The CVM port might still be inaccessible despite being open in the security group for several reasons:
Security Group Rules Conflict: There might be other rules in the security group that conflict with the port opening. For example, if there's a rule that denies all inbound traffic and it's placed above the rule that allows traffic on the specific port, the deny rule will take precedence.
Network ACLs: Even if the security group allows traffic, network ACLs (Access Control Lists) at the subnet level might be blocking the traffic. Network ACLs operate at the subnet level and can override security group settings.
Route Table Issues: Misconfigured route tables can prevent traffic from reaching the CVM instance. If the route table associated with the subnet does not have the correct routes to direct traffic to the instance, it will not be accessible.
Instance Firewall: Some instances have their own firewall settings that can override security group rules. Ensure that the instance's firewall is configured to allow the desired traffic.
Service Not Running: The service you are trying to access might not be running on the CVM instance. For example, if you are trying to access a web server on port 80, but the web server software is not installed or running, the port will appear inaccessible.
Public IP Issues: Ensure that the CVM instance has a public IP address assigned and that it is correctly configured. Without a public IP, external traffic cannot reach the instance.
DNS Resolution: If you are trying to access the CVM using a domain name, ensure that DNS resolution is working correctly. A DNS issue can make the instance appear inaccessible even if the network configuration is correct.
Example: Suppose you have a CVM instance running a web server on port 80, and you have added a rule in the security group to allow inbound traffic on port 80. However, if there is a deny-all rule above this rule in the security group, or if the network ACL for the subnet blocks port 80 traffic, you will not be able to access the web server from outside.
Recommendation: If you are using Tencent Cloud, consider using the Network Security Groups (NSGs) and Virtual Private Cloud (VPC) services to ensure that your network configuration is correct and secure. Additionally, you can use Tencent Cloud's monitoring and logging services to troubleshoot network issues effectively.