Use strace or netstat to troubleshoot socket_accept() listening status issues

When writing Socket-based server applications using PHP, socket_accept() is a key function to receive client connections. However, in some scenarios, even if the program logic is correct, socket_accept() may not receive the connection normally. This article will explain how to use strace and netstat tools to quickly locate and resolve this problem.

1. Prerequisite: A typical PHP Socket server example

The following is a simple PHP server program that listens to port 8080:

 <?php
$host = '0.0.0.0';
$port = 8080;

$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
socket_bind($socket, $host, $port);
socket_listen($socket);

echo "Server listening on $host:$port\n";

while (true) {
    $client = socket_accept($socket);
    if ($client === false) {
        echo "Failed to accept connection\n";
        continue;
    }

    $msg = "Hello from server\n";
    socket_write($client, $msg, strlen($msg));
    socket_close($client);
}
?>

When you find socket_accept() stuck or always return false , you need to use system tools to conduct in-depth troubleshooting.

2. Use strace to track system calls

strace is a system call tracking tool under Linux. You can directly view which system underlying functions are called by PHP scripts at runtime and their execution results.

step:

1. Find the PHP process PID that is running, for example using ps :

    ps aux | grep php
   

2. Use strace to attach to the process:

    strace -p <PID> -e trace=network
   

3. Observe whether the following behavior exists in the output:

   • accept(...) = -1 : System call failed, usually accompanied by error codes (such as EAGAIN , ECONNABORTED , etc.)

   • Listen(...) or bind(...) error: may mean that the port is occupied or the permissions are insufficient

   • No call to accept() : the program may be stuck elsewhere

If strace shows accept() is normal but still has no connection access, the problem may be in the network layer or the firewall.

3. Use netstat or ss to confirm the listening status

Netstat and SS can be used to check whether the port is in the listening state and whether there is a connection established.

Check the listening status:

 netstat -tlnp | grep :8080

or:

 ss -tlnp | grep :8080

You should be able to see outputs similar to the following:

 tcp    LISTEN     0      128    0.0.0.0:8080    0.0.0.0:*     users:(("php",pid=12345,fd=3))

• If there is no listening, socket_bind() or socket_listen() fails;

• If the listening is normal but there is no connection, it may be that the client has not initiated the connection or is intercepted by the firewall;

• If the status is CLOSE_WAIT or TIME_WAIT , it means that the connection life cycle ends abnormally.

4. Further debugging suggestions

1. Check for error message <br> After each Socket call, use socket_strerror(socket_last_error()) to view the specific error information.

2. Firewall or SELinux Limitations <br> Use iptables or firewalld to confirm that the 8080 port is not blocked.

3. The port is occupied by other programs <br> Use lsof -i:8080 to check if any other programs occupy the port.