git » spf » master » tree

[master] / spf.go

   1
   2
   3
   4
   5
   6
   7
   8
   9
  10
  11
  12
  13
  14
  15
  16
  17
  18
  19
  20
  21
  22
  23
  24
  25
  26
  27
  28
  29
  30
  31
  32
  33
  34
  35
  36
  37
  38
  39
  40
  41
  42
  43
  44
  45
  46
  47
  48
  49
  50
  51
  52
  53
  54
  55
  56
  57
  58
  59
  60
  61
  62
  63
  64
  65
  66
  67
  68
  69
  70
  71
  72
  73
  74
  75
  76
  77
  78
  79
  80
  81
  82
  83
  84
  85
  86
  87
  88
  89
  90
  91
  92
  93
  94
  95
  96
  97
  98
  99
 100
 101
 102
 103
 104
 105
 106
 107
 108
 109
 110
 111
 112
 113
 114
 115
 116
 117
 118
 119
 120
 121
 122
 123
 124
 125
 126
 127
 128
 129
 130
 131
 132
 133
 134
 135
 136
 137
 138
 139
 140
 141
 142
 143
 144
 145
 146
 147
 148
 149
 150
 151
 152
 153
 154
 155
 156
 157
 158
 159
 160
 161
 162
 163
 164
 165
 166
 167
 168
 169
 170
 171
 172
 173
 174
 175
 176
 177
 178
 179
 180
 181
 182
 183
 184
 185
 186
 187
 188
 189
 190
 191
 192
 193
 194
 195
 196
 197
 198
 199
 200
 201
 202
 203
 204
 205
 206
 207
 208
 209
 210
 211
 212
 213
 214
 215
 216
 217
 218
 219
 220
 221
 222
 223
 224
 225
 226
 227
 228
 229
 230
 231
 232
 233
 234
 235
 236
 237
 238
 239
 240
 241
 242
 243
 244
 245
 246
 247
 248
 249
 250
 251
 252
 253
 254
 255
 256
 257
 258
 259
 260
 261
 262
 263
 264
 265
 266
 267
 268
 269
 270
 271
 272
 273
 274
 275
 276
 277
 278
 279
 280
 281
 282
 283
 284
 285
 286
 287
 288
 289
 290
 291
 292
 293
 294
 295
 296
 297
 298
 299
 300
 301
 302
 303
 304
 305
 306
 307
 308
 309
 310
 311
 312
 313
 314
 315
 316
 317
 318
 319
 320
 321
 322
 323
 324
 325
 326
 327
 328
 329
 330
 331
 332
 333
 334
 335
 336
 337
 338
 339
 340
 341
 342
 343
 344
 345
 346
 347
 348
 349
 350
 351
 352
 353
 354
 355
 356
 357
 358
 359
 360
 361
 362
 363
 364
 365
 366
 367
 368
 369
 370
 371
 372
 373
 374
 375
 376
 377
 378
 379
 380
 381
 382
 383
 384
 385
 386
 387
 388
 389
 390
 391
 392
 393
 394
 395
 396
 397
 398
 399
 400
 401
 402
 403
 404
 405
 406
 407
 408
 409
 410
 411
 412
 413
 414
 415
 416
 417
 418
 419
 420
 421
 422
 423
 424
 425
 426
 427
 428
 429
 430
 431
 432
 433
 434
 435
 436
 437
 438
 439
 440
 441
 442
 443
 444
 445
 446
 447
 448
 449
 450
 451
 452
 453
 454
 455
 456
 457
 458
 459
 460
 461
 462
 463
 464
 465
 466
 467
 468
 469
 470
 471
 472
 473
 474
 475
 476
 477
 478
 479
 480
 481
 482
 483
 484
 485
 486
 487
 488
 489
 490
 491
 492
 493
 494
 495
 496
 497
 498
 499
 500
 501
 502
 503
 504
 505
 506
 507
 508
 509
 510
 511
 512
 513
 514
 515
 516
 517
 518
 519
 520
 521
 522
 523
 524
 525
 526
 527
 528
 529
 530
 531
 532
 533
 534
 535
 536
 537
 538
 539
 540
 541
 542
 543
 544
 545
 546
 547
 548
 549
 550
 551
 552
 553
 554
 555
 556
 557
 558
 559
 560
 561
 562
 563
 564
 565
 566
 567
 568
 569
 570
 571
 572
 573
 574
 575
 576
 577
 578
 579
 580
 581
 582
 583
 584
 585
 586
 587
 588
 589
 590
 591
 592
 593
 594
 595
 596
 597
 598
 599
 600
 601
 602
 603
 604
 605
 606
 607
 608
 609
 610
 611
 612
 613
 614
 615
 616
 617
 618
 619
 620
 621
 622
 623
 624
 625
 626
 627
 628
 629
 630
 631
 632
 633
 634
 635
 636
 637
 638
 639
 640
 641
 642
 643
 644
 645
 646
 647
 648
 649
 650
 651
 652
 653
 654
 655
 656
 657
 658
 659
 660
 661
 662
 663
 664
 665
 666
 667
 668
 669
 670
 671
 672
 673
 674
 675
 676
 677
 678
 679
 680
 681
 682
 683
 684
 685
 686
 687
 688
 689
 690
 691
 692
 693
 694
 695
 696
 697
 698
 699
 700
 701
 702
 703
 704
 705
 706
 707
 708
 709
 710
 711
 712
 713
 714
 715
 716
 717
 718
 719
 720
 721
 722
 723
 724
 725
 726
 727
 728
 729
 730
 731
 732
 733
 734
 735
 736
 737
 738
 739
 740
 741
 742
 743
 744
 745
 746
 747
 748
 749
 750
 751
 752
 753
 754
 755
 756
 757
 758
 759
 760
 761
 762
 763
 764
 765
 766
 767
 768
 769
 770
 771
 772
 773
 774
 775
 776
 777
 778
 779
 780
 781
 782
 783
 784
 785
 786
 787
 788
 789
 790
 791
 792
 793
 794
 795
 796
 797
 798
 799
 800
 801
 802
 803
 804
 805
 806
 807
 808
 809
 810
 811
 812
 813
 814
 815
 816
 817
 818
 819
 820
 821
 822
 823
 824
 825
 826
 827
 828
 829
 830
 831
 832
 833
 834
 835
 836
 837
 838
 839
 840
 841
 842
 843
 844
 845
 846
 847
 848
 849
 850
 851
 852
 853
 854
 855
 856
 857
 858
 859
 860
 861
 862
 863
 864
 865
 866
 867
 868
 869
 870
 871
 872
 873
 874
 875
 876
 877
 878
 879
 880
 881
 882
 883
 884
 885
 886
 887
 888
 889
 890
 891
 892
 893
 894
 895
 896
 897
 898
 899
 900
 901
 902
 903
 904
 905
 906
 907
 908
 909
 910
 911
 912
 913
 914
 915
 916
 917
 918
 919
 920
 921
 922
 923
 924
 925
 926
 927
 928
 929
 930
 931
 932
 933
 934
 935
 936
 937
 938
 939
 940
 941
 942
 943
 944
 945
 946
 947
 948
 949
 950
 951
 952
 953
 954
 955
 956
 957
 958
 959
 960
 961
 962
 963
 964
 965
 966
 967
 968
 969
 970
 971
 972
 973
 974
 975
 976
 977
 978
 979
 980
 981
 982
 983
 984
 985
 986
 987
 988
 989
 990
 991
 992
 993
 994
 995
 996
 997
 998
 999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
// Package spf implements SPF (Sender Policy Framework) lookup and validation.
//
// Sender Policy Framework (SPF) is a simple email-validation system designed
// to detect email spoofing by providing a mechanism to allow receiving mail
// exchangers to check that incoming mail from a domain comes from a host
// authorized by that domain's administrators [Wikipedia].
//
// This package is intended to be used by SMTP servers to implement SPF
// validation.
//
// All SPF mechanisms, modifiers, and macros are supported.
//
// References:
//
//	https://tools.ietf.org/html/rfc7208
//	https://en.wikipedia.org/wiki/Sender_Policy_Framework
package spf // import "blitiri.com.ar/go/spf"

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/url"
	"regexp"
	"strconv"
	"strings"
)

// The Result of an SPF check. Note the values have meaning, we use them in
// headers.  https://tools.ietf.org/html/rfc7208#section-8
type Result string

// Valid results.
var (
	// https://tools.ietf.org/html/rfc7208#section-8.1
	// Not able to reach any conclusion.
	None = Result("none")

	// https://tools.ietf.org/html/rfc7208#section-8.2
	// No definite assertion (positive or negative).
	Neutral = Result("neutral")

	// https://tools.ietf.org/html/rfc7208#section-8.3
	// Client is authorized to inject mail.
	Pass = Result("pass")

	// https://tools.ietf.org/html/rfc7208#section-8.4
	// Client is *not* authorized to use the domain.
	Fail = Result("fail")

	// https://tools.ietf.org/html/rfc7208#section-8.5
	// Not authorized, but unwilling to make a strong policy statement.
	SoftFail = Result("softfail")

	// https://tools.ietf.org/html/rfc7208#section-8.6
	// Transient error while performing the check.
	TempError = Result("temperror")

	// https://tools.ietf.org/html/rfc7208#section-8.7
	// Records could not be correctly interpreted.
	PermError = Result("permerror")
)

var qualToResult = map[byte]Result{
	'+': Pass,
	'-': Fail,
	'~': SoftFail,
	'?': Neutral,
}

// Errors returned by the library. Note that the errors returned in different
// situations may change over time, and new ones may be added. Be careful
// about over-relying on these.
var (
	// Errors related to an invalid SPF record.
	ErrUnknownField  = errors.New("unknown field")
	ErrInvalidIP     = errors.New("invalid ipX value")
	ErrInvalidMask   = errors.New("invalid mask")
	ErrInvalidMacro  = errors.New("invalid macro")
	ErrInvalidDomain = errors.New("invalid domain")

	// Errors related to DNS lookups.
	// Note that the library functions may also return net.DNSError.
	ErrNoResult               = errors.New("no DNS record found")
	ErrLookupLimitReached     = errors.New("lookup limit reached")
	ErrVoidLookupLimitReached = errors.New("void lookup limit reached")
	ErrTooManyMXRecords       = errors.New("too many MX records")
	ErrMultipleRecords        = errors.New("multiple matching DNS records")

	// Errors returned on a successful match.
	ErrMatchedAll    = errors.New("matched all")
	ErrMatchedA      = errors.New("matched a")
	ErrMatchedIP     = errors.New("matched ip")
	ErrMatchedMX     = errors.New("matched mx")
	ErrMatchedPTR    = errors.New("matched ptr")
	ErrMatchedExists = errors.New("matched exists")
)

const (
	// Default value for the maximum number of DNS lookups while resolving SPF.
	// RFC is quite clear 10 must be the maximum allowed.
	// https://tools.ietf.org/html/rfc7208#section-4.6.4
	defaultMaxLookups = 10

	// Default value for the maximum number of DNS void lookups while
	// resolving SPF.  RFC suggests that implementations SHOULD limit these
	// with a configurable default of 2.
	// https://tools.ietf.org/html/rfc7208#section-4.6.4
	defaultMaxVoidLookups = 2
)

// TraceFunc is the type of tracing functions.
type TraceFunc func(f string, a ...interface{})

var (
	nullTrace    = func(f string, a ...interface{}) {}
	defaultTrace = nullTrace
)

// Option type, for setting options. Users are expected to treat this as an
// opaque type and not rely on the implementation, which is subject to change.
type Option func(*resolution)

// CheckHost fetches SPF records for `domain`, parses them, and evaluates them
// to determine if `ip` is permitted to send mail for it.
// Because it doesn't receive enough information to handle macros well, its
// usage is not recommended, but remains supported for backwards
// compatibility.
//
// The function returns a Result, which corresponds with the SPF result for
// the check as per RFC, as well as an error for debugging purposes. Note that
// the error may be non-nil even on successful checks.
//
// Reference: https://tools.ietf.org/html/rfc7208#section-4
//
// Deprecated: use CheckHostWithSender instead.
func CheckHost(ip net.IP, domain string) (Result, error) {
	r := &resolution{
		ip:           ip,
		maxcount:     defaultMaxLookups,
		maxvoidcount: defaultMaxVoidLookups,
		helo:         domain,
		sender:       "@" + domain,
		ctx:          context.TODO(),
		resolver:     defaultResolver,
		trace:        defaultTrace,
	}
	return r.Check(domain)
}

// CheckHostWithSender fetches SPF records for `sender`'s domain, parses them,
// and evaluates them to determine if `ip` is permitted to send mail for it.
// The `helo` domain is used if the sender has no domain part.
//
// The `opts` optional parameter can be used to adjust some specific
// behaviours, such as the maximum number of DNS lookups allowed.
//
// The function returns a Result, which corresponds with the SPF result for
// the check as per RFC, as well as an error for debugging purposes. Note that
// the error may be non-nil even on successful checks.
//
// Reference: https://tools.ietf.org/html/rfc7208#section-4
func CheckHostWithSender(ip net.IP, helo, sender string, opts ...Option) (Result, error) {
	_, domain := split(sender)
	if domain == "" {
		domain = helo
	}

	r := &resolution{
		ip:           ip,
		maxcount:     defaultMaxLookups,
		maxvoidcount: defaultMaxVoidLookups,
		helo:         helo,
		sender:       sender,
		ctx:          context.TODO(),
		resolver:     defaultResolver,
		trace:        defaultTrace,
	}

	for _, opt := range opts {
		opt(r)
	}

	return r.Check(domain)
}

// OverrideLookupLimit overrides the maximum number of DNS lookups allowed
// during SPF evaluation. Note that using this violates the RFC, which is
// quite explicit that the maximum allowed MUST be 10 (the default). Please
// use with care.
//
// This is EXPERIMENTAL for now, and the API is subject to change.
func OverrideLookupLimit(limit uint) Option {
	return func(r *resolution) {
		r.maxcount = limit
	}
}

// OverrideVoidLookupLimit overrides the maximum number of void DNS lookups allowed
// during SPF evaluation. A void DNS lookup is one that returns an empty
// answer, or a NXDOMAIN.  Note that as per RFC, the default value of 2 SHOULD
// be used. Please use with care.
//
// This is EXPERIMENTAL for now, and the API is subject to change.
func OverrideVoidLookupLimit(limit uint) Option {
	return func(r *resolution) {
		r.maxvoidcount = limit
	}
}

// WithContext is an option to set the context for this operation, which will
// be passed along to the resolver functions and other external calls if
// needed.
//
// This is EXPERIMENTAL for now, and the API is subject to change.
func WithContext(ctx context.Context) Option {
	return func(r *resolution) {
		r.ctx = ctx
	}
}

// DNSResolver implements the methods we use to resolve DNS queries.
// It is intentionally compatible with *net.Resolver.
type DNSResolver interface {
	LookupTXT(ctx context.Context, name string) ([]string, error)
	LookupMX(ctx context.Context, name string) ([]*net.MX, error)
	LookupIPAddr(ctx context.Context, host string) ([]net.IPAddr, error)
	LookupAddr(ctx context.Context, addr string) (names []string, err error)
}

var defaultResolver DNSResolver = net.DefaultResolver

// WithResolver sets the resolver to use for DNS lookups. It can be useful for
// testing, and for customize DNS resolution specifically for this library.
//
// The default is to use net.DefaultResolver, which should be appropriate for
// most users.
//
// This is EXPERIMENTAL for now, and the API is subject to change.
func WithResolver(resolver DNSResolver) Option {
	return func(r *resolution) {
		r.resolver = resolver
	}
}

// WithTraceFunc sets the resolver's trace function.
//
// This can be used for debugging. The trace messages are NOT machine
// parseable, and are NOT stable. They should also NOT be included in
// user-visible output, as they may include sensitive details.
//
// This is EXPERIMENTAL for now, and the API is subject to change.
func WithTraceFunc(trace TraceFunc) Option {
	return func(r *resolution) {
		r.trace = trace
	}
}

// split an user@domain address into user and domain.
func split(addr string) (string, string) {
	ps := strings.SplitN(addr, "@", 2)
	if len(ps) != 2 {
		return addr, ""
	}

	return ps[0], ps[1]
}

type resolution struct {
	ip           net.IP
	count        uint
	maxcount     uint
	voidcount    uint
	maxvoidcount uint

	helo   string
	sender string

	// Result of doing a reverse lookup for ip (so we only do it once).
	ipNames []string

	// Context for this resolution.
	ctx context.Context

	// DNS resolver to use.
	resolver DNSResolver

	// Trace function, used for debugging.
	trace TraceFunc
}

var aField = regexp.MustCompile(`^(a$|a:|a/)`)
var mxField = regexp.MustCompile(`^(mx$|mx:|mx/)`)
var ptrField = regexp.MustCompile(`^(ptr$|ptr:)`)

func (r *resolution) Check(domain string) (Result, error) {
	r.trace("check %q %d %d", domain, r.count, r.voidcount)
	txt, err := r.getDNSRecord(domain)
	if err != nil {
		if isNotFound(err) {
			// NXDOMAIN -> None.
			// https://datatracker.ietf.org/doc/html/rfc7208#section-4.3
			r.trace("dns domain not found: %v", err)
			return None, ErrNoResult
		}
		if isTemporary(err) {
			r.trace("dns temp error: %v", err)
			return TempError, err
		}
		if err == ErrMultipleRecords {
			r.trace("multiple dns records")
			return PermError, err
		}
		// Got another, permanent error.
		// https://datatracker.ietf.org/doc/html/rfc7208#section-2.6.7
		r.trace("dns perm error: %v", err)
		return PermError, err
	}
	r.trace("dns record %q", txt)

	if txt == "" {
		// No record => None.
		// https://tools.ietf.org/html/rfc7208#section-4.5
		return None, ErrNoResult
	}

	fields := strings.Split(txt, " ")

	// Redirects must be handled after the rest; instead of having two loops,
	// we just move them to the end.
	var newfields, redirects []string
	for _, field := range fields {
		if strings.HasPrefix(field, "redirect=") {
			redirects = append(redirects, field)
		} else {
			newfields = append(newfields, field)
		}
	}
	if len(redirects) > 1 {
		// At most a single redirect is allowed.
		// https://tools.ietf.org/html/rfc7208#section-6
		r.trace("too many redirects")
		return PermError, ErrInvalidDomain
	}
	fields = append(newfields, redirects...)

	for _, field := range fields {
		if field == "" {
			continue
		}

		// The version check should be case-insensitive (it's a
		// case-insensitive constant in the standard).
		// https://tools.ietf.org/html/rfc7208#section-12
		if strings.HasPrefix(field, "v=") || strings.HasPrefix(field, "V=") {
			continue
		}

		// Limit the number of resolutions.
		// https://tools.ietf.org/html/rfc7208#section-4.6.4
		if r.count > r.maxcount {
			r.trace("lookup limit reached")
			return PermError, ErrLookupLimitReached
		}

		if r.voidcount > r.maxvoidcount {
			r.trace("void lookup limit reached")
			return PermError, ErrVoidLookupLimitReached
		}

		// See if we have a qualifier, defaulting to + (pass).
		// https://tools.ietf.org/html/rfc7208#section-4.6.2
		result, ok := qualToResult[field[0]]
		if ok {
			field = field[1:]
		} else {
			result = Pass
		}

		// Mechanism and modifier names are case-insensitive.
		// https://tools.ietf.org/html/rfc7208#section-4.6.1
		lfield := strings.ToLower(field)

		if lfield == "all" {
			// https://tools.ietf.org/html/rfc7208#section-5.1
			r.trace("all: %v", result)
			return result, ErrMatchedAll
		} else if strings.HasPrefix(lfield, "include:") {
			if ok, res, err := r.includeField(result, field, domain); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if aField.MatchString(lfield) {
			if ok, res, err := r.aField(result, field, domain); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if mxField.MatchString(lfield) {
			if ok, res, err := r.mxField(result, field, domain); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if strings.HasPrefix(lfield, "ip4:") || strings.HasPrefix(lfield, "ip6:") {
			if ok, res, err := r.ipField(result, field); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if ptrField.MatchString(lfield) {
			if ok, res, err := r.ptrField(result, field, domain); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if strings.HasPrefix(lfield, "exists:") {
			if ok, res, err := r.existsField(result, field, domain); ok {
				r.trace("%q %v, %v", field, res, err)
				return res, err
			}
		} else if strings.HasPrefix(lfield, "exp=") {
			r.trace("exp= ignored")
			continue
		} else if strings.HasPrefix(lfield, "redirect=") {
			res, err := r.redirectField(field, domain)
			r.trace("%q: %v, %v", field, res, err)
			return res, err
		} else {
			r.trace("unknown field, permerror")
			return PermError, ErrUnknownField
		}
	}

	// Got to the end of the evaluation without a result => Neutral.
	// https://tools.ietf.org/html/rfc7208#section-4.7
	r.trace("fallback to neutral")
	return Neutral, nil
}

// getDNSRecord gets TXT records from the given domain, and returns the SPF
// (if any).  Note that at most one SPF is allowed per a given domain:
// https://tools.ietf.org/html/rfc7208#section-3
// https://tools.ietf.org/html/rfc7208#section-3.2
// https://tools.ietf.org/html/rfc7208#section-4.5
func (r *resolution) getDNSRecord(domain string) (string, error) {
	txts, err := r.resolver.LookupTXT(r.ctx, domain)
	if err != nil {
		return "", err
	}

	records := []string{}
	for _, txt := range txts {
		// The version check should be case-insensitive (it's a
		// case-insensitive constant in the standard).
		// https://tools.ietf.org/html/rfc7208#section-12
		if strings.HasPrefix(strings.ToLower(txt), "v=spf1 ") {
			records = append(records, txt)
		}

		// An empty record is explicitly allowed:
		// https://tools.ietf.org/html/rfc7208#section-4.5
		if strings.ToLower(txt) == "v=spf1" {
			records = append(records, txt)
		}
	}

	// 0 records is ok, handled by the parent.
	// 1 record is what we expect, return the record.
	// More than that, it's a permanent error:
	// https://tools.ietf.org/html/rfc7208#section-4.5
	l := len(records)
	if l == 0 {
		return "", nil
	} else if l == 1 {
		return records[0], nil
	}
	return "", ErrMultipleRecords
}

func isTemporary(err error) bool {
	derr, ok := err.(*net.DNSError)
	return ok && derr.Temporary()
}

func isNotFound(err error) bool {
	derr, ok := err.(*net.DNSError)
	return ok && derr.IsNotFound
}

// Check if the given DNS error is a "void lookup" (0 answers, or nxdomain),
// and if so increment the void lookup counter.
func (r *resolution) checkVoidLookup(nanswers int, err error) {
	if err == nil && nanswers == 0 {
		r.voidcount++
		r.trace("void lookup: no answers")
		return
	}

	derr, ok := err.(*net.DNSError)
	if !ok {
		return
	}

	if derr.IsNotFound {
		r.voidcount++
		r.trace("void lookup: nxdomain")
	}
}

// ipField processes an "ip" field.
func (r *resolution) ipField(res Result, field string) (bool, Result, error) {
	fip := field[4:]
	if strings.Contains(fip, "/") {
		_, ipnet, err := net.ParseCIDR(fip)
		if err != nil {
			return true, PermError, ErrInvalidMask
		}
		if ipnet.Contains(r.ip) {
			r.trace("ip match: %v contains %v", ipnet, r.ip)
			return true, res, ErrMatchedIP
		}
	} else {
		ip := net.ParseIP(fip)
		if ip == nil {
			return true, PermError, ErrInvalidIP
		}
		if ip.Equal(r.ip) {
			r.trace("ip match: %v", ip)
			return true, res, ErrMatchedIP
		}
	}

	return false, "", nil
}

// ptrField processes a "ptr" field.
func (r *resolution) ptrField(res Result, field, domain string) (bool, Result, error) {
	// Extract the domain if the field is in the form "ptr:domain".
	ptrDomain := domain
	if len(field) >= 4 {
		ptrDomain = field[4:]

	}
	ptrDomain, err := r.expandMacros(ptrDomain, domain)
	if err != nil {
		return true, PermError, ErrInvalidMacro
	}

	if ptrDomain == "" {
		return true, PermError, ErrInvalidDomain
	}

	if r.ipNames == nil {
		r.ipNames = []string{}
		r.count++
		ns, err := r.resolver.LookupAddr(r.ctx, r.ip.String())
		r.checkVoidLookup(len(ns), err)
		if err != nil {
			// https://tools.ietf.org/html/rfc7208#section-5
			if isNotFound(err) {
				return false, "", err
			}
			return true, TempError, err
		}

		// Only take the first 10 names, ignore the rest.
		// Each A/AAAA lookup in this context is NOT included in the overall
		// count. The RFC defines this separate logic and limits.
		// https://datatracker.ietf.org/doc/html/rfc7208#section-4.6.4
		if len(ns) > 10 {
			r.trace("ptr names trimmed %d down to 10", len(ns))
			ns = ns[:10]
		}

		for _, n := range ns {
			// Validate the record by doing a forward resolution: it has to
			// have some A/AAAA.
			addrs, err := r.resolver.LookupIPAddr(r.ctx, n)
			if err != nil {
				// RFC explicitly says to skip domains which error here.
				continue
			}
			r.trace("ptr forward resolution %q -> %q", n, addrs)
			if len(addrs) > 0 {
				// Append the lower-case variants so we do a case-insensitive
				// lookup below.
				r.ipNames = append(r.ipNames, strings.ToLower(n))
			}
		}
	}

	r.trace("ptr evaluating %q in %q", ptrDomain, r.ipNames)
	ptrDomain = strings.ToLower(ptrDomain)
	for _, n := range r.ipNames {
		if strings.HasSuffix(n, ptrDomain+".") {
			r.trace("ptr match: %q", n)
			return true, res, ErrMatchedPTR
		}
	}

	return false, "", nil
}

// existsField processes a "exists" field.
// https://tools.ietf.org/html/rfc7208#section-5.7
func (r *resolution) existsField(res Result, field, domain string) (bool, Result, error) {
	// The field is in the form "exists:<domain>".
	eDomain := field[7:]
	eDomain, err := r.expandMacros(eDomain, domain)
	if err != nil {
		return true, PermError, ErrInvalidMacro
	}

	if eDomain == "" {
		return true, PermError, ErrInvalidDomain
	}

	r.count++
	ips, err := r.resolver.LookupIPAddr(r.ctx, eDomain)
	r.checkVoidLookup(len(ips), err)
	if err != nil {
		// https://tools.ietf.org/html/rfc7208#section-5
		if isNotFound(err) {
			return false, "", err
		}
		return true, TempError, err
	}

	// Exists only counts if there are IPv4 matches.
	for _, ip := range ips {
		if ip.IP.To4() != nil {
			r.trace("exists match: %v", ip.IP)
			return true, res, ErrMatchedExists
		}
	}
	return false, "", nil
}

// includeField processes an "include" field.
func (r *resolution) includeField(res Result, field, domain string) (bool, Result, error) {
	// https://tools.ietf.org/html/rfc7208#section-5.2
	incdomain := field[len("include:"):]
	incdomain, err := r.expandMacros(incdomain, domain)
	if err != nil {
		return true, PermError, ErrInvalidMacro
	}
	r.count++
	ir, err := r.Check(incdomain)
	switch ir {
	case Pass:
		return true, res, err
	case Fail, SoftFail, Neutral:
		return false, ir, err
	case TempError:
		return true, TempError, err
	case PermError:
		return true, PermError, err
	case None:
		return true, PermError, err
	}

	return false, "", fmt.Errorf("this should never be reached")
}

type dualMasks struct {
	v4 net.IPMask
	v6 net.IPMask
}

func maskToStr(m net.IPMask) string {
	ones, bits := m.Size()
	if ones == 0 && bits == 0 {
		return m.String()
	}
	return fmt.Sprintf("/%d", ones)
}

func (m dualMasks) String() string {
	return fmt.Sprintf("[%v, %v]", maskToStr(m.v4), maskToStr(m.v6))
}

func ipMatch(ip, tomatch net.IP, masks dualMasks) bool {
	mask := net.IPMask(nil)
	if tomatch.To4() != nil && masks.v4 != nil {
		mask = masks.v4
	} else if tomatch.To4() == nil && masks.v6 != nil {
		mask = masks.v6
	}

	if mask != nil {
		ipnet := net.IPNet{IP: tomatch, Mask: mask}
		return ipnet.Contains(ip)
	}

	return ip.Equal(tomatch)
}

var aRegexp = regexp.MustCompile(`^[aA](:([^/]+))?(/(\w+))?(//(\w+))?$`)
var mxRegexp = regexp.MustCompile(`^[mM][xX](:([^/]+))?(/(\w+))?(//(\w+))?$`)

func domainAndMask(re *regexp.Regexp, field, domain string) (string, dualMasks, error) {
	masks := dualMasks{}
	groups := re.FindStringSubmatch(field)
	if groups != nil {
		if groups[2] != "" {
			domain = groups[2]
		}
		if groups[4] != "" {
			i, err := strconv.Atoi(groups[4])
			mask4 := net.CIDRMask(i, 32)
			if err != nil || mask4 == nil {
				return "", masks, ErrInvalidMask
			}
			masks.v4 = mask4
		}
		if groups[6] != "" {
			i, err := strconv.Atoi(groups[6])
			mask6 := net.CIDRMask(i, 128)
			if err != nil || mask6 == nil {
				return "", masks, ErrInvalidMask
			}
			masks.v6 = mask6
		}
	}

	// Test to catch malformed entries: if there's a /, there must be at least
	// one mask.
	if strings.Contains(field, "/") && masks.v4 == nil && masks.v6 == nil {
		return "", masks, ErrInvalidMask
	}

	return domain, masks, nil
}

// aField processes an "a" field.
func (r *resolution) aField(res Result, field, domain string) (bool, Result, error) {
	// https://tools.ietf.org/html/rfc7208#section-5.3
	aDomain, masks, err := domainAndMask(aRegexp, field, domain)
	r.trace("masks on %q, %q: %q %v", field, domain, aDomain, masks)
	if err != nil {
		return true, PermError, err
	}
	aDomain, err = r.expandMacros(aDomain, domain)
	if err != nil {
		return true, PermError, ErrInvalidMacro
	}

	r.count++
	ips, err := r.resolver.LookupIPAddr(r.ctx, aDomain)
	r.checkVoidLookup(len(ips), err)
	if err != nil {
		// https://tools.ietf.org/html/rfc7208#section-5
		if isNotFound(err) {
			return false, "", err
		}
		return true, TempError, err
	}
	for _, ip := range ips {
		if ipMatch(r.ip, ip.IP, masks) {
			r.trace("a match: %v, %v, %v", r.ip, ip.IP, masks)
			return true, res, ErrMatchedA
		}
	}

	return false, "", nil
}

// mxField processes an "mx" field.
func (r *resolution) mxField(res Result, field, domain string) (bool, Result, error) {
	// https://tools.ietf.org/html/rfc7208#section-5.4
	mxDomain, masks, err := domainAndMask(mxRegexp, field, domain)
	r.trace("masks on %q, %q: %q %v", field, domain, mxDomain, masks)
	if err != nil {
		return true, PermError, err
	}
	mxDomain, err = r.expandMacros(mxDomain, domain)
	if err != nil {
		return true, PermError, ErrInvalidMacro
	}

	r.count++
	mxs, err := r.resolver.LookupMX(r.ctx, mxDomain)
	r.checkVoidLookup(len(mxs), err)

	// If we get some results, use them even if we get an error alongisde.
	// This happens when one of the records is invalid, because Go library can
	// be quite strict about it. The RFC is not clear about this specific
	// situation, and other SPF libraries and implementations just skip the
	// invalid value, so we match common practice.
	if err != nil && len(mxs) == 0 {
		// https://tools.ietf.org/html/rfc7208#section-5
		if isNotFound(err) {
			return false, "", err
		}
		return true, TempError, err
	}

	// There's an explicit maximum of 10 MX records per match.
	// https://tools.ietf.org/html/rfc7208#section-4.6.4
	if len(mxs) > 10 {
		return true, PermError, ErrTooManyMXRecords
	}

	mxips := []net.IP{}
	for _, mx := range mxs {
		ips, err := r.resolver.LookupIPAddr(r.ctx, mx.Host)
		if err != nil {
			// If the address of the MX record was not found, we just skip it.
			// https://tools.ietf.org/html/rfc7208#section-5
			if isNotFound(err) {
				continue
			}
			return true, TempError, err
		}
		for _, ipaddr := range ips {
			mxips = append(mxips, ipaddr.IP)
		}
	}

	r.trace("mx ips: %v", mxips)
	for _, ip := range mxips {
		if ipMatch(r.ip, ip, masks) {
			r.trace("mx match: %v, %v, %v", r.ip, ip, masks)
			return true, res, ErrMatchedMX
		}
	}

	return false, "", nil
}

// redirectField processes a "redirect=" field.
func (r *resolution) redirectField(field, domain string) (Result, error) {
	rDomain := field[len("redirect="):]
	rDomain, err := r.expandMacros(rDomain, domain)
	if err != nil {
		return PermError, ErrInvalidMacro
	}

	if rDomain == "" {
		return PermError, ErrInvalidDomain
	}

	// https://tools.ietf.org/html/rfc7208#section-6.1
	r.count++
	result, err := r.Check(rDomain)
	if result == None {
		result = PermError
	}
	return result, err
}

// Group extraction of macro-string from the formal specification.
// https://tools.ietf.org/html/rfc7208#section-7.1
var macroRegexp = regexp.MustCompile(
	`([slodiphcrtvSLODIPHCRTV])([0-9]+)?([rR])?([-.+,/_=]+)?`)

// Expand macros, return the expanded string.
// This expects to be passed the domain-spec within a field, not an entire
// field or larger (that has problematic security implications).
// https://tools.ietf.org/html/rfc7208#section-7
func (r *resolution) expandMacros(s, domain string) (string, error) {
	// Macros/domains shouldn't contain CIDR. Our parsing should prevent it
	// from happening in case where it matters (a, mx), but for the ones which
	// doesn't, prevent them from sneaking through.
	if strings.Contains(s, "/") {
		r.trace("macro contains /")
		return "", ErrInvalidDomain
	}

	// Bypass the complex logic if there are no macros present.
	if !strings.Contains(s, "%") {
		return s, nil
	}

	// Are we processing the character right after "%"?
	afterPercent := false

	// Are we inside a macro definition (%{...}) ?
	inMacroDefinition := false

	// Macro string, where we accumulate the values inside the definition.
	macroS := ""

	var err error
	n := ""
	for _, c := range s {
		if afterPercent {
			afterPercent = false
			switch c {
			case '%':
				n += "%"
				continue
			case '_':
				n += " "
				continue
			case '-':
				n += "%20"
				continue
			case '{':
				inMacroDefinition = true
				continue
			}
			return "", ErrInvalidMacro
		}
		if inMacroDefinition {
			if c != '}' {
				macroS += string(c)
				continue
			}
			inMacroDefinition = false

			// Extract letter, digit transformer, reverse transformer, and
			// delimiters.
			groups := macroRegexp.FindStringSubmatch(macroS)
			r.trace("macro %q: %q", macroS, groups)
			macroS = ""
			if groups == nil {
				return "", ErrInvalidMacro
			}
			letter := groups[1]

			digits := 0
			if groups[2] != "" {
				// Use 0 as "no digits given"; an explicit value of 0 is not
				// valid.
				digits, err = strconv.Atoi(groups[2])
				if err != nil || digits <= 0 {
					return "", ErrInvalidMacro
				}
			}
			reverse := groups[3] == "r" || groups[3] == "R"
			delimiters := groups[4]
			if delimiters == "" {
				// By default, split strings by ".".
				delimiters = "."
			}

			// Uppercase letters indicate URL escaping of the results.
			urlEscape := letter == strings.ToUpper(letter)
			letter = strings.ToLower(letter)

			str := ""
			switch letter {
			case "s":
				str = r.sender
			case "l":
				str, _ = split(r.sender)
			case "o":
				_, str = split(r.sender)
			case "d":
				str = domain
			case "i":
				str = ipToMacroStr(r.ip)
			case "p":
				// This shouldn't be used, we don't want to support it, it's
				// risky. "unknown" is a safe value.
				// https://tools.ietf.org/html/rfc7208#section-7.3
				str = "unknown"
			case "v":
				if r.ip.To4() != nil {
					str = "in-addr"
				} else {
					str = "ip6"
				}
			case "h":
				str = r.helo
			default:
				// c, r, t are allowed in exp only, and we don't expand macros
				// in exp so they are just as invalid as the rest.
				return "", ErrInvalidMacro
			}

			// Split str using the given separators.
			splitFunc := func(r rune) bool {
				return strings.ContainsRune(delimiters, r)
			}
			split := strings.FieldsFunc(str, splitFunc)

			// Reverse if requested.
			if reverse {
				reverseStrings(split)
			}

			// Leave the last $digits fields, if given.
			if digits > 0 {
				if digits > len(split) {
					digits = len(split)
				}
				split = split[len(split)-digits:]
			}

			// Join back, always with "."
			str = strings.Join(split, ".")

			// Escape if requested. Note this doesn't strictly escape ALL
			// unreserved characters, it's the closest we can get without
			// reimplmenting it ourselves.
			if urlEscape {
				str = url.QueryEscape(str)
			}

			n += str
			continue
		}
		if c == '%' {
			afterPercent = true
			continue
		}
		n += string(c)
	}

	r.trace("macro expanded %q to %q", s, n)
	return n, nil
}

func reverseStrings(a []string) {
	for left, right := 0, len(a)-1; left < right; left, right = left+1, right-1 {
		a[left], a[right] = a[right], a[left]
	}
}

func ipToMacroStr(ip net.IP) string {
	if ip.To4() != nil {
		return ip.String()
	}

	// For IPv6 addresses, the "i" macro expands to a dot-format address.
	// https://datatracker.ietf.org/doc/html/rfc7208#section-7.3
	sb := strings.Builder{}
	sb.Grow(64)
	for _, b := range ip.To16() {
		fmt.Fprintf(&sb, "%x.%x.", b>>4, b&0xf)
	}
	// Return the string without the trailing ".".
	return sb.String()[:sb.Len()-1]
}