I'm having trouble to use bind method on Windows (C language). Well. My code is supposed to do the following things:
get 2 addresses from the user: One must be an ip address from the local machine and another a remote address ( a google address for example).
The application i'm writing will bind to the local address
The application will connect to the second address.
I got some code from the Internet. You can see all the sources i had consulted on the comments.
This application will only accepts a string from the client (which specifies the local
address which the application will bind), then, the app will connect to a remote service
google, youtube or whatever. The main purpose that this app serves is to know if, when
binded to a local address, the operative system (in this case Windows Based system) gives priority to:
- SA/DA rule
- Forwarding table
SOURCES:
http://stackoverflow.com/questions/2065495/using-a-specific-network-interface-for-a-socket-in-windows
http://stackoverflow.com/questions/2605182/when-binding-a-client-tcp-socket-to-a-specific-local-port-with-winsock-so-reuse
http://www.delta-search.com/?q=error+on+binding+to+local+address%2Cwindows&s=web&as=0&rlz=0&babsrc=HP_ss
http://social.msdn.microsoft.com/Forums/zh/vcgeneral/thread/763a00ab-0e7f-44c6-9794-840fc6cb2e07
http://www.delta-search.com/?q=add+ws2_32.lib+visual+studio+2010&babsrc=HP_ss&s=web&rlz=0&as=3&ac=0%2C331
http://stackoverflow.com/questions/5220309/why-am-i-getting-linker-errors-for-ws2-32-dll-in-my-c-program
http://msdn.microsoft.com/en-US/windows/desktop/aa904949
http://msdn.microsoft.com/en-us/library/windows/desktop/ms737550(v=vs.85).aspx
#pragma once
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")
#include <Ws2tcpip.h>
#include <stdio.h>
void Abort(char *msg);
void main(int argc,char*argv[]){
int ret, fd;
struct sockaddr_in sa_dst;
struct sockaddr_in sa_loc;
if(argc < 3)
Abort("Syntax: SourceIpAddress(to bind) DestinationIpAddress(to connect)");
fd = socket(AF_INET, SOCK_STREAM, 0);
// Local
memset(&sa_loc, 0, sizeof(struct sockaddr_in));
sa_loc.sin_family = AF_INET;
sa_loc.sin_port = htons(0);
sa_loc.sin_addr.s_addr = inet_addr(argv[1]);
if((ret = bind(fd, (struct sockaddr *)&sa_loc, sizeof(struct sockaddr))) < 0)
Abort("Binding to local address");
// Remote
memset(&sa_dst, 0, sizeof(struct sockaddr_in));
sa_dst.sin_family = AF_INET;
sa_dst.sin_port = htons(80);
sa_dst.sin_addr.s_addr = inet_addr(argv[2]); // google :)
if((ret = connect(fd, (struct sockaddr *)&sa_dst, sizeof(struct sockaddr))) < 0)
Abort("Connect to remote address");
printf("\n\nConnection Successfully made!!\n\n"); }
void Abort(char *msg){
fprintf(stderr,"\n\n<ERROR>: <%s>\n",msg);
perror("\n\nExiting...");
exit(EXIT_FAILURE); }
The problem is that bind returns -1. I've tested already.
Can anyone help me on this?
Thanks in advance
When a Winsock function fails, you can call WSAGetLastError() to find out why.
In this case, you are not calling WSAStartup(), so socket() fails with a WSANOTINITIALISED error that you are ignoring, so you end up passing an invalid socket to bind().
Related
The following server in C doesn't work as expected. Upon running it the first time, no issues occur. Every next time you run it, it fails to bind. The solution of the question that some of you will probably mark as duplicate doesn't work either, regardless of the fact that setsockopt(...) is successful.
#include <stdio.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#define PORT 8080
#define SA struct sockaddr
int main() {
int sockfd, connfd;
struct sockaddr_in servaddr, cli;
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if(sockfd == -1) puts("Socket creation failed."), exit(0);
else puts("Socket created.");
const int optVal = 1;
const socklen_t optLen = sizeof(optVal);
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &optVal, optLen);
servaddr.sin_family = AF_INET, servaddr.sin_port = htons(PORT);
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
if (bind(sockfd, (SA *) &servaddr, sizeof(servaddr))) {
puts("Bind failed.");
shutdown(sockfd, 2);
return 12;
}
else puts("Bound.");
if(listen(sockfd, 5)) puts("Listen failed."), exit(2);
else puts("Listening: ");
int len = sizeof(cli);
connfd = accept(sockfd, (SA*)&cli, &len);
if(connfd < 0) puts("Connection failed."), exit(3);
else puts("Accepted.");
close(connfd);
return 0;
}
Things I've tried so far:
using setsockopt(...) has no effect
shutdown(...) doesn't work either
closesocket(...) is part of the Windows API, which I just have no intention to use
If I decide to ignore the "Address already in use" error, accept(...) fails with Invalid argument
If it matters I'm using CLion with cygwin under Windows 10.
"If it matters I'm using CLion with cygwin under Windows 10."
I strongly suspect that this matters.
When I compile your code exactly as you posted it, it behaves according to your desired / expected behavior.
When I comment out the setsockopt() call, I need to wait until the TIME_WAIT expires before being able to re-bind the same address+port, which is also expected.
I'm using gcc on macOS, so I suspect that your compilation and/or runtime environment has something to do with your code not working as expected for you. One way you could verify this if you don't have access to a physical Linux machine, and if you cannot set up dual-boot on the Windows machine that you do have, would be to spin up a small Linux instance at Digital Ocean, AWS, or some other cloud provider.
EDIT #1
I repeated this on a DO Linux host, and I have confirmed that your original code works as expected.
I suspect this has an easy solution I'm overlooking, probably to do with the client or how this is set up.
Anyways, I'm trying to set up a simple Echo server/client to understand the basics of socket programming. I have a virtual machine running Linux Mint, and the host is running Windows 10. The virtual machine I am setting to run the server c code, and the Windows will be running the client.
I started off making the server code
//Echo Server for UNIX: Using socket programming in C, a client sends a string
//to this server, and the server responds with the same string sent back to the client
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
int main()
{
char stringBuffer[50]; //string buffer for reading incoming and resending
int listener, communicator, c; //store values returned by socket system call
if((listener = socket(AF_INET, SOCK_STREAM, 0)) == -1) //creates a new socket
puts("Could not create socket");
puts("Socket Created");
struct sockaddr_in servAddr, client; //structure from <netinet/in.h> for address of server
servAddr.sin_family = AF_INET; //addressing scheme set to IP
servAddr.sin_port = htons(8888); //server listens to port 5000
servAddr.sin_addr.s_addr = inet_addr("127.0.0.1"); //symbolic constant of server IP address
//binds the socket to the address of the current host and port# the server will run on
if (bind(listener, (struct sockaddr *) &servAddr, sizeof(servAddr)) < 0){
puts("Bind failed");
return 1;
}
puts("Bind Successful");
listen(listener, 5); //listens for up to 5 connections at a time
c = sizeof(struct sockaddr_in);
if ((communicator = accept(listener, (struct sockaddr*)&client, (socklen_t*)&c ))<0)
puts("accept failed");
puts("Connection Accepted");
//wait until someone wants to connect, then whatever is sent can be read from communicator, which can then be sent back
while(1){
bzero(stringBuffer, 50); //sets buffer to 0
read(communicator, stringBuffer, 50); //reads from communicator into buffer
write(communicator, stringBuffer, strlen(stringBuffer)+1); //returns back
}
return 0;
}
after that I tested it out by opening another terminal in the guest machine and typed "telnet localhost 8888" and input whatever strings I wanted.
This test worked so now, onto my Windows machine to create the client side of the socket programming:
#include <winsock.h>
#include <stdio.h>
#include <string.h>
#pragma comment(lib,"ws2_32.lib") //Winsock Library
int main(int argc, char *argv[])
{
WSADATA wsadata; //variable for using sockets in windows
SOCKET sock; //socket variable for network commands
char sendString[50], recieveString[50]; //variables for sending and recieving messages to/from server
//check if WSA initialises correctly
if (WSAStartup(MAKEWORD(2,2), &wsadata) != 0)
printf("Error Code: %d", WSAGetLastError());
//creates new socket and saves into sock
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == INVALID_SOCKET)
printf("Could not create socket: %d", WSAGetLastError());
printf("Socket created\n");
struct sockaddr_in servAddr;
servAddr.sin_addr.s_addr = inet_addr("127.0.0.1"); //sets the IP address to the same machine as the server
servAddr.sin_family = AF_INET; //addressing scheme set to TCP/IP
servAddr.sin_port = htons(8888); //server address is on port 8888
//connects to device with specifications from servAddr
if (connect(sock, (struct sockaddr *)&servAddr, sizeof(servAddr)) < 0) {
printf("Connection Error %d\n", WSAGetLastError());
return 1;
}
printf("Connection Accepted\n");
while(1){
fgets(sendString, 50, stdin); //uses stdin to get input to put into sendString
//sends sendString to server using sock's properties
if (send(sock, sendString, strlen(sendString) + 1, 0) < 0); {
printf("Send Failed");
return 0;
}
//reads from server into recieveString
if ((recv(sock, recieveString, 50, 0)) == SOCKET_ERROR)
printf("Recieve Failed");
printf("%s", recieveString); //prints out recieveString
}
}
Now, with the server still running, when I try out the client-side, I get the response "Connection Error" (from line 35). Having looked at both Unix and WinSock examples, I'm unsure as to why I would be failing the connection. I suspect it might have something to do with a windows to linux VM but I'm not sure.
---UPDATE---
Having updated the accidental semicolon and added the WSAGetLastError, it's showing an error code of 10061; This translates to
"Connection refused.
No connection could be made because the target computer actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host—that is, one with no server application running."
[after the 3rd edit:]
Sry, just re-read your question. The important thing is here:
The virtual machine I am setting to run the server c code, and the Windows will be running the client.
127.0.0.1 is an address always only local to an IP enabled box. So you your server is listening on the interface 127.0.0.1 local to the Linux VM and the client tries to connect to 127.0.0.0 local to the Windows box. Those two interfaces are not the same. The result is the obvious, namely the client does not find anything to connect to.
127.0.0.1 (the so called "IPv4 local loopback interface") can only be used for connections local to exactly one box.
if (connect(sock, (struct sockaddr *)&servAddr, sizeof(servAddr)) < 0); {
printf("Connection Error");
return 1;
}
This is just a trivial syntax mistake. You are entering the block unconditionally. Remove the first semicolon.
However there is a much more important point to be made. When you get an error from a system call such as connect(), you must print the error. Not just some message of your own devising. Otherwise you don't know whether you simply have a bug, or a temporary problem, or a long-lasting problem, or a permanent problem.
Change the printf() to:
printf("Connect error %s\n", WSAGetLastError());
and then don't continue as though the error didn't happen.
Note that this applies to all system calls, specifically including socket(), bind(), listen(), connect(), accept(), recv(), send(), and friends.
I want to get the IP address of the computer my program is launched on, to be able then to send it to a client, but I always get 0.0.0.1 instead of the real IP address (like 127.0.0.1 for instance).
I'm currently able to get the port, but not the IP address.
How can I get it?
The best solution would be to be able to get it with a sockaddr_in. Here's what I'm currently doing:
int open_connection(char* ip, int* port)
{
int sock;
struct sockaddr_in sin;
socklen_t len;
int i;
i = 0;
len = sizeof(sin);
if ((sock = socket(AF_INET, SOCK_STREAM, 0)) == -1)
return (-1);
bzero(&sin, sizeof(struct sockaddr_in));
sin.sin_family = AF_INET;
if (bind(sock, (struct sockaddr *) &sin, sizeof(sin)) != 0)
perror("Error on bind");
if (getsockname(sock, (struct sockaddr *)&sin, &len) != 0)
perror("Error on getsockname");
strcpy(ip, inet_ntoa(sin.sin_addr)); // IP = 0.0.0.0
*port = sin.sin_port;
return (sock);
}
EDIT: I understand I was going on the wrong way with my way of thinking. So my question is: What's the best way to get your own IP address?
When you bind() a socket to 0.0.0.0, that is the only IP the socket has available when calling getsockname(). It means the socket is bound to all local interfaces. In order to get a specific IP from a socket, it has to be bound to a specific IP.
Using the socket API to get the machine's local IP(s) is the wrong approach anyway. A common mistake is to use gethostname() with gethostbyname() or getaddrinfo() to get the local IP list. Usually that works, but it has some hidden gotchas that can cause false information, but people tend to ignore that fact, or don't even know about it in the first place (I didn't know about it for years, but then I learned better).
Instead, you really should use platform-specific APIs for enumerating the local networking interfaces. That will provide more reliable information. Windows has GetAdaptersInfo() and GetAdaptersAddresses(). Other platforms have getifaddrs(). Those will tell you what local IPs are available. You can then bind() a socket to 0.0.0.0 in order to accept clients on any of those IPs, or bind() to a specific IP to accept clients only on that IP.
The sockets API allows you to enumerate the IP addresses assigned to your network interfaces, but it will not tell you what you "real IP" is if you are connecting to the Internet from behind a router.
The only way to know it is by asking someone outside. Thats how servers like FileZilla FTP Server do that. They instruct you to configure the URL to a "ip.php" script like this one in the server's settings so it can ask the Internet whats its public IP address, to use in Passive Mode.
You can also consider using STUN, a protocol widely used in VoIP to discover public IP.
You could call ioctl(sock, SIOCGIFADDR, adr)
see netdevice(7)
Following #Remy Lebeau's answer I wrote a function that return current machine's address. I have only tested this on macOS High Sierra.
interfaec can be anything among lo0, en0, etc.
ipVersion can be AF_INET or AF_INET6.
long int getInternalAddress(char* interface, sa_family_t ipVersion)
{
struct ifaddrs *ifaddrHead, *ifaddr;
/* int_8 */
sa_family_t family;
int n;
char *interfaceName;
if (getifaddrs(&ifaddrHead) != 0)
{
fprintf(stderr, "ifaddrs error");
}
/* iterate through address list */
for (ifaddr = ifaddrHead, n = 0; ifaddr != NULL; ifaddr = ifaddr->ifa_next, n++)
{
family = ifaddr->ifa_addr->sa_family;
interfaceName = ifaddr->ifa_name;
if (!family || family != ipVersion || strcmp(interfaceName, interface)) continue;
struct sockaddr *addr = ifaddr->ifa_addr;
struct sockaddr_in* addr_in = (struct sockaddr_in*) addr;
long int address = addr_in->sin_addr.s_addr;
freeifaddrs(ifaddrHead);
return address;
}
freeifaddrs(ifaddrHead);
return 0;
}
To use it,
int main()
{
long int address = getInternalAddress((char*) &"en0", AF_INET);
printf("%li\n", address);
return 0;
}
I'm still a beginner in C, if there is anything wrong please tell me.
I'm learning from the book Hacking, the Art of Exploitation by Jon Erickson, and I am confused regarding to a simple code sample that he provided. The code is to set up a simple server, but when I complied it (no error) and ran the code, it hangs
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include "hacking.h"
#define PORT 7890 // the port users will be connecting to
int main(void) {
int sockfd, new_sockfd; // listen on sock_fd, new connection on new_fd
struct sockaddr_in host_addr, client_addr; // my address information
socklen_t sin_size;
int recv_length=1, yes=1;
char buffer[1024];
if ((sockfd = socket(PF_INET, SOCK_STREAM, 0)) == -1)
fatal("in socket");
if (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int)) == -1)
fatal("setting socket option SO_REUSEADDR");
host_addr.sin_family = AF_INET; // host byte order
host_addr.sin_port = htons(PORT); // short, network byte order
host_addr.sin_addr.s_addr = INADDR_ANY; // automatically fill with my IP
memset(&(host_addr.sin_zero), '\0', 8); // zero the rest of the struct
if (bind(sockfd, (struct sockaddr *)&host_addr, sizeof(struct sockaddr)) == -1)
fatal("binding to socket");
if (listen(sockfd, 5) == -1)
fatal("listening on socket");
while(1) { // Accept loop
sin_size = sizeof(struct sockaddr_in);
new_sockfd = accept(sockfd, (struct sockaddr *)&client_addr, &sin_size);
if(new_sockfd == -1)
fatal("accepting connection");
printf("server: got connection from %s port %d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
send(new_sockfd, "Hello World!\n", 13, 0);
recv_length = recv(new_sockfd, &buffer, 1024, 0);
while(recv_length > 0) {
printf("RECV: %d bytes\n", recv_length);
dump(buffer, recv_length);
recv_length = recv(new_sockfd, &buffer, 1024, 0);
}
close(new_sockfd);
}
return 0;
}
I did a little printf() to find out where I hangs, and it turns out to be on this line
sin_size = sizeof(struct sockaddr_in);
I'm not sure if it has to do with my environment, or there is something that I am missing. The environment that the book uses can no longer be updated (some old verson of Ubuntu). So I am currently using the latest one.
Can someone please explain to me why the program does not work?
And if there is some basic that need to know before learning the network chapter, please do tell.
This program won't proceed until accept on the line after the sizeof receives an incoming connection from a client program. Your printf shows that accept was called but was blocked.
You need to compile and run the client with the right options (IP / Port) to connect to this server program.
Update
If 192.168.42.248 is from the book, then you're probably trying to connect to the wrong IP. Try telnet 127.0.0.1 7890.
It's a server, it will "hang" until you make a connection to port 7890. That's the whole point of the program (for more detais, it blocks since accept() is waiting for a connection)
Assuming you are running unix, try to type echo "hi there" | nc localhost 7890 in a terminal from the same machine while you run it, and you will see how it "unblocks"
Telling by all the comments flying around in your thread I'd recommend to connect with telnet using the following command line: telnet localhost 7890
telnet takes as arguments the host to connect to and the port to connect to on this host. Using "localhost" is similar to using the loopback IP 127.0.0.1.
Why does connecting to the server solve the "hang"? accept is blocking as you can read in the man page or any other documentation of your programming environment. This means the function won't return until a client connects. After connecting the function returns a handle to the socket created for the connecting client which can be used to communicate.
I am writing a server application that will provide a service on an ephemeral port that I only want accessible on the loopback interface. In order to do this, I am writing code like the following:
struct sockaddr_in bind_addr;
memset(&bind_addr,0,sizeof(bind_addr));
bind_addr.sin_family = AF_INET;
bind_addr.sin_port = 0;
bind_addr.sin_addr.s_addr = htonl(inet_addr("127.0.0.1"));
rcd = ::bind(
socket_handle,
reinterpret_cast<struct sockaddr *>(&bind_addr),
sizeof(bind_addr));
The return value for this call to bind() is -1 and the value of errno is 99 (Cannot assign requested address). Is this failing because inet_addr() already returns its result in network order or is there some other reason?
inet_addr should be avoided, for there is a much saner method of constructing struct sockaddrs (which means it also obsoletes gethostby*):
#include <netdb.h>
/* Error checking omitted for brevity */
struct addrinfo hints = {.ai_flags = AI_PASSIVE};
struct addrinfo *res;
getaddrinfo("::1", NULL, &hints, &res); /* or 127.0.0.1 if you are 60+ */
bind(fd, res->ai_addr, res->ai_addrlen);
freeaddrinfo(res);
Is this failing because inet_addr() already returns its result in network order
Yes.
So remove the htonl call.