Improve validation check for Azure configuration (elastic#20389)

A blob container name must be between 3 and 63 characters in length;
start with a letter or number; and contain only letters, numbers, and
the hyphen. All letters used in blob container names must be
lowercase.  Added validation to make sure the storage container name
meets those requirements.

Closes elastic#20369

CHANGELOG.next.asciidoc

@@ -235,6 +235,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix `okta` geoip lookup in pipeline for `destination.ip` {pull}20454[20454]
 - Fix mapping exception in the `googlecloud/audit` dataset pipeline. {issue}18465[18465] {pull}20465[20465]
 - Fix `cisco` asa and ftd parsing of messages 106102 and 106103. {pull}20469[20469]
+- Improve validation checks for Azure configuration {issue}20369[20369] {pull}20389[20389]
 
 *Heartbeat*
 

x-pack/filebeat/input/azureeventhub/config.go

@@ -7,6 +7,7 @@ package azureeventhub
 import (
 	"errors"
 	"fmt"
+	"unicode"
 )
 
 type azureInputConfig struct {
@@ -36,6 +37,32 @@ func (conf *azureInputConfig) Validate() error {
 	}
 	if conf.SAContainer == "" {
 		conf.SAContainer = fmt.Sprintf("%s-%s", ephContainerName, conf.EventHubName)
+
+	}
+	err := storageContainerValidate(conf.SAContainer)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func storageContainerValidate(name string) error {
+	runes := []rune(name)
+	length := len(runes)
+	if length < 3 {
+		return fmt.Errorf("storage_account_container (%s) must be 3 or more characters", name)
+	}
+	if length > 63 {
+		return fmt.Errorf("storage_account_container (%s) must be less than 63 characters", name)
+	}
+	if !unicode.IsLower(runes[0]) && !unicode.IsNumber(runes[0]) {
+		return fmt.Errorf("storage_account_container (%s) must start with a lowercase letter or number", name)
+	}
+	for i := 0; i < length; i++ {
+		if !unicode.IsLower(runes[i]) && !unicode.IsNumber(runes[i]) && !('-' == runes[i]) {
+			return fmt.Errorf("rune %d of storage_account_container (%s) is not a lowercase letter, number or dash", i, name)
+		}
 	}
 	return nil
 }

x-pack/filebeat/input/azureeventhub/config_test.go

@@ -0,0 +1,29 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package azureeventhub
+
+import (
+	"testing"
+)
+
+func TestStorageContainerValidate(t *testing.T) {
+	var tests = []struct {
+		input    string
+		errIsNil bool
+	}{
+		{"a-valid-name", true},
+		{"a", false},
+		{"a-name-that-is-really-too-long-to-be-valid-and-should-never-be-used-no-matter-what", false},
+		{"-not-valid", false},
+		{"capital-A-not-valid", false},
+		{"no_underscores_either", false},
+	}
+	for _, test := range tests {
+		err := storageContainerValidate(test.input)
+		if (err == nil) != test.errIsNil {
+			t.Errorf("storageContainerValidate(%s) = %v", test.input, err)
+		}
+	}
+}