Advanced Forwarding

Overview

Advanced forwarding policies are available only for dedicated load balancers. If you have enabled Advanced Forwarding, you can add advanced forwarding policies to HTTP and HTTPS listeners of dedicated load balancers.

You can add advanced forwarding policies to HTTP or HTTPS listeners to forward requests to different backend server groups based on HTTP request method, HTTP header, query string, or CIDR block in addition to domain names and URLs. Table 1 describes the rules and actions that you can configure for request forwarding.

Figure 1 How advanced forwarding works

The following describes how an advanced forwarding policy works:

The client sends a request to the load balancer.
The load balancer matches the request based on the forwarding rule you configure.
The load balancer forwards the request to the corresponding backend server or returns a fixed response to the client based on the action you configure.
The load balancer sends a response to the client.

**Table 1** Rules and actions supported by an advanced forwarding policy
Forwarding Policy	Description
Forwarding rule	There are six types of forwarding rules: domain name, URL, HTTP request method, HTTP header, query string, and CIDR block For details, see Forwarding Rule.
Action	The following actions are supported: forward to a backend server group, redirect to another listener, redirect to another URL, and return a specific response body. For details, see Action Types.

How Requests Are Matched

After you add an HTTP or HTTPS listener to a load balancer, a default forwarding policy is generated. This policy uses the protocol and port specified for the listener to match requests and forward the requests to the backend server group you specified when adding the listener.

The default forwarding policy has the lowest priority and is not included when you sort forwarding policies. It can be edited but cannot be deleted.

Each request is matched based on the forwarding policy priority (a smaller value indicates a higher priority). Once a forwarding policy is matched, the request is forwarded based on this forwarding policy.

If the request is matched with any forwarding policy of the listener, it is forwarded based on this forwarding policy.
If the request is not matched with any forwarding policy, it is forwarded based on the default forwarding policy.

Forwarding Rule

Advanced forwarding policies support the following types of forwarding rules: domain name, URL, HTTP request method, HTTP header, query string, and CIDR block (source IP addresses).

**Table 2** Forwarding rules
Forwarding Rule	Description
Domain name	Description Route requests based on the domain name. You can configure multiple domain names in a forwarding policy. Each domain name contains at least two labels separated by periods (.). Max total: 100 characters. Max label: 63 characters. Each label can contain letters, digits, hyphens (-), periods (.), and asterisks (). A label must start with a letter, digit, or asterisk () and cannot end with a hyphen (-). An asterisk () must be used as the leftmost label if you want to configure a wildcard domain name. Matching rules Exact match domains and wildcard domains are supported. Example Request URL: https://www.example.com/login.php?locale=en-us=#videos Domain name in the forwarding rule: www.example.com*
URL	Description Route requests based on URLs. You can configure multiple URLs in a forwarding policy. A URL can contain letters, digits, and special characters _~';@^-%#$.+?,=!:\|\/()[]{}. If the URL contains special characters such as question marks (?) or pound keys (#), escape the special characters before configuring the forwarding rule. Matching rules Exact match: The request URL must exactly match that specified in the forwarding policy. The URL must start with a slash (/) and can use asterisks () and question marks (?) as wildcards. Prefix match: The requested URL starts with the specified URL string. The URL must start with a slash (/) and can use asterisks () and question marks (?) as wildcard characters. Regular expression match: The URLs are matched using a regular expression. For more information about URL matching rules, see URL Matching. Example Request URL: https://www.example.com/login.php?locale=en-us#videos URL in the forwarding rule: /login.php*
Query string	Route requests based on the query string. A query string consists of a key and one or more values. You need to set the key and values separately. The key can contain only letters, digits, and special characters !$'()+,./:;=?@^-_' A key can have one or more values. The value can contain letters, digits, and special characters !$'()+,./:;=?@^-_' Asterisks () and question marks (?) can be used as wildcard characters. Example Request URL: https://www.example.com/login.php?locale=en-us#videos A query string needs to be configured for the forwarding rule: Key: locale* Value: en-us
HTTP request method	Route requests based on the HTTP method. You can configure multiple request methods in a forwarding policy. The following methods are available: GET, POST, PUT, DELETE, PATCH, HEAD, and OPTIONS. Example GET
HTTP header	Route requests based on the HTTP header. An HTTP header consists of a key and one or more values. You need to configure the key and values separately. The key can contain only letters, digits, underscores (_), and hyphens (-). A key can have one or more values. The value can contain letters, digits, and special characters !#$%&'()+,.\/:;<=>?@[]^-_'{\|}~ Asterisks () and question marks (?) can be used as wildcard characters. Example Key: Accept-Language Value: en-us
CIDR block	Route requests based on the source IP addresses from where requests originate. Example 192.168.1.0/24 or 2020:50::44/127

Action Types

Advanced forwarding policies support the following actions: forward to a backend server group, redirect to another listener, redirect to another URL, and return a specific response body.

**Table 3** Actions of an advanced forwarding policy
Action	Description
Forward to a backend server group	Requests are forwarded to the specified backend server group.
Redirect to another listener	Requests are redirected to another listener, which then routes the requests to its associated backend server group. NOTE: If you select Redirect to another listener and create a redirect for the listener, it will redirect the requests to the specified HTTPS listener, but access control configured for the listener will still take effect. For example, if you configure a redirect for an HTTP listener, HTTP requests to access a web page will be redirected to the HTTPS listener you select and handled by the backend servers associated with the HTTPS listener. As a result, the clients access the web page over HTTPS. The configuration of the HTTP listener will become invalid.
Redirect to another URL	Requests are redirected to the configured URL. When clients access website A, the load balancer returns 302 or any other 3xx status code and automatically redirects the clients to website B. You can custom the redirection URL that will be returned to the clients. Configure at least one of the following components: Protocol: ${protocol}, HTTP, or HTTPS ${protocol}: retains the protocol of the request. Domain Name: A domain name consists of at least two labels separated by periods (.). Each label can contain only letters, digits, hyphens (-), and periods (.), must start with a letter, digit, or asterisk (), and cannot end with a hyphen (-). ${host}: retains the domain name of the request. Port: ranges from 1 to 65535. ${port}: retains the port number of the request. Path: A path can contain letters, digits, and special characters _~';@^-%#&$.+?,=!:\|\/()[]{} and must start with a slash (/). ${path}: retains the path of the request. NOTE: If you select regular expression match, the request path will be overwritten by the variables that match the regular expressions. For details, see URL Matching Based on Regular Expressions. Query String: A query string can contain only letters, digits, and special characters !$'()+,./:;=?@&^-_'. Ampersand (&) can only be used as separators. HTTP Status Code*: 301, 302, 303, 307, or 308 Example URL for redirection: http://www.example1.com/index.html?locale=en-us#videos Protocol: HTTP Domain name: www.example1.com Port: 8081 Path: /index.html Query String: locale=en-us HTTP Status Code: 301
Return a specific response body	Load balancers return a fixed response to the clients. You can custom the status code and response body that load balancers directly return to the clients without the need to route the requests to backend servers. Configure the following components: HTTP Status Code: By default, 2xx, 4xx, and 5xx status codes are supported. Content-Type: text/plain, text/css, text/html, application/javascript, or application/json Message Body: This parameter is optional. The value is a string of 0 to 1,024 characters. Example text/plain Sorry, the language is not supported. text/css <head><style type="text/css">div {background-color:red}#div {font-size:15px;color:red}</style></head> text/html <form action="/" method="post" enctype="multipart/form-data"><input type="text" name="description" value="some text"><input type="file" name="myFile"><button type="submit">Submit</button></form> application/javascript String.prototype.trim = function() {var reExtraSpace = /^\s(.?)\s+$/;return this.replace(reExtraSpace, "$1")} application/json { "publicip": { "type": "5_bgp","ip_version": 4},"bandwidth": {"name": "bandwidth123","size": 10,"share_type": "PER"}} NOTE: Ensure that the response body does not contain carriage return characters. Otherwise, it cannot be saved.

URL Matching

Table 4 shows how URLs configured in the forwarding policies match the URLs in the requests.

**Table 4** URL matching examples
Request URL	Forwarding Policy	URL in the Forwarding Policy	Matching Mode	Forwarding Policy Priority	Destination Backend Server Group
/elb/abc.html	Forwarding policy 01	/elb/abc.html	Prefix match	1	Backend server group 01
/elb/abc.html	Forwarding policy 02	/elb	Prefix match	2	Backend server group 02
/exa/index.html	Forwarding policy 03	/exa[^\s]*	Regular expression match	3	Backend server group 03
/exa/index.html	Forwarding policy 04	/exa/index.html	Regular expression match	4	Backend server group 04
/mpl/index.html	Forwarding policy 05	/mpl/index.html	Exact match	5	Backend server group 05

URLs are matched as follows:

When the request URL is /elb/abc.html, it matches both forwarding policy 01 and forwarding policy 02. However, the priority of forwarding policy 01 is higher than that of forwarding policy 02. Forwarding policy 01 is used, and requests are forwarded to backend server group 01.
When the request URL is /exa/index.html, it matches both forwarding policy 03 and forwarding policy 04. However, the priority of forwarding policy 03 is higher than that of forwarding policy 04. Forwarding policy 03 is used, and requests are forwarded to backend server group 03.
If the request URL is /mpl/index.html, it matches forwarding policy 05 exactly, and requests are forwarded to backend server group 05.

URL Matching Based on Regular Expressions

A path can contain letters, digits, and special characters _~';@^-%#&$.*+?,=!:|\/()[]{} and must start with a slash (/). ${path} retains the path of the request.

If you select regular expression match, the request path will be overwritten by the variables that match the regular expressions.

How Request Paths Are Overwritten

URL matching: The client sends a request, and the request matches a regular expression in the forwarding rule. You can specify one or more regular expressions as the match conditions and set multiple capture groups represented by parentheses ( ) for one regular expression.
Extraction and replacement: extracts the content from the capture groups.
Destination path: writes them to $1, $2, all the way to $9 configured for the path.

Example

When a client requests to access /test/ELB/elb/index, which matches the regular expression /test/(.*)/(.*)/index, $1 will be replaced by ELB and $2 by elb, and then the request will be redirected to /ELB/elb.

**Table 5** URL matching based on regular expressions
Matching Step		Description
Forwarding rule: URL	Regular expression match	Matching condition: /test/(.)/(.)/index Request URL: /test/ELB/elb/index
Action: redirect to another URL	Path	Path: /$1/$2 Extracting content $1: ELB $2: elb Destination path: /ELB/elb