Configure domain name- and URL path-based forwarding rules for ALB - Server Load Balancer

This topic describes how to configure domain name- and URL path-based forwarding rules for Application Load Balancer (ALB) listeners, and how to configure advanced URL path-based forwarding rules for rewrites and redirects.

Domain name-based forwarding rules

You can specify a specific domain name, a wildcard domain name, or a regular expression as the match condition in a domain name-based forwarding rule. For more information about how to create a forwarding rule, see the Create a forwarding rule section of the Manage forwarding rules for a listener topic.

Match type	Description
Exact match and wildcard match	Match conditions Exact match: The requested domain name must be the same as the specified domain name. Wildcard match: The requested domain name must match the wildcard pattern of the specified domain name. Requirements The domain name must be 3 to 128 characters in length and can contain only letters, digits, and the following special characters: `. - ? = ~ _ + \ ^ * ! $ & \| ( ) [ ]`. You can use asterisks (``) and question marks (`?`) as wildcard characters. Examples* Requested domain name: www.example.com Exact match: The requested domain name is matched only if `www.example.com` is specified in the forwarding rule. Wildcard match: The requested domain name is matched if `.example.com` or `www.example.` is specified in the forwarding rule.
Regular expression match	Match conditions The requested domain name must match the specified regular expression. Requirements The domain name must be 3 to 128 characters in length and can contain only letters, digits, and the following special characters: `. - ? = ~ _ - + \ ^ * ! $ & \| ( ) [ ]`. Examples Requested domain name: www.example.com Case-insensitive regular expression: The requested domain name is matched if `^www.example.com$` is specified in the forwarding rule.

URL-based forwarding rules

You can specify a specific URL, a wildcard URL, or a regular expression as the match condition in a URL-based forwarding rule. For more information about how to create a forwarding rule, see the Create a forwarding rule section of the " Manage forwarding rules for a listener" topic.

Match type	Description
Exact match and wildcard match	Match conditions Exact match: The requested path must be the same as the specified path. Wildcard match: The requested path must match the wildcard pattern of the specified path. Requirements The path must start with a forward slash (`/`) and can contain only letters, digits, and the following special characters: `$ - _ . + / & ~ @ :`. You can use asterisks (``) and question marks (`?`) as wildcard characters. Examples* Requested path: /example/text Exact match: The requested path is matched only if `/example/text` is specified in the forwarding rule. Wildcard match: The requested path is matched if `/example/` is specified in the forwarding rule. Note* The path matching rules of ALB are different from the rules of NGINX. ALB does not support the longest prefix match rule. For example, a common path configured for NGINX is `location /abc`. The longest prefix match rule is used to match the path. To perform longest prefix match, ALB uses wildcard characters. You can configure `/abc/*` in ALB to perform longest prefix match.
Regular expression match	Match conditions The requested path matches the specified regular expression. Requirements The regular expression can contain only letters, digits, and the following special characters: `. - _ / = ? ~ ^ * $ : ( ) [ ] + \|`. Example Requested path: `/sys/aaa/HOST` Case-sensitive regular expression: The requested path is matched if `^/sys/(.)/HOST$` is specified in the forwarding rule. Case-insensitive regular expression: The requested path is matched if `^/sys/(.)/host$` is specified in the forwarding rule.

Advanced URL-based forwarding rule settings for rewrites and redirects

If you specify a regular expression as the match condition in a rewrite or redirect rule, the original URL is overwritten by the variables that match the regular expression. For more information about how to create a forwarding rule, see the Create a forwarding rule section of the Manage forwarding rules for a listener topic.

Note

For more information about how to configure regular expressions in URL-based forwarding conditions, see the URL-based forwarding rules section of this topic.

Usage notes
- You can specify one or more regular expressions as the match conditions in a rewrite or redirect rule and use parentheses ( ) to define them as capture groups. The number of capture groups must be the same as the number of variables in the rewrite or redirect rule.
- You can use up to three variables (${1}, ${2}, and ${3}) to form the URL that overwrites the original URL. The variables cannot be replaced with other characters.

Procedure

URL matching: The client sends a request. The request matches a regular expression that is specified in a forwarding rule.
Extraction and replacement: extracts the content from the capture groups ( ) and writes the content to ${1}, ${2}, and ${3}. The content is used to replace the rewriting or redirection path of the forwarding action.
Splicing: splices the strings that overwrote the variables ${1}, ${2}, and ${3} to form the new URL.

No.	Step	Example
1	Configure the forwarding condition and the forwarding action in a forwarding rule.	Path in the forwarding condition: `/sys/(.)/(.)/aaa` Path to which requests are rewritten or redirected: `/${1}/${2}`
2	A client request matches the path in the forwarding rule.	Requested path: `/sys/ccc/bbb/aaa` Matched path: `/sys/(.)/(.)/aaa`
3	The system extracts values from the regular expression and replaces the variables with the values.	`ccc` and `bbb` are extracted from `(.*)` in the regular expression and replace the variables ${1} and ${2} in the path to which requests are rewritten or redirected. `${1}` is replaced by `ccc`. `${2}` is replaced by `bbb`.
4	The values are concatenated to form the actual path.	The path received by the backend server is `/ccc/bbb`.

Examples

You can create forwarding rules in the ALB console based on the usage notes and procedure.

Example 1: Configure the Rewrite or Forward action

In this example, a forwarding rule is configured to rewrite the path of requests destined for sys/ccc/bbb/aaa to /ccc/bbb and forward the requests accordingly. The following figure shows the sample configuration.

示例1

Parameter		Description
If Match All Conditions	Regular expression match	Match conditions The requested path matches the specified regular expression. Requirements The regular expression can contain only letters, digits, and the following special characters: `. - _ / = ? ~ ^ * $ : ( ) [ ] + \|`. Examples Requested path: `/sys/ccc/bbb/aaa` Case-sensitive regular expression: The requested path is matched if `/sys/(.)/(.)/aaa` is specified in the forwarding rule. Case-insensitive regular expression: The requested path is matched if `/sys/(.)/(.)/aaa` is specified in the forwarding rule.
Action	Rewrite	Domain Name: `${host}` Path: `/${1}/${2}` Search: `${query}` Note Set Search to the part follows the question mark (?) in a URL. Example: To rewrite URLs to `www.example.com/test/test1?x=1`, set Search to `x=1`.
Action	Forward	Select a server group from the drop-down list.

Example 2: Configure the Redirect action

In this example, a forwarding rule is configured to redirect requests originally destined for sys/ccc/bbb/aaa to /ccc/bbb. The following figure shows the sample configuration.

示例2

Parameter		Description
If Match All Conditions	Regular expression match	Match conditions The requested path matches the specified regular expression. Requirements The regular expression can contain only letters, digits, and the following special characters: `. - _ / = ? ~ ^ * $ : ( ) [ ] + \|`. Examples Requested path: `/sys/ccc/bbb/aaa` Case-sensitive regular expression: The requested path is matched if `/sys/(.)/(.)/aaa` is specified in the forwarding rule. Case-insensitive regular expression: The requested path is matched if `/sys/(.)/(.)/aaa` is specified in the forwarding rule.
Action	Redirect	Protocol: `$protocol` Domain Name: `${host}` Port: `${port}` Path: `/${1}/${2}` Search: `${query}` Status Code: `301`

FAQ

How can I configure a forwarding rule to remove a specific prefix from the request URL path and forward the request based on the subsequent directories?

Solution:

Assume you want ALB to remove /api from the URL of requests destined for www.example.com/api/aaa/bbb/... and forward the requests based on the directories following /api.

In the forwarding rule, set Add Condition to Path, select Regular Expression Matching, and set the path to /api/(.*) to match all requests whose URL path starts with /api. Set Action to Rewrite or Redirect, and set the action Path to /${1} to use the part following /api in the original URL path.

Sample configurations are as follows:

Rewrite
Redirect

References

For more information about how to configure other forwarding rules for an ALB listener, see Manage forwarding rules for a listener.