/* vi: set sw=4 ts=4: */
/*
 * bare bones sendmail
 *
 * Copyright (C) 2008 by Vladimir Dronnikov <dronnikov@gmail.com>
 *
 * Licensed under GPLv2, see file LICENSE in this source tree.
 */
//config:config SENDMAIL
//config:       bool "sendmail (14 kb)"
//config:       default y
//config:       help
//config:       Barebones sendmail.

//applet:IF_SENDMAIL(APPLET(sendmail, BB_DIR_USR_SBIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_SENDMAIL) += sendmail.o mail.o

//usage:#define sendmail_trivial_usage
//usage:       "[-tv] [-f SENDER] [-amLOGIN 4<user_pass.txt | -auUSER -apPASS]"
//usage:     "\n                [-w SECS] [-H 'PROG ARGS' | -S HOST] [RECIPIENT_EMAIL]..."
//usage:#define sendmail_full_usage "\n\n"
//usage:       "Read email from stdin and send it\n"
//usage:     "\nStandard options:"
//usage:     "\n        -t              Read additional recipients from message body"
//usage:     "\n        -f SENDER       For use in MAIL FROM:<sender>. Can be empty string"
//usage:     "\n                        Default: -auUSER, or username of current UID"
//usage:     "\n        -o OPTIONS      Various options. -oi implied, others are ignored"
//usage:     "\n        -i              -oi synonym, implied and ignored"
//usage:     "\n"
//usage:     "\nBusybox specific options:"
//usage:     "\n        -v              Verbose"
//usage:     "\n        -w SECS         Network timeout"
//usage:     "\n        -H 'PROG ARGS'  Run connection helper. Examples:"
//usage:     "\n                openssl s_client -quiet -tls1 -starttls smtp -connect smtp.gmail.com:25"
//usage:     "\n                openssl s_client -quiet -tls1 -connect smtp.gmail.com:465"
//usage:     "\n                        $SMTP_ANTISPAM_DELAY: seconds to wait after helper connect"
//usage:     "\n        -S HOST[:PORT]  Server (default $SMTPHOST or 127.0.0.1)"
//usage:     "\n        -amLOGIN        Log in using AUTH LOGIN"
//usage:     "\n        -amPLAIN        or AUTH PLAIN"
//usage:     "\n                        (-amCRAM-MD5 not supported)"
//usage:     "\n        -auUSER         Username for AUTH"
//usage:     "\n        -apPASS         Password for AUTH"
//usage:     "\n"
//usage:     "\nIf no -a options are given, authentication is not done."
//usage:     "\nIf -amLOGIN is given but no -au/-ap, user/password is read from fd #4."
//usage:     "\nOther options are silently ignored; -oi is implied."
//usage:        IF_MAKEMIME(
//usage:     "\nUse makemime to create emails with attachments."
//usage:        )

/* Currently we don't sanitize or escape user-supplied SENDER and RECIPIENT_EMAILs.
 * We may need to do so. For one, '.' in usernames seems to require escaping!
 *
 * From http://cr.yp.to/smtp/address.html:
 *
 * SMTP offers three ways to encode a character inside an address:
 *
 * "safe": the character, if it is not <>()[].,;:@, backslash,
 *  double-quote, space, or an ASCII control character;
 * "quoted": the character, if it is not \012, \015, backslash,
 *   or double-quote; or
 * "slashed": backslash followed by the character.
 *
 * An encoded box part is either (1) a sequence of one or more slashed
 * or safe characters or (2) a double quote, a sequence of zero or more
 * slashed or quoted characters, and a double quote. It represents
 * the concatenation of the characters encoded inside it.
 *
 * For example, the encoded box parts
 *      angels
 *      \a\n\g\e\l\s
 *      "\a\n\g\e\l\s"
 *      "angels"
 *      "ang\els"
 * all represent the 6-byte string "angels", and the encoded box parts
 *      a\,comma
 *      \a\,\c\o\m\m\a
 *      "a,comma"
 * all represent the 7-byte string "a,comma".
 *
 * An encoded address contains
 *      the byte <;
 *      optionally, a route followed by a colon;
 *      an encoded box part, the byte @, and a domain; and
 *      the byte >.
 *
 * It represents an Internet mail address, given by concatenating
 * the string represented by the encoded box part, the byte @,
 * and the domain. For example, the encoded addresses
 *     <God@heaven.af.mil>
 *     <\God@heaven.af.mil>
 *     <"God"@heaven.af.mil>
 *     <@gateway.af.mil,@uucp.local:"\G\o\d"@heaven.af.mil>
 * all represent the Internet mail address "God@heaven.af.mil".
 */

#include "libbb.h"
#include "mail.h"

// limit maximum allowed number of headers to prevent overflows.
// set to 0 to not limit
#define MAX_HEADERS 256

static void send_r_n(const char *s)
{
        if (verbose)
                bb_error_msg("send:'%s'", s);
        printf("%s\r\n", s);
}

static int smtp_checkp(const char *fmt, const char *param, int code)
{
        char *answer;
        char *msg = send_mail_command(fmt, param);
        // read stdin
        // if the string has a form NNN- -- read next string. E.g. EHLO response
        // parse first bytes to a number
        // if code = -1 then just return this number
        // if code != -1 then checks whether the number equals the code
        // if not equal -> die saying msg
        while ((answer = xmalloc_fgetline(stdin)) != NULL) {
                if (verbose)
                        bb_error_msg("recv:'%.*s'", (int)(strchrnul(answer, '\r') - answer), answer);
                if (strlen(answer) <= 3 || '-' != answer[3])
                        break;
                free(answer);
        }
        if (answer) {
                int n = atoi(answer);
                if (timeout)
                        alarm(0);
                free(answer);
                if (-1 == code || n == code) {
                        free(msg);
                        return n;
                }
        }
        bb_error_msg_and_die("%s failed", msg);
}

static int smtp_check(const char *fmt, int code)
{
        return smtp_checkp(fmt, NULL, code);
}

// strip argument of bad chars
static char *sane_address(char *str)
{
        char *s;

        trim(str);
        s = str;
        while (*s) {
                /* Standard allows these chars in username without quoting:
                 * /!#$%&'*+-=?^_`{|}~
                 * and allows dot (.) with some restrictions.
                 * I chose to only allow a saner subset.
                 * I propose to expand it only on user's request.
                 */
                if (!isalnum(*s) && !strchr("=+_-.@", *s)) {
                        bb_error_msg("bad address '%s'", str);
                        /* returning "": */
                        str[0] = '\0';
                        return str;
                }
                s++;
        }
        return str;
}

// check for an address inside angle brackets, if not found fall back to normal
static char *angle_address(char *str)
{
        char *s, *e;

        e = trim(str);
        if (e != str && *--e == '>') {
                s = strrchr(str, '<');
                if (s) {
                        *e = '\0';
                        str = s + 1;
                }
        }
        return sane_address(str);
}

static void rcptto(const char *s)
{
        if (!*s)
                return;
        // N.B. we don't die if recipient is rejected, for the other recipients may be accepted
        if (250 != smtp_checkp("RCPT TO:<%s>", s, -1))
                bb_error_msg("Bad recipient: <%s>", s);
}

// send to a list of comma separated addresses
static void rcptto_list(const char *list)
{
        char *free_me = xstrdup(list);
        char *str = free_me;
        char *s = free_me;
        char prev = 0;
        int in_quote = 0;

        while (*s) {
                char ch = *s++;

                if (ch == '"' && prev != '\\') {
                        in_quote = !in_quote;
                } else if (!in_quote && ch == ',') {
                        s[-1] = '\0';
                        rcptto(angle_address(str));
                        str = s;
                }
                prev = ch;
        }
        if (prev != ',')
                rcptto(angle_address(str));
        free(free_me);
}

int sendmail_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int sendmail_main(int argc UNUSED_PARAM, char **argv)
{
        char *opt_connect;
        char *opt_from = NULL;
        char *s;
        llist_t *list = NULL;
        char *host = sane_address(safe_gethostname());
        unsigned nheaders = 0;
        int code;
        enum {
                HDR_OTHER = 0,
                HDR_TOCC,
                HDR_BCC,
        } last_hdr = 0;
        int check_hdr;
        int has_to = 0;

        enum {
        //--- standard options
                OPT_t = 1 << 0,         // read message for recipients, append them to those on cmdline
                OPT_f = 1 << 1,         // sender address
                OPT_o = 1 << 2,         // various options. -oi IMPLIED! others are IGNORED!
                OPT_i = 1 << 3,         // IMPLIED!
        //--- BB specific options
                OPT_w = 1 << 4,         // network timeout
                OPT_H = 1 << 5,         // use external connection helper
                OPT_S = 1 << 6,         // specify connection string
                OPT_a = 1 << 7,         // authentication tokens
                OPT_v = 1 << 8,         // verbosity
        //--- for -amMETHOD
                OPT_am_plain = 1 << 9,  // AUTH PLAIN
        };

        // init global variables
        INIT_G();

        // default HOST[:PORT] is $SMTPHOST, or localhost
        opt_connect = getenv("SMTPHOST");
        if (!opt_connect)
                opt_connect = (char *)"127.0.0.1";

        // save initial stdin since body is piped!
        xdup2(STDIN_FILENO, 3);
        G.fp0 = xfdopen_for_read(3);

        // parse options
        // N.B. since -H and -S are mutually exclusive they do not interfere in opt_connect
        // -a is for ssmtp (http://downloads.openwrt.org/people/nico/man/man8/ssmtp.8.html) compatibility,
        // it is still under development.
        opts = getopt32(argv, "^"
                        "tf:o:iw:+H:S:a:*:v"
                        "\0"
                        // -v is a counter, -H and -S are mutually exclusive, -a is a list
                        "vv:H--S:S--H",
                        &opt_from, NULL,
                        &timeout, &opt_connect, &opt_connect, &list, &verbose
        );
        //argc -= optind;
        argv += optind;

        // process -a[upm]<token> options
        if ((opts & OPT_a) && !list)
                bb_show_usage();
        while (list) {
                char *a = (char *) llist_pop(&list);
                if ('u' == a[0])
                        G.user = xstrdup(a+1);
                if ('p' == a[0])
                        G.pass = xstrdup(a+1);
                if ('m' == a[0]) {
                        if ((a[1] | 0x20) == 'p') // PLAIN
                                opts |= OPT_am_plain;
                        else if ((a[1] | 0x20) == 'l') // LOGIN
                                ; /* do nothing (this is the default) */
                        else
                                bb_error_msg_and_die("unsupported AUTH method %s", a+1);
                }
        }
        // N.B. list == NULL here
        //bb_error_msg("OPT[%x] AU[%s], AP[%s], AM[%s], ARGV[%s]", opts, au, ap, am, *argv);

        // connect to server

        // connection helper ordered? ->
        if (opts & OPT_H) {
                const char *delay;
                const char *args[] = { "sh", "-c", opt_connect, NULL };
                // plug it in
                launch_helper(args);
                // Now:
                // our stdout will go to helper's stdin,
                // helper's stdout will be available on our stdin.

                // Wait for initial server message.
                // If helper (such as openssl) invokes STARTTLS, the initial 220
                // is swallowed by helper (and not repeated after TLS is initiated).
                // We will send NOOP cmd to server and check the response.
                // We should get 220+250 on plain connection, 250 on STARTTLSed session.
                //
                // The problem here is some servers delay initial 220 message,
                // and consider client to be a spammer if it starts sending cmds
                // before 220 reached it. The code below is unsafe in this regard:
                // in non-STARTTLSed case, we potentially send NOOP before 220
                // is sent by server.
                //
                // If $SMTP_ANTISPAM_DELAY is set, we pause before sending NOOP.
                //
                delay = getenv("SMTP_ANTISPAM_DELAY");
                if (delay)
                        sleep(atoi(delay));
                code = smtp_check("NOOP", -1);
                if (code == 220)
                        // we got 220 - this is not STARTTLSed connection,
                        // eat 250 response to our NOOP
                        smtp_check(NULL, 250);
                else
                if (code != 250)
                        bb_error_msg_and_die("SMTP init failed");
        } else {
                // vanilla connection
                int fd;
                fd = create_and_connect_stream_or_die(opt_connect, 25);
                // and make ourselves a simple IO filter
                xmove_fd(fd, STDIN_FILENO);
                xdup2(STDIN_FILENO, STDOUT_FILENO);

                // Wait for initial server 220 message
                smtp_check(NULL, 220);
        }

        // we should start with modern EHLO
        if (250 != smtp_checkp("EHLO %s", host, -1))
                smtp_checkp("HELO %s", host, 250);

        // perform authentication
        if (opts & OPT_a) {
                // read credentials unless they are given via -a[up] options
                if (!G.user || !G.pass)
                        get_cred_or_die(4);
                if (opts & OPT_am_plain) {
                        // C: AUTH PLAIN
                        // S: 334
                        // C: base64encoded(auth<NUL>user<NUL>pass)
                        // S: 235 2.7.0 Authentication successful
//Note: a shorter format is allowed:
// C: AUTH PLAIN base64encoded(auth<NUL>user<NUL>pass)
// S: 235 2.7.0 Authentication successful
                        smtp_check("AUTH PLAIN", 334);
                        {
                                unsigned user_len = strlen(G.user);
                                unsigned pass_len = strlen(G.pass);
                                unsigned sz = 1 + user_len + 1 + pass_len;
                                char plain_auth[sz + 1];
                                // the format is:
                                // "authorization identity<NUL>username<NUL>password"
                                // authorization identity is empty.
                                plain_auth[0] = '\0';
                                strcpy(stpcpy(plain_auth + 1, G.user) + 1, G.pass);
                                printbuf_base64(plain_auth, sz);
                        }
                } else {
                        // C: AUTH LOGIN
                        // S: 334 VXNlcm5hbWU6
                        //        ^^^^^^^^^^^^ server says "Username:"
                        // C: base64encoded(user)
                        // S: 334 UGFzc3dvcmQ6
                        //        ^^^^^^^^^^^^ server says "Password:"
                        // C: base64encoded(pass)
                        // S: 235 2.7.0 Authentication successful
                        smtp_check("AUTH LOGIN", 334);
                        printstr_base64(G.user);
                        smtp_check("", 334);
                        printstr_base64(G.pass);
                }
                smtp_check("", 235);
        }

        // set sender
        // N.B. we have here a very loosely defined algorythm
        // since sendmail historically offers no means to specify secrets on cmdline.
        // 1) server can require no authentication ->
        //      we must just provide a (possibly fake) reply address.
        // 2) server can require AUTH ->
        //      we must provide valid username and password along with a (possibly fake) reply address.
        //      For the sake of security username and password are to be read either from console or from a secured file.
        //      Since reading from console may defeat usability, the solution is either to read from a predefined
        //      file descriptor (e.g. 4), or again from a secured file.

        // got no sender address? use auth name, then UID username as a last resort
        if (!opt_from) {
                opt_from = xasprintf("%s@%s",
                                     G.user ? G.user : xuid2uname(getuid()),
                                     xgethostbyname(host)->h_name);
        }
        free(host);

        smtp_checkp("MAIL FROM:<%s>", opt_from, 250);

        // process message

        // read recipients from message and add them to those given on cmdline.
        // this means we scan stdin for To:, Cc:, Bcc: lines until an empty line
        // and then use the rest of stdin as message body
        code = 0; // set "analyze headers" mode
        while ((s = xmalloc_fgetline(G.fp0)) != NULL) {
 dump:
                // put message lines doubling leading dots
                if (code) {
                        // escape leading dots
                        // N.B. this feature is implied even if no -i (-oi) switch given
                        // N.B. we need to escape the leading dot regardless of
                        // whether it is single or not character on the line
                        if ('.' == s[0] /*&& '\0' == s[1] */)
                                bb_putchar('.');
                        // dump read line
                        send_r_n(s);
                        free(s);
                        continue;
                }

                // analyze headers
                // To: or Cc: headers add recipients
                check_hdr = (0 == strncasecmp("To:", s, 3));
                has_to |= check_hdr;
                if (opts & OPT_t) {
                        if (check_hdr || 0 == strncasecmp("Bcc:" + 1, s, 3)) {
                                rcptto_list(s+3);
                                last_hdr = HDR_TOCC;
                                goto addheader;
                        }
                        // Bcc: header adds blind copy (hidden) recipient
                        if (0 == strncasecmp("Bcc:", s, 4)) {
                                rcptto_list(s+4);
                                free(s);
                                last_hdr = HDR_BCC;
                                continue; // N.B. Bcc: vanishes from headers!
                        }
                }
                check_hdr = (list && isspace(s[0]));
                if (strchr(s, ':') || check_hdr) {
                        // other headers go verbatim
                        // N.B. RFC2822 2.2.3 "Long Header Fields" allows for headers to occupy several lines.
                        // Continuation is denoted by prefixing additional lines with whitespace(s).
                        // Thanks (stefan.seyfried at googlemail.com) for pointing this out.
                        if (check_hdr && last_hdr != HDR_OTHER) {
                                rcptto_list(s+1);
                                if (last_hdr == HDR_BCC)
                                        continue;
                                        // N.B. Bcc: vanishes from headers!
                        } else {
                                last_hdr = HDR_OTHER;
                        }
 addheader:
                        // N.B. we allow MAX_HEADERS generic headers at most to prevent attacks
                        if (MAX_HEADERS && ++nheaders >= MAX_HEADERS)
                                goto bail;
                        llist_add_to_end(&list, s);
                } else {
                        // a line without ":" (an empty line too, by definition) doesn't look like a valid header
                        // so stop "analyze headers" mode
 reenter:
                        // put recipients specified on cmdline
                        check_hdr = 1;
                        while (*argv) {
                                char *t = sane_address(*argv);
                                rcptto(t);
                                //if (MAX_HEADERS && ++nheaders >= MAX_HEADERS)
                                //      goto bail;
                                if (!has_to) {
                                        const char *hdr;

                                        if (check_hdr && argv[1])
                                                hdr = "To: %s,";
                                        else if (check_hdr)
                                                hdr = "To: %s";
                                        else if (argv[1])
                                                hdr = "To: %s," + 3;
                                        else
                                                hdr = "To: %s" + 3;
                                        llist_add_to_end(&list,
                                                        xasprintf(hdr, t));
                                        check_hdr = 0;
                                }
                                argv++;
                        }
                        // enter "put message" mode
                        // N.B. DATA fails iff no recipients were accepted (or even provided)
                        // in this case just bail out gracefully
                        if (354 != smtp_check("DATA", -1))
                                goto bail;
                        // dump the headers
                        while (list) {
                                send_r_n((char *) llist_pop(&list));
                        }
                        // stop analyzing headers
                        code++;
                        // N.B. !s means: we read nothing, and nothing to be read in the future.
                        // just dump empty line and break the loop
                        if (!s) {
                                send_r_n("");
                                break;
                        }
                        // go dump message body
                        // N.B. "s" already contains the first non-header line, so pretend we read it from input
                        goto dump;
                }
        }
        // odd case: we didn't stop "analyze headers" mode -> message body is empty. Reenter the loop
        // N.B. after reenter code will be > 0
        if (!code)
                goto reenter;

        // finalize the message
        smtp_check(".", 250);
 bail:
        // ... and say goodbye
        smtp_check("QUIT", 221);
        // cleanup
        if (ENABLE_FEATURE_CLEAN_UP)
                fclose(G.fp0);

        return EXIT_SUCCESS;
}