All Products
Search

This Product

    Server Load Balancer:Add a TCP listener

    Document Center

    Server Load Balancer:Add a TCP listener

    Last Updated:Jun 02, 2026

    Add a TCP listener to a CLB instance to forward TCP traffic to backend servers. TCP listeners suit reliability-first workloads such as file transfers, email, and remote logon.

    Prerequisites

    Step 1: Configure the listener

    1. Log on to the CLB console.

    2. Select the region of the CLB instance.

    3. Open the listener configuration wizard:

      • On the Instances page, find the CLB instance and click Configure Listener in the Actions column.

      • On the Instances page, click the instance ID to open the details page. Click the Listener tab, then click Add Listener.

    4. In the Protocol & Listener step, configure the following parameters, then click Next.

      Listener parameters

      Parameter

      Description

      Select Listener Protocol

      Select TCP.

      Backend Protocol

      Automatically set to TCP when the listener protocol is TCP.

      Listener Port

      The port that receives and forwards requests. Valid values: 1 to 65535.

      Tags

      Select or enter a Tag Key and Tag Value.

      Advanced settings

      Click Modify next to Advanced Settings to configure the following parameters.

      Scheduling algorithm

      Select a scheduling algorithm. Default value: Round Robin.

      Algorithm

      Description

      Weighted Round-robin (WRR)

      Distributes requests by weight. Higher-weight servers receive more requests.

      Round Robin (RR)

      Distributes requests sequentially to backend servers.

      Consistent Hashing (CH)

      Distributes requests by hash key. Two modes: Four-element (source IP, destination IP, source port, destination port) and Source IP (source IP only). Requests with the same hash key go to the same backend server.
      Note

      Only high-performance CLB instances support Consistent Hashing (CH). Existing TCP listeners using Weighted Round-robin (WRR) or Round Robin (RR) cannot switch to Consistent Hashing (CH). To use Consistent Hashing (CH), create a new TCP listener.

      Session persistence

      Disabled by default. When enabled, CLB forwards requests from the same client IP to the same backend server.

      Access control

      Disabled by default. When enabled, select an ACL type and an ACL:

      ACL Type

      Behavior

      Whitelist: Allows Specified IP Addresses to Access the SLB Instance

      Forwards requests only from IP addresses or CIDR blocks in the ACL. An empty whitelist forwards all requests. Improper configuration may block legitimate traffic.

      Blacklist: Forbids Specified IP Addresses to Access the SLB Instance

      Blocks requests from IP addresses or CIDR blocks in the ACL. An empty blacklist forwards all requests.
      Note

      IPv6 instances can only be associated with IPv6 ACLs. IPv4 instances can only be associated with IPv4 ACLs. For more information, see Access control.

      Bandwidth throttling for listeners

      Disabled by default. Available only for bandwidth-billed CLB instances. Set a maximum bandwidth per listener to control traffic distribution. The total bandwidth of all listeners cannot exceed the instance bandwidth.

      Important

      Connection timeout period

      Maximum idle time for a TCP connection between CLB and the client. Valid values: 10 to 900 seconds. Default value: 900.

      Note

      This timeout applies to all server groups on the listener. To use a different timeout for a specific server, create a separate listener.

      Proxy Protocol

      Passes client IP addresses to backend servers. Retrieve client IP addresses on backend servers through a CLB Layer-4 listener.

      Important

      • Proxy Protocol is not compatible with PrivateLink.

      • Both proxy and backend servers must support this protocol. Enabling it without backend support causes packet parsing failures and affects availability.

      Obtain client source IP address

      Enabled by default. Preserves client IP addresses. Only Layer 4 listeners support this feature.

      Automatically enable listener

      Enabled by default. The listener starts immediately after creation.

    Step 2: Add backend servers

    Add backend servers to process client requests. You can use the default server group, a vServer group, or a primary/secondary server group. CLB server groups. This example uses the default server group.

    1. In the Backend Servers step, select Default Server Group and click Add More.

    2. In the Select Servers step, select the backend servers, then click Next.

    3. In the Ports/Weights step, configure weights for the backend servers.

      Note

      Default weight: 100. Higher weight means more requests. Weight 0 stops traffic to that server.

    4. Click Add and specify the port for each backend server. Valid values: 1 to 65535. Multiple backend servers in the same CLB instance can use the same port.

    5. Click Next.

    Step 3: Configure health checks

    CLB health checks monitor backend server availability and reduce the impact of server failures.

    Note

    Health checks cannot be disabled for listeners using a primary/secondary server group.

    1. (Optional) In the Health Check step, click Modify to change the health check configuration, then click Next. For more information, see Configure and manage CLB health checks.

    2. In the Confirm step, review the listener configurations. Click Modify to modify if needed.

    3. Click Submit. In the Configuration Successful message, click OK.

    After submission, the listener appears on the Listener tab of the instance details page.

    FAQ

    How do I route traffic to a CLB instance with a TCP listener using a domain name?

    Create an A record to resolve your domain name to the CLB instance IP address. Use an A record to resolve a domain name to an IP address.

    References