Creating a basic C/C++ TCP socket writer - c++

Below is the following basic socket code I came up with:
//General includes:
#include <iostream>
#include <stdio.h>
#include <string>
//Network related includes:
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>
//Target host details:
#define PORT 1234
#define HOST "74.74.74.74"
using namespace std;
//Function prototypes:
string MessageFormat(int, char**);
void MessageSend(string);
int main(int argc, char *argv[])
{
//Parse arguments and format message:
string message = MessageFormat(argc, argv);
//Send the message out:
MessageSend(message);
return 0;
}
string MessageFormat(int argc, char *argv[])
{
//Massage the command line parameters
// into my desired payload format.
return message;
}
void MessageSend(string message)
{
int sd, ret;
struct sockaddr_in server;
struct in_addr ipv4addr;
struct hostent *hp;
sd = socket(AF_INET,SOCK_DGRAM,0);
server.sin_family = AF_INET;
inet_pton(AF_INET, HOST, &ipv4addr);
hp = gethostbyaddr(&ipv4addr, sizeof ipv4addr, AF_INET);
//hp = gethostbyname(HOST);
bcopy(hp->h_addr, &(server.sin_addr.s_addr), hp->h_length);
server.sin_port = htons(PORT);
connect(sd, (const sockaddr *)&server, sizeof(server));
send(sd, (char *)message.c_str(), strlen((char *)message.c_str()), 0);
}
This is quite basic, and does in fact work. HOWEVER, it's sending UDP packets instead of TCP packets, so the target host expecting TCP rejects these. Also, by inspecting connect/send values and watching my interfaces with ngrep I can 100% verify the packet is going out, so that's not the issue.
I'm only interested in modifying what I have, not creating a full featured server with boost asio. How can I tweak this so that it operates in terms of TCP instead of UDP?

Following are changes you need to make to transfer data via TCP
While creating socket pass correct parameters .In above example you passed SOCK_DGRAM instead pass SOCK_STREAM.
After binding server should go into listen mode (check the manual page of listen)
while Client Side should connect after socket creation.
Then accept in server side after listen.
Final Read and write to transfer data
Diagram attached will give you a clear picture of TCP connection
You can check manual pages for detailed info on all functions or refer beej's guide for socket programming ( use this link )

Replace SOCK_DGRAM with SOCK_STREAM.
Also, read the manual or get a good book.

Related

binding errors: "cannot assign requested address" and "permission denied" [duplicate]

Following code is TCP server program just send back “HELLO!!” to client.
When I run server with port 80, bind() is returned Permission denied.
Port 12345 is OK.
How can I use port 80 for this server program?
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int
main(){
int sock0;
struct sockaddr_in addr;
struct sockaddr_in client;
int len;
int sock;
char *message;
message = "HELLO !!";
sock0 = socket(AF_INET,SOCK_STREAM,0);
addr.sin_family = AF_INET;
addr.sin_port = htons(80);
inet_pton(AF_INET,"127.0.0.1",&addr,sizeof(addr));
bind(sock0,(struct sockaddr *)&addr,sizeof(addr));
perror("bind");
len = sizeof(client);
sock = accept(sock0,(struct sockaddr *)&client,&len);
perror("accept");
write(sock,message,sizeof(message));
perror("write");
close(sock);
return 0;
}
Ports below 1024 are considered "privileged" and can only be bound to with an equally privileged user (read: root).
Anything above and including 1024 is "free to use" by anyone.
OT: you may know this already, but the port in your example is that for HTTP web servers. Anything listening to this port should speak HTTP, too. A simple "hello world" does not suffice. ;-)
Only the root user is allowed to bind to ports <= 1024. Every ports > 1024 can be bound to by normal users.
Try executing your program as root or with sudo.
you have to run your application with super user account (root)
Run your application with sudo command

(Winsock) UDP receive works but send fails for same socket

I'm having issues working with a UDP socket in Windows. I have a separate application I'm trying to communicate with that outputs on port 1625 and receives on port 26027. I tried to make a simple executable that reads one message and sends one message. The read works fine, but the send ends up with a WSAEADDRNOTAVAIL (10049) error.
To troubleshoot I also tried the equivalent code in Linux with (using Windows Subsystem for Linux) on the same machine and it works fine. So I can't figure out what the issue is. I also tried disabling Windows Firewall but that didn't make a difference. Any suggestions would be appreciated.
The Windows Visual C++ code:
#pragma comment(lib, "Ws2_32.lib")
#include <iostream>
#include <WS2tcpip.h>
#define MAXLINE 1024
int main()
{
WSADATA wsaData;
WSAStartup(MAKEWORD(2, 2), &wsaData);
// Define local port address.
sockaddr_in local_port;
memset(&local_port, 0, sizeof(local_port));
local_port.sin_family = AF_INET;
local_port.sin_addr.s_addr = INADDR_ANY;
local_port.sin_port = htons(1625);
// Bind local socket.
int socket_id = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
bind(socket_id, (const struct sockaddr *)&local_port, sizeof(local_port));
// Receive UDP Port message.
char in_buffer[MAXLINE];
int num_bytes = recv(socket_id, (char *)in_buffer, MAXLINE, 0);
in_buffer[num_bytes] = '\0';
printf("Received : %s\n", in_buffer);
// Set up send destination port.
sockaddr_in dest_port;
memset(&dest_port, 0, sizeof(dest_port));
dest_port.sin_family = AF_INET;
dest_port.sin_addr.s_addr = INADDR_ANY;
dest_port.sin_port = htons(26027);
// Send UDP message to specific UDP port.
char out_buffer[] = "Test message";
int result = sendto(
socket_id, out_buffer, strlen(out_buffer), 0, (struct sockaddr *)&dest_port, sizeof(dest_port));
printf("Send result : %d -- WSA Error : %d\n", result, WSAGetLastError());
closesocket(socket_id);
return 0;
}
Terminal output from running this executable is:
Received : 5e4009df*755=-0.0028:761=0.6942
Send result : -1 -- WSA Error : 10049
The WSL linux C++ code (the same source code except for WSA includes and error output):
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#define MAXLINE 1024
int main()
{
// Define local port address.
sockaddr_in local_port;
memset(&local_port, 0, sizeof(local_port));
local_port.sin_family = AF_INET;
local_port.sin_addr.s_addr = INADDR_ANY;
local_port.sin_port = htons(1625);
// Bind local socket.
int socket_id = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
bind(socket_id, (const struct sockaddr *)&local_port, sizeof(local_port));
// Receive UDP Port message.
char in_buffer[MAXLINE];
int num_bytes = recv(socket_id, (char *)in_buffer, MAXLINE, 0);
in_buffer[num_bytes] = '\0';
printf("Received : %s\n", in_buffer);
// Set up send destination port.
sockaddr_in dest_port;
memset(&dest_port, 0, sizeof(dest_port));
dest_port.sin_family = AF_INET;
dest_port.sin_addr.s_addr = INADDR_ANY;
dest_port.sin_port = htons(26027);
// Send UDP message to specific UDP port.
char out_buffer[] = "Test message";
int result = sendto(
socket_id, out_buffer, strlen(out_buffer), 0, (struct sockaddr *) &dest_port, sizeof(dest_port));
printf("Send result : %d\n", result);
close(socket_id);
return 0;
}
Terminal output from running this executable is:
Received : 5e4009df*755=-0.0028:761=0.6942
Send result : 12
I can also validate that the output to port 26027 via this Linux implementation is received by the other application and can also see it in Wireshark.
EDIT:
After Remy's answer below I was able to get this working as per the comments below. To clarify my network:
My network if I view it with Wireshark now looks like:
127.0.0.1 UDP 50223 → 1625 Len=32
127.0.0.1 UDP 1625 → 26027 Len=12
Where my node binds to 1625 where it can recv() UDP from some unknown port number (50223 in this case), and sendto() port 26027.
You can't use recv() with a UDP socket unless you first call connect() to statically assign the peer's IP/port to the socket, which you are not doing. So recv() will fail, but you are not checking for that. You need to use recvfrom() instead.
Also, no matter what, you can't send packets to INADDR_ANY (0.0.0.0) as you are. That is why you are getting the send error.
sendto Function
WSAEADDRNOTAVAIL
The remote address is not a valid address, for example, ADDR_ANY.
Windows Sockets Error Codes
WSAEADDRNOTAVAIL
10049
Cannot assign requested address. The requested address is not valid in its context. This normally results from an attempt to bind to an address that is not valid for the local computer. This can also result from connect, sendto, WSAConnect, WSAJoinLeaf, or WSASendTo when the remote address or port is not valid for a remote computer (for example, address or port 0).
You need to send to an actual IP/port, such as to the peer's IP/port that is reported by recvfrom() when it receives a packet.

Asio UDP socket receive failed

Tried this code to send/receive with Asio UDP sockets (boost less version)
asio::io_service service;
asio::ip::udp::socket sock(service);
asio::ip::udp::endpoint endpoint(asio::ip::address::from_string("127.0.0.1"), 20100);
sock.connect(endpoint);
sock.send(buffer("testing\n"));
std::string buffer;
size_t length = sock.receive(asio::buffer(buffer)); <--- spawn exception
but got following error:
An existing connection was forcibly closed by the remote host
Something wrong here? Thanks for any help!
I don't know how you build your udp server, but I guess something wrong with it. I write an example program to explain the error message you get:
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
int main()
{
struct sockaddr_in addr;
int fd, cnt, ret;
if ((fd=socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
printf("error");
exit(1);
}
memset(&addr, 0, sizeof(addr));
addr.sin_family=AF_INET;
addr.sin_addr.s_addr=inet_addr("127.0.0.1");
addr.sin_port=htons(9090); // address which is not bound
ret = connect(fd,(struct sockaddr *)&addr,sizeof(addr));
char buf[512];
cnt = send(fd, "hello", sizeof("hello"), 0);
if(cnt < 0)
perror("send:");
cnt = recv(fd, buf, 512, 0); // error: Connection refused
if(cnt < 0)
perror("recv:");
return 0;
}
I try to send data to udp port 9090 at localhost or any host which no udp socket is bound to port 9090. the send() succeeds but the recv() fails. According to man page for udp:
All fatal errors will be passed to the
user as an error return even when the
socket is not connected. This includes
asynchronous errors received from the
network. You may get an error for an
earlier packet that was sent on the
same socket. This behaviour differs
from many other BSD socket
implementations which don't pass any
errors unless the socket is connected.
Linux's behaviour is mandated by
According to the rfc 1122
which says:
UDP MUST pass to the application layer all ICMP error messages that it receives from the IP layer. Conceptually at least, this may be accomplished with an upcall to the ERROR_REPORT routine
the send() succeeds, but it causes an ICMP error message(you can see it with tcpdump), then recv() sees this error(You may get an error for an earlier packet that was sent on the same socket), so it fails.

Raspbian C++ Error: expected primary expression before 'struct'

I am trying to write a program for my raspberry pi that changes its system time to the time from a GPS unit on the same network. The GPS sends out a 72 byte UDP packet across port 3000. I am new to socket programming so I am unsure where I am going wrong.
The trouble that I am having is that I can't seem to get it to build with g++. I am getting the following error:
So the main error seems to be in the line
char A = struct sockaddr_in address;
Here is the start of my program and the method where I create the socket and where the error is located, if you would like the main method of my program then I will add it too.
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <errno.h>
#include <math.h>
// defines the socket used by the GPS
#define PORT 3000
/****************************/
int CreateSocket(int port)
/****************************/
{
// Create an UDP-socket
int sock = socket(AF_INET, SOCK_DGRAM, 0);
// Check if UDP-socket was created
if(sock==-1)
{
fprintf(stderr, "1CreateSocket: socket failed\n");
return -1;
}
// Bind it to the local IP-address
struct sockaddr_in address;
char A = struct sockaddr_in address;
fprintf(stderr, A);
// Pointer to the block of memory to fill with address data
memset(&address, 0, sizeof(address));
address.sin_family = AF_INET; // Address family for IP-address
address.sin_addr.s_addr = htonl(INADDR_ANY); // converts the unsigned integer hostlong from host byte order to network byte order
address.sin_port = htons(port); // converts the unsigned short integer hostshort from host byte order to network byte order
// Check if IP-address is correct, if not Socket failed. Otherwise it returns the socket
if(bind(sock, (struct sockaddr *) &address, sizeof(address))==-1)
{
fprintf(stderr, "2CreateSocket: bind failed\n");
close(sock);
return -1;
}
return sock;
}
Can anyone see any obvious errors here? Thanks
You don't really need these two lines:
char A = struct sockaddr_in address;
fprintf(stderr, A);
You can delete them, since they don't do anything useful, and they have a syntax error.
And to do some extra cleanup, the comment of the binding above those lines that can be deleted should actually go above the call to bind().

recvfrom works with INADDR_ANY, but specifying certain interface doesn't work

I wrote a program that join source specific multicast group and receive udp multicast packets:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netdb.h>
#include <unistd.h>
#include <net/if.h>
typedef unsigned int UINT32;
int join_ssm_group(int s, UINT32 group, UINT32 source, UINT32 inter) {
struct ip_mreq_source imr;
imr.imr_multiaddr.s_addr = group;
imr.imr_sourceaddr.s_addr = source;
imr.imr_interface.s_addr = inter;
return setsockopt(s, IPPROTO_IP, IP_ADD_SOURCE_MEMBERSHIP, (char *) &imr, sizeof(imr));
}
UINT32 LISTEN_INTERFACE = inet_addr("10.10.1.2");
int main(int argc, char *argv[]) {
if (argc<3) {
printf(" Use: %s <group> <source> <port>", argv[0]);
return 1;
}
// Make socket
int sd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
struct sockaddr_in Sender;
socklen_t SenderAddrSize = sizeof( Sender );
struct sockaddr_in binda;
// Bind it to listen appropriate UDP port
binda.sin_family = AF_INET;
binda.sin_port = htons( atoi(argv[3]));
= INADDR_ANY;
// binda.sin_addr.s_addr = LISTEN_INTERFACE;
bind(sd,(struct sockaddr*)&binda, sizeof(binda));
// Join to group
join_ssm_group( sd, inet_addr(argv[1]),
inet_addr(argv[2]),
INADDR_ANY );
char buf[65536];
UINT32 seq;
while(1) {
printf("try receive\n");
int res=recvfrom(sd,(char*)buf,sizeof(buf),0, (struct sockaddr *)& Sender, &SenderAddrSize);
printf("received\n");
seq = *(UINT32*)buf;
printf("scr=:%12s;\tseq=%6d;\tlen=%4d\n", inet_ntoa(Sender.sin_addr), seq, res);
}
return 0;
}
It works fine but note that I'm using binda.sin_addr.s_addr = INADDR_ANY;. netstat shows this:
netstat -a | grep 16002
udp 0 0 0.0.0.0:16002 0.0.0.0:*
When I change it to binda.sin_addr.s_addr = LISTEN_INTERFACE; program stops working - it can not recieve packets, it hangs in recvfrom. netstat shows this:
netstat -a | grep 16002
udp 0 0 localhost.localdo:16002 0.0.0.0:*
In both cases tcpdump shows that data is online, so the problem is that I can not receive data on the specific interface, only on ALL interfaces. I'm using RHEL 7, teaming, and LISTEN_INTERFACE is the IP of the corresponding VLAN. Why my code doesn't work and how to troubleshoot it? I do not want to use INADDR_ANY for performance reasons - listening ALL interfaces would be more expensive than listeining certain interface.
upd passing LISTEN_INTERFACE to both join_ssm_group and and binda.sin_addr.s_addr doesn't work too. BTW similar Windows version of such code works on the same PC under Windows Server 2008 R2, but it doesn't work in RHEL 7. I guess I should check these:
if RHEL 7 receives data on the requreid interface on the required port (answer is Yes, proved by tcpdump)
if socket is listening on the required interface on the required port (answer is Yes, proved by netstat?)
if both answers above are Yes then how is it possible that call to recvfrom doesn't receive data?
Well probably this question more about RHEL 7 now, than about c++.
When you join the multicast group you need to specify the same interface that you are listening on, or join it via all interfaces in a loop.
However listening on all interfaces is the norm. It is not 'slow', and it is a 'good idea', unless you have a specific reason to restrict who can connect.