Sending mail via smtp and libcurl with tls

I was writing a daemon that monitored users on a shared system, which sent an email to them if they were using too much CPU. The problem was that in a daemon you don't want to kick off a new process by calling a commandline tool. I wanted to send it from within the daemon process itself. I could get the email address of the user from ldap easy enough, but how to actually send the email?

The only way to do it via open source software I found was to use libcurl. Unfortunately I wanted to send to an outlook server, which uses NTLM for authentication process. And libcurl at least on redhat linux uses NSS, not openssl, for the TLS (SSL) transport. And nss does not support NTLM. That meant I had to recompile the libcurl library using openssl not nss. I read lots of postings on this, none of which resolved the problem. The libcurl people simply refused to provide an openssl version of libcurl. But with the proper configuration you can rebuild libcurl. Here's what I used, after getting the source code from the libcurl github account:

./configure --with-ssl=/usr/lib64 --without-nss

Then, make, make install worked OK but installed into /usr/local/lib, not /lib/64. There was already a copy of libcurl there so I deleted it and swapped it over. with the fresh copy and voila: it worked. Here's the test code in case someone else has the same problem. The original code I found here. It has more comments than my version. You have to fill in your own credentials of course.

#include <stdio.h> #include <string.h> #include <curl/curl.h> #define FROM "<forename.surname@myplace.com>" #define TO "<muemail@email-server.com>" #define CC "<anotheremail@other-server.com>" static const char *payload_text[] = { "Date: Mon, 22 Mar 2017 16:20:29 +1000\r\n", "To: " TO "\r\n", "From: " FROM "(Forename Surname)\r\n", "Cc: " CC "(Fore Surname)\r\n", "Subject: SMTP TLS example message\r\n", "\r\n", /* empty line to divide headers from body, see RFC5322 */ "The body of the message starts here.\r\n", "\r\n", "It could be a lot of lines, could be MIME encoded, whatever.\r\n", "Check RFC5322.\r\n", NULL }; struct upload_status { int lines_read; }; static size_t payload_source(void *ptr, size_t size, size_t nmemb, void *userp) { struct upload_status *upload_ctx = (struct upload_status *)userp; const char *data; if((size == 0) || (nmemb == 0) || ((size*nmemb) < 1)) { return 0; } data = payload_text[upload_ctx->lines_read]; if(data) { size_t len = strlen(data); memcpy(ptr, data, len); upload_ctx->lines_read++; return len; } return 0; } int main(void) { CURL *curl; CURLcode res = CURLE_OK; struct curl_slist *recipients = NULL; struct upload_status upload_ctx; upload_ctx.lines_read = 0; curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_USERNAME, "myid@myplace.com"); curl_easy_setopt(curl, CURLOPT_PASSWORD, "mysecret"); curl_easy_setopt(curl, CURLOPT_URL, "smtp://outlook.office365.com:587"); curl_easy_setopt(curl, CURLOPT_USE_SSL, (long)CURLUSESSL_ALL); curl_easy_setopt(curl, CURLOPT_CAINFO, "/etc/pki/tls/certs/ca-bundle.crt"); curl_easy_setopt(curl, CURLOPT_CAPATH, "/etc/pki/tls/certs"); curl_easy_setopt(curl, CURLOPT_MAIL_FROM, FROM); recipients = curl_slist_append(recipients, TO); recipients = curl_slist_append(recipients, CC); curl_easy_setopt(curl, CURLOPT_MAIL_RCPT, recipients); curl_easy_setopt(curl, CURLOPT_READFUNCTION, payload_source); curl_easy_setopt(curl, CURLOPT_READDATA, &upload_ctx); curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L); curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L); res = curl_easy_perform(curl); if(res != CURLE_OK) fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res)); curl_slist_free_all(recipients); curl_easy_cleanup(curl); } return (int)res; }