package io

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/url"
	"strings"
)

// ErrSSRF is returned when a requested URL targets a loopback, private or
// link-local address or a localhost hostname that is not on the allowlist.
// Use errors.Is to test for it.
var ErrSSRF = errors.New("io: disallowed url")

// validateRequestURL parses rawURL and refuses SSRF-prone targets: it returns an
// error wrapping ErrSSRF for loopback/private/link-local/unspecified addresses
// and for localhost hostnames, unless the host (or hostname) is on the explicit
// allowlist. A hostname that resolves to any denied address is also refused.
func validateRequestURL(ctx context.Context, rawURL string, allowed []string) (*url.URL, error) {
	if strings.TrimSpace(rawURL) == "" {
		return nil, fmt.Errorf("io: url is required")
	}
	u, err := url.Parse(rawURL)
	if err != nil {
		return nil, fmt.Errorf("io: parse url: %w", err)
	}
	if u.Scheme != "http" && u.Scheme != "https" {
		return nil, fmt.Errorf("io: unsupported scheme %q", u.Scheme)
	}
	host := hostOnly(u.Host)
	if host == "" {
		return nil, fmt.Errorf("io: url has no host")
	}
	if isHostAllowed(host, allowed) {
		return u, nil
	}
	if ip := net.ParseIP(host); ip != nil {
		if isDeniedIP(ip) {
			return nil, fmt.Errorf("%w: %s", ErrSSRF, host)
		}
		return u, nil
	}
	addrs, err := net.DefaultResolver.LookupHost(ctx, host)
	if err != nil {
		return nil, fmt.Errorf("io: resolve %q: %w", host, err)
	}
	for _, a := range addrs {
		if ip := net.ParseIP(a); ip != nil && isDeniedIP(ip) {
			return nil, fmt.Errorf("%w: %q resolves to %q", ErrSSRF, host, a)
		}
	}
	return u, nil
}

// isDeniedIP reports whether ip is a loopback, private, link-local, multicast or
// unspecified address — the targets SSRF protection refuses by default.
func isDeniedIP(ip net.IP) bool {
	if ip == nil {
		return false
	}
	return ip.IsLoopback() ||
		ip.IsPrivate() ||
		ip.IsLinkLocalUnicast() ||
		ip.IsLinkLocalMulticast() ||
		ip.IsUnspecified()
}

// isHostAllowed reports whether the host (with any port stripped) matches an
// entry on the allowlist.
func isHostAllowed(host string, allowed []string) bool {
	h := strings.ToLower(hostOnly(host))
	if h == "" {
		return false
	}
	for _, a := range allowed {
		if strings.EqualFold(hostOnly(a), h) {
			return true
		}
	}
	return false
}

// hostOnly strips the port (and any IPv6 brackets) from a host:port string.
func hostOnly(hostport string) string {
	h := strings.TrimSpace(hostport)
	if h == "" {
		return ""
	}
	if strings.HasPrefix(h, "[") {
		if j := strings.Index(h, "]"); j >= 0 {
			return h[1:j]
		}
		return h
	}
	if i := strings.LastIndex(h, ":"); i >= 0 {
		return h[:i]
	}
	return h
}