Am trying my hand at sockets programming and I need some help because I can't build any of the programs I write. Whenever I try to build any sockets application, the compiler reports the following error:
error: expected specifier-qualifier-list before '_uid32_t'
This variable is inside a struct, which in turn is located in the header file "socket.h", and looks like this:
struct ucred {
pid_t pid;
__uid32_t uid;
__gid32_t gid;
};
There aren't any other errors in the other files and this is the only error the compiler returns.
I don't know if this is of any consequence but the header file is the one that comes with cygwin because the guides and tutorials am using are on Unix sockets and am running Windows XP. Am also using Code::Blocks running a gcc compiler.
I really hope that it's possible to run a program using the Unix sockets API on Windows because I'd really hate to confine myself to winsock only. Also, most of the freely available and comprehensive tutorials on sockets and network programming use Unix sockets.
Here's the code, though I don't think it really matters because I get the exact same error no matter program as long as it's using socket.h.
#include <stdio.h>
#include <string.h>
#include <sys\types.h>
#include <sys\socket.h>
#include <netdb.h>
#include <arpa\inet.h>
int main(int argc, char *argv[])
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2)
{
fprintf(stderr, "Usage: showip hostname\n");
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC;//AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0)
{
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
return 2;
}
printf("IP addresses for %s:\n\n", argv[1]);
for (p = res; p != NULL; p = p ->ai_next)
{
void *addr;
char *ipver;
//get the pointer to the address itself
//different fields in IPv4 and IPv6:
if (p ->ai_family == AF_INET)
{
//IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p ->ai_addr;
addr = &(ipv4 ->sin_addr);
ipver = "IPv4";
}
else//IPv6
{
struct sockaddr_in *ipv6 = (struct sockaddr_in6 *)p ->ai_addr;
ipver = "IPv6";
}
//convert the IP to a string and print it;
inet_ntop(p ->ai_family, addr, ipstr, sizeof ipstr);
printf(" %s: %s\n", ipver, ipstr);
}
freeaddrinfo(res);//free linked list
return 0;
}
There's also other source code from other programs I could have added but I chose this one instead because I got it from the internet and not a textbook so anyone can look it up here.
Try including <unistd.h> and <sys/types.h> before your socket.h.
Related
I found this piece of C code:
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,"usage: showip hostname\n");
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
return 2;
}
printf("IP addresses for %s:\n\n", argv[1]);
for(p = res;p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = "IPv4";
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = "IPv6";
}
// convert the IP to a string and print it:
inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
printf(" %s: %s\n", ipver, ipstr);
}
freeaddrinfo(res); // free the linked list
return 0;
}
There are two lines that I don't know how to interpret:
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
Why does the author of this program use parentheses?
I tried following a wonderful guide about interpreting complex declarations but I still don't understand.
It's called casting. It converts from one type to another, but the theory behind it is a lot more complicated. Casting doesn't mean you have an object describing a Dog (for example a struct) and you can cast it to a Human. Obviously you can't.
Casting means having identical memory blocks that can map on another block. A simpler explanation is when you have a complex structure and you cast from a pointer to the structure or from the data structure to the pointer.
Also note, that a data structure doesn't necessarily mean a struct. It can also mean an array. And, an array is a concept so it can mean something like int some_data[20]; or int** some_data;.
Now, as for why the author uses the parenthesis, it's because this is the syntax. You can't use any character.
Along with further reading about casting, please look up static_cast, dynamic_cast and reinterpret_cast. These 3 functions (well, almost, but I'm trying to not complicate it even more) use native casting. static_cast is similar to (type_a*)type_b.
For some examples, look up casting from int to enum and vice-versa to get an idea.
If you want to keep it simple, don't check any C++ concepts about casting just yet until you get a hang of the concepts in C first. They are common, but C++ will fork a lot from C because of classes, polymorphism and inheritance in general.
Where am I going wrong with my experimentation:
I am trying to experimentally deduce the details of creating a simple shellcode. My first impression was I probably would not be able to use imports, as my shellcode is not linked by the compiler of the target program. I then began wondering how small I can make a simple shell command interface over a socket without using imports, so I wrote up some code; and started ignoring implicit call warnings:
// socket_family
#define AF_INET 2
#define AF_PACKET 17
// socket_type
#define SOCK_STREAM 1
typedef unsigned short sa_family_t;
struct sockaddr {
sa_family_t sa_family;
char sa_data[14];
};
struct in_addr {
unsigned long s_addr; // load with inet_pton()
};
struct sockaddr_in {
short sin_family; // e.g. AF_INET, AF_INET6
unsigned short sin_port; // e.g. htons(3490)
struct in_addr sin_addr; // see struct in_addr, above
char sin_zero[8]; // zero this if you want to
};
int main(void) {
int sfd;
const short family = AF_INET;
const char host[] = "127.0.0.1";
struct sockaddr addr;
struct sockaddr_in *addr_full = (struct sockaddr_in*)&addr;
if (sfd = socket(family, SOCK_STREAM, 0) < 0) return 1;
memset(&addr, 0, sizeof(struct sockaddr));
addr_full->sin_family = family;
addr_full->sin_port = htons(8000);
inet_pton(family, host, &(addr_full->sin_addr.s_addr));
if (connect(sfd, &addr, sizeof(struct sockaddr)) < 0) return 2;
close(sfd);
return 0;
}
Somewhere along the line I am not connecting to my python -m SimpleHTTPServer properly; which reports Serving HTTP on 0.0.0.0 port 8000.
$ gcc my_program.c -o my_program
$ ./my_program
$ echo $?
2
I am taking a Coursera course on software security; and a lot of this topic is new to me.
EDIT:
After removing redefinitions and adding includes:
#include <sys/socket.h>
#include <string.h>
#include <netinet/in.h>
int main(void) {
int sfd;
const short family = AF_INET;
const char host[] = "127.0.0.1";
struct sockaddr_in addr_full; // = (struct sockaddr_in*)&addr;
if (sfd = socket(family, SOCK_STREAM, 0) < 0) return 1;
memset(&addr_full, 0, sizeof(struct sockaddr));
addr_full.sin_family = family;
addr_full.sin_port = htons(8000);
inet_pton(family, host, &(addr_full.sin_addr.s_addr));
if (connect(sfd, (struct sockaddr*)&addr_full, sizeof(struct sockaddr)) < 0) return 2;
close(sfd);
return 0;
}
The program still exits with 2.
The socket file descriptor was not being assigned properly. Adding parenthesis, I should be assigning the descriptor, then checking its value:
if ((sfd = socket(family, SOCK_STREAM, 0)) < 0) return 1;
Motoko,
please grab C language book and check operations priority. And do it regulary, even experienced developers make errors here. And don't write this crazy style, please learn how to write for humans:
if (sfd = socket(family, SOCK_STREAM, 0) < 0) return 1;
You should instead do something like this:
sfd = socket(family, SOCK_STREAM, 0);
if (sfd < 0)
{
/* perror() is pretty old style but some good check is recommended. */
perror("socket");
return 1;
}
SPOILER: < just has higher priority here so sfd is always 0 in your code.
Your assumption about the include files is wrong. The include files only contain the function declarations, not the code itself. They don't determine how your program will get linked to shared libraries.
Include files only tell the compiler that the functions you call inside your program exist somewhere, in some external library or in another source file you wrote. After compilation the linker tries to find all the functions that your program needs, and if they are not found, linking fails.
Try compiling your code with the necessary files included (socket.h etc.). This will give you useful warnings, for example when you pass an argument of the wrong type.
I am reading Beej's guide to network programming and in chapter 5.1, in the showip.c program I see the following line of code:
memset(&hints, 0, sizeof hints);
After a discussion on the ##c channel on freenode I deducted that the reasoning of that memset call could be to set the value of hints.ai_flags to 0(note that the program works fine I remove that line and I explicitly initialize hints.ai_flags to 0). If this is true, why does he need to set the whole struct to 0?
This is the full source:
/*
** showip.c -- show IP addresses for a host given on the command line
*/
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <netinet/in.h>
int main(int argc, char *argv[])
{
struct addrinfo hints, *res, *p;
int status;
char ipstr[INET6_ADDRSTRLEN];
if (argc != 2) {
fprintf(stderr,"usage: showip hostname\n");
return 1;
}
memset(&hints, 0, sizeof hints);
hints.ai_family = AF_UNSPEC; // AF_INET or AF_INET6 to force version
hints.ai_socktype = SOCK_STREAM;
if ((status = getaddrinfo(argv[1], NULL, &hints, &res)) != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(status));
return 2;
}
printf("IP addresses for %s:\n\n", argv[1]);
for(p = res;p != NULL; p = p->ai_next) {
void *addr;
char *ipver;
// get the pointer to the address itself,
// different fields in IPv4 and IPv6:
if (p->ai_family == AF_INET) { // IPv4
struct sockaddr_in *ipv4 = (struct sockaddr_in *)p->ai_addr;
addr = &(ipv4->sin_addr);
ipver = "IPv4";
} else { // IPv6
struct sockaddr_in6 *ipv6 = (struct sockaddr_in6 *)p->ai_addr;
addr = &(ipv6->sin6_addr);
ipver = "IPv6";
}
// convert the IP to a string and print it:
inet_ntop(p->ai_family, addr, ipstr, sizeof ipstr);
printf(" %s: %s\n", ipver, ipstr);
}
freeaddrinfo(res); // free the linked list
return 0;
}
It's required by getaddrinfo() function documentation (where you pass your hints variable as parameter). From man getaddrinfo:
All the other fields in the structure pointed to by hints must contain either 0 or a NULL pointer, as appropriate.
It's because you are only going to fill/use/initialize some of the fields of the struct, giving 0 to the other fields prevents reading an uninitialzied variable, and sometimes 0 is the default value for those variables.
note that the program works fine I remove that line and I explicitly initialize hints.ai_flags to 0
Not necessarily, if you are on linux, I suggest using valgrind to detect reads to uninitialized variables, since that causes undefined behavior, the behavior could be that nothing wierd happens.
I'm either very tired and not noticing something simple , or this is completely screwing with me. I'm getting a segmentation fault ( core dumped ) and I've managed to pinpoint it to the sendto() in the worker function. (in the server)
Server code:
//UDPServer.c
/*
* gcc -o server UDPServer.c
* ./server <port> <buffersize>
*/
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
void err(char *str)
{
perror(str);
exit(1);
}
int sock;
typedef struct
{
struct sockaddr_in client;
int buffsize;
char *msg;
} data;
void *worker (void* asd)
{
int len;
FILE *fp;
data d;
d = *(data*) asd;
char buff[d.buffsize];
printf("Received packet from %s:%d\nData:%sSize:%d\n",
inet_ntoa(d.client.sin_addr), ntohs(d.client.sin_port)
,d.msg,d.buffsize);
char * fn;
memcpy (fn,d.msg,strlen(d.msg)-1);
fp = fopen(fn,"rb");
int bytes;
len = sizeof(d.client);
printf ("%d\n",len);
while (bytes=fread(buff,sizeof(char),d.buffsize,fp))
{
printf ("Server sent %d bytes.\n",bytes);
-> this if right here. this causes the core dump when attempting to send
if(sendto(sock , &buff , sizeof(buff),0,(struct sockaddr *)&d.client,len)<0)
err("Error sending.");
}
fclose(fp);
}
int main(int argc, char** argv)
{
struct sockaddr_in server, client;
int port, i;
socklen_t slen=sizeof(client);
if(argc != 3)
{
printf("Usage: <Port> <Bytes>\n");
exit(0);
}
else
sscanf(argv[1],"%d",&port);
int buffsize = atoi(argv[2]);
char buff[buffsize];
if ((sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))==-1)
err("socket");
else
printf("Server : Socket() successful\n");
bzero(&server, sizeof(server));
server.sin_family = AF_INET;
server.sin_port = htons(port);
server.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sock, (struct sockaddr* ) &server, sizeof(server))==-1)
err("bind");
else
printf("Server : bind() successful\n");
while(1)
{
memset(&buff,0,sizeof(buff));
if (recvfrom(sock, &buff, sizeof(buff), 0, (struct sockaddr*)&client, &slen)==-1)
err("recvfrom()");
data d;
d.client = client;
d.buffsize = buffsize;
d.msg = buff;
pthread_t t;
pthread_create(&t,NULL,worker,&d);
pthread_join(t,NULL);
}
return 0;
}
I don't think the client is relevant here since it's only job is to send the filename. The read works btw , I've tested.
Anyway , I'm just trying to send the content of the file for the moment.I've been trying to figure this out for the past hour and for the life of me I can't find out what's it's problem. The segmentation fault makes no sense to me.
Any suggestions are greatly appreciated.
I'd be nervous about the sizeof(buff) in the sendto. buff's size is fixed at runtime based on the argument. But sizeof is a compile-time operation. (Or at least it was back in the good old days - I'm not sure about C99) Oh, nevermind - I see that has changed
Still, why not use d.buffsize there instead? Or maybe bytes, since you might not have filled the buffer.
Although #21Zoo is wrong about dynamic arrays in C99, I think he found the root problem
char * fn;
memcpy (fn,d.msg,strlen(d.msg)-1);
fn has no memory allocated to copy into, so you are writing to a random point in memory.
Something in the sendto is probably stumbling over that memory which now contains garbage.
You either need to malloc(strlen(d.msg)+1) or use strdup instead.
Hi I am trying to make a simple server that takes in an IP address from getaddrinfo() and binds to it. Using ifconfig, I've found that I have an ip address of wlan0 192.168.2.10 which I would like to bind to. Unfortunately the address I seem to be binding to is my lo device. For some reason when I initialize getaddrinfo("192.168.2.10","3490",&hings,&res); res gets returned to a NULL pointer. I will show off my code bellow.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <signal.h>
#define MAXDATASIZE 500;
int main(int argc, char *argv[]){
// dealing with client socket
struct sockaddr_storage their_addr;
socklen_t addr_size;
// server socket
struct addrinfo serverSide,*serverInfo,*sortIP;
int optValRet;
int listenSock, newSock;
// this is for reading in information
char buf[501];
char point[INET6_ADDRSTRLEN];
char compare[INET6_ADDRSTRLEN] = "192.168.2.10";
// this is for handeling child processes and signals
struct sigaction sa;
sa.sa_handler = NULL;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_RESTART;
if(sigaction(SIGCHLD, &sa, NULL) == -1){
printf("We have a problem, sigaction is not working.\n");
perror("\n");
exit(1);
}
// this sets up addrinfo
memset(&serverSide, 0, sizeof serverSide);
serverSide.ai_family = AF_UNSPEC;
serverSide.ai_socktype = SOCK_STREAM;
serverSide.ai_flags = INADDR_ANY;
// set up the address
if(getaddrinfo("192.168.2.10","3490",&serverSide,&serverInfo)!=0){
printf("get addr not success\n");
perror("\n");
return 1;
}
printf("Got address lists\n");
for(sortIP = serverInfo; sortIP = sortIP->ai_next; sortIP != NULL){
if((listenSock = socket(sortIP->ai_family, sortIP->ai_socktype, sortIP->ai_protocol))==-1){
continue;
}
if(setsockopt(listenSock,SOL_SOCKET,SO_REUSEADDR,&optValRet,sizeof(int))==-1){
perror("\n");
exit(1);
}
if(bind(listenSock,sortIP->ai_addr,sortIP->ai_addrlen) == -1 ){
perror("\n");
close(listenSock);
continue;
}
break;
}
if(sortIP == NULL){printf("sort ip is null.");}
inet_ntop(sortIP->ai_family,sortIP->ai_addr,point,sizeof point);
printf("Tell the clients connect to ip address %s on port 3490\n",point);
listen(listenSock, 10);
addr_size = sizeof their_addr;
newSock = accept(listenSock,(struct sockaddr *)&their_addr,&addr_size);
recv(newSock, buf, 500, 0);
printf("%s\n",buf);
close(listenSock);
close(newSock);
freeaddrinfo(serverInfo);
return 0;
}
Now I have some other questions beside the fact that I'm returning null. Since the wifi router has assigned me the ip address 192.168.2.10 for my subnet, how do I find out what my ip address is if I'm outside the network and trying to contact my server? I'm assuming the inside network ip is different from the outside network ip ... am I wrong? Anyways those are my two questions.
Thanks for any help!
This is wrong and is your immediate problem:
for (sortIP = serverInfo; sortIP = sortIP->ai_next; sortIP != NULL)
You want something like:
for (sortIP = serverInfo; sortIP != NULL; sortIP = sortIP->ai_next)
but I would go with a while loop personally.
To your main question, you should just bind to INADDR_ANY. That avoids that whole mess. Also:
recv(newSock, buf, 500, 0);
printf("%s\n",buf);
The %s format specifier is only for C-style strings, it's not for arbitrary binary data. Also, you throw away the return value from recv. There is no other way to know how many bytes you received.
As for finding your dynamic IP address from outside your network, use any of the dozens of IP posting services that assign you a host name and map it to your dynamic IP address.