trying to talk to email server via SMTP with c program - c

Question

I'm trying to create a little C program that allow me to talk to mail server via smtp, I created this little program:
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>
// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "25"
#define DEFAULT_ADDRESS "smtp.live.com"
int __cdecl main(int argc, char **argv)
{
WSADATA wsaData;
SOCKET ConnectSocket = INVALID_SOCKET;
struct addrinfo *result = NULL,
*ptr = NULL,
hints;
char messaggio[100];
char recvbuf[DEFAULT_BUFLEN];
int iResult;
int recvbuflen = DEFAULT_BUFLEN;
while(1){// Initialize Winsock
iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
if (iResult != 0) {
printf("WSAStartup failed with error: %d\n", iResult);
return 1;
}
ZeroMemory( &hints, sizeof(hints) );
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
// Resolve the server address and port
iResult = getaddrinfo(DEFAULT_ADDRESS, DEFAULT_PORT, &hints, &result);
if ( iResult != 0 ) {
printf("getaddrinfo failed with error: %d\n", iResult);
WSACleanup();
return 1;
}
// Attempt to connect to an address until one succeeds
for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) {
// Create a SOCKET for connecting to server
ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype,
ptr->ai_protocol);
if (ConnectSocket == INVALID_SOCKET) {
printf("socket failed with error: %ld\n", WSAGetLastError());
WSACleanup();
return 1;
}
// Connect to server.
iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
if (iResult == SOCKET_ERROR) {
closesocket(ConnectSocket);
ConnectSocket = INVALID_SOCKET;
continue;
}
break;
}
freeaddrinfo(result);
if (ConnectSocket == INVALID_SOCKET) {
printf("Unable to connect to server!\n");
WSACleanup();
return 1;
}
printf("messaggio: ");
gets(messaggio);
fflush(stdin);
printf("%s\n", messaggio);
// Send an initial buffer
iResult = send( ConnectSocket, messaggio, (int)strlen(messaggio), 0 );
if (iResult == SOCKET_ERROR) {
printf("send failed with error: %d\n", WSAGetLastError());
closesocket(ConnectSocket);
WSACleanup();
return 1;
}
printf("Bytes Sent: %ld\n", iResult);
// shutdown the connection since no more data will be sent
iResult = shutdown(ConnectSocket, SD_SEND);
if (iResult == SOCKET_ERROR) {
printf("shutdown failed with error: %d\n", WSAGetLastError());
closesocket(ConnectSocket);
WSACleanup();
return 1;
}
// Receive until the peer closes the connection
// do {
iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0);
if ( iResult > 0 ){
printf("Bytes received: %d\n", iResult);
printf("messaggio: %s\n", recvbuf);}
else if ( iResult == 0 )
printf("Connection closed\n");
else
printf("recv failed with error: %d\n", WSAGetLastError());
// } while( iResult > 0 );
// cleanu
closesocket(ConnectSocket);
WSACleanup();
}
system("PAUSE");
return 0;
}
When I launch it, it does connect to the smtp server, then send the message (for example Hello message) and receive 220 reply code meaning that the services is ready. When I continue typing command it does continue to send me 220 code without going forward, and when I miss type a command it just crush.
may any of you please help me?
p.s. I take no credit for code, I just modified some socket client documentation of microsoft to talk to smtp.live.com and apparently it does work...
[EDIT] x2:
ok, thanks to Remy Lebeau i apparently managed to resolve those problem, hope this is the last one.
edited code:
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>
// Need to link with Ws2_32.lib, Mswsock.lib, and Advapi32.lib
#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "25"
#define DEFAULT_ADDRESS "smtp.live.com"
int __cdecl main(int argc, char **argv){
WSADATA wsaData;
SOCKET ConnectSocket = INVALID_SOCKET;
struct addrinfo *result = NULL,
*ptr = NULL,
hints;
char messaggio[100];
char recvbuf[DEFAULT_BUFLEN];
int iResult;
int recvbuflen = DEFAULT_BUFLEN;
int success = 1;
iResult = WSAStartup(MAKEWORD(2,2), &wsaData); // Initialize Winsock
if (iResult != 0) {
printf("WSAStartup failed with error: %d\n", iResult);
return 1;
}
ZeroMemory( &hints, sizeof(hints) );
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
iResult = getaddrinfo(DEFAULT_ADDRESS, DEFAULT_PORT, &hints, &result); // Resolve the server address and port
if ( iResult != 0 ) {
printf("getaddrinfo failed with error: %d\n", iResult);
WSACleanup();
return 1;
}
for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) { // Attempt to connect to an address until one succeeds
ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
if (ConnectSocket == INVALID_SOCKET) {
printf("socket failed with error: %ld\n", WSAGetLastError());
WSACleanup();
return 1;
}
iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen); //Connect to server.
if (iResult == SOCKET_ERROR) {
closesocket(ConnectSocket);
ConnectSocket = INVALID_SOCKET;
continue;
}
break;
}
freeaddrinfo(result);
if (ConnectSocket == INVALID_SOCKET) {
printf("Unable to connect to server!\n");
WSACleanup();
return 1;
}
while(success){
printf("messaggio: ");
scanf(" %[^\n]", messaggio);
printf("%s\n", messaggio);
iResult = send( ConnectSocket, strcat(messaggio,"\r\n"), (int)strlen(messaggio)+2, 0 ); //Send an initial buffer
if (iResult < 0 || iResult == SOCKET_ERROR) {
printf("send failed with error: %d\n", WSAGetLastError());
closesocket(ConnectSocket);
WSACleanup();
system("PAUSE");
return 1;
}
printf("Bytes Sent: %ld\n", iResult);
do{ //riceve tutto il possibile dal server
iResult = recv(ConnectSocket, recvbuf, recvbuflen-1, 0);
recvbuf[iResult]='\0';
if ( iResult > 0 ){
printf("Bytes received: %d\n", iResult);
printf("messaggio: %s\n", recvbuf);
}
else{
printf("recv failed with error: %d\n", WSAGetLastError());
success=0;
}
}while(recvbuf[iResult-3]=='\r'&& recvbuf[iResult-2]=='\n');
printf("stop receive\n");
}
iResult = shutdown(ConnectSocket, SD_SEND);
closesocket(ConnectSocket);
WSACleanup();
system("PAUSE");
return 0;
}
Now it does take my command and it does respond, i do helo, mail from: <mail> and to: <mail> and at this point it says: "530 (code for auth problem) Must issue a STARTTLS command first.
Is there a way to resolve this? i've to encode my sended command, is there a way to do this with winsock?
Did i almost got the point of the problem or it isn't this the problem?

Answer 1

Your code logic is incomplete for an SMTP client.
TCP is a byte stream. There is no guarantee that you will be able to send an entire command in a single send() call, or that you will be able to receive an entire response in a single recv() call. You have to keep sending until you have nothing left to send, and you have to keep reading until there is nothing more to read. Which, in the case of SMTP, means when you encounter the \r\n that terminates each line.
Once you have your underlying socket I/O working, you are still not implementing the actual SMTP protocol. You are just throwing arbitrary data at the server and reading whatever arbitrary data it returns. SMTP has rules that you need to take into account. For example, an SMTP server sends a greeting before accepting commands. You are not reading that greeting. Also, an SMTP response may have 1 or more lines, so you have to parse each line individually (the format is outlined in RFC 5321) to discover how many lines need to be read. Keep reading until you encounter the last line of a response. Don't send the next command until the previous response has been fully read (unless you want to implement SMTP pipelining, which I won't get into here).
As for the "Must issue a STARTTLS command first" error, that is pretty self-explanatory. You sent an SMTP command over an unsecure connection, but the server wants that command to be sent over a secure connection instead (such as when sending an AUTH command containing authentication credentials in plain text). The STARTTLS command is used to activate a new SSL/TLS session to secure a connection that is currently unsecure.
You can use libraries like OpenSSL, or APIs like Microsoft's SChannel, to implement the actual SSL/TLS on top of your socket. That is outside the scope of this question. If you want help with implementing SSL/TLS, there are plenty of books and tutorials on that subject.
With all of that said, try something more like this instead:
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdlib.h>
#include <stdio.h>
#pragma comment(lib, "ws2_32.lib")
#pragma command(lib, "mswsock.lib")
//#pragma command(lib, "advapi32.lib")
#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "25"
#define DEFAULT_ADDRESS "smtp.live.com"
char *recvbuf = NULL;
int recvbuflen = 0;
int recvbufused = 0;
int sendData(SOCKET s, char *buffer, int buflen)
{
int iResult;
while (buflen > 0)
{
// TODO: if the connection is secure, encrypt the data being sent...
iResult = send(s, buffer, buflen, 0);
if (iResult == SOCKET_ERROR)
{
printf("send failed with error: %d\n", WSAGetLastError());
return -1;
}
buffer += iResult;
buflen -= iResult;
}
return 0;
}
int sendLine(SOCKET s, char *line)
{
int iResult = sendData(s, line, strlen(line));
if (iResult == 0)
iResult = sendData(s, "\r\n", 2);
return iResult;
}
int readLine(SOCKET s, char **line)
{
char buf[DEFAULT_BUFLEN];
int iResult;
int offset = 0, len, total;
char *p;
*line = NULL;
do
{
// check if the line terminator is already in the buffer
p = memchr(recvbuf+offset, '\n', recvbufused-offset);
if (p != NULL)
{
++p;
total = (p - recvbuf);
// check if the line terminator is actually "\r\n"
len = total - 1;
if ((len > 0) && (recvbuf[len-1] == '\r'))
--len;
// extract the line and remove it from the buffer
*line = malloc(len+1);
if (*line == NULL)
{
printf("Unable to allocate memory! Bytes: %d\n", len+1);
return -1;
}
memcpy(*line, recvbuf, len);
(*line)[len] = 0;
memmove(recvbuf, recvbuf+total, recvbufused-total);
recvbufused -= total;
printf("%s\n", *line);
return len;
}
// line terminator is not in the buffer, keep reading
// do not search the buffer that was already searched
offset = recvbufused;
// read more data from the socket
iResult = recv(s, buf, sizeof(buf), 0);
if (iResult <= 0)
{
if (iResult == 0)
printf("server closed the connection\n");
else
printf("recv failed with error: %d\n", WSAGetLastError());
break;
}
// TODO: if the connection is secure, decrypt the data that was received...
// if the buffer is too small for the new data, grow it
if ((recvbufused+iResult) > recvbuflen)
{
len = (recvbufused+iResult) + DEFAULT_BUFLEN;
p = realloc(recvbuf, len);
if (p == NULL)
{
printf("Unable to allocate memory! Bytes: %d\n", len);
break;
}
recvbuf = p;
recvbuflen = len;
}
// now add the data to the buffer
memcpy(recvbuf+recvbufused, buf, iResult);
recvbufused += iResult;
}
while (1);
return -1;
}
int readResponse(SOCKET s)
{
char *line, term[5];
int len, code = 0;
// read the first line
len = readLine(s, &line);
if (len == -1)
return -1;
// check if the response has multiple lines
if ((len > 3) && (line[3] == '-'))
{
// keep reading until the last line is received
memcpy(term, line, 3);
term[3] = ' ';
term[4] = '\0';
do
{
free(line);
len = readLine(s, &line);
if (len == -1)
return -1;
}
while ((len > 3) && (strncmp(line, term, 4) != 0));
}
// now extract the response code and return it
if (len > 3) line[3] = '\0';
code = strtol(line, NULL, 10);
free(line);
if (code == 0)
return -1;
return code;
}
int __cdecl main(int argc, char **argv)
{
WSADATA wsaData;
SOCKET ConnectSocket = INVALID_SOCKET;
struct addrinfo hints, *result, *ptr;
char messaggio[100];
int iResult;
// Initialize Winsock
iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
if (iResult != 0)
{
printf("WSAStartup failed with error: %d\n", iResult);
return 1;
}
ZeroMemory( &hints, sizeof(hints) );
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
// Resolve the server address and port
iResult = getaddrinfo(DEFAULT_ADDRESS, DEFAULT_PORT, &hints, &result);
if (iResult != 0)
{
printf("getaddrinfo failed with error: %d\n", iResult);
WSACleanup();
return 1;
}
// Attempt to connect to an address until one succeeds
for(ptr=result; ptr != NULL; ptr=ptr->ai_next)
{
ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
if (ConnectSocket == INVALID_SOCKET)
{
printf("socket failed with error: %ld\n", WSAGetLastError());
break;
}
//Connect to server.
iResult = connect(ConnectSocket, ptr->ai_addr, ptr->ai_addrlen);
if (iResult == 0)
break;
closesocket(ConnectSocket);
ConnectSocket = INVALID_SOCKET;
}
freeaddrinfo(result);
if (ConnectSocket == INVALID_SOCKET)
{
printf("Unable to connect to server!\n");
WSACleanup();
return 1;
}
printf("Connected to server\n");
// read the greeting
iResult = readResponse(ConnectSocket);
if (iResult == 220)
{
do
{
// ask the user for a command
printf("messaggio: ");
if (!fgets(messaggio, sizeof(messaggio), stdin))
break;
// remove the line terminator if present
iResult = strlen(messaggio);
if ((iResult > 0) && (messaggio[iResult-1] == '\n'))
messaggio[iResult-1] = '\0';
// send the command
iResult = sendLine(ConnectSocket, messaggio);
if (iResult == -1)
break;
// read the response
iResult = readResponse(ConnectSocket);
if (iResult == -1)
break;
// need to stop now?
if (strcmp(messaggio, "QUIT") == 0)
break;
// need to turn on SSL/TLS now?
if ((strcmp(messaggio, "STARTTLS") == 0) && (iResult == 220))
{
// TODO: activate SSL/TLS, stop if failed...
}
}
while(1);
}
shutdown(ConnectSocket, SD_SEND);
do
{
iResult = recv(ConnectSocket, messaggio, sizeof(messaggio), 0);
}
while (iResult > 0);
closesocket(ConnectSocket);
WSACleanup();
free(recvbuf);
system("PAUSE");
return 0;
}