yaid.c: Time out idle connections after 30s.

[yaid] / yaid.c

/* -*-c-*-
 *
 * Main daemon
 *
 * (c) 2012 Straylight/Edgeware
 */

/*----- Licensing notice --------------------------------------------------*
 *
 * This file is part of Yet Another Ident Daemon (YAID).
 *
 * YAID is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * YAID is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with YAID; if not, write to the Free Software Foundation,
 * Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 */

/*----- Header files ------------------------------------------------------*/

#include "yaid.h"

/*----- Data structures ---------------------------------------------------*/

/* A write buffer is the gadget which keeps track of our output and writes
 * portions of it out as and when connections are ready for it.
 */
#define WRBUFSZ 1024
struct writebuf {
  size_t o;                             /* Offset of remaining data */
  size_t n;                             /* Length of remaining data */
  sel_file wr;                          /* Write selector */
  void (*func)(int /*err*/, void *);    /* Function to call on completion */
  void *p;                              /* Context for `func' */
  unsigned char buf[WRBUFSZ];           /* Output buffer */
};

/* Structure for a listening socket.  There's one of these for each address
 * family we're looking after.
 */
struct listen {
  const struct addrops *ao;             /* Address family operations */
  sel_file f;                           /* Watch for incoming connections */
};

/* The main structure for a client. */
struct client {
  int fd;                               /* The connection to the client */
  selbuf b;                             /* Accumulate lines of input */
  struct query q;                       /* The clients query and our reply */
  struct sel_timer t;                   /* Timeout for idle or doomed conn */
  struct listen *l;                     /* Back to the listener (and ops) */
  struct writebuf wb;                   /* Write buffer for our reply */
  struct proxy *px;                     /* Proxy if conn goes via NAT */
};

/* A proxy connection. */
struct proxy {
  int fd;                               /* Connection; -1 if in progress */
  struct client *c;                     /* Back to the client */
  conn cn;                              /* Nonblocking connection */
  selbuf b;                             /* Accumulate the response line */
  struct writebuf wb;                   /* Write buffer for query */
  char nat[ADDRLEN];                    /* Server address, as text */
};

/*----- Static variables --------------------------------------------------*/

static sel_state sel;                   /* I/O multiplexer state */

static const struct policy default_policy = POLICY_INIT(A_NAME);
static policy_v policy = DA_INIT;       /* Vector of global policy rules */
static fwatch polfw;                    /* Watch policy file for changes */

static unsigned char tokenbuf[4096];    /* Random-ish data for tokens */
static size_t tokenptr = sizeof(tokenbuf); /* Current read position */
static int randfd;                      /* File descriptor for random data */

/*----- Ident protocol parsing --------------------------------------------*/

/* Advance *PP over whitespace characters. */
static void skipws(const char **pp)
  { while (isspace((unsigned char )**pp)) (*pp)++; }

/* Copy a token of no more than N bytes starting at *PP into Q, advancing *PP
 * over it.
 */
static int idtoken(const char **pp, char *q, size_t n)
{
  const char *p = *pp;

  skipws(&p);
  n--;
  for (;;) {
    if (*p == ':' || *p <= 32 || *p >= 127) break;
    if (!n) return (-1);
    *q++ = *p++;
    n--;
  }
  *q++ = 0;
  *pp = p;
  return (0);
}

/* Read an unsigned decimal number from *PP, and store it in *II.  Check that
 * it's between MIN and MAX, and advance *PP over it.  Return zero for
 * success, or nonzero if something goes wrong.
 */
static int unum(const char **pp, unsigned *ii, unsigned min, unsigned max)
{
  char *q;
  unsigned long i;
  int e;

  skipws(pp);
  if (!isdigit((unsigned char)**pp)) return (-1);
  e = errno; errno = 0;
  i = strtoul(*pp, &q, 10);
  if (errno) return (-1);
  *pp = q;
  errno = e;
  if (i < min || i > max) return (-1);
  *ii = i;
  return (0);
}

/*----- Asynchronous writing ----------------------------------------------*/

/* Callback for actually writing stuff from a `writebuf'. */
static void write_out(int fd, unsigned mode, void *p)
{
  ssize_t n;
  struct writebuf *wb = p;

  /* Try to write something. */
  if ((n = write(fd, wb->buf + wb->o, wb->n)) < 0) {
    if (errno == EAGAIN || errno == EWOULDBLOCK) return;
    wb->n = 0;
    sel_rmfile(&wb->wr);
    wb->func(errno, wb->p);
  }
  wb->o += n;
  wb->n -= n;

  /* If there's nothing left then restore the buffer to its empty state. */
  if (!wb->n) {
    wb->o = 0;
    sel_rmfile(&wb->wr);
    wb->func(0, wb->p);
  }
}

/* Queue N bytes starting at P to be written. */
static int queue_write(struct writebuf *wb, const void *p, size_t n)
{
  /* Maybe there's nothing to actually do. */
  if (!n) return (0);

  /* Make sure it'll fit. */
  if (wb->n - wb->o + n > WRBUFSZ) return (-1);

  /* If there's anything there already, then make sure it's at the start of
   * the available space.
   */
  if (wb->o) {
    memmove(wb->buf, wb->buf + wb->o, wb->n);
    wb->o = 0;
  }

  /* If there's nothing currently there, then we're not requesting write
   * notifications, so set that up, and force an initial wake-up.
   */
  if (!wb->n) {
    sel_addfile(&wb->wr);
    sel_force(&wb->wr);
  }

  /* Copy the new material over. */
  memcpy(wb->buf + wb->n, p, n);
  wb->n += n;

  /* Done. */
  return (0);
}

/* Release resources allocated to WB. */
static void free_writebuf(struct writebuf *wb)
  { if (wb->n) sel_rmfile(&wb->wr); }

/* Initialize a writebuf in *WB, writing to file descriptor FD.  On
 * completion, call FUNC, passing it P and an error indicator: either 0 for
 * success or an `errno' value on failure.
 */
static void init_writebuf(struct writebuf *wb,
                          int fd, void (*func)(int, void *), void *p)
{
  sel_initfile(&sel, &wb->wr, fd, SEL_WRITE, write_out, wb);
  wb->func = func;
  wb->p = p;
  wb->n = wb->o = 0;
}

/*----- General utilities -------------------------------------------------*/

/* Format and log MSG somewhere sensible, at the syslog(3) priority PRIO.
 * Prefix it with a description of the query Q, if non-null.
 */
void logmsg(const struct query *q, int prio, const char *msg, ...)
{
  va_list ap;
  dstr d = DSTR_INIT;

  va_start(ap, msg);
  if (q) {
    dputsock(&d, q->ao, &q->s[L]);
    dstr_puts(&d, " <-> ");
    dputsock(&d, q->ao, &q->s[R]);
    dstr_puts(&d, ": ");
  }
  dstr_vputf(&d, msg, &ap);
  va_end(ap);
  fprintf(stderr, "yaid: %s\n", d.buf);
  dstr_destroy(&d);
}

/* Fix up a socket FD so that it won't bite us.  Returns zero on success, or
 * nonzero on error.
 */
static int fix_up_socket(int fd, const char *what)
{
  int yes = 1;

  if (fdflags(fd, O_NONBLOCK, O_NONBLOCK, 0, 0)) {
    logmsg(0, LOG_ERR, "failed to set %s connection nonblocking: %s",
           what, strerror(errno));
    return (-1);
  }

  if (setsockopt(fd, SOL_SOCKET, SO_OOBINLINE, &yes, sizeof(yes))) {
    logmsg(0, LOG_ERR,
           "failed to disable `out-of-band' data on %s connection: %s",
           what, strerror(errno));
    return (-1);
  }

  return (0);
}

/*----- Client output functions -------------------------------------------*/

static void disconnect_client(struct client *c);

/* Notification that output has been written.  If successful, re-enable the
 * input buffer and prepare for another query.
 */
static void done_client_write(int err, void *p)
{
  struct client *c = p;

  if (!err)
    selbuf_enable(&c->b);
  else {
    logmsg(&c->q, LOG_ERR, "failed to send reply: %s", strerror(err));
    disconnect_client(c);
  }
}

/* Format the message FMT and queue it to be sent to the client.  Client
 * input will be disabled until the write completes.
 */
static void write_to_client(struct client *c, const char *fmt, ...)
{
  va_list ap;
  char buf[WRBUFSZ];
  ssize_t n;

  va_start(ap, fmt);
  n = vsnprintf(buf, sizeof(buf), fmt, ap);
  if (n < 0) {
    logmsg(&c->q, LOG_ERR, "failed to format output: %s", strerror(errno));
    disconnect_client(c);
    return;
  } else if (n > sizeof(buf)) {
    logmsg(&c->q, LOG_ERR, "output too long for client send buffer");
    disconnect_client(c);
    return;
  }

  selbuf_disable(&c->b);
  if (queue_write(&c->wb, buf, n)) {
    logmsg(&c->q, LOG_ERR, "write buffer overflow");
    disconnect_client(c);
  }
}

/* Format a reply to the client, with the form LPORT:RPORT:TY:TOK0[:TOK1].
 * Typically, TY will be `ERROR' or `USERID'.  In the former case, TOK0 will
 * be the error token and TOK1 will be null; in the latter case, TOK0 will be
 * the operating system and TOK1 the user name.
 */
static void reply(struct client *c, const char *ty,
                  const char *tok0, const char *tok1)
{
  write_to_client(c, "%u,%u:%s:%s%s%s\r\n",
                  c->q.s[L].port, c->q.s[R].port, ty,
                  tok0, tok1 ? ":" : "", tok1 ? tok1 : "");
}

/* Mapping from error codes to their protocol tokens. */
const char *const errtok[] = {
#define DEFTOK(err, tok) tok,
  ERROR(DEFTOK)
#undef DEFTOK
};

/* Report an error with code ERR to the client. */
static void reply_error(struct client *c, unsigned err)
{
  assert(err < E_LIMIT);
  reply(c, "ERROR", errtok[err], 0);
}

/*----- NAT proxy functions -----------------------------------------------*/

/* Cancel the proxy operation PX, closing the connection and releasing
 * resources.  This is used for both normal and unexpected closures.
 */
static void cancel_proxy(struct proxy *px)
{
  if (px->fd == -1)
    conn_kill(&px->cn);
  else {
    close(px->fd);
    selbuf_destroy(&px->b);
    free_writebuf(&px->wb);
  }
  selbuf_enable(&px->c->b);
  px->c->px = 0;
  xfree(px);
}

/* Notification that a line (presumably a reply) has been received from the
 * server.  We should check it, log it, and propagate the answer back.
 * Whatever happens, this proxy operation is now complete.
 */
static void proxy_line(char *line, size_t sz, void *p)
{
  struct proxy *px = p;
  char buf[1024];
  const char *q = line;
  unsigned lp, rp;

  /* Trim trailing space. */
  while (sz && isspace((unsigned char)line[sz - 1])) sz--;

  /* Parse the port numbers.  These should match the request. */
  if (unum(&q, &lp, 1, 65535)) goto syntax;
  skipws(&q); if (*q != ',') goto syntax; q++;
  if (unum(&q, &rp, 1, 65535)) goto syntax;
  skipws(&q); if (*q != ':') goto syntax; q++;
  if (lp != px->c->q.u.nat.port || rp != px->c->q.s[R].port) goto syntax;

  /* Find out what kind of reply this is. */
  if (idtoken(&q, buf, sizeof(buf))) goto syntax;
  skipws(&q); if (*q != ':') goto syntax; q++;

  if (strcmp(buf, "ERROR") == 0) {

    /* Report the error without interpreting it.  It might be meaningful to
     * the client.
     */
    skipws(&q);
    logmsg(&px->c->q, LOG_ERR, "proxy error from %s: %s", px->nat, q);
    reply(px->c, "ERROR", q, 0);

  } else if (strcmp(buf, "USERID") == 0) {

    /* Parse out the operating system and user name, and pass them on. */
    if (idtoken(&q, buf, sizeof(buf))) goto syntax;
    skipws(&q); if (*q != ':') goto syntax; q++;
    skipws(&q);
    logmsg(&px->c->q, LOG_ERR, "user `%s'; proxy = %s, os = %s",
           q, px->nat, buf);
    reply(px->c, "USERID", buf, q);

  } else
    goto syntax;
  goto done;

syntax:
  /* We didn't understand the message from the client. */
  logmsg(&px->c->q, LOG_ERR, "failed to parse response from %s", px->nat);
  reply_error(px->c, E_UNKNOWN);
done:
  /* All finished, no matter what. */
  cancel_proxy(px);
}

/* Notification that we have written the query to the server.  Await a
 * response if successful.
 */
static void done_proxy_write(int err, void *p)
{
  struct proxy *px = p;

  if (err) {
    logmsg(&px->c->q, LOG_ERR, "failed to proxy query to %s: %s",
           px->nat, strerror(errno));
    reply_error(px->c, E_UNKNOWN);
    cancel_proxy(px);
    return;
  }
  selbuf_enable(&px->b);
}

/* Notification that the connection to the server is either established or
 * failed.  In the former case, queue the right query.
 */
static void proxy_connected(int fd, void *p)
{
  struct proxy *px = p;
  char buf[16];
  int n;

  /* If the connection failed then report the problem and give up. */
  if (fd < 0) {
    logmsg(&px->c->q, LOG_ERR,
           "failed to make %s proxy connection to %s: %s",
           px->c->l->ao->name, px->nat, strerror(errno));
    reply_error(px->c, E_UNKNOWN);
    cancel_proxy(px);
    return;
  }

  /* We're now ready to go, so set things up. */
  px->fd = fd;
  selbuf_init(&px->b, &sel, fd, proxy_line, px);
  selbuf_setsize(&px->b, 1024);
  selbuf_disable(&px->b);
  init_writebuf(&px->wb, fd, done_proxy_write, px);

  /* Write the query.  This buffer is large enough because we've already
   * range-checked the remote the port number and the local one came from the
   * kernel, which we trust not to do anything stupid.
   */
  n = sprintf(buf, "%u,%u\r\n", px->c->q.u.nat.port, px->c->q.s[R].port);
  queue_write(&px->wb, buf, n);
}

/* Proxy the query through to a client machine for which we're providing NAT
 * disservice.
 */
static void proxy_query(struct client *c)
{
  struct socket s;
  struct sockaddr_storage ss;
  size_t ssz;
  struct proxy *px;
  int fd;

  /* Allocate the context structure for the NAT. */
  px = xmalloc(sizeof(*px));

  /* We'll use the client host's address in lots of log messages, so we may
   * as well format it once and use it over and over.
   */
  inet_ntop(c->q.ao->af, &c->q.u.nat.addr, px->nat, sizeof(px->nat));

  /* Create the socket for the connection. */
  if ((fd = socket(c->q.ao->af, SOCK_STREAM, 0)) < 0) {
    logmsg(&c->q, LOG_ERR, "failed to make %s socket for proxy: %s",
           c->l->ao->name, strerror(errno));
    goto err_0;
  }
  if (fix_up_socket(fd, "proxy")) goto err_1;

  /* Set up the connection to the client host.  The connection interface is a
   * bit broken: if the connection completes immediately, then the callback
   * function is called synchronously, and that might decide to shut
   * everything down.  So we must have fully initialized our context before
   * calling `conn_init', and mustn't touch it again afterwards -- since the
   * block may have been freed.
   */
  s = c->q.u.nat;
  s.port = 113;
  c->l->ao->socket_to_sockaddr(&s, &ss, &ssz);
  selbuf_disable(&c->b);
  c->px = px; px->c = c;
  px->fd = -1;
  if (conn_init(&px->cn, &sel, fd, (struct sockaddr *)&ss, ssz,
                proxy_connected, px)) {
    logmsg(&c->q, LOG_ERR, "failed to make %s proxy connection to %s: %s",
           c->l->ao->name, px->nat, strerror(errno));
    goto err_2;
  }

  /* All ready to go. */
  return;

  /* Tidy up after various kinds of failures. */
err_2:
  selbuf_enable(&c->b);
err_1:
  close(px->fd);
err_0:
  xfree(px);
  reply_error(c, E_UNKNOWN);
}

/*----- Client connection functions ---------------------------------------*/

/* Disconnect a client, freeing up any associated resources. */
static void disconnect_client(struct client *c)
{
  close(c->fd);
  selbuf_destroy(&c->b);
  sel_rmtimer(&c->t);
  free_writebuf(&c->wb);
  if (c->px) cancel_proxy(c->px);
  xfree(c);
}

/* Time out a client because it's been idle for too long. */
static void timeout_client(struct timeval *tv, void *p)
{
  struct client *c = p;
  logmsg(&c->q, LOG_NOTICE, "timing out idle or stuck client");
  sel_addtimer(&sel, &c->t, tv, timeout_client, 0);
  disconnect_client(c);
}

/* Reset the client idle timer, as a result of activity.  Set EXISTP if
 * there is an existing timer which needs to be removed.
 */
static void reset_client_timer(struct client *c, int existp)
{
  struct timeval tv;

  gettimeofday(&tv, 0);
  tv.tv_sec += 30;
  if (existp) sel_rmtimer(&c->t);
  sel_addtimer(&sel, &c->t, &tv, timeout_client, c);
}

/* Write a pseudorandom token into the buffer at P, which must have space for
 * at least TOKENSZ bytes.
 */
#define TOKENRANDSZ 8
#define TOKENSZ ((4*TOKENRANDSZ + 5)/3)
static void user_token(char *p)
{
  unsigned a = 0;
  unsigned b = 0;
  int i;
  static const char tokmap[64] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789.-";

  /* If there's not enough pseudorandom stuff lying around, then read more
   * from the kernel.
   */
  if (tokenptr + TOKENRANDSZ >= sizeof(tokenbuf)) {
    if (read(randfd, tokenbuf, sizeof(tokenbuf)) < sizeof(tokenbuf))
      die(1, "unexpected short read or error from `/dev/urandom'");
    tokenptr = 0;
  }

  /* Now encode the bytes using a slightly tweaked base-64 encoding.  Read
   * bytes into the accumulator and write out characters while there's
   * enough material.
   */
  for (i = 0; i < TOKENRANDSZ; i++) {
    a = (a << 8) | tokenbuf[tokenptr++]; b += 8;
    while (b >= 6) {
      b -= 6;
      *p++ = tokmap[(a >> b) & 0x3f];
    }
  }

  /* If there's anything left in the accumulator then flush it out. */
  if (b)
    *p++ = tokmap[(a << (6 - b)) & 0x3f];

  /* Null-terminate the token. */
  *p++ = 0;
}

/* Notification that a line has been received from the client.  Parse it,
 * find out about the connection it's referring to, apply the relevant
 * policy rules, and produce a response.  This is where almost everything
 * interesting happens.
 */
static void client_line(char *line, size_t len, void *p)
{
  struct client *c = p;
  const char *q;
  struct passwd *pw = 0;
  const struct policy *pol;
  dstr d = DSTR_INIT;
  struct policy upol = POLICY_INIT(A_LIMIT);
  struct policy_file pf;
  char buf[16];
  int i;

  /* If the connection has closed, then tidy stuff away. */
  c->q.s[L].port = c->q.s[R].port = 0;
  if (!line) {
    disconnect_client(c);
    return;
  }

  /* Client activity, so update the timer. */
  reset_client_timer(c, 1);

  /* See if the policy file has changed since we last looked.  If so, try to
   * read the new version.
   */
  if (fwatch_update(&polfw, "yaid.policy")) {
    logmsg(0, LOG_INFO, "reload master policy file `%s'", "yaid.policy");
    load_policy_file("yaid.policy", &policy);
  }

  /* Read the local and remote port numbers into the query structure. */
  q = line;
  if (unum(&q, &c->q.s[L].port, 1, 65535)) goto bad;
  skipws(&q); if (*q != ',') goto bad; q++;
  if (unum(&q, &c->q.s[R].port, 1, 65535)) goto bad;
  skipws(&q); if (*q) goto bad;

  /* Identify the connection.  Act on the result. */
  identify(&c->q);
  switch (c->q.resp) {

    case R_UID:
      /* We found a user.  Track down the user's password entry, because
       * we'll want that later.  Most of the processing for this case is
       * below.
       */
      if ((pw = getpwuid(c->q.u.uid)) == 0) {
        logmsg(&c->q, LOG_ERR, "no passwd entry for user %d", c->q.u.uid);
        reply_error(c, E_NOUSER);
        return;
      }
      break;

    case R_NAT:
      /* We've acted as a NAT for this connection.  Proxy the query through
       * to the actal client host.
       */
      proxy_query(c);
      return;

    case R_ERROR:
      /* We failed to identify the connection for some reason.  We should
       * already have logged an error, so there's not much to do here.
       */
      reply_error(c, c->q.u.error);
      return;

    default:
      /* Something happened that we don't understand. */
      abort();
  }

  /* Search the table of policy rules to find a match. */
  for (i = 0; i < DA_LEN(&policy); i++) {
    pol = &DA(&policy)[i];
    if (!match_policy(pol, &c->q)) continue;

    /* If this is something simple, then apply the resulting policy rule. */
    if (pol->act.act != A_USER) goto match;

    /* The global policy has decided to let the user have a say, so we must
     * parse the user file.
     */
    DRESET(&d);
    dstr_putf(&d, "%s/.yaid.policy", pw->pw_dir);
    if (open_policy_file(&pf, d.buf, "user policy file", &c->q))
      continue;
    while (!read_policy_file(&pf)) {

      /* Give up after 100 lines.  If the user's policy is that complicated,
       * something's gone very wrong.  Or there's too much commentary or
       * something.
       */
      if (pf.lno > 100) {
        logmsg(&c->q, LOG_ERR, "%s:%d: user policy file too long",
               pf.name, pf.lno);
        break;
      }

      /* If this isn't a match, go around for the next rule. */
      if (!match_policy(&pf.p, &c->q)) continue;

      /* Check that the user is allowed to request this action.  If not, see
       * if there's a more acceptable action later on.
       */
      if (!(pol->act.u.user & (1 << pf.p.act.act))) {
        logmsg(&c->q, LOG_ERR,
               "%s:%d: user action forbidden by global policy",
               pf.name, pf.lno);
        continue;
      }

      /* We've found a match, so grab it, close the file, and say we're
       * done.
       */
      upol = pf.p; pol = &upol;
      init_policy(&pf.p);
      close_policy_file(&pf);
      DDESTROY(&d);
      goto match;
    }
    close_policy_file(&pf);
    DDESTROY(&d);
  }

  /* No match: apply the built-in default policy. */
  pol = &default_policy;

match:
  switch (pol->act.act) {

    case A_NAME:
      /* Report the actual user's name. */
      logmsg(&c->q, LOG_INFO, "user `%s' (%d)", pw->pw_name, c->q.u.uid);
      reply(c, "USERID", "UNIX", pw->pw_name);
      break;

    case A_TOKEN:
      /* Report an arbitrary token which we can look up in our log file. */
      user_token(buf);
      logmsg(&c->q, LOG_INFO, "user `%s' (%d); token = %s",
             pw->pw_name, c->q.u.uid, buf);
      reply(c, "USERID", "OTHER", buf);
      break;

    case A_DENY:
      /* Deny that there's anyone there at all. */
      logmsg(&c->q, LOG_INFO, "user `%s' (%d); denying",
             pw->pw_name, c->q.u.uid);
      break;

    case A_HIDE:
      /* Report the user as being hidden. */
      logmsg(&c->q, LOG_INFO, "user `%s' (%d); hiding",
             pw->pw_name, c->q.u.uid);
      reply_error(c, E_HIDDEN);
      break;

    case A_LIE:
      /* Tell an egregious lie about who the user is. */
      logmsg(&c->q, LOG_INFO, "user `%s' (%d); lie = `%s'",
             pw->pw_name, c->q.u.uid, pol->act.u.lie);
      reply(c, "USERID", "UNIX", pol->act.u.lie);
      break;

    default:
      /* Something has gone very wrong. */
      abort();
  }

  /* All done. */
  free_policy(&upol);
  return;

bad:
  logmsg(&c->q, LOG_ERR, "failed to parse query from client");
  disconnect_client(c);
}

/* Notification that a new client has connected.  Prepare to read a query. */
static void accept_client(int fd, unsigned mode, void *p)
{
  struct listen *l = p;
  struct client *c;
  struct sockaddr_storage ssr, ssl;
  size_t ssz = sizeof(ssr);
  int sk;

  /* Accept the new connection. */
  if ((sk = accept(fd, (struct sockaddr *)&ssr, &ssz)) < 0) {
    if (errno != EAGAIN && errno == EWOULDBLOCK) {
      logmsg(0, LOG_ERR, "failed to accept incoming %s connection: %s",
             l->ao->name, strerror(errno));
    }
    return;
  }
  if (fix_up_socket(sk, "incoming client")) { close(sk); return; }

  /* Build a client block and fill it in. */
  c = xmalloc(sizeof(*c));
  c->l = l;
  c->q.ao = l->ao;

  /* Collect the local and remote addresses. */
  l->ao->sockaddr_to_addr(&ssr, &c->q.s[R].addr);
  ssz = sizeof(ssl);
  if (getsockname(sk, (struct sockaddr *)&ssl, &ssz)) {
    logmsg(0, LOG_ERR,
           "failed to read local address for incoming %s connection: %s",
           l->ao->name, strerror(errno));
    close(sk);
    xfree(c);
    return;
  }
  l->ao->sockaddr_to_addr(&ssl, &c->q.s[L].addr);
  c->q.s[L].port = c->q.s[R].port = 0;

  /* Set stuff up for reading the query and sending responses. */
  selbuf_init(&c->b, &sel, sk, client_line, c);
  selbuf_setsize(&c->b, 1024);
  reset_client_timer(c, 0);
  c->fd = sk;
  c->px = 0;
  init_writebuf(&c->wb, sk, done_client_write, c);
}

/*----- Main code ---------------------------------------------------------*/

/* Set up a listening socket for the address family described by AO,
 * listening on PORT.
 */
static int make_listening_socket(const struct addrops *ao, int port)
{
  int fd;
  int yes = 1;
  struct socket s;
  struct sockaddr_storage ss;
  struct listen *l;
  size_t ssz;

  /* Make the socket. */
  if ((fd = socket(ao->af, SOCK_STREAM, 0)) < 0) {
    if (errno == EAFNOSUPPORT) return (-1);
    die(1, "failed to create %s listening socket: %s",
        ao->name, strerror(errno));
  }

  /* Build the appropriate local address. */
  s.addr = *ao->any;
  s.port = port;
  ao->socket_to_sockaddr(&s, &ss, &ssz);

  /* Perform any initialization specific to the address type. */
  if (ao->init_listen_socket(fd)) {
    die(1, "failed to initialize %s listening socket: %s",
        ao->name, strerror(errno));
  }

  /* Bind to the address. */
  setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));
  if (bind(fd, (struct sockaddr *)&ss, ssz)) {
    die(1, "failed to bind %s listening socket: %s",
        ao->name, strerror(errno));
  }

  /* Avoid unpleasant race conditions. */
  if (fdflags(fd, O_NONBLOCK, O_NONBLOCK, 0, 0)) {
    die(1, "failed to set %s listening socket nonblocking: %s",
        ao->name, strerror(errno));
  }

  /* Prepare to listen. */
  if (listen(fd, 5))
    die(1, "failed to listen for %s: %s", ao->name, strerror(errno));

  /* Make a record of all of this. */
  l = xmalloc(sizeof(*l));
  l->ao = ao;
  sel_initfile(&sel, &l->f, fd, SEL_READ, accept_client, l);
  sel_addfile(&l->f);

  /* Done. */
  return (0);
}

int main(int argc, char *argv[])
{
  int port = 113;
  const struct addrops *ao;
  int any = 0;

  ego(argv[0]);

  fwatch_init(&polfw, "yaid.policy");
  init_sys();
  if (load_policy_file("yaid.policy", &policy))
    exit(1);
  { int i;
    for (i = 0; i < DA_LEN(&policy); i++)
      print_policy(&DA(&policy)[i]);
  }

  if ((randfd = open("/dev/urandom", O_RDONLY)) < 0) {
    die(1, "failed to open `/dev/urandom' for reading: %s",
        strerror(errno));
  }

  sel_init(&sel);
  for (ao = addroptab; ao->name; ao++)
    if (!make_listening_socket(ao, port)) any = 1;
  if (!any)
    die(1, "no IP protocols supported");

  for (;;)
    if (sel_select(&sel)) die(1, "select failed: %s", strerror(errno));

  return (0);
}

/*----- That's all, folks -------------------------------------------------*/