Let's assume I'm in computer A, I have a few servers running on different ports, but all are basically an instance of the same program (just binding to different ports). Now, computer B, a client, does he need to know what port is the software he wishes to connect to on computer A?
The point is, I am implementing some sort of communication similar to sockets. Everything should work fine but I'm not sure how to create the initial-message from a computer to another - I just don't know to what port to send it to. Does the client know the port he's sending to on the server?
Say here (client): clientsocket.connect(('localhost', 8089)), does the client connect a server running on port 8089? If so, what port is his socket on (what port is he using for the client?
Yes. The only way for the network stack on computer A to know which process to deliver an incoming packet is for computer B to set the correct port in the packet. A web server runs on port 80 by default, but a machine running several distinct web servers will run them on distinct ports, and a client must be specific about which server they want to connect to. http://example.com, http://example.com:8080, and http://example.com:12345 would refer to the servers running on example.com on ports 80, 8080, and 12345, respectively.
In order to know which port to use in your client, you need to read the documentation for the server you want to connect to.
Going in the other direction, the port used by the client to receive responses is typically set by the networking stack automatically. The client doesn't need to do anything special to set it, and the server simply sends packets back to the address/port found in the source portion of the incoming packet.
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.
Usually a web server is listening to any incoming connection through port 80. So, my question is that shouldn't it be that in general concept of socket programming is that port 80 is for listen for incoming connection. But then after the server accepted the connection, it will use another port e.g port 12345 to communicate with the client. But, when I look into the wireshark, the server is always using port 80 during the communication. I am confused here.
So what if https://www.facebook.com:443, it has hundreds of thousands of connection to the it at a second. Is it possible for a single port to handle such a large amount of traffic?
A particular socket is uniquely identified by a 5-tuple (i.e. a list of 5 particular properties.) Those properties are:
Source IP Address
Destination IP Address
Source Port Number
Destination Port Number
Transport Protocol (usually TCP or UDP)
These parameters must be unique for sockets that are open at the same time. Where you're probably getting confused here is what happens on the client side vs. what happens on the server side in TCP. Regardless of the application protocol in question (HTTP, FTP, SMTP, whatever,) TCP behaves the same way.
When you open a socket on the client side, it will select a random high-number port for the new outgoing connection. This is required, otherwise you would be unable to open two separate sockets on the same computer to the same server. Since it's entirely reasonable to want to do that (and it's very common in the case of web servers, such as having stackoverflow.com open in two separate tabs) and the 5-tuple for each socket must be unique, a random high-number port is used as the source port. However, each of those sockets will connect to port 80 at stackoverflow.com's webserver.
On the server side of things, stackoverflow.com can already distinguish between those two different sockets from your client, again, because they already have different client-side port numbers. When it sees an incoming request packet from your browser, it knows which of the sockets it has open with you to respond to because of the different source port number. Similarly, when it wants to send a response packet to you, it can send it to the correct endpoint on your side by setting the destination port number to the client-side port number it got the request from.
The bottom line is that it's unnecessary for each client connection to have a separate port number on the server's side because the server can already uniquely identify each client connection by its client IP address and client-side port number. This is the way TCP (and UDP) sockets work regardless of application-layer protocol.
shouldn't it be that in general concept of socket programming is that port 80 is for listen for incoming connection. But then after the server accepted the connection, it will use another port e.g port 12345 to communicate with the client.
No.
But, when I look into the wireshark, the server is always using port 80 during the communication.
Yes.
I am confused here.
Only because your 'general concept' isn't correct. An accepted socket uses the same local port as the listening socket.
So what if https://www.facebook.com:443, it has hundreds of thousands of connection to the it at a second. Is it possible for a single port to handle such a large amount of traffic?
A port is only a number. It isn't a physical thing. It isn't handling anything. TCP is identifying connections based on the tuple {source IP, source port, target IP, target port}. There's no problem as long as the entire tuple is unique.
Ports are a virtual concept, not a hardware ressource, it's no harder to handle 10 000 connection on 1 port than 1 connection each on 10 000 port (it's probably much faster even)
Not all servers are web servers listening on port 80, nor do all servers maintain lasting connections. Web servers in particular are stateless.
Your suggestion to open a new port for further communication is exactly what happens when using the FTP protocol, but as you have seen this is not necessary.
Ports are not a physical concept, they exist in a standardised form to allow multiple servers to be reachable on the same host without specialised multiplexing software. Such software does still exist, but for entirely different reasons (see: sshttp). What you see as a response from the server on port 80, the server sees as a reply to you on a not-so-random port the OS assigned your connection.
When a server listening socket accepts a TCP request in the first time ,the function such as Socket java.net.ServerSocket.accept() will return a new communication socket whoes port number is the same as the port from java.net.ServerSocket.ServerSocket(int port).
Here are the screen shots.
Is it possible to use port 80 for non http traffic ? For example I'm making a small script that will communicate with a friends computer through the internet, however they must port forward it to get past the router. Is there a problem with using port 80 in the script so it will be let through automatically ? Is there some part of this i don't understand that will not let non http data through ? Please explain :)
there is no problem doing that. in fact, skype's default behaviour is to use port 80 and port 443 to transport voice!
There are a lot of ISPs that actually block port 80, so you might want to try a different port if you are having a problem (still needs to be forwarded)
The firewall on the computer also needs to be set to allow the incoming traffic.
This will work fine, but your friend may still need to setup port forwarding
If your friend's PC is the one listening on Port 80, he will need to setup port forwarding. Otherwise, how would the router/NAT know which computer in the house to bridge the connection to?
But if your friend's PC is the one making the outbound connection, then likely no port forwarding is needed at all on his end.
In other words, port forwarding (for TCP) is only for inbound connections. The router/NAT will automatically setup a port mapping scheme for outbound connections (as it does it with all web traffic).
Can two applications on the same machine bind to the same port and IP address? Taking it a step further, can one app listen to requests coming from a certain IP and the other to another remote IP?
I know I can have one application that starts off two threads (or forks) to have similar behavior, but can two applications that have nothing in common do the same?
The answer differs depending on what OS is being considered. In general though:
For TCP, no. You can only have one application listening on the same port at one time. Now if you had 2 network cards, you could have one application listen on the first IP and the second one on the second IP using the same port number.
For UDP (Multicasts), multiple applications can subscribe to the same port.
Edit: Since Linux Kernel 3.9 and later, support for multiple applications listening to the same port was added using the SO_REUSEPORT option. More information is available at this lwn.net article.
Yes (for TCP) you can have two programs listen on the same socket, if the programs are designed to do so. When the socket is created by the first program, make sure the SO_REUSEADDR option is set on the socket before you bind(). However, this may not be what you want. What this does is an incoming TCP connection will be directed to one of the programs, not both, so it does not duplicate the connection, it just allows two programs to service the incoming request. For example, web servers will have multiple processes all listening on port 80, and the O/S sends a new connection to the process that is ready to accept new connections.
SO_REUSEADDR
Allows other sockets to bind() to this port, unless there is an active listening socket bound to the port already. This enables you to get around those "Address already in use" error messages when you try to restart your server after a crash.
Yes.
Multiple listening TCP sockets, all bound to the same port, can co-exist, provided they are all bound to different local IP addresses. Clients can connect to whichever one they need to. This excludes 0.0.0.0 (INADDR_ANY).
Multiple accepted sockets can co-exist, all accepted from the same listening socket, all showing the same local port number as the listening socket.
Multiple UDP sockets all bound to the same port can all co-exist provided either the same condition as at (1) or they have all had the SO_REUSEADDR option set before binding.
TCP ports and UDP ports occupy different namespaces, so the use of a port for TCP does not preclude its use for UDP, and vice versa.
Reference: Stevens & Wright, TCP/IP Illustrated, Volume II.
In principle, no.
It's not written in stone; but it's the way all APIs are written: the app opens a port, gets a handle to it, and the OS notifies it (via that handle) when a client connection (or a packet in UDP case) arrives.
If the OS allowed two apps to open the same port, how would it know which one to notify?
But... there are ways around it:
As Jed noted, you could write a 'master' process, which would be the only one that really listens on the port and notifies others, using any logic it wants to separate client requests.
On Linux and BSD (at least) you can set up 'remapping' rules that redirect packets from the 'visible' port to different ones (where the apps are listening), according to any network related criteria (maybe network of origin, or some simple forms of load balancing).
Yes Definitely. As far as i remember From kernel version 3.9 (Not sure on the version) onwards support for the SO_REUSEPORT was introduced. SO_RESUEPORT allows binding to the exact same port and address, As long as the first server sets this option before binding its socket.
It works for both TCP and UDP. Refer to the link for more details: SO_REUSEPORT
No. Only one application can bind to a port at a time, and behavior if the bind is forced is indeterminate.
With multicast sockets -- which sound like nowhere near what you want -- more than one application can bind to a port as long as SO_REUSEADDR is set in each socket's options.
You could accomplish this by writing a "master" process, which accepts and processes all connections, then hands them off to your two applications who need to listen on the same port. This is the approach that Web servers and such take, since many processes need to listen to 80.
Beyond this, we're getting into specifics -- you tagged both TCP and UDP, which is it? Also, what platform?
You can have one application listening on one port for one network interface. Therefore you could have:
httpd listening on remotely accessible interface, e.g. 192.168.1.1:80
another daemon listening on 127.0.0.1:80
Sample use case could be to use httpd as a load balancer or a proxy.
When you create a TCP connection, you ask to connect to a specific TCP address, which is a combination of an IP address (v4 or v6, depending on the protocol you're using) and a port.
When a server listens for connections, it can inform the kernel that it would like to listen to a specific IP address and port, i.e., one TCP address, or on the same port on each of the host's IP addresses (usually specified with IP address 0.0.0.0), which is effectively listening on a lot of different "TCP addresses" (e.g., 192.168.1.10:8000, 127.0.0.1:8000, etc.)
No, you can't have two applications listening on the same "TCP address," because when a message comes in, how would the kernel know to which application to give the message?
However, you in most operating systems you can set up several IP addresses on a single interface (e.g., if you have 192.168.1.10 on an interface, you could also set up 192.168.1.11, if nobody else on the network is using it), and in those cases you could have separate applications listening on port 8000 on each of those two IP addresses.
Just to share what #jnewton mentioned.
I started an nginx and an embedded tomcat process on my mac. I can see both process runninng at 8080.
LT<XXXX>-MAC:~ b0<XXX>$ sudo netstat -anp tcp | grep LISTEN
tcp46 0 0 *.8080 *.* LISTEN
tcp4 0 0 *.8080 *.* LISTEN
Another way is use a program listening in one port that analyses the kind of traffic (ssh, https, etc) it redirects internally to another port on which the "real" service is listening.
For example, for Linux, sslh: https://github.com/yrutschle/sslh
If at least one of the remote IPs is already known, static and dedicated to talk only to one of your apps, you may use iptables rule (table nat, chain PREROUTING) to redirect incomming traffic from this address to "shared" local port to any other port where the appropriate application actually listen.
Yes.
From this article:
https://lwn.net/Articles/542629/
The new socket option allows multiple sockets on the same host to bind to the same port
Yes and no. Only one application can actively listen on a port. But that application can bequeath its connection to another process. So you could have multiple processes working on the same port.
You can make two applications listen for the same port on the same network interface.
There can only be one listening socket for the specified network interface and port, but that socket can be shared between several applications.
If you have a listening socket in an application process and you fork that process, the socket will be inherited, so technically there will be now two processes listening the same port.
I have tried the following, with socat:
socat TCP-L:8080,fork,reuseaddr -
And even though I have not made a connection to the socket, I cannot listen twice on the same port, in spite of the reuseaddr option.
I get this message (which I expected before):
2016/02/23 09:56:49 socat[2667] E bind(5, {AF=2 0.0.0.0:8080}, 16): Address already in use
If by applications you mean multiple processes then yes but generally NO.
For example Apache server runs multiple processes on same port (generally 80).It's done by designating one of the process to actually bind to the port and then use that process to do handovers to various processes which are accepting connections.
Short answer:
Going by the answer given here. You can have two applications listening on the same IP address, and port number, so long one of the port is a UDP port, while other is a TCP port.
Explanation:
The concept of port is relevant on the transport layer of the TCP/IP stack, thus as long as you are using different transport layer protocols of the stack, you can have multiple processes listening on the same <ip-address>:<port> combination.
One doubt that people have is if two applications are running on the same <ip-address>:<port> combination, how will a client running on a remote machine distinguish between the two? If you look at the IP layer packet header (https://en.wikipedia.org/wiki/IPv4#Header), you will see that bits 72 to 79 are used for defining protocol, this is how the distinction can be made.
If however you want to have two applications on same TCP <ip-address>:<port> combination, then the answer is no (An interesting exercise will be launch two VMs, give them same IP address, but different MAC addresses, and see what happens - you will notice that some times VM1 will get packets, and other times VM2 will get packets - depending on ARP cache refresh).
I feel that by making two applications run on the same <op-address>:<port> you want to achieve some kind of load balancing. For this you can run the applications on different ports, and write IP table rules to bifurcate the traffic between them.
Also see #user6169806's answer.