验证端口是否打开

Question

I'm trying to create a connection to a MySQL database hosted on a raspberrypi through a java program running on my computer. So far when I run my program I receive a communications link failure.

As part of my trouble shooting I am trying to test if port 3306 is open or not.

I have used websites like this - http://www.mynetworktest.com/ports.php However I don't understand how port 80 can be closed if I can surf the web and I'm unsure if the rest of information presented is reliable.

What is the best way of testing if a port is open and if also if port 80 genuinely is closed how come I can surf the web?

Answer 1

Two things (you're probably more interested in the second point):

1. The port's you're quoting are incoming port numbers. Your device will not use port 80 to "browse the web" (connect to a web server). It will select a port at random (for all intents and purposes) for its outgoing connection, and its the web server your requesting files from that uses port 80. To that end, assuming you don't have any over aggressive firewall rules on your computer, you simply need to make sure that the incoming ports you require are open in the PI's firewall (3306 by default for MySQL, but feel free to double check my.cnf if you want to satisfy yourself of the fact)

2. What is probably the problem for you is that most mysql default configurations come with the default setting of having MySQL bind to the 127.0.0.1 ip. Which means only devices connecting via the local loopback interface can connect (which means only those clients running on the same device). Edit your my.cnf and delete the bind_address line, or set to:

   bind_address=0.0.0.0

Then restart the mysql service

What your doing is telling MySQL to bind to any network interface (0.0.0.0) rather than the local loopback interface. You could also set it to your locally assigned ip address which would only allow those on your local network to connect, but for the sake of ease (and not having to worry about DHCP) i'd just set it to 0.0.0.0 for now.

Answer 2

Sorry I am not sure what you mean. Are there 2 separate computers 1) raspberrpi 2) "my computer"? If so, how is "my computer" connected to "raspberrypi"? Are they connected to each other, or are they both connected independently to the internet?

Typically, you create direct connection between the 2 machines (EG an ethernet cable). You then make sure both machines have a "route" set to the other machine. Your mysql connection string (setting) would include either the ip address or host name of your raspberry computer.

You are correct, mysql is running on port 3306 by default (not port 80).

What operating systems are you running?

Sorry, more question than answer.

Hope that helps :)

Answer 3

you can use

nc -l 3306

if it fails with "port is already open" , you can make sure the port is already open :-)
also you can telnet to that port from other computer.

telnet 192.168.1.92 3306

to determine if the port is accessible from other computer or not.
you should do this to change mysql config as well:
here is the link: how to Configure mysql
you should change the line

[mysqld]
bind-address = 127.0.0.1

and set it to 0.0.0.0