@@ -4290,3 +4290,21 @@ func (s *DockerSuite) TestRunVolumeCopyFlag(c *check.C) {

 	out, _, err = dockerCmdWithError("run", "-v", "/foo:/bar:nocopy", "busybox", "true")

 	c.Assert(err, checker.NotNil, check.Commentf(out))

 }

+

+func (s *DockerSuite) TestRunTooLongHostname(c *check.C) {

+	// Test case in #21445

+	hostname1 := "this-is-a-way-too-long-hostname-but-it-should-give-a-nice-error.local"

+	out, _, err := dockerCmdWithError("run", "--hostname", hostname1, "busybox", "echo", "test")

+	c.Assert(err, checker.NotNil, check.Commentf("Expected docker run to fail!"))

+	c.Assert(out, checker.Contains, "invalid hostname format for --hostname:", check.Commentf("Expected to have 'invalid hostname format for --hostname:' in the output, get: %s!", out))

+

+	// HOST_NAME_MAX=64 so 65 bytes will fail

+	hostname2 := "this-is-a-hostname-with-65-bytes-so-it-should-give-an-error.local"

+	out, _, err = dockerCmdWithError("run", "--hostname", hostname2, "busybox", "echo", "test")

+	c.Assert(err, checker.NotNil, check.Commentf("Expected docker run to fail!"))

+	c.Assert(out, checker.Contains, "invalid hostname format for --hostname:", check.Commentf("Expected to have 'invalid hostname format for --hostname:' in the output, get: %s!", out))

+

+	// 64 bytes will be OK

+	hostname3 := "this-is-a-hostname-with-64-bytes-so-will-not-give-an-error.local"

+	dockerCmd(c, "run", "--hostname", hostname3, "busybox", "echo", "test")

+}

@@ -244,8 +244,9 @@ func Parse(cmd *flag.FlagSet, args []string) (*container.Config, *container.Host

 	// Validate if the given hostname is RFC 1123 (https://tools.ietf.org/html/rfc1123) compliant.

 	hostname := *flHostname

 	if hostname != "" {

+		// Linux hostname is limited to HOST_NAME_MAX=64, not not including the terminating null byte.

 		matched, _ := regexp.MatchString("^(([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])\\.)*([[:alnum:]]|[[:alnum:]][[:alnum:]\\-]*[[:alnum:]])$", hostname)

-		if !matched {

+		if len(hostname) > 64 || !matched {

 			return nil, nil, nil, cmd, fmt.Errorf("invalid hostname format for --hostname: %s", hostname)

 		}

 	}

@@ -390,6 +390,7 @@ func TestParseHostname(t *testing.T) {

 		"host-name":   "host-name",

 		"hostname123": "hostname123",

 		"123hostname": "123hostname",

+		"hostname-of-64-bytes-long-should-be-valid-and-without-any-errors": "hostname-of-64-bytes-long-should-be-valid-and-without-any-errors",

 	}

 	invalidHostnames := map[string]string{

 		"^hostname": "invalid hostname format for --hostname: ^hostname",

@@ -397,6 +398,7 @@ func TestParseHostname(t *testing.T) {

 		"host&name": "invalid hostname format for --hostname: host&name",

 		"-hostname": "invalid hostname format for --hostname: -hostname",

 		"host_name": "invalid hostname format for --hostname: host_name",

+		"hostname-of-65-bytes-long-should-be-invalid-and-be-given-an-error": "invalid hostname format for --hostname: hostname-of-65-bytes-long-should-be-invalid-and-be-given-an-error",

 	}

 	hostnameWithDomain := "--hostname=hostname.domainname"

 	hostnameWithDomainTld := "--hostname=hostname.domainname.tld"