/*

Copyright (c) 2004 Carl Byington - 510 Software Group, released under
the GPL version 2 or any later version at your choice available at
http://www.fsf.org/licenses/gpl.txt

Based on a sample milter Copyright (c) 2000-2003 Sendmail, Inc. and its
suppliers.  Inspired by the DCC by Rhyolite Software

-p port  The port through which the MTA will connect to this milter.
-t sec   The timeout value.
-c       Check the config, and print a copy to stderr. Don't start the
         milter or do anything with the socket.

*/


// from sendmail sample
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sysexits.h>
#include <unistd.h>

// needed for socket io
#include <sys/ioctl.h>
#include <net/if.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <netdb.h>
#include <sys/socket.h>

// needed for thread
#include <pthread.h>

// needed for std c++ collections
#include <set>
#include <map>
#include <list>

// for the dns resolver
#include <netinet/in.h>
#include <arpa/nameser.h>
#include <resolv.h>

// misc stuff needed here
#include <ctype.h>
#include <fstream>
#include <syslog.h>


static char* version="$Id$";

using namespace std;


extern "C" {
    #include "libmilter/mfapi.h"
    sfsistat mlfi_connect(SMFICTX *ctx, char *hostname, _SOCK_ADDR *hostaddr);
    sfsistat mlfi_envfrom(SMFICTX *ctx, char **argv);
    sfsistat mlfi_envrcpt(SMFICTX *ctx, char **argv);
    sfsistat mlfi_eom_or_abort(SMFICTX *ctx);
    sfsistat mlfi_close(SMFICTX *ctx);
}

struct ltstr {
    bool operator()(char* s1, char* s2) const {
        return strcmp(s1, s2) < 0;
    }
};

struct DNSBL {
    char    *suffix;    // blacklist suffix like blackholes.five-ten-sg.com
    char    *message;   // error message with one or two %s operators for the ip address replacement
    DNSBL(char *s, char *m);
};
DNSBL::DNSBL(char *s, char *m) {
    suffix  = s;
    message = m;
}

typedef DNSBL *                           DNSBLP;
typedef list<DNSBLP>                      DNSBLL;
typedef DNSBLL *                          DNSBLLP;
typedef map<char *, char *, ltstr>        string_map;
typedef map<char *, string_map *, ltstr>  from_map;
typedef map<char *, DNSBLP, ltstr>        dnsblp_map;
typedef map<char *, DNSBLLP, ltstr>       dnsbllp_map;
typedef set<char *, ltstr>                string_set;
typedef list<char *>                      string_list;

struct CONFIG {
    // the only mutable stuff once it has been loaded from the config file
    int         reference_count;    // protected by the global config_mutex
    // all the rest is constant after loading from the config file
    time_t      load_time;
    string_list config_files;
    dnsblp_map  dnsbls;
    dnsbllp_map dnsblls;
    from_map    env_from;
    string_map  env_to_dnsbll;      // map recipient to a named dnsbll
    string_map  env_to_chkfrom;     // map recipient to a named from map
    CONFIG();
    ~CONFIG();
};
CONFIG::CONFIG() {
    reference_count = 0;
    load_time       = 0;
}
CONFIG::~CONFIG() {
    for (dnsblp_map::iterator i=dnsbls.begin(); i!=dnsbls.end(); i++) {
        DNSBLP d = (*i).second;
        delete d;
    }
    for (dnsbllp_map::iterator i=dnsblls.begin(); i!=dnsblls.end(); i++) {
        DNSBLLP d = (*i).second;
        delete d;
    }
    for (from_map::iterator i=env_from.begin(); i!=env_from.end(); i++) {
        string_map *d = (*i).second;
        delete d;
    }
}

static string_set all_strings;      // owns all the strings, only modified by the config loader thread
static CONFIG * config = NULL;      // protected by the config_mutex

static pthread_mutex_t  config_mutex;
static pthread_mutex_t  syslog_mutex;
static pthread_mutex_t  resolve_mutex;



////////////////////////////////////////////////
// predefined names
//
#define DEFAULT "default"
#define WHITE   "white"
#define BLACK   "black"
#define OK      "ok"
#define MANY    "many"


////////////////////////////////////////////////
// mail filter private data, held for us by sendmail
//
enum status {oksofar,   // not rejected yet
             white,     // whitelisted by envelope from
             black,     // blacklisted by envelope from or to
             reject};   // rejected by a dns list
struct mlfiPriv
{
    CONFIG  *pc;            // global context with our maps
    int     ip;             // ip4 address of the smtp client
    char    *mailaddr;      // envelope from value
    bool    authenticated;  // client authenticated? if so, suppress all dnsbl checks
    map<DNSBLP, status> checked;    // status from those lists
    mlfiPriv();
    ~mlfiPriv();
};
mlfiPriv::mlfiPriv() {
    pthread_mutex_lock(&config_mutex);
        pc = config;
        pc->reference_count++;
    pthread_mutex_unlock(&config_mutex);
    ip       = 0;
    mailaddr = NULL;
}
mlfiPriv::~mlfiPriv() {
    pthread_mutex_lock(&config_mutex);
        pc->reference_count--;
    pthread_mutex_unlock(&config_mutex);
    if (mailaddr) free(mailaddr);
}

#define MLFIPRIV    ((struct mlfiPriv *) smfi_getpriv(ctx))


////////////////////////////////////////////////
// syslog a message
//
static void my_syslog(char *text);
static void my_syslog(char *text) {
    pthread_mutex_lock(&syslog_mutex);
        openlog("dnsbl", LOG_PID, LOG_MAIL);
        syslog(LOG_NOTICE, "%s", text);
        closelog();
    pthread_mutex_unlock(&syslog_mutex);
}


////////////////////////////////////////////////
// register a global string
//
static char* register_string(char *name);
static char* register_string(char *name) {
    string_set::iterator i = all_strings.find(name);
    if (i != all_strings.end()) return *i;
    char *x = strdup(name);
    all_strings.insert(x);
    return x;
}


static char* next_token(char *delim);
static char* next_token(char *delim) {
    char *name = strtok(NULL, delim);
    if (!name) return name;
    return register_string(name);
}


////////////////////////////////////////////////
// lookup an email address in the env_from or env_to maps
//
static char* lookup1(char *email, string_map map);
static char* lookup1(char *email, string_map map) {
    string_map::iterator i = map.find(email);
    if (i != map.end()) return (*i).second;
    char *x = strchr(email, '@');
    if (!x) return DEFAULT;
    x++;
    i = map.find(x);
    if (i != map.end()) return (*i).second;
    return DEFAULT;
}


////////////////////////////////////////////////
// lookup an email address in the env_from or env_to maps
// this email address is passed in from sendmail, and will
// always be enclosed in <>. It may have mixed case, just
// as the mail client sent it.
//
static char* lookup(char* email, string_map map);
static char* lookup(char* email, string_map map) {
    int n = strlen(email)-2;
    if (n < 1) return DEFAULT;  // malformed
    char *key = strdup(email+1);
    key[n] = '\0';
    for (int i=0; i<n; i++) key[i] = tolower(key[i]);
    char *rc = lookup1(key, map);
    free(key);
    return rc;
}


////////////////////////////////////////////////
//  find the dnsbl with a specific name
//
static DNSBLP find_dnsbl(CONFIG &dc, char *name);
static DNSBLP find_dnsbl(CONFIG &dc, char *name) {
    dnsblp_map::iterator i = dc.dnsbls.find(name);
    if (i == dc.dnsbls.end()) return NULL;
    return (*i).second;
}


////////////////////////////////////////////////
//  find the dnsbll with a specific name
//
static DNSBLLP find_dnsbll(CONFIG &dc, char *name);
static DNSBLLP find_dnsbll(CONFIG &dc, char *name) {
    dnsbllp_map::iterator i = dc.dnsblls.find(name);
    if (i == dc.dnsblls.end()) return NULL;
    return (*i).second;
}


////////////////////////////////////////////////
//  find the envfrom map with a specific name
//
static string_map* find_from_map(CONFIG &dc, char *name);
static string_map* find_from_map(CONFIG &dc, char *name) {
    from_map::iterator i = dc.env_from.find(name);
    if (i == dc.env_from.end()) return NULL;
    return (*i).second;
}


static string_map& really_find_from_map(CONFIG &dc, char *name);
static string_map& really_find_from_map(CONFIG &dc, char *name) {
    string_map *sm = find_from_map(dc, name);
    if (!sm) {
        sm = new string_map;
        dc.env_from[name] = sm;
    }
    return *sm;
}


////////////////////////////////////////////////
//  check a single dnsbl - we don't try very hard, just
//  using the default resolver retry settings. If we cannot
//  get an answer, we just accept the mail. The caller
//  must ensure thread safety.
//
static status check_single(int ip, DNSBL &bl);
static status check_single(int ip, DNSBL &bl) {
    // make a dns question
    const u_char *src = (const u_char *)&ip;
    if (src[0] == 127) return oksofar;  // don't do dns lookups on localhost
    char question[NS_MAXDNAME];
    snprintf(question, sizeof(question), "%u.%u.%u.%u.%s.", src[3], src[2], src[1], src[0], bl.suffix);
    // ask the question
    u_char answer[NS_PACKETSZ];
    int length = res_search(question, ns_c_in, ns_t_a, answer, sizeof(answer));
    if (length < 0) return oksofar;     // error in getting answer
    // parse the answer
    ns_msg handle;
    ns_rr  rr;
    if (ns_initparse(answer, length, &handle) != 0) return oksofar;
    int rrnum = 0;
    while (ns_parserr(&handle, ns_s_an, rrnum++, &rr) == 0) {
        if (ns_rr_type(rr) == ns_t_a) {
            // we see an A record, implies blacklisted ip address
            return reject;
        }
    }
    return oksofar;
}


////////////////////////////////////////////////
//  check the dnsbls specified for this recipient
//
static status check_dnsbl(mlfiPriv &priv, DNSBLLP dnsbllp, DNSBLP &rejectlist);
static status check_dnsbl(mlfiPriv &priv, DNSBLLP dnsbllp, DNSBLP &rejectlist) {
    if (priv.authenticated) return oksofar;
    if (!dnsbllp)           return oksofar;
    DNSBLL &dnsbll = *dnsbllp;
    for (DNSBLL::iterator i=dnsbll.begin(); i!=dnsbll.end(); i++) {
        DNSBLP dp = *i;     // non null by construction
        status st;
        map<DNSBLP, status>::iterator f = priv.checked.find(dp);
        if (f == priv.checked.end()) {
            // have not checked this list yet
            pthread_mutex_lock(&resolve_mutex);
                st = check_single(priv.ip, *dp);
            pthread_mutex_unlock(&resolve_mutex);
            rejectlist = dp;
            priv.checked[dp] = st;
        }
        else {
            st = (*f).second;
            rejectlist = (*f).first;
        }
        if (st == reject) return st;
    }
    return oksofar;
}


////////////////////////////////////////////////
// start of sendmail milter interfaces
//
sfsistat mlfi_connect(SMFICTX *ctx, char *hostname, _SOCK_ADDR *hostaddr)
{
    // allocate some private memory
    mlfiPriv *priv = new mlfiPriv;
    if (hostaddr->sa_family == AF_INET) {
        priv->ip = ((struct sockaddr_in *)hostaddr)->sin_addr.s_addr;
    }

    // save the private data
    smfi_setpriv(ctx, (void*)priv);

    // continue processing
    return SMFIS_CONTINUE;
}

sfsistat mlfi_envfrom(SMFICTX *ctx, char **from)
{
    mlfiPriv &priv = *MLFIPRIV;
    priv.mailaddr      = strdup(from[0]);
    priv.authenticated = (smfi_getsymval(ctx, "{auth_authen}") != NULL);
    return SMFIS_CONTINUE;
}

sfsistat mlfi_envrcpt(SMFICTX *ctx, char **rcpt)
{
    DNSBLP rejectlist = NULL;   // list that caused the reject
    status st = oksofar;
    mlfiPriv &priv = *MLFIPRIV;
    CONFIG &dc = *priv.pc;
    char *rcptaddr = rcpt[0];
    char *dnsname  = lookup(rcptaddr, dc.env_to_dnsbll);
    char *fromname = lookup(rcptaddr, dc.env_to_chkfrom);
    if ((strcmp(dnsname,  BLACK) == 0) ||
        (strcmp(fromname, BLACK) == 0)) {
        st = black; // two options to blacklist this recipient
    }
    else if (strcmp(fromname, WHITE) == 0) {
        st = white;
    }
    else {
        // check an env_from map
        string_map *sm = find_from_map(dc, fromname);
        if (sm != NULL) {
            fromname = lookup(priv.mailaddr, *sm);  // returns default if name not in map
            if (strcmp(fromname, BLACK) == 0) {
                st = black; // blacklist this envelope from value
            }
            if (strcmp(fromname, WHITE) == 0) {
                st = white; // blacklist this envelope from value
            }
        }
    }
    if ((st == oksofar) && (strcmp(dnsname, WHITE) != 0)) {
        // check dns lists
        st = check_dnsbl(priv, find_dnsbll(dc, dnsname), rejectlist);
    }

    if (st == reject) {
        // reject the recipient based on some dnsbl
        char adr[sizeof "255.255.255.255"];
        adr[0] = '\0';
        const char *rc = inet_ntop(AF_INET, (const u_char *)&priv.ip, adr, sizeof(adr));
        char buf[2000];
        snprintf(buf, sizeof(buf), rejectlist->message, adr, adr);
        smfi_setreply(ctx, "550", "5.7.1", buf);
        return SMFIS_REJECT;
    }
    else if (st == black) {
        // reject the recipient based on blacklisting either from or to
        smfi_setreply(ctx, "550", "5.7.1", "no such user");
        return SMFIS_REJECT;
    }
    else {
        // accept the recipient
        return SMFIS_CONTINUE;
    }
}

sfsistat mlfi_eom_or_abort(SMFICTX *ctx)
{
    mlfiPriv &priv = *MLFIPRIV;
    if (priv.mailaddr) {
        free(priv.mailaddr);
        priv.mailaddr = NULL;
    }
    return SMFIS_CONTINUE;
}

sfsistat mlfi_close(SMFICTX *ctx)
{
    mlfiPriv *priv = MLFIPRIV;
    if (!priv) return SMFIS_CONTINUE;
    delete priv;
    smfi_setpriv(ctx, NULL);
    return SMFIS_CONTINUE;
}

struct smfiDesc smfilter =
{
    "DNSBL",            // filter name
    SMFI_VERSION,       // version code -- do not change
    SMFIF_DELRCPT,      // flags
    mlfi_connect,       // connection info filter
    NULL,               // SMTP HELO command filter
    mlfi_envfrom,       // envelope sender filter
    mlfi_envrcpt,       // envelope recipient filter
    NULL,               // header filter
    NULL,               // end of header
    NULL,               // body block filter
    mlfi_eom_or_abort,  // end of message
    mlfi_eom_or_abort,  // message aborted
    mlfi_close,         // connection cleanup
};


static void dumpit(char *name, string_map map);
static void dumpit(char *name, string_map map) {
    fprintf(stderr, "\n");
    for (string_map::iterator i=map.begin(); i!=map.end(); i++) {
        fprintf(stderr, "%s %s->%s\n", name, (*i).first, (*i).second);
    }
}


static void dumpit(from_map map);
static void dumpit(from_map map) {
    for (from_map::iterator i=map.begin(); i!=map.end(); i++) {
        char buf[2000];
        snprintf(buf, sizeof(buf), "envelope from map for %s", (*i).first);
        string_map *sm = (*i).second;
        dumpit(buf, *sm);
    }
}


static void dumpit(CONFIG &dc);
static void dumpit(CONFIG &dc) {
    dumpit(dc.env_from);
    dumpit("envelope to (dnsbl list)", dc.env_to_dnsbll);
    dumpit("envelope to (from map)", dc.env_to_chkfrom);
    fprintf(stderr, "\ndnsbls\n");
    for (dnsblp_map::iterator i=dc.dnsbls.begin(); i!=dc.dnsbls.end(); i++) {
        fprintf(stderr, "%s %s %s\n", (*i).first, (*i).second->suffix, (*i).second->message);
    }
    fprintf(stderr, "\ndnsbl_lists\n");
    for (dnsbllp_map::iterator i=dc.dnsblls.begin(); i!=dc.dnsblls.end(); i++) {
        char *name = (*i).first;
        DNSBLL &dl = *((*i).second);
        fprintf(stderr, "%s", name);
        for (DNSBLL::iterator j=dl.begin(); j!=dl.end(); j++) {
            DNSBL &d = **j;
            fprintf(stderr, " %s", d.suffix);
        }
        fprintf(stderr, "\n");
    }
    fprintf(stderr, "\nfiles\n");
    for (string_list::iterator i=dc.config_files.begin(); i!=dc.config_files.end(); i++) {
        char *f = *i;
        fprintf(stderr, "config includes %s\n", f);
    }
}


////////////////////////////////////////////////
//  check for redundant or recursive include files
//
static bool ok_to_include(CONFIG &dc, char *fn);
static bool ok_to_include(CONFIG &dc, char *fn) {
    if (!fn) return false;
    bool ok = true;
    for (string_list::iterator i=dc.config_files.begin(); i!=dc.config_files.end(); i++) {
        char *f = *i;
        if (strcmp(f, fn) == 0) {
            my_syslog("redundant or recursive include file detected");
            ok = false;
            break;
        }
    }
    return ok;
}


////////////////////////////////////////////////
//  load a single config file
//
static void load_conf_dcc(CONFIG &dc, char *name, char *fn);
static void load_conf_dcc(CONFIG &dc, char *name, char *fn) {
    dc.config_files.push_back(fn);
    char *list = BLACK;
    const int LINE_SIZE = 2000;
    ifstream is(fn);
    if (is.fail()) return;
    char line[LINE_SIZE];
    char *delim = " \t";
    int curline = 0;
    while (!is.eof()) {
        is.getline(line, LINE_SIZE);
        curline++;
        int n = strlen(line);
        if (!n) continue;
        for (int i=0; i<n; i++) line[i] = tolower(line[i]);
        if (line[0] == '#') continue;
        char *head = line;
        if (strspn(line, delim) == 0) {
            // have a leading ok/many tag to fetch
            char *cmd = strtok(line, delim);
                 if (strcmp(cmd, MANY) == 0) list = BLACK;
            else if (strcmp(cmd, OK) == 0)   list = WHITE;
            head = cmd + strlen(cmd) + 1;
        }
        char *cmd = strtok(head, delim);
        if (!cmd) continue;
        if (strcmp(cmd, "env_from") == 0) {
            char *from = next_token(delim);
            if (from) {
                string_map &fm = really_find_from_map(dc, name);
                fm[from] = list;
            }
        }
        else if (strcmp(cmd, "env_to") == 0) {
            char *to = next_token(delim);
            if (to) {
                dc.env_to_dnsbll[to]  = list;
                dc.env_to_chkfrom[to] = list;
            }
        }
        else if (strcmp(cmd, "substitute") == 0) {
            char *tag = next_token(delim);
            if (tag && (strcmp(tag, "mail_host") == 0)) {
                char *from = next_token(delim);
                if (from) {
                    string_map &fm = really_find_from_map(dc, name);
                    fm[from] = list;
                }
            }
        }
        else if (strcmp(cmd, "include") == 0) {
            char *fn = next_token(delim);
            if (ok_to_include(dc, fn)) {
                load_conf_dcc(dc, name, fn);
            }
        }

    }
    is.close();
}


static void load_conf(CONFIG &dc, char *fn);
static void load_conf(CONFIG &dc, char *fn) {
    dc.config_files.push_back(fn);
    map<char*, int, ltstr> commands;
    enum {dummy, dnsbl, dnsbll, envfrom, envto, include, includedcc};
    commands["dnsbl"      ] = dnsbl;
    commands["dnsbl_list" ] = dnsbll;
    commands["env_from"   ] = envfrom;
    commands["env_to"     ] = envto;
    commands["include"    ] = include;
    commands["include_dcc"] = includedcc;
    const int LINE_SIZE = 2000;
    ifstream is(fn);
    if (is.fail()) return;
    char line[LINE_SIZE];
    char orig[LINE_SIZE];
    char *delim = " \t";
    int curline = 0;
    while (!is.eof()) {
        is.getline(line, LINE_SIZE);
        snprintf(orig, sizeof(orig), "%s", line);
        curline++;
        int n = strlen(line);
        for (int i=0; i<n; i++) line[i] = tolower(line[i]);
        char *cmd = strtok(line, delim);
        if (cmd && (cmd[0] != '#') && (cmd[0] != '\0')) {
            // have a decent command
            bool processed = false;
            switch (commands[cmd]) {
                case dnsbl: {
                    // have a new dnsbl to use
                    char *name = next_token(delim);
                    if (!name) break;                           // no name name
                    if (find_dnsbl(dc, name)) break;            // duplicate entry
                    char *suff = strtok(NULL, delim);
                    if (!suff) break;                           // no dns suffic
                    char *msg = suff + strlen(suff);
                    if ((msg - line) >= strlen(orig)) break;    // line ended with the dns suffix
                    msg  = strchr(msg+1, '\'');
                    if (!msg) break;                            // no reply message template
                    msg++; // move over the leading '
                    if ((msg - line) >= strlen(orig)) break;    // line ended with the leading quote
                    char *last = strchr(msg, '\'');
                    if (!last) break;                           // no trailing quote
                    *last = '\0';                               // make it a null terminator
                    dc.dnsbls[name] = new DNSBL(register_string(suff), register_string(msg));
                    processed = true;
                    } break;

                case dnsbll: {
                    // define a new combination of dnsbls
                    char *name = next_token(delim);
                    if (!name) break;
                    if (find_dnsbll(dc, name)) break;               // duplicate entry
                    char *list = next_token(delim);
                    if (!list || (*list == '\0') || (*list == '#')) break;
                    DNSBLLP d = new DNSBLL;
                    DNSBLP p = find_dnsbl(dc, list);
                    if (p) d->push_back(p);
                    while (true) {
                        list = next_token(delim);
                        if (!list || (*list == '\0') || (*list == '#')) break;
                        DNSBLP p = find_dnsbl(dc, list);
                        if (p) d->push_back(p);
                    }
                    dc.dnsblls[name] = d;
                    processed = true;
                    } break;

                case envfrom: {
                    // add an entry into the named string_map
                    char *name = next_token(delim);
                    if (!name) break;
                    char *from = next_token(delim);
                    if (!from) break;
                    char *list = next_token(delim);
                    if (!list) break;
                    if ((strcmp(list, WHITE) == 0) ||
                        (strcmp(list, BLACK) == 0)) {
                        string_map &fm = really_find_from_map(dc, name);
                        fm[from] = list;
                        processed = true;
                    }
                    else {
                        // list may be the name of a previously defined from_map
                        string_map *m = find_from_map(dc, list);
                        if (m && (strcmp(list,name) != 0)) {
                            string_map &pm = *m;
                            string_map &fm = really_find_from_map(dc, name);
                            fm.insert(pm.begin(), pm.end());
                            processed = true;
                        }
                    }
                    } break;

                case envto: {
                    // define the dnsbl_list and env_from maps to use for this recipient
                    char *to   = next_token(delim);
                    if (!to) break;
                    char *list = next_token(delim);
                    if (!list) break;
                    char *from = next_token(delim);
                    if (!from) break;
                    dc.env_to_dnsbll[to]  = list;
                    dc.env_to_chkfrom[to] = from;
                    processed = true;
                    } break;

                case include: {
                    char *fn = next_token(delim);
                    if (ok_to_include(dc, fn)) {
                        load_conf(dc, fn);
                        processed = true;
                    }
                    } break;

                case includedcc: {
                    char *name = next_token(delim);
                    if (!name) break;
                    char *fn = next_token(delim);
                    if (ok_to_include(dc, fn)) {
                        load_conf_dcc(dc, name, fn);
                        processed = true;
                    }
                    } break;

                default: {
                    } break;
            }
            if (!processed) {
                pthread_mutex_lock(&syslog_mutex);
                    openlog("dnsbl", LOG_PID, LOG_MAIL);
                    syslog(LOG_ERR, "ignoring file %s line %d : %s\n", fn, curline, orig);
                    closelog();
                pthread_mutex_unlock(&syslog_mutex);
            }
        }
    }
    is.close();
}


////////////////////////////////////////////////
//  reload the config
//
static CONFIG* new_conf();
static CONFIG* new_conf() {
    my_syslog("loading new configuration");
    CONFIG *newc = new CONFIG;
    load_conf(*newc, "dnsbl.conf");
    newc->load_time = time(NULL);
    return newc;
}


////////////////////////////////////////////////
//  thread to watch the old config files for changes
//  and reload when needed. we also cleanup old
//  configs whose reference count has gone to zero.
//
static void* config_loader(void *arg);
static void* config_loader(void *arg) {
    typedef set<CONFIG *> configp_set;
    configp_set old_configs;
    while (true) {
        sleep(180);  // look for modifications every 3 minutes
        CONFIG &dc = *config;
        time_t then = dc.load_time;
        struct stat st;
        bool reload = false;
        for (string_list::iterator i=dc.config_files.begin(); i!=dc.config_files.end(); i++) {
            char *fn = *i;
            if (stat(fn, &st))           reload = true; // file disappeared
            else if (st.st_mtime > then) reload = true; // file modified
            if (reload) break;
        }
        if (reload) {
            CONFIG *newc = new_conf();
            // replace the global config pointer
            pthread_mutex_lock(&config_mutex);
                CONFIG *old = config;
                config = newc;
            pthread_mutex_unlock(&config_mutex);
            if (old) old_configs.insert(old);
        }
        // now look for old configs with zero ref counts
        for (configp_set::iterator i=old_configs.begin(); i!=old_configs.end(); ) {
            CONFIG *old = *i;
            if (!old->reference_count) {
                delete old; // destructor does all the work
                old_configs.erase(i++);
            }
            else i++;
        }
    }
}


static void usage(char *prog);
static void usage(char *prog)
{
    fprintf(stderr, "Usage: %s  -p socket-addr [-t timeout]\n", prog);
    fprintf(stderr, "where socket-addr is for the connection to sendmail and should be one of\n");
    fprintf(stderr, "   inet:port@local-ip-address\n");
    fprintf(stderr, "   local:local-domain-socket-file-name\n");
}


int main(int argc, char**argv)
{
    bool check   = false;
    bool setconn = false;
    int c;
    const char *args = "p:t:hc";
    extern char *optarg;

    // Process command line options
    while ((c = getopt(argc, argv, args)) != -1) {
        switch (c) {
            case 'p':
                if (optarg == NULL || *optarg == '\0') {
                    fprintf(stderr, "Illegal conn: %s\n", optarg);
                    exit(EX_USAGE);
                }
                if (smfi_setconn(optarg) == MI_FAILURE) {
                    fprintf(stderr, "smfi_setconn failed\n");
                    exit(EX_SOFTWARE);
                }

                     if (strncasecmp(optarg, "unix:", 5) == 0)  unlink(optarg + 5);
                else if (strncasecmp(optarg, "local:", 6) == 0) unlink(optarg + 6);
                setconn = true;
                break;

            case 't':
                if (optarg == NULL || *optarg == '\0') {
                    fprintf(stderr, "Illegal timeout: %s\n", optarg);
                    exit(EX_USAGE);
                }
                if (smfi_settimeout(atoi(optarg)) == MI_FAILURE) {
                    fprintf(stderr, "smfi_settimeout failed\n");
                    exit(EX_SOFTWARE);
                }
                break;

            case 'c':
                check = true;
                break;

            case 'h':
            default:
                usage(argv[0]);
                exit(EX_USAGE);
        }
    }

    if (check) {
        CONFIG &dc = *new_conf();
        dumpit(dc);
        return 0;
    }

    if (!setconn) {
        fprintf(stderr, "%s: Missing required -p argument\n", argv[0]);
        usage(argv[0]);
        exit(EX_USAGE);
    }

    if (smfi_register(smfilter) == MI_FAILURE) {
        fprintf(stderr, "smfi_register failed\n");
        exit(EX_UNAVAILABLE);
    }

    // switch to background mode
    if (daemon(1,0) < 0) {
        fprintf(stderr, "daemon() call failed\n");
        exit(EX_UNAVAILABLE);
    }

    // initialize the thread sync objects
    pthread_mutex_init(&config_mutex, 0);
    pthread_mutex_init(&syslog_mutex, 0);
    pthread_mutex_init(&resolve_mutex, 0);

    // load the initial config
    config = new_conf();

    // only create threads after the fork() in daemon
    pthread_t tid;
    if (pthread_create(&tid, 0, config_loader, 0))
        my_syslog("failed to create config loader thread");
    if (pthread_detach(tid))
        my_syslog("failed to detach config loader thread");

    // write the pid
    const char *pidpath = "/var/run/dnsbl.pid";
    unlink(pidpath);
    FILE *f = fopen(pidpath, "w");
    if (f) {
        fprintf(f, "-%d\n", (u_int)getpgrp());
        fclose(f);
    }

    return smfi_main();
}