Hii All,
While running a socket programme (server side ) am getting message like
Address already in use
Am trying to connect to port 80 since port 80 is reserved for https application So before running server side programme i am closing all application that uses https application ,is it enough...
or am doing it wrong??
Am trying to make a communication between browser and termial...
You must run your application as super user(root) on Linux or administrator privileges on Windows in order to bind to port 80. This is the case for all service ports, which is < 1024. Either that or there still is another program binded to that port.
Try using netstat to find out what programs might be listening on port 80.
Example:
on Linux:
netstat -punta
on Windows:
netstat -ban
Both must be run with super user/admin privileges in order to see the program names that bind to specific ports.
If you just closed another process listening on 80 port, this port will be blocked for a certain timespan depending on your OS. This behavior is here to prevent an attacker to crash a service on your machine and immediately restart a malicious service on the same port.
This behavior can be disabled by using SO_REUSEADDR (by using setsockopt).
If your main problem is to communicate from a custom server to your broswer, you can use any port in your server for providing HTTP (8080 is common for that), just specify the port in the url http://server:port/ (ie. http://localhost:8080/)
Related
Please forgive me for being naive on the subject, however, I do not understand ports and how they work in the slightest. I am trying to make a program where two computers could communicate given their IP addresses and I am using TCP protocol. I don't, however, know what port(s) I would be able to use for this application, because when I look up TCP ports I get a list of ports each with their own function. Any help is useful.
P.S. I am using C to create my program
The short answer is you can choose any port you like - although the safe range is generally considered to be between 1024 and 65535. The only problem that you will encounter is when some other program installed on the device is already listening on that port. Unfortunately, there is no port that is guaranteed to be available to listen on.
One possible solution to this is to have a primary listening port and a fallback secondary port. You can then first try to connect on the primary port and, if a satisfactory response is not received, try to connect on the secondary port. However, even this is not infallible, as there is a chance that the secondary post could also be in use.
The easiest approach is to try to create your listener on the port that you have chosen, and if the port fails to create, let the user know that some other application is preventing execution of your application.
I want to connect to my home computer from the library via a VNCServer located on my home computer, but I'm pretty sure the library blocks the port 5900 and higher. Can I change the port which my vncserver listens to get around the blocked port? Which port probably would allow me to get around the library's port blocking? Is there anyway I can test which ports are open at the library?
The VNCServer is running on your home machine, it shouldn't matter what ports are blocked in the library. You are in the library(with your laptop?). The library blocking ports 5900+ only stops incoming connections (coming into the library) from using those ports not from you connecting to other device's port on that device's port number.
You can check from the library if your port/IP is visible using things like : http://www.canyouseeme.org/
Do reserved TCP/IP ports require that a program is running and bound to the port? If no such program is running or exists, can another program use this port? For example, on Linux, port 7 is reserved for an echo server. I assume there is some program running and is bound to port 7 of the machine. The program basically echos back input. If this program is stopped, will port 7 be released?
If I wrote my own echo server and bound it to some other port, wouldn't this port be released once my custom echo server program is killed?
Does the same thing happen for reserved ports?
Also, if all these programs are running on reserved ports, wouldn't they consume system resources even if they are blocked listening for a connection? Are these programs running at all times?
Do reserved TCP/IP ports require that a program is running and bound to the port?
No.
If no such program is running or exists, can another program use this port?
Nothing to stop you, but it's still reserved, and users are entitled to complain to you if you misuse ports reserved for something else.
For example, on Linux, port 7 is reserved for an echo server. I assume there is some program running and is bound to port 7 of the machine. The program basically echos back input. If this program is stopped, will port 7 be released?
Yes.
If I wrote my own echo server and bound it to some other port, wouldn't this port be released once my custom echo server program is killed?
Yes.
Does the same thing happen for reserved ports?
Yes, of course.
Also, if all these programs are running on reserved ports, wouldn't they consume system resources even if they are blocked listening for a connection?
Yes.
Are these programs running at all times?
Either they are running or they aren't running. You're asking about both situations at the same time. If you mean 'executing', i.e. consuming CPU, the answer is no, they are blocked waiting for connections while there are no connections.
ports 1-65535 are available, and ports in range 1-1023 are the privileged ones and using for standard applications.
And there will be Ephemeral port range also exists in your system and it can be found as follows:
sysctl -A | grep ip_local_port_range
Epeheral port range is available for all your client sockets.
When there is a server, client communication most of the time communication is happening using a socket. Socket is nothing but a pair of IP address and a port number. All the port referred usually with configuration will be a server port configuration and client port is dynamically choosing from the Epeheral port range. In case of epeheral ports, system may not release the port until this range is exhausted.
You can check port availability by using command:
netstat -a | grep <port number>
[EDIT]
The idea of a reserved port is that any custom software you write should take care to avoid binding to them, to avoid interfering with an established service. Further, non-root users can't bind any ports below 1024 anyway, many of which are registered with IANA (aka reserved).
There's no requirement that a particular process be bound to any port. A port is just a system resource. Typically a master inetd starts early in the system boot sequence, binds to some low-numbered ports, and handles the trivial services such as echo itself. These algorithms are so simple, and so infrequently used in practice, that very few resources are consumed. That's why you'll not find a separate "echo server" process. If you read the inetd.conf manual page:
http://www.freebsd.org/cgi/man.cgi?query=inetd.conf&sektion=5&manpath=FreeBSD%209.2-RELEASE
The inetd utility also provides several other 'trivial' services inter-
nally by use of routines within itself. These services are 'echo',
'discard', 'chargen' (character generator), 'daytime' (human read-
able time), and 'time' (machine readable time, in the form of the number of seconds since midnight, January 1, 1900).
I have a windows application on my PC which connects to a device over telnet. It sends a series of commands to the device, and the device responds to it.
Is there any way I can listen to what that program is writing to the device?
I tried using win32 socket programming to create a client that connects to the application. But, I get an error saying connection refused. When I analyze the traffic between the application and device on wireshark, I can see that the application uses different ports each time it reads from or writes to the telnet port(23) of the device.
Is there any way I can read the commands sent by the program to the device?
If you want to capture program's output to the device
programmatically, the right way is not to connect to the program (you
can't tap into an existing connection), but instead provide a server
that will stand for target device from the program's point of view.
It's going to work if the program can be configured to connect to the
device at different address and port. Write a "proxy" that listens on
some fixed port, and for any accepted connection, opens a client
connection to the real device. Then it should forward data in both
directions between the accepted connection from the program and the
client connection to the device. During this, you can also parse data
or do whatever you want with them, e.g. forward them to yet another
connection to another real device.
I'm learning C socket programming. When would you use bind() on the client-side? What types of program will need it and why? Where can I find an example?
On the client side, you would only use bind if you want to use a specific client-side port, which is rare. Usually on the client, you specify the IP address and port of the server machine, and the OS will pick which port you will use. Generally you don't care, but in some cases, there may be a firewall on the client that only allows outgoing connections on certain port. In that case, you will need to bind to a specific port before the connection attempt will work.
An example would be the data connection of an active FTP connection. In this case, the server connects from its port 20 to the IP and port specified by a PORT or EPRT command.
A classic example of a client program using bind() is the (obsolete) rlogin / rsh family of network clients. These clients were intended to be used within networks with strong trust relationships - in some cases the server machine trusts the client machine to tell it the username of the user that is connecting. This required that the client program connect from a low port (a port less than 1024), because such ports are restricted to the root user and thus (in theory) prove that the client being used is authorised by the system administrator.
The NFS protocol has similar trust relationships, and similarly the client makes connections from a low port number, using bind().
Another example is IRC clients that allow the user to specify a particular source IP address to connect from. This is to accomodate users with many IP addresses assigned to their machine, each with a different "vanity" domain name assigned to it. Choosing which IP to connect from (with bind()) allows the user to choose which domain name to appear as on IRC.
A good situation would be in a p2p case, you’re communicating with a STUN Server with a bound socket, and the STUN Server tells you the port on which he is receiving messages from your socket (that can be different from the one you specified when you bound your socket depending on your network and more specifically on your NAT type). This will allow you to be aware of the real port translation that your NAT is doing, and you’ll be able to give this information to potential peers that want to connect to you.
Binding the socket is useful as some NATs are dynamically giving you ports (binding on port x twice might not give you the same “real” port). So you’ll be able to directly use the socket you bound to listen on the port.
I suppose you should bind() in the case of UDP sockets.
bind function is one of "key" functions. It associates your socket (server or client) with address (ip + port). As for Windows you must use bind for WinSockets. There is good book about it "Network Programming for Microsoft Windows" by Anthony Jones and Jim Ohlund.
Bind can be used to attach names to a sockets. Thus, say you create a software that uses a particular TCP port, you need to bind it and then, you will know the TCP port it is using.