.0.0.0/8",
		"172.16.0.0/12",
		"192.168.0.0/16",
		"169.254.0.0/16",
		"0.0.0.0/8",
		"100.64.0.0/10",
		"192.0.0.0/24",
		"192.0.2.0/24",
		"198.18.0.0/15",
		"198.51.100.0/24",
		"203.0.113.0/24",
		"224.0.0.0/4",
		"240.0.0.0/4",
		"::1/128",
		"fc00::/7",
		"fe80::/10",
	}

	for _, cidr := range cidrs {
		_, network, err := net.ParseCIDR(cidr)
		if err != nil {
			panic(fmt.Sprintf(
				"ssrf: failed to parse CIDR %q: %v",
				cidr, err,
			))
		}

		blockedNetworks = append(
			blockedNetworks, network,
		)
	}
}

// isBlockedIP checks whether an IP address falls within
// any blocked private/reserved network range.
func isBlockedIP(ip net.IP) bool {
	for _, network := range blockedNetworks {
		if network.Contains(ip) {
			return true
		}
	}

	return false
}

// ValidateTargetURL checks that an HTTP delivery target
// URL is safe from SSRF attacks.
func ValidateTargetURL(
	ctx context.Context, targetURL string,
) error {
	parsed, err := url.Parse(targetURL)
	if err != nil {
		return fmt.Errorf("invalid URL: %w", err)
	}

	err = validateScheme(parsed.Scheme)
	if err != nil {
		return err
	}

	host := parsed.Hostname()
	if host == "" {
		return errNoHostname
	}

	if ip := net.ParseIP(host); ip != nil {
		return checkBlockedIP(ip)
	}

	return validateHostname(ctx, host)
}

func validateScheme(scheme string) error {
	if scheme != "http" && scheme != "https" {
		return fmt.Errorf(
			"unsupported URL scheme %q: %w",
			scheme, errInvalidScheme,
		)
	}

	return nil
}

func checkBlockedIP(ip net.IP) error {
	if isBlockedIP(ip) {
		return fmt.Errorf(
			"target IP %s is in a blocked "+
				"private/reserved range: %w",
			ip, errBlockedIP,
		)
	}

	return nil
}

func validateHostname(
	ctx context.Context, host string,
) error {
	dnsCtx, cancel := context.WithTimeout(
		ctx, dnsResolutionTimeout,
	)
	defer cancel()

	ips, err := net.DefaultResolver.LookupIPAddr(
		dnsCtx, host,
	)
	if err != nil {
		return fmt.Errorf(
			"failed to resolve hostname %q: %w",
			host, err,
		)
	}

	if len(ips) == 0 {
		return fmt.Errorf(
			"hostname %q: %w", host, errNoIPs,
		)
	}

	for _, ipAddr := range ips {
		if isBlockedIP(ipAddr.IP) {
			return fmt.Errorf(
				"hostname %q resolves to blocked "+
					"IP %s: %w",
				host, ipAddr.IP, errBlockedIP,
			)
		}
	}

	return nil
}

// NewSSRFSafeTransport creates an http.Transport with a
// custom DialContext that blocks connections to
// private/reserved IP addresses.
func NewSSRFSafeTransport() *http.Transport {
	return &http.Transport{
		DialContext: ssrfDialContext,
	}
}

func ssrfDialContext(
	ctx context.Context,
	network, addr string,
) (net.Conn, error) {
	host, port, err := net.SplitHostPort(addr)
	if err != nil {
		return nil, fmt.Errorf(
			"ssrf: invalid address %q: %w",
			addr, err,
		)
	}

	ips, err := net.DefaultResolver.LookupIPAddr(
		ctx, host,
	)
	if err != nil {
		return nil, fmt.Errorf(
			"ssrf: DNS resolution failed for %q: %w",
			host, err,
		)
	}

	for _, ipAddr := range ips {
		if isBlockedIP(ipAddr.IP) {
			return nil, fmt.Errorf(
				"ssrf: connection to %s (%s) "+
					"blocked: %w",
				host, ipAddr.IP, errBlockedIP,
			)
		}
	}

	var dialer net.Dialer

	return dialer.DialContext(
		ctx, network,
		net.JoinHostPort(ips[0].IP.String(), port),
	)
}