Set weights_Public Authoritative DNS Resolution - Alibaba Cloud DNS

Overview

Alibaba Cloud DNS allows you to set weights for IP addresses when you configure multiple IP addresses for a host record on a Domain Name System (DNS) server. When Alibaba Cloud DNS responds to DNS requests, Alibaba Cloud DNS returns different DNS records based on the preset weights of IP addresses. This way, traffic is forwarded to different servers, and load balancing is achieved.

Prerequisites

The domain name is configured with multiple A records, AAAA records, and canonical name (CNAME) records that contain the same hostname and DNS request source.

Note

If weight settings is disabled:

A/AAAA/NS/MX/TXT records: Alibaba Cloud DNS returns all records for the line, and the local DNS returns result (if local DNS returns all addresses, the terminal randomly selects one).
CNAME record: Alibaba Cloud DNS directly returns one of the records.

Limits

Weight settings apply only to DNS records that have the same type, hostname, and line. The supported types are A, CNAME, and AAAA. The following table describes these limits in detail.

Limit	Supported	Not supported
Record type	A, CNAME, and AAAA	Other types
Record status	Enabled records	Paused records, locked records, and wildcard DNS records
Number of DNS records supported	The free edition supports weight settings for a maximum of 10 DNS records that have the same type, host record, and line. The paid edition supports weight settings for a maximum of 90 DNS records that have the same type, host record, and line.	N/A
Weight	The weight for a DNS record can be set to a value in the range of 0 to 100. The default weight ratio of three DNS records is 1:1. If the weight for a DNS record is set to 0, Alibaba Cloud DNS does not return the DNS record.	N/A
Line	You can set it to a default line or a specific line for A records. Note The weights are independent of each other in different lines.	Enabling or disabling load balancing across different lines

Terms

No.	Term	Description
1	Load Balancing Policy	This column displays the load balancing policy applied by the current account. Load balancing policies include Return all addresses and Return addresses by weight.
2	Return All Addresses	This value indicates that the domain name is resolved to multiple IP addresses and access traffic is evenly forwarded to each IP address.
3	Return Addresses by Weight	This value indicates that the domain name is resolved to multiple IP addresses and access traffic is forwarded to the IP addresses by weight.
4	EnableWeight	Enabling weight settings is a global operation. This operation takes effect for all lines of a subdomain.
5	DisableWeight	Disabling weight settings is a global operation. This operation takes effect for all lines of a subdomain.
6	Set weight	You can use this feature to enable weight settings and set weights for one or specific resolution lines.

Procedure

Log on to the Alibaba Cloud DNS console.
On the Authoritative Domain Names tab of the Authoritative DNS Resolution page, find the desired domain name and click DNS Settings in the Actions column.
On the DNS Settings page, click the Weight Settings tab to go to the Weighted Round-robin page. Click the Enable Weight in the Actions column, and click OK in the dialog box that appears. The default weight ratio of three DNS records is 1:1:1. If the default weight ratio is retained, Alibaba Cloud DNS returns IP addresses to DNS requests based on this ratio.
Note
Enabling weight settings is a global operation. This operation takes effect for all lines of a subdomain.
On the Weighted Round-robin page, click Weight Settings tab. Click the Set Weight in the Actions column. After you set weights, Alibaba Cloud DNS returns IP addresses to DNS requests based on the preset weights.

Enable weight settings for a specific line

To enable weight settings for a specific line, you can perform the global operation or separately enable weight settings for the line. Example:

Find the subdomain for which the Load Balancing Policy is set to Return All Addresses and click Set Weight.
Select the line for which you want to enable weight settings.
Note
If the Load Balancing Policy is Return All Addresses and Weighted Round-robin is disabled, the weights cannot be modified.
Enable Weighted Round-robin, and specify weight in the Weight column based on your business needs.

Result: The line with Weighted Round-robin enabled returns IP addresses based on the weight ratio of 1:1:1 and other lines return all IP addresses.

Result

Result of disabling weight settings

Assume that you have three servers whose IP addresses are 1.1.XX.XX, 2.2.XX.XX, and 3.3.XX.XX. The IP addresses of these servers map to one domain name. The following table describes the DNS records of the domain name.

Record type	Host record	Line	Record value
A	www	Default	1.1.XX.XX
A	www	Default	2.2.XX.XX
A	www	Default	3.3.XX.XX

When a local DNS accesses Alibaba Cloud DNS, Alibaba Cloud DNS returns all the three DNS records to the local DNS. The local DNS sends all the IP addresses to the browser of a website visitor and the browser randomly accesses one of the IP addresses.

The preceding method can reduce the workloads of an authoritative DNS server to some extent. However, the method cannot differentiate between the servers or reflect the current status of each server.

Result of default weight settings

If the weight settings are enabled, the default weight ratio of three DNS records is 1:1:1. Alibaba Cloud DNS polls three A records and returns three IP addresses in sequence in response to DNS requests. Alibaba Cloud DNS returns the following DNS records:

Alibaba Cloud DNS returns 192.0.2.0 in response to a DNS request from Region 1.
Alibaba Cloud DNS returns 192.0.2.1 in response to a DNS request from Region 2.
Alibaba Cloud DNS returns 192.0.2.2 in response to a DNS request from Region 3.
Alibaba Cloud DNS returns 192.0.2.0 in response to a DNS request from Region 4.
Alibaba Cloud DNS returns 192.0.2.1 in response to a DNS request from Region 5.
Alibaba Cloud DNS returns 192.0.2.2 in response to a DNS request from Region 6.
...

Result of specified weight settings

After the weight settings are enabled, you can set weights. Alibaba Cloud DNS returns IP addresses to DNS requests based on the weights. This way, traffic is forwarded to different servers based on the weights. For example, if the weight ratio of the preceding three DNS records is set to 2:1:1, the following DNS records are returned:

Alibaba Cloud DNS returns 1.1.XX.XX in response to a DNS request from Region 1.
Alibaba Cloud DNS returns 2.2.XX.XX in response to a DNS request from Region 2.
Alibaba Cloud DNS returns 3.3.XX.XX in response to a DNS request from Region 3.
Alibaba Cloud DNS returns 1.1.XX.XX in response to a DNS request from Region 4.
Alibaba Cloud DNS returns 1.1.XX.XX in response to a DNS request from Region 5.
Alibaba Cloud DNS returns 2.2.XX.XX in response to a DNS request from Region 6.
...

Note

You may find that DNS records are not returned based on the weight settings during the test. This is a common occurrence. The cause is that weighted round-robin is a coarse-grained method to schedule traffic based on the weight of an IP address in a DNS record. Weighted round-robin is set to process requests from local DNSs. However, local DNSs request the authoritative DNS, which is Alibaba Cloud DNS, only once within the time to live (TTL) period.

For example, a domain name is accessed by users in both the China (Shanghai) and China (Beijing) regions. Assume that users in the China (Shanghai) region use local DNS A and users in the China (Beijing) region use local DNS B. When local DNS A and local DNS B initiate requests to Alibaba Cloud DNS, Alibaba Cloud DNS returns IP addresses based on the weight settings. However, all users who use the same local DNS obtain the same IP address within the TTL period.