Why does opening socket on port 25 on Android cause a permission denied error

Question

I am trying to open port 25 on Android but for some reason I keep getting the following error

java.net.BindException: bind failed: EACCES (Permission denied)

Below is the code that I am using

ServerSocket ss = new ServerSocket(smtpPort);
                    Log.d("SMTPSocketHandler", "Socket created");
                    while (!end)
                    {
                        Socket s = ss.accept();
                        Log.d("SMTPSocketHandler", "Client Accepted");
                        BufferedReader input = new BufferedReader(new InputStreamReader(s.getInputStream()));
                        PrintWriter output = new PrintWriter(s.getOutputStream());

                        String stringData = input.readLine();
                        output.println("FROM Server - " + stringData.toUpperCase());
                        output.flush();
                        end = true;
                        output.close();
                        s.close();
                        break;
                    }
                    ss.close();

If I use another port though such as 5555 it then works. Is there a particular reason this isn't supported or is there something I can do to make it work?

You need superuser permission to bind to any port below 1024. — user207421, Nov 14 '19 at 23:17
port 25 is the Standard SMTP Port, my guess is, that they blocked all Standard ports which could be used by the system — Dominik Wuttke, Nov 14 '19 at 23:17
@user207421, that looks to be the reason 1024 is fine but 1023 gets the permission denied error. That's annoying — Boardy, Nov 14 '19 at 23:22

score 2 · Accepted Answer · answered Nov 14 '19 at 23:37

As was mentioned in a comment, ports below 1024 require superuser/root permissions. This is not unique to Android — this is standard Unix/Linux behavior, and has been that way for a couple of decades.

See:

Why are ports below 1024 privileged?
https://www.w3.org/Daemon/User/Installation/PrivilegedPorts.html (reporting on this from ~25 years ago)

Why does opening socket on port 25 on Android cause a permission denied error

1 Answers1