Many website owners using Hong Kong cloud servers may encounter a common network connectivity problem: after modifying the security group, the server, which was working normally, suddenly becomes inaccessible, possibly manifesting as an inability to connect via SSH, access to the website, or connect to the database. For some novice website owners, this type of problem is often confusing, even leading them to mistakenly believe that the server is broken or that the cloud service provider's system is malfunctioning.

　　In reality, most problems with Hong Kong cloud server connectivity are related to improper security group configuration or network policies. A Hong Kong cloud server's "security group" can be viewed as firewall rules; it controls which traffic can enter (inbound rules) and which traffic can leave the server (outbound rules). Improperly configuring security group rules can cause the server to become inaccessible, or even unable to access external resources. So, what factors cause the problem of "server inaccessibility after modifying the security group"?

　　Basic Concepts and Functions of Security Groups:

　　Before understanding the cause of the problem, we first need to understand the basic concepts of security groups. A security group is a virtual firewall provided by the cloud service platform, used to control data traffic entering and leaving the cloud server. It specifies which IPs can access which ports, which services can be accessed externally, and which are blocked.

　　Inbound rules: Rules that control external traffic accessing the cloud server.

　　Outbound rules: Rules that control the cloud server's access to external resources.

　　Security group rules are typically set based on IP addresses and ports. By configuring security group rules, users can implement measures such as allowing only a specific IP to access the SSH service, or only allowing HTTP and HTTPS ports to be open.

　　Common connection problems after modifying security group rules:

　　The most common server connection problems after modifying security group rules typically manifest in the following ways:

　　SSH connection failure: Modifying security group rules may cause SSH (port 22) to become inaccessible externally.

　　Web access problems: If port configurations are modified, HTTP (port 80) and HTTPS (port 443) may be incorrectly blocked, resulting in inaccessibility to websites.

　　Database connection errors: Database connection ports (such as MySQL's 3306) may be disabled, causing database connection failures.

　　Unable to access external resources: Modifying outbound rules may prevent the cloud server from accessing external network resources, such as remote updates or external API services.

　　The root cause of these problems is often due to "missing to allow necessary ports" or "misconfiguring security rules" when modifying security group rules.

　　Troubleshooting and solutions:

　　When encountering connection problems after modifying the security group on a Hong Kong cloud server, a clear troubleshooting approach is crucial. Below are some common troubleshooting steps and solutions:

　　1. Check inbound rules to confirm that SSH and HTTP/HTTPS ports are open.

　　When the cloud server cannot connect, the first step is to confirm that the inbound rules are configured correctly. Especially if you modified port rules in the security group, you might have accidentally blocked commonly used ports (such as SSH, HTTP, and HTTPS).

　　SSH port: If SSH connection fails, first check if port 22 is allowed. By default, SSH connections are made through port 22.

　　Solution: Log in to the cloud platform's management console and go to the "Security Group" settings page. Locate the currently used security group and check if there are any inbound rules allowing access to port 22 (allowing all IPs or allowing specific IPs). Source IP: 0.0.0.0/0 (or some specified IP addresses).

　　Web Ports: If the website is inaccessible, check if ports 80 (HTTP) and 443 (HTTPS) are blocked. Many website administrators often accidentally close these ports when modifying security groups.

　　Solution: Ensure that web service ports (such as HTTP and HTTPS) are not blocked, especially ports 80 and 443. Configure inbound rules to ensure that HTTP requests from external sources can successfully reach the web server.

　　2. Check outbound rules to confirm whether the server can access external resources.

　　Besides inbound rules, outbound rules can also affect server connectivity. If outbound traffic is accidentally restricted when modifying security groups, the server will be unable to connect to external networks or APIs.

　　Outbound Traffic Settings: Cloud servers typically allow all outbound traffic by default. However, modifying outbound rules may prevent the server from accessing update sources, external APIs, or databases.

　　Solution: Ensure that outbound rules do not restrict the server's access to external resources. If a specific service is unable to connect, check if the corresponding port is open (e.g., port 3306 for MySQL, port 6379 for Redis).

　　3. Check Server Firewall

　　Besides the cloud platform's security groups, operating system-level firewalls can also affect connectivity, especially on Linux systems. Most Linux servers have firewalls enabled by default (such as iptables or firewalld), which may override security group rules and block connections to certain ports.

　　Solution: Check the firewall status on the server to ensure that no additional firewall rules are blocking port access.

　　If firewall rules are blocking ports 22, 80, or 443, use the following command to unblock them:

sudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -j ACCEPT

　　4. Verify that there are no incorrect rules in the security group

　　Sometimes, incorrect rule configurations can cause problems. For example, misconfigured rules may prevent traffic from being forwarded correctly or be blocked by the firewall. This is especially true after modifying multiple security groups, where configuration conflicts are common.

　　Solution: Carefully check each security group rule to ensure there are no conflicts or duplicate configurations. If you are unsure about the rule settings, you can temporarily relax the inbound rules of the security group, setting them to "Allow all traffic," and then gradually tighten the rules.

　　5. Check server logs

　　If the above steps do not resolve the issue, check the server's error logs to determine if other factors are causing the connection problems.

　　Solution:

　　Check the SSH login logs for any connection rejection records:

sudo tail -f /var/log/auth.log

　　Check the web service logs for errors such as 403, 404, and 500.

sudo tail -f /var/log/nginx/access.log

　　In general, connection problems after modifying the security group on a Hong Kong cloud server are usually caused by incorrect security group rule configurations, blocked ports, or improper firewall settings. With proper troubleshooting steps, website administrators can easily restore the connection and ensure the server continues to operate normally. The most critical points to check are whether inbound rules have allowed the necessary ports, whether outbound rules are affecting external access, and whether the firewall has any additional restrictions.