Kernel Korner - Network Programming in the Kernel

Take a tour of the kernel's networking functionality by writing a network client that runs in kernel space.
A User-Space Program

Now, let's take a look at a user-space program that invokes our system call to transfer a file. We explain the relevant details for calling a new system call:

#define __NR_my_sys_call 223
_syscall1(long long int, my_sys_call,
          struct params *, p);

int main(int argc, char **argv)
  struct params pm;
  /* fill pm with appropriate values */
  r =  my_sys_call(&pm);

#define __NR_my_sys_call 223 assigns a number to our system call. _syscall1() is a macro that creates a stub for the system call. It shows the type and number of arguments that our system call expects. With this in place, my_sys_call can be invoked just like any other system call. Upon running the program, with correct values for the source and destination files, a file from a remote FTP server is downloaded onto the client machine. Here is a transcript of a sample run:

# make
make -C /lib/modules/2.6.9/build SUBDIRS=/home/ppadala/ftp modules
make[1]: Entering directory `/home/ppadala/linux-2.6.9'
  CC [M]  /home/ppadala/ftp/ftp.o
  Building modules, stage 2.
  CC      /home/ppadala/ftp/ftp.mod.o
  LD [M]  /home/ppadala/ftp/ftp.ko
make[1]: Leaving directory `/home/ppadala/linux-2.6.9'
# gcc do_ftp.c
# ./a.out <local host's IP address> /README /tmp/README anonymous
Connection from
return = 215 (length of file copied)


We have seen a basic implementation of an FTP client within the kernel. This article explains various issues of socket programming in the kernel. Interested readers can follow these ideas to write various network applications, such as an HTTP client or even a Web server in the kernel. Kernel applications, such as the TUX Web server are used for high-performance content serving and are well suited for environments that demand data transfer at high rates. Careful attention has to be paid to the design, implementation and security issues of such applications.

Resources for this article: /article/8453.

Pradeep Padala is a PhD student at the University of Michigan. His general interests are in distributed systems with specific emphasis on scheduling and fault tolerance. He is the author of the NCurses Programming HOWTO and contributes to various open-source projects. More about him can be found on his Web site at

Ravi Parimi has a Master's degree in Computer Engineering and currently works in Silicon Valley, California. His main interests are in operating systems, networking and Internet security. He has been using Linux since 1998 and aspires to be a kernel hacker. In his free time, he pursues Vedic studies and Chess.



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Fail to create dynamic system call

Kunsheng's picture

Hi everyone,

Has anyone tried this code themselves? I failed to insert that system call dynamically (but succeeded to compile and insmod the module).

I am wondering if anyone know what's going on there.

PS: I am using Linux 2.6.29 in Ubuntu.

Thanks in advance,


kernel raw sockets

Anonymous's picture

I am new to linux kernel development. so if any mistakes you find, pls
frgive it and correct me.

I wanted to send raw packets through ethernet, from kernel level.
So i use PF_PACKET family. & SOCK_RAW.And i used sock_create()
function to create socket.
But I found that when i create socket with
sock_create(PF_PACKET,SOCK_RAW,.....) the program always fails in
bind. (when i do sock->ops->bind(.....))
why is it so ? but when I use PF_INET & SOCK_PACKET to create socket.
bind happens successfully.
Can any one help me to come out of this issue?? OR direct me to create
raw packets and send from kernel??

thanks in advance

create_address doesn't exist

Derek's picture

The function create_address doesn't exist in my kernel header (2.6.17). Where should I get the definition?


It's true.Create_address is

Prafulla's picture

It's true.Create_address is not in Kernel Header.
Has anyone found how to get it?
Please reply

insmod 'ing the module

Nathan's picture

In your example you don't actually use insmod after building the module, does that mean its not necessary? If not then how does the userland program see the system call. If so then do you know why it insmod'ing it would freeze my system? Cause it does. I did fiddle with the code a bit, mostly stripped it down to just connect, send a message, and close.


Problem was redefining the

Nathan's picture

Problem was redefining the system call. Seems linux doesn't appreciate it none too much and freezes. I've read its not really done anymore anyway.