If I understand right, applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems. But how does that work without conflicting with other applications such as web-browsers? In fact how do multiple browsers running at once not conflict? Do they all monitor the port and get notified... can you share a port in this way?
I have a feeling this is a dumb question, but not something I ever thought of before, and in other cases I've seen problems when 2 apps are configured to use the same port.
There are 2 ports: a source port (browser) and a destination port (server). The browser asks the OS for an available source port (let's say it receives 33123) then makes a socket connection to the destination port (usually 80/HTTP, 443/HTTPS).
When the web server receives the answer, it sends a response that has 80 as source port and 33123 as destination port.
So if you have 2 browsers concurrently accessing stackoverflow.com, you'd have something like this:
Firefox (localhost:33123) <-----------> stackoverflow.com (69.59.196.211:80)
Chrome (localhost:33124) <-----------> stackoverflow.com (69.59.196.211:80)
Outgoing HTTP requests don't happen on port 80. When an application requests a socket, it usually receives one at random. This is the Source port.
Port 80 is for serving HTTP content (by the server, not the client). This is the Destination port.
Each browser uses a different Source to generate requests. That way, the packets make it back to the correct application.
It is the 5-tuple of (IP protocol, local IP address, local port, remote IP address, remote port) that identifies a connection. Multiple browsers (or in fact a single browser loading multiple pages simultaneously) will each use destination port 80, but the local port (which is allocated by the O/S) is distinct in each case. Therefore there is no conflict.
Clients usually pick a port between 1024 and 65535.
It depends on the operating system how to handle this. I think Windows Clients increment the value for each new connection, Unix Clients pick a random port no.
Some services rely on a static client port like NTP (123 UDP)
A browser is a client application that you use in order to see content on a web server which is usually on a different machine.
The web server is the one listening on port 80, not the browser on the client.
You need to be careful in making the distinction between "listening on port 80" and "connecting to port 80".
When you say "applications sometimes use HTTP to send messages, since using other ports is liable to cause firewall problems", you actually mean "applications sometimes send messages to port 80".
The server is listening on port 80, and can accept multiple connections on that port.
Port 80 you're talking about here is the remote port on the server, locally browser opens high port for each connection established.
Each connection has port numbers on both ends, one is called local port, other remote port.
Firewall will allow traffic to high port for browser, because it knows that connection has been established from you computer.
Related
When we open a TCP Listening, we use a fixed port, like "9870".
But the clients which connect to this listening, use different ports like "1024, 1025" or other. I don't know what is the name of this port, "client port", "dynamic port" or "ephemeral port"... But I need to know if is possible to change this client port.
Because, like in the second image, it shows the error "Port numbers reused", and I think this is related to this port configuration.
I think if I could configure these ports, the connections of the equipments on my network will be stabilized.
TL;TR: there is usually no need to configure the clients source ports and you can definitely not set the clients source port at the server.
The client can bind to a address+port the same way the server can do and this port is then used as the source port for the connection. But usually this is not done and instead the socket is not specifically bound and a free source port is automatically assigned by the system. The client source port can only be set by the client itself and can not be changed by the server.
Usually it is not possible that a port number gets reused by the client since the OS will not let the client do this. But what you see can happen if the client crashes . After the restart the client is not aware of any connections which were established (and never closed) before the reboot so it will happily use the same source port again. In this case it gets a RST from the server since the new data do not match the old connection.
This can also happen if the client is connected with some router doing NAT and the router crashes. After restart the router is not aware of any previous connections and will thus create new translations which might conflict with old connections.
I am confused that if ports serve to distinguish the applications , so as to tell to which particular application data packet belongs , then if i have two browsers both downloading file using FTP , how would packet be distinguished to which brower it is going, as FTP uses port 21 ?
FTP is not the best example as it's behaviour change depending on Active/Passive mode.
Consider HTTP instead, which is simplier.
Browser_A initiates a TCP connection to ServerIP:80
PC assigns a random port, example HostIP:55123
TCP Connextion between HostIP:55123 and ServerIP:80 is established
Server receives a request and responds to HostIP:55123
PC knows every TCP packet from ServerIP:80 to 55123 should be delivered to Browser_A
First of all port 21 is COMMAND port, so when downloading file for sure FTP clients will not be connected to port 21.
Secondly, a connection is established from a high port (> 1024) to the FTP data port, so your browsers will open (in PASSIVE mode) a connection from a high port.
Answering to your question, if you open 2 browsers and start downloading data from FTP server, every browser will connect from a different port.
I've been investigating networking for use in a two-player game I'm writing, and I'm still not clear on when a device must have a port forwarded in order to communicate with the outside world.
From what I've seen in other games, port forwarding is always required in order to host a server, but is not required on the client. In addition, there are other situations, such as skype (which, to my understanding is ultimately client to client), where neither end must forward a port.
So my question is, in over-the-Internet communication, when is and isn't port forwarding necessary, and what steps can i take as a developer to make it so my users don't have to worry about it? Thanks in advance!
Port forwarding is needed when a machine on the Internet needs to initiate a connection to a machine that's behind a firewall or NAT router. If the connection is initiated by the machine behind the firewall, the firewall/router automatically recognizes the reply traffic and sends it to the machine that opened the connection.
But if a packet arrives on the external interface, and it's not a part of such a connection, the router needs to know what to do with it. By default, it will reject it. But if forwarding is configured for the port, that tells it what internal machine to send it to.
Put another way: you need port forwarding if you want to run a server behind the NAT firewall/router, you don't need it if you're just running a client.
There is reason why Skype don't (not always) need manual setting of port forwarding:
When you install Skype, a port above 1024 is chosen at random as the
port for incoming connections. You can configure Skype to use a
different port for incoming connections if you wish, but if you do,
you must open the alternative port manually.
If the port chosen for incoming connections becomes unavailable, by
default ports 80 and 443 will be used as alternatives. If another
application (such as Apache HTTP server or IIS) uses these ports, you
can either configure the application to use other ports, or you can
configure Skype to not use these ports.
Port forwarding is must if you host a server.
You can use same technique as Skype...
I am not sure if there is any other option...
Port forwarding (occurs) when a NAT, firewall or some other device blocks communication on all or some ports.
To answer your question as an example, most commercial routers use NAT to allow multiple people to use the same IP(As view from the outside world) provided by ISPs. Most ISP's use NAT to allow multiple customers to use the same IP(As viewed from the outside world). To get this to work, the NAT changes the internal IP and the port number of a communication to THE(there is only one for the entire sub network) external IP and a new port number. By doing this, the router/isp/ect can tell which internal IP and port each external communication goes to.
Anytime one of the computers communicating over the internet are behind a NAT, port forwarding is required. I'm sure there are way more situations than this, and the solution to each can be quite complicated. But this covers the vast majority.
What port does httpclient use?
80, 8080, ....?
There are always two ports involved in a connection - one at the server, and another at the client.
The server-side port is specified (and is known to the client) via the HTTP URL (the default is 80 for HTTP connections and 443 for HTTPS) and is specified after the hostname using hostname:port notation.
On the client-side, things are a bit more interesting. Clients open a temporary or dynamic port that lasts only as long as the connection is made. Client-side ports are always opened on the dynamic range (49152 through 65535) reserved for them by IANA, which is usually enforced on the newer operating systems (I'm not sure if Windows XP follows the IANA directive to the letter, but Vista and Windows Server 2008 happen to).
If you mean the destination port, the standard for the non-secure (HTTP) protocol is 80, whereas the standard HTTPS port is 443.
In terms of the outbound client port, that's largely irrelevant and will be chosen based on what ports are available, etc. (i.e.: There's no set outbound port.)
The standard port is 80. If you see something like http://example.com:8080, it means the site is using a different port, in this case 8080.
From what I understand, each HTTP request uses its own TCP connection (please correct me if i'm wrong). So, let's say that there are two current connections to the same server. For example, client side javascript code triggering a couple of AJAX POST requests using the XMLHttpRequest object, one right after the other, before getting the response to the first one. So we're talking about two connections to the same server, each waiting for a response in order to route it to each separate callback function.
Now here's the thing that I don't understand: The TCP packet includes source and destination ip and port, but won't both of these connections have the same src and dest ip addresses, and port 80? How can the packets be differentiated and routed to appropriately? Does it have anything to do with the packet sequence number which is different for each connection?
When your browser creates a new connection to the HTTP server, it uses a different source port.
For example, say your browser creates two connections to a server and that your IP address is 60.12.34.56. The first connection might originate from source port 60123 and the second from 60127. This is embedded in the TCP header of each packet sent to the server. When the server replies to each connection, it uses the appropriate port (e.g. 60123 or 60127) so that the packet makes it back to the right spot.
One of the best ways to learn about this is to download Wireshark and just observe traffic on your own network. It will show you this and much more.
Additionally, this gives insight into how Network Address Translation (NAT) works on a router. You can have many computers share the same IP address and the router will rewrite the request to use a different port so that two computers can simultaneously connect to places like AOL Instant Messenger.
They're differentiated by the source port.
The main reason for each HTTP request to not generate a separate TCP connection is called keepalives, incidentally.
A socket, in packet network communications, is considered to be the combination of 4 elements: server IP, server port, client IP, client port. The second one is usually fixed in a protocol, e.g. http usually listen in port 80, but the client port is a random number usually in the range 1024-65535. This is because the operating system could use those ports for known server protocols (e.g. 21 for FTP, 22 for SSH, etc.). The same network device can not use the same client port to open two different connections even to different servers and if two different clients use the same port, the server can tell them apart by their IP addresses. If a port is being used in a system either to listen for connection or to establish a connection, it can not be used for anything else. That's how the operating system can dispatch packets to the correct process once received by the network card.