Add digip pkg

pkg/digip/digip.go

@@ -0,0 +1,124 @@
+// 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
+}