Running the top-command top -c on an Ubuntu server shows all the commands that are currently running. Concerning PostgreSQL commands, what do the values in the brackets mean? I am speaking of the values right next to the red box in the picture.
I can not find any documentation to support this, but I believe it is the client's remote port.
The PostgreSQL server is listening on a specific port (5432 by default) and when a client contacts it via TCP the client provides an arbitrary port for the server to talk back to the client. The red box in your example is covering the IP address of the client, that along with the port plus the IP:Port of the server makes up the unique connection.
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I need to write my first socket program involving TCP connections. In the program I have created there is a client and server, both of which are the machine I am coding on.However,it requires that I pass the port number as a command line argument. How do I accomplish this?
The answer is simple : Make sure your server and your client agree on the port to use. As long as the port is available and can be used, set up the connected so that the client and server use that same port.
Here's a link that explain the different ranges available for TCP and UDP ports.
As an exemple, the port 3074 is used by microsoft for its Xbox live service. Making an application using this port might interfere with the service.
The port used will be defined either in a configuration file or hard-coded in the source code of both the server and the client. You should easily be able to find it with a quick look at the code or the directory which contains the application.
For example, when you make an ssh connection, you are connected to port 22. What happens then? On a very high level brief overview, I know that if port 22 is open on the other end and if you can authenticate to it as a certain user, then you get a shell on that machine.
But I don't understand how ports tie into this model of services and connections to different services from remote machines? Why is there a need for so many specific ports running specific services? And what exactly happens when you try to connect to a port?
I hope this question isn't too confusing due to my naive understanding. Thanks.
Imagine your server as a house with 65536 doors. If you want to visit family "HTTP", you go to door 80. If you were to visit family "SMTP", you would visit door no. 25.
Technically, a port is just one of multiple possible endpoints for outgoing/incomming connections. Many of the port numbers are assigned to certain services by convention.
Opening/establishing a connection means (when the transport protocol is TCP, which are most of the “classical” services like HTTP, SMTP, etc.) that you are performing a TCP handshake. With UDP (used for things like streaming and VoIP), there's no handshake.
Unless you want to understand the deeper voodoo of IP networks, you could just say, that's about it. Nothing overly special.
TCP-IP ports on your machine are essentially a mechanism to get messages to the right endpoints.
Each of the possible 65536 ports (16 total bits) fall under certain categories as designated by the Internet Assigned Numbers Authority (IANA).
But I don't understand how ports tie into this model of services and
connections to different services from remote machines? Why is there a
need for so many specific ports running specific services?
...
And what exactly happens when you try to connect to a port?
Think of it this way: How many applications on your computer communicate with other machines? Web browser, e-mail client, SSH client, online games, etc. Not to mention all of the stuff running under the hood.
Now think: how many physical ports do you have on your machine? Most desktop machines have one. Occasionally two or three. If a single application had to take complete control over your network interface nothing else would be able to use it! So TCP ports are a way of turning 1 connection into 65536 connections.
For example, when you make an ssh connection, you are connected to
port 22. What happens then?
Think of it like sending a package. Your SSH client in front of you needs to send information to a process running on the other machine. So you supply the destination address in the form of "user#[ip or hostname]" (so that it knows which machine on the network to send it to), and "port 22" (so it gets to the right application running on the machine). Your application then packs up a TCP parcel and stamps a destination and a return address and sends it to the network.
The network finds the destination computer and delivers the package. So now it's at the right machine, but it still needs to get to the right application. What do you think would happen if your SSH packet got delivered to an e-mail client? That's what the port number is for. It effectively tells your computer's local TCP mailman where to make the final delivery. Then the application does whatever it needs to with the data (such as verify authentication) and sends a response packet using your machine's return address. The back and forth continues as long as the connection is active.
Hope that helps. :)
The port is meant to allow applications on TCP/IP to exchange data. Each machine on the internet has one single address which is its IP. The port allows different applications on one machine to send and receive data with multiple servers on the network/internet. Common application like ftp and http servers communicate on default ports like 21 and 80 unless network administrators change those default ports for security reasons
I have few doubts regarding telnet and echo which I haven't got so far lucky with google (also to say I have done my homework)
Does the telnet specification says that there are specific echo port and specific non-echo port?
If yes, Do ports 23 and 10001 differs on that specifically?
If conditional yes, is it some version of telnet protocol specific. In other words were those added after some specific iteration of telnet and/or are their fully compliant telnet clients and partially compliant one (regrading echo)?
also whats the difference if any between localecho and remote echo? I believe I am talking about echo from server side (remote echo?) but in any case which is what?
I am not sure if there is telnet / protocol specific child stackoverflow site? If yes, can somebody point me there?
The IANA Service Name and Transport Protocol Port Number Registry contains all port number registrations and for Telnet it cites RFC 854 as specification which defines
Port Assignment
When used for remote user access to service hosts (i.e., remote
terminal access) this protocol is assigned server port 23
(27 octal). That is L=23.
For the Echo Protocol the RFC number RFC 862 which similarily defines
TCP Based Echo Service
One echo service is defined as a connection based application on TCP.
A server listens for TCP connections on TCP port 7. Once a
connection is established any data received is sent back. This
continues until the calling user terminates the connection.
The port number 10001 is registered for "SCP Configuration".
When a true Telnet server starts, it sends a number of IAC commands to the client telling it how to behave. The server can tell the client to do its own character echo (IAC DO ECHO) or it can tell the client to act dumb (IAC DONT ECHO). The client then confirms its action (IAC WILL ECHO) or inaction (IAC WONT ECHO). If the client responds the opposite of what the server requests then the server can either adapt or continue to insist (and likely eventually fail).
https://www.rfc-editor.org/rfc/rfc857
So to answer your specific question, no, there are not different echo/noecho ports. It's a result of server configuration and client capability. Though technically it is possible to have the telnet server have different configuration on different ports.
You can find a list of special Telnet codes and available do/dont/will/wont options here:
http://www.laynetworks.com/telnet.htm
I have a simple node.js client and server programs running on one machine and when I try to connect to the server with second instance of client program simultaneously I get EADDRINUSE, Address already in use error. Is it possible to have two or more TCP based socket client connections (created with createConnection) to one server (created with createServer) on the same machine, or only one client program can be connected to the server at the same time?
Yes, it's possible. Infact, very common. Many applications open dozens, or hundreds of connections to the same server. It sounds like your client program is binding on a port. Only the server should be binding on a port. You should verify.
The client will usually use a random port between 1024-65535, as assigned by your OS. You don't need to worry about it. Since the client is initiating a connection to the server, the server port must unique to one program. Which is why your problem signifies you are trying to start the server twice. Please see http://www.tcpipguide.com/free/t_TCPIPClientEphemeralPortsandClientServerApplicatio.htm
We have a asp.net 3.5 web application running on a hosting company's web server that's got a very strict firewall setup. The web application communicates with our sql server express 2005 which is on a completely different network.
The problem is that the web applications sends our sql server a random port number to connect to when it connects to our sql server on port 1433. Hence the web application can not connect to our sql server because then they have to open a whole range off port numbers on the firewall which they are not prepared to do.
My question is the following. Can you specify either in your connection string or somewhere in your web.config the port the client sends to sql server to connect back to on?
Thanks
If I'm getting you right, you want to do something that goes against how TCP/IP port handling is supposed to work.
You want the web application to always use the same port when connecting to your SQL server? This would mean you can only have one connection open at any given time, and so only one user at any given time. The client is supposed to communicate with a server from a "random" port. It's the basics of networking. At least how I learned it.
I'm not sure you can even force it to use the same port every time, and even if you can I'd advice against it.
I'd say the problem is the firewall, and not your application. I can understand that the SQL server only wants to speak to the IP address that your site is on, but it shouldn't really care which port the client wants to talk over (something like [website-ip]:* -> [sql-ip]:1433). To me, it seems like who ever set up the firewall has some kind of crazy firewall ideas, but maybe that's just me.
Is it possible that your SQL Server Express is configured to listen on dynamic ports? If the 'TCP Dynamic Ports' value is 0, I think it will be dynamic even if you have a specific port configured. That value should be cleared completely.
You can specify port if you are using TCP/IP.
For ex:
Data Source=190.190.200.100,1433;Network Library=DBMSSOCN;Initial Catalog=myDataBase;User ID=myUsername;Password=myPassword;
where DBMSSOCN=TCP/IP