helper/acctest: Add function to return random IP address

helper/acctest/random.go

@@ -10,10 +10,13 @@ import (
 	"fmt"
 	"math/big"
 	"math/rand"
+	"net"
 	"strings"
 	"time"
 
 	"golang.org/x/crypto/ssh"
+
+	"github.com/apparentlymart/go-cidr/cidr"
 )
 
 func init() {
@@ -105,6 +108,39 @@ func RandTLSCert(orgName string) (string, string, error) {
 	return certPEM, privateKeyPEM, nil
 }
 
+// RandIpAddress returns a random IP address in the specified CIDR block.
+// The prefix length must be less than 31.
+func RandIpAddress(s string) (string, error) {
+	_, network, err := net.ParseCIDR(s)
+	if err != nil {
+		return "", err
+	}
+
+	firstIp, lastIp := cidr.AddressRange(network)
+	first := &big.Int{}
+	first.SetBytes([]byte(firstIp))
+	last := &big.Int{}
+	last.SetBytes([]byte(lastIp))
+	r := &big.Int{}
+	r.Sub(last, first)
+	if len := r.BitLen(); len > 31 {
+		return "", fmt.Errorf("CIDR range is too large: %d", len)
+	}
+
+	max := int(r.Int64())
+	if max == 0 {
+		// panic: invalid argument to Int31n
+		return firstIp.String(), nil
+	}
+
+	host, err := cidr.Host(network, RandIntRange(0, max))
+	if err != nil {
+		return "", err
+	}
+
+	return host.String(), nil
+}
+
 func genPrivateKey() (*rsa.PrivateKey, string, error) {
 	privateKey, err := rsa.GenerateKey(crand.Reader, 1024)
 	if err != nil {

helper/acctest/random_test.go

@@ -0,0 +1,58 @@
+package acctest
+
+import (
+	"regexp"
+	"testing"
+)
+
+func TestRandIpAddress(t *testing.T) {
+	testCases := []struct {
+		s           string
+		expected    *regexp.Regexp
+		expectedErr string
+	}{
+		{
+			s:        "1.1.1.1/32",
+			expected: regexp.MustCompile(`^1\.1\.1\.1$`),
+		},
+		{
+			s:        "10.0.0.0/8",
+			expected: regexp.MustCompile(`^10\.\d{1,3}\.\d{1,3}\.\d{1,3}$`),
+		},
+		{
+			s:           "0.0.0.0/0",
+			expectedErr: "CIDR range is too large: 32",
+		},
+		{
+			s:        "449d:e5f1:14b1:ddf3:8525:7e9e:4a0d:4a82/128",
+			expected: regexp.MustCompile(`^449d:e5f1:14b1:ddf3:8525:7e9e:4a0d:4a82$`),
+		},
+		{
+			s:        "2001:db8::/112",
+			expected: regexp.MustCompile(`^2001:db8::[[:xdigit:]]{1,4}$`),
+		},
+		{
+			s:           "2001:db8::/64",
+			expectedErr: "CIDR range is too large: 64",
+		},
+		{
+			s:           "abcdefg",
+			expectedErr: "invalid CIDR address: abcdefg",
+		},
+	}
+
+	for i, tc := range testCases {
+		v, err := RandIpAddress(tc.s)
+		if err != nil {
+			msg := err.Error()
+			if tc.expectedErr == "" {
+				t.Fatalf("expected test case %d to succeed but got error %q, ", i, msg)
+			}
+			if msg != tc.expectedErr {
+				t.Fatalf("expected test case %d to fail with %q but got %q", i, tc.expectedErr, msg)
+			}
+		} else if !tc.expected.MatchString(v) {
+			t.Fatalf("expected test case %d to return %q but got %q", i, tc.expected, v)
+		}
+	}
+}