I'm developing a SIP parser in C (client only) and i have doubt about, do i need to bind a socket with a specific port (5060) on double nat?. What i'm sure about it's that is really important in the server side but i'm not really sure about the client side
You don't have to use port 5060 on the client side regardless of the NAT type. There is no any disadvantage if you just pickup a random port. The only recommendation is that once you pickup a port, keep that across sessions to help NAT bypassing a bit in some circumstances and to not overflow NAT's with various binding.
Even on the server side you can use any port, but there is a big disadvantage: users need to type also the port part as the server address (yourdomain:port) if you are not using the standard 5060 port.
Think about it like in case of http. On the web server the standard port is 80. However none of the clients (web browsers) are using port 80 on the client side.
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Very short qustion. Lets say user1 is connected to internet and running a http server # local. he needs to set port forwarding to work this. redirecting all incoming requests from public ip to local ip's port 80.
my doubt is that, User1 opens mozilla firefox , lets say, port 12343 , assigned by the os.
from this, (192.168.0.14:12343) to google.com:80... sometimes our router changes the incoming port to another port # NAT . clear..
My question: is there is any port forwarding set at the router to handle to route the packet.. ie, requests from google:80 to :12343 . plz correct me if am wrong at any protocol suite layers. i am new to this.
When connection is established through NAT, NAT maintains mapping between inside port and outside port. That is, when the packet comes from outside to the port 54321, NAT knows to forwward it to internal network IP 192.168.0.1., port 12345.
To explain further, let's dwell into details. Let's talk about transparent NAT. Transparent NAT's are ones which do not require any special configuration on locla software (unlike HTTP proxy servers, for instance). They usually serve as network gateways, so that OS knows to route network trafic to such a gateway (almost all home routers work in this mode).
When someone opens web page from desktop - local address 192.169.1.1, local port 12345, remote address stackoverflow.com, remote port 80 - OS directs trafic to network gateway (192.168.1.0).
Gateway sees the trafic as coming from 192.168.1.1, port 12345. On the packet, it replaces 192.168.1.1 with it's outside IP (say, 2.2.2.2) and gives it a port - say, 54321. It also creates an entry in it's mapping tables, indicating that all trafic incoming from outside for port 54321 is to be forwarded to 192.168.1.1, port 12345. StackOverflow server sees the trafic as coming from gateway, and responds back to the gateway address and port. Gatewat sees response, consults mapping table and forwards it to the local machine, where it is seen by the browser - and thus my answer is displayed on your screen.
I think there is nothing to do with NAT here. NAT just translates the internal local address(like 192.168.1.1) to an external global address(like 139.130.4.5). I hope you have adequate knowledge on OSI model. Let me explain it. When a packet reaches the transport layer,it is assigned a random port number(ranging from 0-65535),either TCP or UDP, by the OS. However, the OS can only port numbers from 49152 to 65535, as several ports are registered or is used for some specific process. A port is used to identify a service or a process. After adding port number, the packets are given to the network layer, which adds the source address and destination address of the packet. Switching is a process that happens in the network layer. This switching mechanism is responsible for the source to destination delivery of packets. Internet uses packet switching. When you are sending a packet in this switching mechanism, the packets get routed to several switches between the source and destination. Every packet that is sent through these switches are routed based on a switching table or routing table. This table contains details such as the MAC address and a physical port of the switch through which the packet is received and sent.
This is the only port forwading that happens inside a router or a switch. Delivering the packet to a specified MAC address is the only duty of a switch.
Every packet you sent through a router goes to its destination based on the routing table. Several protocols work in this layer to make the source to destination delivery possible and some of them are ARP,IP,RARP etc.
Additionally, a packet is encapsulated with information from top layers as it moves down the layers. So, at the receiver side, the packet will comes at network layer and then gets decapsulated and it is moved to the transport layer, which then decapsulate the packed and send it to the corresponding process base on the port.
So, what I told is that there is no connection with a process (port number) and the physical port of the router. It is true that the packet travels through the physical port of the router but it doesn't know anything about the process that sends the packet.
A socket is combination of an IP address, a transport protocol and a port number.
For example: two hosts, on different LANs behind NAT, can have the same IP (let's say 192.168.0.2).
When these hosts connect to a web server, could two identical sockets be established?
I know that ports are randomly generated, but on different hosts it could be generated the same one.
Or it is server which tells that this port number is already in use?
Or is it NAT device, which changing SRC IP in IP packet?
How does NAT device knows then, where to send packet back to host on LAN?
Thanks
As you correctly guessed it is NAT who assigns ephemeral port number, so they can't coincide. Therefore, from server perspective, destination duplet of host #1 would be something like 192.168.0.2:46812, and for host #2 - 192.168.0.2:51378. When NAT receives these packets, it knows which host behind the NAT the packet belongs to based on the port number. If you have access to machine's console you can check the numbers with netstat -anp
I an writing a small application that needs to connect through one of multiple network interfaces on the machine. The interface is not the "default" one (the one with the default route). Is it possible to bind an outbound TCP socket directly to a specific interface?
Here is an example:
eth0: 192.168.1.10, gateway 192.168.1.1
eth1: 192.168.2.10, gateway 192.168.2.1
default gateway: 192.168.1.1
(both interfaces can reach the Internet through different external IPs)
Now, I want my application to use eth1 to connect to an external server, even if the system is configured to use eth0 for external traffic.
(The question is probably trivial, but I just wanted to know if it is possible at all before spending time on it)
Currently, I am using Python with Twisted, but if I have to use BSD sockets then so be it.
From: http://linux.about.com/od/commands/l/blcmdl7_socket.htm
SO_DONTROUTE - Don't send via a gateway, only send to directly connected hosts. The same effect can be achieved by setting the MSG_DONTROUTE flag on a socket send(2) operation. Expects an integer boolean flag.
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.