source4/auth/kerberos/krb5_init_context.c

   1 /*
   2    Unix SMB/CIFS implementation.
   3    Wrapper for krb5_init_context
   4
   5    Copyright (C) Andrew Bartlett <abartlet@samba.org> 2005
   6    Copyright (C) Andrew Tridgell 2005
   7    Copyright (C) Stefan Metzmacher 2004
   8
   9    This program is free software; you can redistribute it and/or modify
  10    it under the terms of the GNU General Public License as published by
  11    the Free Software Foundation; either version 3 of the License, or
  12    (at your option) any later version.
  13
  14    This program is distributed in the hope that it will be useful,
  15    but WITHOUT ANY WARRANTY; without even the implied warranty of
  16    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  17    GNU General Public License for more details.
  18
  19    You should have received a copy of the GNU General Public License
  20    along with this program.  If not, see <http://www.gnu.org/licenses/>.
  21 */
  22
  23 #include "includes.h"
  24 #include "system/kerberos.h"
  25 #include "system/gssapi.h"
  26 #include <tevent.h>
  27 #include "auth/kerberos/kerberos.h"
  28 #include "lib/socket/socket.h"
  29 #include "lib/stream/packet.h"
  30 #include "system/network.h"
  31 #include "param/param.h"
  32 #include "libcli/resolve/resolve.h"
  33 #include "../lib/tsocket/tsocket.h"
  34 #include "krb5_init_context.h"
  35 /*
  36   context structure for operations on cldap packets
  37 */
  38 struct smb_krb5_socket {
  39         struct socket_context *sock;
  40
  41         /* the fd event */
  42         struct tevent_fd *fde;
  43
  44         NTSTATUS status;
  45         DATA_BLOB request, reply;
  46
  47         struct packet_context *packet;
  48
  49         size_t partial_read;
  50
  51         krb5_krbhst_info *hi;
  52 };
  53
  54 static krb5_error_code smb_krb5_context_destroy(struct smb_krb5_context *ctx)
  55 {
  56 #ifdef SAMBA4_USES_HEIMDAL
  57         if (ctx->pvt_log_data) {
  58                 /* Otherwise krb5_free_context will try and close what we
  59                  * have already free()ed */
  60                 krb5_set_warn_dest(ctx->krb5_context, NULL);
  61                 krb5_closelog(ctx->krb5_context,
  62                                 (krb5_log_facility *)ctx->pvt_log_data);
  63         }
  64 #else
  65         krb5_set_trace_callback(ctx->krb5_context, NULL, NULL);
  66 #endif
  67         krb5_free_context(ctx->krb5_context);
  68         return 0;
  69 }
  70
  71 /* We never close down the DEBUG system, and no need to unreference the use */
  72 static void smb_krb5_debug_close(void *private_data) {
  73         return;
  74 }
  75
  76 #ifdef SAMBA4_USES_HEIMDAL
  77 static void smb_krb5_debug_wrapper(const char *timestr, const char *msg, void *private_data)
  78 {
  79         DEBUG(3, ("Kerberos: %s\n", msg));
  80 }
  81 #else
  82 static void smb_krb5_debug_wrapper(krb5_context context,
  83                                    const struct krb5_trace_info *info,
  84                                    void *cb_data)
  85 {
  86         DEBUG(3, ("Kerberos: %s\n", info->message));
  87 }
  88 #endif
  89
  90 #ifdef SAMBA4_USES_HEIMDAL
  91 /*
  92   handle recv events on a smb_krb5 socket
  93 */
  94 static void smb_krb5_socket_recv(struct smb_krb5_socket *smb_krb5)
  95 {
  96         TALLOC_CTX *tmp_ctx = talloc_new(smb_krb5);
  97         DATA_BLOB blob;
  98         size_t nread, dsize;
  99
 100         smb_krb5->status = socket_pending(smb_krb5->sock, &dsize);
 101         if (!NT_STATUS_IS_OK(smb_krb5->status)) {
 102                 talloc_free(tmp_ctx);
 103                 return;
 104         }
 105
 106         blob = data_blob_talloc(tmp_ctx, NULL, dsize);
 107         if (blob.data == NULL && dsize != 0) {
 108                 smb_krb5->status = NT_STATUS_NO_MEMORY;
 109                 talloc_free(tmp_ctx);
 110                 return;
 111         }
 112
 113         smb_krb5->status = socket_recv(smb_krb5->sock, blob.data, blob.length, &nread);
 114         if (!NT_STATUS_IS_OK(smb_krb5->status)) {
 115                 talloc_free(tmp_ctx);
 116                 return;
 117         }
 118         blob.length = nread;
 119
 120         if (nread == 0) {
 121                 smb_krb5->status = NT_STATUS_UNEXPECTED_NETWORK_ERROR;
 122                 talloc_free(tmp_ctx);
 123                 return;
 124         }
 125
 126         DEBUG(4,("Received smb_krb5 packet of length %d\n",
 127                  (int)blob.length));
 128
 129         talloc_steal(smb_krb5, blob.data);
 130         smb_krb5->reply = blob;
 131         talloc_free(tmp_ctx);
 132 }
 133
 134 static NTSTATUS smb_krb5_full_packet(void *private_data, DATA_BLOB data)
 135 {
 136         struct smb_krb5_socket *smb_krb5 = talloc_get_type(private_data, struct smb_krb5_socket);
 137         talloc_steal(smb_krb5, data.data);
 138         smb_krb5->reply = data;
 139         smb_krb5->reply.length -= 4;
 140         smb_krb5->reply.data += 4;
 141         return NT_STATUS_OK;
 142 }
 143
 144 /*
 145   handle request timeouts
 146 */
 147 static void smb_krb5_request_timeout(struct tevent_context *event_ctx,
 148                                   struct tevent_timer *te, struct timeval t,
 149                                   void *private_data)
 150 {
 151         struct smb_krb5_socket *smb_krb5 = talloc_get_type(private_data, struct smb_krb5_socket);
 152         DEBUG(5,("Timed out smb_krb5 packet\n"));
 153         smb_krb5->status = NT_STATUS_IO_TIMEOUT;
 154 }
 155
 156 static void smb_krb5_error_handler(void *private_data, NTSTATUS status)
 157 {
 158         struct smb_krb5_socket *smb_krb5 = talloc_get_type(private_data, struct smb_krb5_socket);
 159         smb_krb5->status = status;
 160 }
 161
 162 /*
 163   handle send events on a smb_krb5 socket
 164 */
 165 static void smb_krb5_socket_send(struct smb_krb5_socket *smb_krb5)
 166 {
 167         NTSTATUS status;
 168
 169         size_t len;
 170
 171         len = smb_krb5->request.length;
 172         status = socket_send(smb_krb5->sock, &smb_krb5->request, &len);
 173
 174         if (!NT_STATUS_IS_OK(status)) return;
 175
 176         TEVENT_FD_READABLE(smb_krb5->fde);
 177
 178         TEVENT_FD_NOT_WRITEABLE(smb_krb5->fde);
 179         return;
 180 }
 181
 182
 183 /*
 184   handle fd events on a smb_krb5_socket
 185 */
 186 static void smb_krb5_socket_handler(struct tevent_context *ev, struct tevent_fd *fde,
 187                                  uint16_t flags, void *private_data)
 188 {
 189         struct smb_krb5_socket *smb_krb5 = talloc_get_type(private_data, struct smb_krb5_socket);
 190         switch (smb_krb5->hi->proto) {
 191         case KRB5_KRBHST_UDP:
 192                 if (flags & TEVENT_FD_READ) {
 193                         smb_krb5_socket_recv(smb_krb5);
 194                         return;
 195                 }
 196                 if (flags & TEVENT_FD_WRITE) {
 197                         smb_krb5_socket_send(smb_krb5);
 198                         return;
 199                 }
 200                 /* not reached */
 201                 return;
 202         case KRB5_KRBHST_TCP:
 203                 if (flags & TEVENT_FD_READ) {
 204                         packet_recv(smb_krb5->packet);
 205                         return;
 206                 }
 207                 if (flags & TEVENT_FD_WRITE) {
 208                         packet_queue_run(smb_krb5->packet);
 209                         return;
 210                 }
 211                 /* not reached */
 212                 return;
 213         case KRB5_KRBHST_HTTP:
 214                 /* can't happen */
 215                 break;
 216         }
 217 }
 218
 219 krb5_error_code smb_krb5_send_and_recv_func(krb5_context context,
 220                                             void *data,
 221                                             krb5_krbhst_info *hi,
 222                                             time_t timeout,
 223                                             const krb5_data *send_buf,
 224                                             krb5_data *recv_buf)
 225 {
 226         krb5_error_code ret;
 227         NTSTATUS status;
 228         const char *name;
 229         struct addrinfo *ai, *a;
 230         struct smb_krb5_socket *smb_krb5;
 231
 232         DATA_BLOB send_blob;
 233
 234         struct tevent_context *ev;
 235         TALLOC_CTX *tmp_ctx = talloc_new(NULL);
 236         if (!tmp_ctx) {
 237                 return ENOMEM;
 238         }
 239
 240         if (!data) {
 241                 /* If no event context was available, then create one for this loop */
 242                 ev = tevent_context_init(tmp_ctx);
 243                 if (!ev) {
 244                         talloc_free(tmp_ctx);
 245                         return ENOMEM;
 246                 }
 247         } else {
 248                 ev = talloc_get_type_abort(data, struct tevent_context);
 249         }
 250
 251         send_blob = data_blob_const(send_buf->data, send_buf->length);
 252
 253         ret = krb5_krbhst_get_addrinfo(context, hi, &ai);
 254         if (ret) {
 255                 talloc_free(tmp_ctx);
 256                 return ret;
 257         }
 258
 259         for (a = ai; a; a = a->ai_next) {
 260                 struct socket_address *remote_addr;
 261                 smb_krb5 = talloc(tmp_ctx, struct smb_krb5_socket);
 262                 if (!smb_krb5) {
 263                         talloc_free(tmp_ctx);
 264                         return ENOMEM;
 265                 }
 266                 smb_krb5->hi = hi;
 267
 268                 switch (a->ai_family) {
 269                 case PF_INET:
 270                         name = "ipv4";
 271                         break;
 272 #ifdef HAVE_IPV6
 273                 case PF_INET6:
 274                         name = "ipv6";
 275                         break;
 276 #endif
 277                 default:
 278                         talloc_free(tmp_ctx);
 279                         return EINVAL;
 280                 }
 281
 282                 status = NT_STATUS_INVALID_PARAMETER;
 283                 switch (hi->proto) {
 284                 case KRB5_KRBHST_UDP:
 285                         status = socket_create(name, SOCKET_TYPE_DGRAM, &smb_krb5->sock, 0);
 286                         break;
 287                 case KRB5_KRBHST_TCP:
 288                         status = socket_create(name, SOCKET_TYPE_STREAM, &smb_krb5->sock, 0);
 289                         break;
 290                 case KRB5_KRBHST_HTTP:
 291                         talloc_free(tmp_ctx);
 292                         return EINVAL;
 293                 }
 294                 if (!NT_STATUS_IS_OK(status)) {
 295                         talloc_free(smb_krb5);
 296                         continue;
 297                 }
 298
 299                 talloc_steal(smb_krb5, smb_krb5->sock);
 300
 301                 remote_addr = socket_address_from_sockaddr(smb_krb5, a->ai_addr, a->ai_addrlen);
 302                 if (!remote_addr) {
 303                         talloc_free(smb_krb5);
 304                         continue;
 305                 }
 306
 307                 status = socket_connect_ev(smb_krb5->sock, NULL, remote_addr, 0, ev);
 308                 if (!NT_STATUS_IS_OK(status)) {
 309                         talloc_free(smb_krb5);
 310                         continue;
 311                 }
 312
 313                 /* Setup the FDE, start listening for read events
 314                  * from the start (otherwise we may miss a socket
 315                  * drop) and mark as AUTOCLOSE along with the fde */
 316
 317                 /* Ths is equivilant to EVENT_FD_READABLE(smb_krb5->fde) */
 318                 smb_krb5->fde = tevent_add_fd(ev, smb_krb5->sock,
 319                                               socket_get_fd(smb_krb5->sock),
 320                                               TEVENT_FD_READ,
 321                                               smb_krb5_socket_handler, smb_krb5);
 322                 /* its now the job of the event layer to close the socket */
 323                 tevent_fd_set_close_fn(smb_krb5->fde, socket_tevent_fd_close_fn);
 324                 socket_set_flags(smb_krb5->sock, SOCKET_FLAG_NOCLOSE);
 325
 326                 tevent_add_timer(ev, smb_krb5,
 327                                  timeval_current_ofs(timeout, 0),
 328                                  smb_krb5_request_timeout, smb_krb5);
 329
 330                 smb_krb5->status = NT_STATUS_OK;
 331                 smb_krb5->reply = data_blob(NULL, 0);
 332
 333                 switch (hi->proto) {
 334                 case KRB5_KRBHST_UDP:
 335                         TEVENT_FD_WRITEABLE(smb_krb5->fde);
 336                         smb_krb5->request = send_blob;
 337                         break;
 338                 case KRB5_KRBHST_TCP:
 339
 340                         smb_krb5->packet = packet_init(smb_krb5);
 341                         if (smb_krb5->packet == NULL) {
 342                                 talloc_free(smb_krb5);
 343                                 return ENOMEM;
 344                         }
 345                         packet_set_private(smb_krb5->packet, smb_krb5);
 346                         packet_set_socket(smb_krb5->packet, smb_krb5->sock);
 347                         packet_set_callback(smb_krb5->packet, smb_krb5_full_packet);
 348                         packet_set_full_request(smb_krb5->packet, packet_full_request_u32);
 349                         packet_set_error_handler(smb_krb5->packet, smb_krb5_error_handler);
 350                         packet_set_event_context(smb_krb5->packet, ev);
 351                         packet_set_fde(smb_krb5->packet, smb_krb5->fde);
 352
 353                         smb_krb5->request = data_blob_talloc(smb_krb5, NULL, send_blob.length + 4);
 354                         RSIVAL(smb_krb5->request.data, 0, send_blob.length);
 355                         memcpy(smb_krb5->request.data+4, send_blob.data, send_blob.length);
 356                         packet_send(smb_krb5->packet, smb_krb5->request);
 357                         break;
 358                 case KRB5_KRBHST_HTTP:
 359                         talloc_free(tmp_ctx);
 360                         return EINVAL;
 361                 }
 362                 while ((NT_STATUS_IS_OK(smb_krb5->status)) && !smb_krb5->reply.length) {
 363                         if (tevent_loop_once(ev) != 0) {
 364                                 talloc_free(tmp_ctx);
 365                                 return EINVAL;
 366                         }
 367
 368                         /* After each and every event loop, reset the
 369                          * send_to_kdc pointers to what they were when
 370                          * we entered this loop.  That way, if a
 371                          * nested event has invalidated them, we put
 372                          * it back before we return to the heimdal
 373                          * code */
 374                         ret = krb5_set_send_to_kdc_func(context,
 375                                                         smb_krb5_send_and_recv_func,
 376                                                         data);
 377                         if (ret != 0) {
 378                                 talloc_free(tmp_ctx);
 379                                 return ret;
 380                         }
 381                 }
 382                 if (NT_STATUS_EQUAL(smb_krb5->status, NT_STATUS_IO_TIMEOUT)) {
 383                         talloc_free(smb_krb5);
 384                         continue;
 385                 }
 386
 387                 if (!NT_STATUS_IS_OK(smb_krb5->status)) {
 388                         struct tsocket_address *addr = socket_address_to_tsocket_address(smb_krb5, remote_addr);
 389                         const char *addr_string = NULL;
 390                         if (addr) {
 391                                 addr_string = tsocket_address_inet_addr_string(addr, smb_krb5);
 392                         } else {
 393                                 addr_string = NULL;
 394                         }
 395                         DEBUG(2,("Error reading smb_krb5 reply packet: %s from %s\n", nt_errstr(smb_krb5->status),
 396                                  addr_string));
 397                         talloc_free(smb_krb5);
 398                         continue;
 399                 }
 400
 401                 ret = krb5_data_copy(recv_buf, smb_krb5->reply.data, smb_krb5->reply.length);
 402                 if (ret) {
 403                         talloc_free(tmp_ctx);
 404                         return ret;
 405                 }
 406                 talloc_free(smb_krb5);
 407
 408                 break;
 409         }
 410         talloc_free(tmp_ctx);
 411         if (a) {
 412                 return 0;
 413         }
 414         return KRB5_KDC_UNREACH;
 415 }
 416 #endif
 417
 418 krb5_error_code
 419 smb_krb5_init_context_basic(TALLOC_CTX *tmp_ctx,
 420                             struct loadparm_context *lp_ctx,
 421                             krb5_context *_krb5_context)
 422 {
 423         krb5_error_code ret;
 424         char **config_files;
 425         const char *config_file, *realm;
 426         krb5_context krb5_ctx;
 427
 428         initialize_krb5_error_table();
 429
 430         ret = krb5_init_context(&krb5_ctx);
 431         if (ret) {
 432                 DEBUG(1,("krb5_init_context failed (%s)\n",
 433                          error_message(ret)));
 434                 return ret;
 435         }
 436
 437         config_file = lpcfg_config_path(tmp_ctx, lp_ctx, "krb5.conf");
 438         if (!config_file) {
 439                 krb5_free_context(krb5_ctx);
 440                 return ENOMEM;
 441         }
 442
 443         /* Use our local krb5.conf file by default */
 444         ret = krb5_prepend_config_files_default(config_file, &config_files);
 445         if (ret) {
 446                 DEBUG(1,("krb5_prepend_config_files_default failed (%s)\n",
 447                          smb_get_krb5_error_message(krb5_ctx, ret, tmp_ctx)));
 448                 krb5_free_context(krb5_ctx);
 449                 return ret;
 450         }
 451
 452         ret = krb5_set_config_files(krb5_ctx, config_files);
 453         krb5_free_config_files(config_files);
 454         if (ret) {
 455                 DEBUG(1,("krb5_set_config_files failed (%s)\n",
 456                          smb_get_krb5_error_message(krb5_ctx, ret, tmp_ctx)));
 457                 krb5_free_context(krb5_ctx);
 458                 return ret;
 459         }
 460
 461         realm = lpcfg_realm(lp_ctx);
 462         if (realm != NULL) {
 463                 ret = krb5_set_default_realm(krb5_ctx, realm);
 464                 if (ret) {
 465                         DEBUG(1,("krb5_set_default_realm failed (%s)\n",
 466                                  smb_get_krb5_error_message(krb5_ctx, ret, tmp_ctx)));
 467                         krb5_free_context(krb5_ctx);
 468                         return ret;
 469                 }
 470         }
 471
 472         *_krb5_context = krb5_ctx;
 473         return 0;
 474 }
 475
 476 krb5_error_code smb_krb5_init_context(void *parent_ctx,
 477                                       struct tevent_context *ev,
 478                                       struct loadparm_context *lp_ctx,
 479                                       struct smb_krb5_context **smb_krb5_context)
 480 {
 481         krb5_error_code ret;
 482         TALLOC_CTX *tmp_ctx;
 483         krb5_context kctx;
 484 #ifdef SAMBA4_USES_HEIMDAL
 485         krb5_log_facility *logf;
 486 #endif
 487
 488         initialize_krb5_error_table();
 489
 490         tmp_ctx = talloc_new(parent_ctx);
 491         *smb_krb5_context = talloc_zero(tmp_ctx, struct smb_krb5_context);
 492
 493         if (!*smb_krb5_context || !tmp_ctx) {
 494                 talloc_free(tmp_ctx);
 495                 return ENOMEM;
 496         }
 497
 498         ret = smb_krb5_init_context_basic(tmp_ctx, lp_ctx, &kctx);
 499         if (ret) {
 500                 DEBUG(1,("smb_krb5_context_init_basic failed (%s)\n",
 501                          error_message(ret)));
 502                 talloc_free(tmp_ctx);
 503                 return ret;
 504         }
 505         (*smb_krb5_context)->krb5_context = kctx;
 506
 507         talloc_set_destructor(*smb_krb5_context, smb_krb5_context_destroy);
 508
 509 #ifdef SAMBA4_USES_HEIMDAL
 510         /* TODO: Should we have a different name here? */
 511         ret = krb5_initlog(kctx, "Samba", &logf);
 512
 513         if (ret) {
 514                 DEBUG(1,("krb5_initlog failed (%s)\n",
 515                          smb_get_krb5_error_message(kctx, ret, tmp_ctx)));
 516                 talloc_free(tmp_ctx);
 517                 return ret;
 518         }
 519         (*smb_krb5_context)->pvt_log_data = logf;
 520
 521         ret = krb5_addlog_func(kctx, logf, 0 /* min */, -1 /* max */,
 522                                smb_krb5_debug_wrapper,
 523                                 smb_krb5_debug_close, NULL);
 524         if (ret) {
 525                 DEBUG(1,("krb5_addlog_func failed (%s)\n",
 526                          smb_get_krb5_error_message(kctx, ret, tmp_ctx)));
 527                 talloc_free(tmp_ctx);
 528                 return ret;
 529         }
 530         krb5_set_warn_dest(kctx, logf);
 531
 532         /* Set use of our socket lib */
 533         if (ev) {
 534                 struct tevent_context *previous_ev;
 535                 ret = smb_krb5_context_set_event_ctx(*smb_krb5_context,
 536                                                      ev, &previous_ev);
 537                 if (ret) {
 538                         talloc_free(tmp_ctx);
 539                         return ret;
 540                 }
 541         }
 542
 543         /* Set options in kerberos */
 544
 545         krb5_set_dns_canonicalize_hostname(kctx,
 546                         lpcfg_parm_bool(lp_ctx, NULL, "krb5",
 547                                         "set_dns_canonicalize", false));
 548 #else
 549         ret = krb5_set_trace_callback(kctx, smb_krb5_debug_wrapper, NULL);
 550         if (ret && ret != KRB5_TRACE_NOSUPP) {
 551                 DEBUG(1, ("krb5_set_trace_callback failed (%s)\n"
 552                           smb_get_krb5_error_message(kctx, ret, tmp_ctx)));
 553                 talloc_free(tmp_ctx);
 554                 return ret;
 555         }
 556 #endif
 557         talloc_steal(parent_ctx, *smb_krb5_context);
 558         talloc_free(tmp_ctx);
 559
 560         return 0;
 561 }
 562
 563 #ifdef SAMBA4_USES_HEIMDAL
 564 krb5_error_code smb_krb5_context_set_event_ctx(struct smb_krb5_context *smb_krb5_context,
 565                                                struct tevent_context *ev,
 566                                                struct tevent_context **previous_ev)
 567 {
 568         int ret;
 569         if (!ev) {
 570                 return EINVAL;
 571         }
 572
 573         *previous_ev = smb_krb5_context->current_ev;
 574
 575         smb_krb5_context->current_ev = talloc_reference(smb_krb5_context, ev);
 576         if (!smb_krb5_context->current_ev) {
 577                 return ENOMEM;
 578         }
 579
 580         /* Set use of our socket lib */
 581         ret = krb5_set_send_to_kdc_func(smb_krb5_context->krb5_context,
 582                                         smb_krb5_send_and_recv_func,
 583                                         ev);
 584         if (ret) {
 585                 TALLOC_CTX *tmp_ctx = talloc_new(NULL);
 586                 DEBUG(1,("krb5_set_send_recv_func failed (%s)\n",
 587                          smb_get_krb5_error_message(smb_krb5_context->krb5_context, ret, tmp_ctx)));
 588                 talloc_free(tmp_ctx);
 589                 talloc_unlink(smb_krb5_context, smb_krb5_context->current_ev);
 590                 smb_krb5_context->current_ev = NULL;
 591                 return ret;
 592         }
 593         return 0;
 594 }
 595
 596 krb5_error_code smb_krb5_context_remove_event_ctx(struct smb_krb5_context *smb_krb5_context,
 597                                                   struct tevent_context *previous_ev,
 598                                                   struct tevent_context *ev)
 599 {
 600         int ret;
 601         talloc_unlink(smb_krb5_context, ev);
 602         /* If there was a mismatch with things happening on a stack, then don't wipe things */
 603         smb_krb5_context->current_ev = previous_ev;
 604         /* Set use of our socket lib */
 605         ret = krb5_set_send_to_kdc_func(smb_krb5_context->krb5_context,
 606                                         smb_krb5_send_and_recv_func,
 607                                         previous_ev);
 608         if (ret) {
 609                 TALLOC_CTX *tmp_ctx = talloc_new(NULL);
 610                 DEBUG(1,("krb5_set_send_recv_func failed (%s)\n",
 611                          smb_get_krb5_error_message(smb_krb5_context->krb5_context, ret, tmp_ctx)));
 612                 talloc_free(tmp_ctx);
 613                 return ret;
 614         }
 615         return 0;
 616 }
 617 #endif