I'm looking at ZeroMQ Realtime Exchange Protocol (ZRE) as inspiration for building an auto-discovery of peers in a distributed application.
I've built a simple prototype application using UDP in Python following this model. It seems it has the (obivious, in retrospect) limitation that it only works for detecting peers if all peers are on other machines. This due to the socket bind operation on the discovery port.
Reading up on SO_REUSEADDR and SO_REUSEPORT tells me that I can't exactly do this with the UDP broadcast scheme as described in ZRE.
If you needed to build an auto-discovery mechanism for distributed applications such that multiple application instances (possibly with different versioN) can run on the same machine, how would you build it?
You should be able to bind each server instance to a different address. The entire subnet 127.0.0.0/8 should resolve to your localhost, so you can set up - for example - one service listening on 127.0.0.1, another listening on 127.0.0.2, etc. Anything from 127.0.0.1 to 127.255.255.254.
# works as expected
nc -l 127.0.0.100 3000 &
nc -l 127.0.0.101 3000 &
# shows error "nc: Address already in use"
nc -l 127.0.0.1 3000 &
nc -l 127.0.0.1 3000 &
Related
When enabling JMX remote connection, I saw these two properties, java.rmi.server.hostname and com.sun.management.jmxremote.host. What the differences between these two?
The initial connection from the client (e.g. jconsole or visualvm) is established to com.sun.management.jmxremote.host on port
com.sun.management.jmxremote.port. Then the connecting client obtains com.sun.management.jmxremote.rmi.port (it is dynamically assigned by Java from a pool of unused ports if not explicitly specified) and further communication and data exchange goes over Java RMI connecting to java.rmi.server.hostname on port com.sun.management.jmxremote.rmi.port.
If you don't use JMX authentication or SSL I'd recommend configuring both hosts as localhost (see configuration example below).
java.rmi.server.hostname=127.0.0.1
com.sun.management.jmxremote
com.sun.management.jmxremote.port=9091
com.sun.management.jmxremote.host=127.0.0.1
com.sun.management.jmxremote.rmi.port=9092
com.sun.management.jmxremote.ssl=false
com.sun.management.jmxremote.authenticate=false
You can connect to such process either from the same machine or via SSH with port forwarding e.g.
ssh user#host -L 9091:localhost:9091 -L 9092:localhost:9092
I'm trying to open port TCP 28016 and UDP 28015 for a game server in my compute engine VM running on Microsoft Windows Server 2016.
I've tried opening the opening inside my server using RDP, going to Windows Firewall setting and creating new inbound rules for both TCP 28016 and UDP 28015.
Also done setting firewall rules on my Cloud Platform Firewall Rules for both port.
When running my game server application, running netstat didn't show any of the port being used / not listening . Not even shows up. What did i do wrong ?
Edit : it now shows up on netstat -a -b , but didn't have LISTENING
If it doesn't show as LISTENING, it's not a firewall or "port forwarding" issue; rather, the application either isn't running, or is running but isn't configured to listen for connections on that port.
Currently I need to establish remote connection with my server (Ubuntu 16.04 LTS).
I Install Postgresql and I made the following settings:
/etc/postgresql/9.5/main/postgresql.conf:
listen_addresses='*'
/etc/postgresql/9.5/main/pg_hba.conf:
host all all 0.0.0.0/0 md5
If run this command: netstat -anpt | grep LISTEN
shows the port is listening
but when I try to establish the connection, I have this error:
And this tool tells me that the port is closed:
Allowing only on Configurations of Postgresql server is not enough. You need to add a firewall rule in google compute engine. Check this
Firewall rules control incoming or outgoing traffic to an instance. By default, incoming traffic from outside your network is blocked.
When I publish and subscribe at localhost its work fine.
When I try from my PC at home I just can't connect to the broker.
open TCP port in/out at security group - 1883 8883 8080
open the ports also at my ec2 instance firewall...
what is the problem? I use the public DNS by amazon as I think I should...
This is an exercise at diagnosing network problems:
1) netstat -a -n | grep 1883
will tell you whether your broker is configured correctly
2) wireshark packet capture will tell you whether your system is receiving packets at the specific port
You will not get an answer until you at least do those.
How I can say a port is open or closed. What's the exact meaning of Open port and closed port.
My favorite tool to check if a specific port is open or closed is telnet. You'll find this tool on all of the operating systems.
The syntax is: telnet <hostname/ip> <port>
This is what it looks like if the port is open:
telnet localhost 3306
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
This is what it looks like if the port is closed:
telnet localhost 9999
Trying 127.0.0.1...
telnet: connect to address 127.0.0.1: Connection refused
telnet: Unable to connect to remote host
Based on your use case, you may need to do this from a different machine, just to rule out firewall rules being an issue. For example, just because I am able to telnet to port 3306 locally doesn't mean that other machines are able to access port 3306. They may see it as closed due to firewall rules.
As far as what open/closed ports means, an open port allows data to be sent to a program listening on that port. In the examples above, port 3306 is open. MySQL server is listening on that port. That allows MySQL clients to connect to the MySQL database and issue queries and so on.
There are other tools to check the status of multiple ports. You can Google for Port Scanner along with the OS you are using for additional options.
A port that's opened is a port to which you can connect (TCP)/ send data (UDP). It is open because a process opened it.
There are many different types of ports. These used on the Internet are TCP and UDP ports.
To see the list of existing connections you can use netstat (available under Unix and MS-Windows). Under Linux, we have the -l (--listen) command line option to limit the list to opened ports (i.e. listening ports).
> netstat -n64l
...
tcp 0 0 0.0.0.0:6000 0.0.0.0:* LISTEN
...
udp 0 0 0.0.0.0:53 0.0.0.0:*
...
raw 0 0 0.0.0.0:1 0.0.0.0:* 7
...
In my example, I show a TCP port 6000 opened. This is generally for X11 access (so you can open windows between computers.)
The other port, 53, is a UDP port used by the DNS system. Notice that UDP port are "just opened". You can always send packets to them. You cannot create a client/server connection like you do with TCP/IP. Hence, in this case you do not see the LISTEN state.
The last entry here is "raw". This is a local type of port which only works between processes within one computer. It may be used by processes to send RPC events and such.
Update:
Since then netstat has been somewhat deprecated and you may want to learn about ss instead:
ss -l4n
-- or --
ss -l6n
Unfortunately, at the moment you have to select either -4 or -6 for the corresponding stack (IPv4 or IPv6).
If you're interested in writing C/C++ code or alike, you can read that information from /proc/net/.... For example, the TCP connections are found here:
/proc/net/tcp (IPv4)
/proc/net/tcp6 (IPv6)
Similarly, you'll see UDP files and a Unix file.
Programmatically, if you are only checking one port then you can just attempt a connection. If the port is open, then it will connect. You can then close the connection immediately.
Finally, there is the Kernel direct socket connection for socket diagnostics like so:
int s = socket(
AF_NETLINK
, SOCK_RAW | SOCK_CLOEXEC | SOCK_NONBLOCK
, NETLINK_SOCK_DIAG);
The main problem I have with that one is that it does not really send you events when something changes. But you can read the current state in structures which is safer than attempting to parse files in /proc/....
I have some code handling such a socket in my eventdispatcher library. Only it still has to do a poll to get the data since the kernel does not generate events on its own (i.e. a push is much better since it only has to happen once when an event actually happens).