Does the accept function returns error(-1) if the Ethernet interface it is attached goes out of network?if not how does the application(TCP server) will know that its interface is not active any more ??
I am using one thread for accepting the connection and not using any "select" statement for doing so.Directly calling accept() function but somehow it is not retuning error if I remove the IP address from Ethernet interface.
using C and working environment is linux.
Usually you don't bind your server socket to a specific IP address (you use INADDR_ANY). So even if you remove your ethernet IP address, you could still contact your server using the loopback interface. Or some other interface with an IP address.
If you want to make sure that your server is reachable from the net, checking the interface status does not get you much. It's just a single hop on a long path through the network. You'd need a testing client somewhere else to check the reachability of your server.
Related
On a Windows machine there are different network interfaces available. One of these interfaces is the TAP interface connected to a VPN-server, with a specific IP (e.g. 10.25.1.9). To reach the host 10.2.1.1 the route leads through this interface.
I want to start a local python server program and bind it to this interface, so I need to know the IP address to bind to. Since this local server program shall be installed on several machines from which I don't know the IP addresses, I want to find this IP automagically.
It can be that there are several VPN connections installed on the machine, so neither a IP address prefix is unambiguous nor the interface name can safely assumed to be unique.
The only safe thing I know about the interface is the fact that through it the host 10.2.1.1 can be reached (probably I will test some simple service on that host to be sure that this is the right one).
I had a look at netifaces but it does not offer this. I also had a look at PyRoute2 but this is not available on Windows machines.
So, my question remains: how can I find out which interface the route to my VPN base host 10.2.1.1 leads through, so I can take this IP to bind to?
Provided that there is any known service to connect to on the remote host, use socket.getsockname() to find out which local IP is used for connecting:
import socket
# connect to known destination, e.g. via UDP port 80
test_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
test_sock.connect(("10.2.1.1", 80))
# check which local IP was used to connect
with test_sock:
private_ip, *_ = test_sock.getsockname()
Which is the source IP address in tcp socket if bind is called on a multihomed client host? Client has two interfaces eth0(IP0) and eth1(IP1) and the client tcp socket is bound to IP0. After socket, bind, connect in client, it sends a packet to server.The destination IP isservIP. But servIP and IP0 are not in a same subnet(Maybe servIP and IP1 are). Which is the source IP in the packet sent to server? And what will getsockname return?
There are two separate issues here:
1) Which IP to bind on?
When calling bind() you have an option to specify and address to bind on or you can leave this decision to TCP/IP stack on your computer. You can pass a specific address in 'addr' parameter or leave it as INADDR_ANY. You can find more information how to do it in manual page of ip(7). If you call bind() providing the valid IP address and call to bind() succeeds, then datagrams sent using the binded socket will have their source address set to the value provided in call to bind().
2) How the packet is routed?
The way your packet is routed depend only on the destination address and not the source address. It can be that your source address will be the one from eth0 and it will go out through eht1. This is because the routing system in your OS is using destination based routing as opposed to source based routing. You can always see which adapter will be used by issuing "route" command in the console of your OS and comparing the output with the destination address
I am creating a tcp connection using the function socket(), bind(), and then listen().
Our customers would like to be able to define an IP address of the server at runtime. Is there a way of changing the IP at runtime or must it be done in the BIOS?
Thanks for any tips
I've changed the IP address using ifAddrSet(..) many times. Usually I call this function from within the startup script before my application is running so I have no idea how calling this function affects already connected sockets.
But have a look at the functions provided by ifLib.h. I'm sure you'll find something that suits your needs (ifAddrAdd(..) looks promising).
I am not sure what you mean by defining 'IP address of the server at runtime?'. Obviously for a given socket it's IP address cannot be changed. It's an endpoint of a connection, it cannot be changed run time. If you just want to assign multiple IP addresses to a host that's possible.
In general - you can add as many IP addresses as you want to your machine (ok not exactly) but certainly a hundred or so (ie. statically allocated). That's not the problem (management of that is a nightmare, but sure not impossible). The problem is how those IP addresses are reached, that is not in your control, that depends upon the settings on client especially the routing entries. eg. you could use all of the IP addresses in a Subnet (say 10.1.2/24).
Not recommended - but possible.
Once you have those IP addresses - you bind on the port and address as INADDR_ANY, which says accept connection on 'any ' of the local addresses. On which address the connection was made to can be determined on server using getsockname.
I'm writing a distributed system and I want to test it on my machine. I created several ip addresses on the interface lo using ip addr add ip_add dev lo.
I have binded all servers to their specific addresses and now I want my servers to connect to each other such that each server would think that it connects from his own ip. But when I use connect I get connection from my localhost. How is it possible to connect from a specific ip address?
It turned out that calling bind() on my socket does all necessary work.
I have a Linux server with multiple ips (so, multiple eth0, eth0:0, eth0:1 etc).
The script I'm trying to start is a php CLI script which is downloading stuff from an another server API, and I would like to change the IP based on different parameters. Once the script is started, I don't need anymore to change the ip OF THAT SPECIFIC script until his end.
Do you have any clue if it is possible to achieve it?
My other solution was to install Xen or OpenVZ and create N different VPS per each IP, but as you can see is definitely a PITA :-)
You don't specify how you connect to the other server, but with sockets you can try socket_bind.
EDIT:
With curl you can try curl_setopt.
CURLOPT_INTERFACE The name of the outgoing network interface to use. This can be an interface name, an IP address or a host name.
I know how to do it in C - you use bind() on your socket before you call connect(), and you bind to the IP address assigned to the desired interface, passing 0 for port. I don't know how to do it in PHP.