Linux API Hooking

Had some success today hooking APIs on linux.
Don't think that this is going to be a techy blog-site. Just pasting here for later references. Looks like blog site is quite handy repository.

Keywords: LD_PRELOAD, dlsym
http://sourceware.org/ml/libc-alpha/2001-05/msg00321.html

One more note:
LD_PRELOAD is supported for most of the UNIX flavors. I had tried it on HPUX in patni and it worked there.
read that it will work for SOLARIS and AIX also.

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follow these 3 simple steps to hook APIs in Linux.

Step1: create a file with following code..say preload.c

//the blogger treats < as tags..removing < from include statements
#include dlfcn.h
#include stdio.h
#include sys/types.h
#include sys/socket.h

#if defined(RTLD_NEXT)
#define REAL_LIBC RTLD_NEXT
#else
#define REAL_LIBC ((void *) -1L)
#endif


int connect(int sockfd, const struct sockaddr *serv_addr, socklen_t
addrlen)
{
printf("NEOACCEL: connect hacked\n");
static int (*o_dlconnect) ( int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen )=0;

printf( "dlopen was called\n" );
o_dlconnect = (int(*)( int sockfd, const struct sockaddr *serv_addr, socklen_t addrlen )) dlsym(REAL_LIBC,"connect");
return (*o_dlconnect)( sockfd, serv_addr, addrlen );

}

Step2: compile it using cmd line
gcc -Wall -fPIC -shared -o preload.so preload.c -ldl

Step3: Define env variable LD_PRELOAD as:
export LD_PRELOAD=absolute file-path of preload.so, inclding filename

and you are done....


some good respources:
http://neworder.box.sk/newsread.php?newsid=13857

http://www.security.nnov.ru/articles/reveng/

www.phrack.org