Using c++ to send smtp mail - c++

Question

I was wondering if you could help me understand what is going wrong here. I am trying to write a little client that engages in SMTP dialogue on port 25.
If you recall SMTP, there's a few things you need to send, and then you write the email after the DATA message, ending with a period on it's own line to send the email.
There is something problematic in the way the program handles this. It handles the dialogue fine until after the DATA message. It will recognize the period only if I type it first. After any subsequent line, any code execution seems to be lost. The if statement fails to recognize if a period has been entered. Thank you again. Attached is the relevant code..
void readstuff(int sock, char* buf) {
int r = read (sock, buf, BUFSIZE -1);
buf[r] = NULL;
cout << buf << endl;
}
void doit(int sock, string arg, char* buf) {
int r = write(sock, arg.c_str(), arg.length());
readstuff(sock, buf);
}
int main(int argc, char *argv[]) {
char buf[BUFSIZE];
// Make a socket
int sock = MakeSocket(argv[1], argv[2]);
cout << "socket is " << sock << endl;
assert(sock != -1);
// Begin dialogue
doit(sock, "HELO " + org.substr(org.find("#") + 1) + "\r\n", buf);
doit(sock, "MAIL FROM: <" + org + "> \r\n", buf);
doit(sock, "RCPT TO: <" + dest + "> \r\n", buf);
doit(sock, "DATA \r\n", buf);
readstuff(sock, buf); //should say "go ahead"
//User writes email here
while (true) {
string line = "";
getline(cin, line);
doit(sock, line + "\r\n", buf);
if (line == ".") {
readstuff(sock, buf); //should say "email cleared to send"
return 0;
}
}
}

Answer 1

Please read the SMTP specification, RFC 5321, particularly sections 4.1.1.4 DATA and 4.5.2 Transparency:
4.1.1.4. DATA (DATA)
The receiver normally sends a 354 response to DATA, and then treats
the lines (strings ending in <CRLF> sequences, as described in
Section 2.3.7) following the command as mail data from the sender.
This command causes the mail data to be appended to the mail data
buffer. The mail data may contain any of the 128 ASCII character
codes, although experience has indicated that use of control
characters other than SP, HT, CR, and LF may cause problems and
SHOULD be avoided when possible.
The mail data are terminated by a line containing only a period, that
is, the character sequence "<CRLF>.<CRLF>", where the first <CRLF> is
actually the terminator of the previous line (see Section 4.5.2).
This is the end of mail data indication. The first <CRLF> of this
terminating sequence is also the <CRLF> that ends the final line of
the data (message text) or, if there was no mail data, ends the DATA
command itself (the "no mail data" case does not conform to this
specification since it would require that neither the trace header
fields required by this specification nor the message header section
required by RFC 5322 [4] be transmitted). An extra <CRLF> MUST NOT
be added, as that would cause an empty line to be added to the
message. The only exception to this rule would arise if the message
body were passed to the originating SMTP-sender with a final "line"
that did not end in <CRLF>; in that case, the originating SMTP system
MUST either reject the message as invalid or add <CRLF> in order to
have the receiving SMTP server recognize the "end of data" condition.
The custom of accepting lines ending only in <LF>, as a concession to
non-conforming behavior on the part of some UNIX systems, has proven
to cause more interoperability problems than it solves, and SMTP
server systems MUST NOT do this, even in the name of improved
robustness. In particular, the sequence "<LF>.<LF>" (bare line
feeds, without carriage returns) MUST NOT be treated as equivalent to
<CRLF>.<CRLF> as the end of mail data indication.
Receipt of the end of mail data indication requires the server to
process the stored mail transaction information. This processing
consumes the information in the reverse-path buffer, the forward-path
buffer, and the mail data buffer, and on the completion of this
command these buffers are cleared. If the processing is successful,
the receiver MUST send an OK reply. If the processing fails, the
receiver MUST send a failure reply. The SMTP model does not allow
for partial failures at this point: either the message is accepted by
the server for delivery and a positive response is returned or it is
not accepted and a failure reply is returned. In sending a positive
"250 OK" completion reply to the end of data indication, the receiver
takes full responsibility for the message (see Section 6.1). Errors
that are diagnosed subsequently MUST be reported in a mail message,
as discussed in Section 4.4.
When the SMTP server accepts a message either for relaying or for
final delivery, it inserts a trace record (also referred to
interchangeably as a "time stamp line" or "Received" line) at the top
of the mail data. This trace record indicates the identity of the
host that sent the message, the identity of the host that received
the message (and is inserting this time stamp), and the date and time
the message was received. Relayed messages will have multiple time
stamp lines. Details for formation of these lines, including their
syntax, is specified in Section 4.4.
Additional discussion about the operation of the DATA command appears
in Section 3.3.
Syntax:
data = "DATA" CRLF
4.5.2. Transparency
Without some provision for data transparency, the character sequence
"<CRLF>.<CRLF>" ends the mail text and cannot be sent by the user.
In general, users are not aware of such "forbidden" sequences. To
allow all user composed text to be transmitted transparently, the
following procedures are used:
o Before sending a line of mail text, the SMTP client checks the
first character of the line. If it is a period, one additional
period is inserted at the beginning of the line.
o When a line of mail text is received by the SMTP server, it checks
the line. If the line is composed of a single period, it is
treated as the end of mail indicator. If the first character is a
period and there are other characters on the line, the first
character is deleted.
...
Your DATA command needs to account for that:
Your doit() function is expecting a response after sending a string. That is the wrong logic to use while sending the email data. You can't read a response until after the final terminating <CRLF>.<CRLF> has been sent.
you have to apply transparency to any line in the email that begins with a . character.
With that said, try something more like this:
int readLine(int sock, string &line)
{
// read a line from sock until CRLF is reached.
// I leave this as an exercise for you to implement...
line = ...;
return -1 on error, else 0;
}
int readResponse(int sock)
{
// Please read RFC 5321 section 4.2 for the PROPER format
// of an SMTP response. You should be reading from the
// socket until you receive the terminating
// "Reply-code [ SP textstring ] CRLF" line...
string line;
int r = readLine(sock, line);
if (r < 0) return r;
string code = line.substr(0, 3);
string text = line.substr(4);
if ((line.length() >= 4) && (line[3] = '-'))
{
do
{
r = readLine(sock, line);
if (r < 0) return r;
text += (" " + line.substr(4));
}
while (line.compare(0, 4, code+"-") == 0);
}
cout << code << ": " << text << endl;
return stoi(code);
}
int sendText(int sock, const string &arg)
{
const char *p = arg.c_str();
int len = arg.length();
while (len > 0)
{
int r = write(sock, p, len);
if (r <= 0) return -1;
p += r;
len -= r;
}
return 0;
}
int sendCmd(int sock, const string &arg)
{
int r = sendText(sock, arg + "\r\n");
if (r < 0) return r;
return readResponse(sock);
}
int main(int argc, char *argv[])
{
// Make a socket
int sock = MakeSocket(argv[1], argv[2]);
cout << "socket is " << sock << endl;
assert(sock != -1);
// Begin dialogue
// read the server greeting first...
if (readResponse(sock) != 220) {
// failed, do something...
}
if (sendCmd(sock, "HELO " + org.substr(org.find("#") + 1)) != 250) {
// failed, do something...
}
if (sendCmd(sock, "MAIL FROM: <" + org + ">") != 250) {
// failed, do something...
}
int r = sendCmd(sock, "RCPT TO: <" + dest + ">");
if ((r != 250) && (r != 251)) {
// failed, do something...
}
if (sendCmd(sock, "DATA") != 354) {
// failed, do something...
}
//User writes email here
while (true) {
string line;
getline(cin, line);
// A line consisting of only "." is a valid line in an email
// message, so you should not use that as a terminator in your
// input, use something else, like an EOF marker, or CTRL-C,
// or something...
if (some termination condition)
break;
// DO NOT call readResponse() here!
if (!line.empty() && (line[0] == '.')) {
if (sendText(sock, ".") < 0) {
// failed, do something...
}
}
if (sendText(sock, line) < 0) {
// failed, do something...
}
if (sendText(sock, "\r\n") < 0) {
// failed, do something...
}
}
// NOW call readResponse() here!
if (sendCmd(sock, ".") != 250) {
// failed, do something...
}
sendCmd(sock, "QUIT");
close(sock);
return 0;
}