I need to get the file descriptor to use in ioctl() calls for an ethernet port in Linux. Not sure how to do this.
Just use the file descriptor of an open socket, using the name of the device in the ifreq structure passed to ioctl(), assuming your program has adequate permissions to do so.
From the docs:
Linux supports some standard ioctls to
configure network devices. They can
be used on any socket's file
descriptor regardless of the family or
type. They pass an ifreq structure:
The socket need not be bound to the target device, or be of any specific family. Any open socket fd will do (again, with appropriate privileges), just open one for your specific task, wait for ioctl() to return and close it.
See man 7 netdevice for more, or here if you don't have the appropriate documentation packages installed (hint, the package is usually named manpages-dev or manpages-devel, depending on your distro)
You can also take a look at the source to the net-tools package, which may be named differently depending on your distro. That's the source to ifconfig (Debian / Ubuntu here).
Sorry for the original ambiguity, I thought you were trying to configure a special multi function device (not sure why now, perhaps lack of sleep).
You can do something like this fd = socket(PF_INET, SOCK_DGRAM, IPPROTO_IP)
Use strace to see what functions ifconfig calls.
Related
I found out that, under 'everything is a file' philosophy in Unix, even sockets are considered as files, and can be stored in a path in file system. So, I was trying to create a TCP-IP socket file out of curiosity.
This answer shows how to create a named Unix domain socket file. But struct sockaddr_in doesn't have sin_path field. So I have no idea how to create a named TCP-IP socket file. Can anyone tell me how to do this?
... even sockets are considered as files, and can be stored in a path in file system.
Not really. There are some sockets where the endpoint is represented by a file. These are UNIX domain sockets (AF_UNIX) and their is a variant which has a message semantic (SOCK_DGRAM) similar to UDP and one which has a stream semantic (SOCK_STREAM) similar to TCP.
But, UDP and TCP sockets have a no file representation in UNIX. They have a file descriptor though similar to normal files (and read and write system calls work with these), but they are not represented by a path in the file system.
There is also something like /dev/tcp/.. to deal with sockets inside the bash shell. But this is not an actual path on the file system either, but only a fancy syntax specific to the bash shell.
For more on this see also Wikipedia: Everything is a file which specifically notes:
... Therefore, a more accurate description of this feature is Everything is a file descriptor.
I like to write tcp packets to socket file of my program by opening/or not opening(I dont care) and just write packets to it. so my program socket can read packets and proceed with where ever it is in connection. Is it possible in C and Linux
On Man page from linux it says
O_ASYNC
Enable ... This
feature is available only for terminals, pseudoterminals,
sockets[SO ITS AVAILABLE TO OPEN A SOCKET --HOW?], and (since Linux 2.6) pipes and FIFOs. See
fcntl(2)[DO I NEED TO USE fcntl] for further details. See also BUGS, below.
The above is for open system call in Linux on Man page
So I like to know how to open socket file on my system in /proc/my_pid/fd/3 or any number. Does anyone know this please tell
There is an application running on a FreeBSD 10.1 release operating system and i need to figure out how to find the sockets it has created and is using. I know that i'm looking for an AF_MAP socket which should be similar to a AF_UNIX socket.
How do i see what sockets are open, and once i find the one im looking for i need to see what information passes through. how is this also done?
Thankyou
I'm not sure on FreeBSD specifically but you can use lsof in a way like:
$ lsof -p $(pidof your-appname)
This will give you all the files it has opened.
For AF_UNIX sockets you may refer to this.
Also, you may want to use netstat -xa to see CONNECTED state sockets (if the application uses stream oriented sockets)
Can anyone help me in tracing bind() system call in socket programming. I would like to know what happens when bind() is called, in kernel space. Like which are the structures it updates and what functions are invoked in lower level
The bind(2) system call just configures the local side's address parameters that a socket will use once you have connected (or sendto(2)). If you don't use it, the kernel selects defaults for it, depending on the underlying protocol.
The exact procedure bind(2) follows depends on the protocol family you are working on, as bind will behave differently depending if you are using PF_UNIX, PF_INET, PF_PACKET, PF_XNS, etc.
For example, in Unix sockets, you'll get your socket associated to an inode in the filesystem (an inode that supports unix sockets, of course), so clients have a path to connect to (in Unix sockets, addresses are paths in the filesystem). In TCP/IP sockets, you can fix the local IP address or the local IP port your socket can listen on (to accept connections) or you can force a IP address and/or port to connect from, to a server.
For a deeper understanding of networking sockets internals, I recommend you reading the excellent book from W.R. Stevens "TCP/IP Illustrated Vol 2. The implementation," describing the implementation of BSD sockets in NET2. It's old, but still the best explanation ever made. For a good introduction of the BSD socket system calls use, there's also an excellent book (for a long time it was indeed also the best system call reference for BSD unix system calls) by W.R.Stevens: "UNIX network programming, Vol 1 (2ND Ed): The sockets API." Both are two jewels everyone should have available at work.
I have a project which uses poll() to handle the network communication including TCP and Ethernet. Poll is used to support TCP or UDP conneciton. Recenctly I want to use libpcap to receive frames and integrate libpcap into this code with poll().
The poll() needs to know the file descriptor, but the pcap does not returns the file descriptor. How to manage pcap with poll() system.
The project is in C code on linux. The reason I do not want to use raw socket is for the pcap's high efficiency.
If, on a UN*X, you have a pcap_t * from a call such as pcap_open_live() or pcap_create()/pcap_activate(), you can get a file descriptor to hand to poll() or select() or something such as that by calling pcap_get_selectable_fd(). Note that it can return -1, which means that select()/poll()/etc. aren't supported on that particular device on the OS version you're using. (The OS version shouldn't matter for Linux, but it might matter for some BSDs.)
According to Linux manual. You can use pcap_get_selectable_fd. But it's not reliable on most BSDs. Some network devices opened with pcap_create() and pcap_activate(), or with pcap_open_live(), do not support select() or poll() (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace DAG devices), so -1 is returned for those devices. Refer to manual for more details.