// Package digip runs "dig" to find the public IP address of the local host.
//
// Each function sends a DNS query directly to a third-party name server that
// returns the source IP address of the query. Because the query does not go
// through the local DNS resolver, the answer is the public IP address of the
// host (or its NAT gateway), not the IP address of the local resolver.
//
// The "dig" executable must be in PATH.
package digip

import (
	"context"
	"fmt"
	"net"
	"os/exec"
	"strings"
)

// Version is a dig flag that selects IPv4 or IPv6.
type Version string

const (
	// V4 is the dig flag for IPv4.
	V4 = "-4"
	// V6 is the dig flag for IPv6.
	V6 = "-6"
)

// Record is a DNS record type to ask for.
type Record string

const (
	// A is the DNS record type for an IPv4 address.
	A = "A"
	// AAAA is the DNS record type for an IPv6 address.
	AAAA = "AAAA"
)

const (
	// googleDomain is a hostname that resolves to the client's IP address.
	// A TXT query for this name returns the IP address as a quoted string.
	googleDomain = "o-o.myaddr.l.google.com"
	// googleServer is the name server that answers queries for
	// googleDomain.
	// The "@" prefix tells dig to query this server directly.
	googleServer = "@ns1.google.com"

	// opendnsDomain is a hostname that resolves to the client's IP address.
	// An A query returns the IPv4 address, and an AAAA query returns the
	// IPv6 address.
	opendnsDomain = "myip.opendns.com"
	// opendnsServer is the name server that answers queries for
	// opendnsDomain.
	// The "@" prefix tells dig to query this server directly.
	opendnsServer = "@resolver1.opendns.com"
)

// Google queries the Google name server for the host's public IP address.
// Because Google returns the address in a TXT record, v selects IPv4 (V4) or
// IPv6 (V6) transport.
//
//	dig TXT +short -4 o-o.myaddr.l.google.com @ns1.google.com
//	dig TXT +short -6 o-o.myaddr.l.google.com @ns1.google.com
func Google(ctx context.Context, v Version) (net.IP, error) {
	cmd := exec.CommandContext(
		ctx,
		"dig",
		"TXT",
		"+short",
		string(v),
		googleDomain,
		googleServer,
	)
	output, err := cmd.CombinedOutput()
	if err != nil {
		return nil, fmt.Errorf("exec dig: %v", err)
	}
	if string(output) == "" {
		return nil, fmt.Errorf("exec dig: no output")
	}
	if len(output) < 2 {
		return nil, fmt.Errorf(
			"exec dig: unexpected output: %q", string(output),
		)
	}

	// Remove the quotes from the Google TXT answer.
	trimmed := strings.TrimSpace(string(output))
	ip := net.ParseIP(trimmed[1 : len(trimmed)-1])

	return ip, nil
}

// OpenDNS queries the OpenDNS name server for the host's public IP address.
// Use r to request the IPv4 address (A) or the IPv6 address (AAAA).
//
//	dig +short A myip.opendns.com @resolver1.opendns.com
//	dig +short AAAA myip.opendns.com @resolver1.opendns.com
func OpenDNS(ctx context.Context, r Record) (net.IP, error) {
	cmd := exec.CommandContext(
		ctx,
		"dig",
		"+short",
		string(r),
		opendnsDomain,
		opendnsServer,
	)
	output, err := cmd.CombinedOutput()
	if err != nil {
		return nil, fmt.Errorf("exec dig: %v", err)
	}
	if string(output) == "" {
		return nil, fmt.Errorf("exec dig: no output")
	}
	if len(output) < 2 {
		return nil, fmt.Errorf(
			"exec dig: unexpected output: %q", string(output),
		)
	}

	ip := net.ParseIP(strings.TrimSpace(string(output)))

	return ip, nil
}