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domain-spec      = macro-string domain-end
domain-end       = ( "." toplabel [ "." ] ) / macro-expand
toplabel         = ( *alphanum ALPHA *alphanum ) /
                   ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
                   ; LDH rule plus additional TLD restrictions
                   ; (see [RFC3696], Section 2)
alphanum         = ALPHA / DIGIT
explain-string   = *( macro-string / SP )
macro-string     = *( macro-expand / macro-literal )
macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                   / "%%" / "%_" / "%-"
macro-literal    = %x21-24 / %x26-7E
                   ; visible characters except "%"
macro-letter     = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                   "c" / "r" / "t"
transformers     = *DIGIT [ "r" ]
delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
                   [ key-value-list ] CRLF
result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
                   "None" / "TempError" / "PermError"
key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
                   [";"]
key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
key              = "client-ip" / "envelope-from" / "helo" /
                   "problem" / "receiver" / "identity" /
                    mechanism / "x-" name / name
identity         = "mailfrom"   ; for the "MAIL FROM" identity
                   / "helo"     ; for the "HELO" identity
                   / name       ; other identities
dot-atom         = <unquoted word as per [RFC2822]>
quoted-string    = <quoted string as per [RFC2822]>
comment          = <comment string as per [RFC2822]>
CFWS             = <comment or folding white space as per [RFC2822]>FWS
                 = <folding white space as per [RFC2822]>CRLF
                 = <standard end-of-line token as per [RFC2822]>

record           = version terms *SP
version          = "v=spf1"
terms            = *( 1*SP ( directive / modifier ) )
directive        = [ qualifier ] mechanism
qualifier        = "+" / "-" / "?" / "~"
mechanism        = ( all / include
                   / A / MX / PTR / IP4 / IP6 / exists )
all              = "all"
include          = "include"  ":" domain-spec
A                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
MX               = "mx"     [ ":" domain-spec ] [ dual-cidr-length ]
PTR              = "ptr"    [ ":" domain-spec ]
IP4              = "ip4"      ":" ip4-network   [ ip4-cidr-length ]
IP6              = "ip6"      ":" ip6-network   [ ip6-cidr-length ]
exists           = "exists"   ":" domain-spec
modifier         = redirect / explanation / unknown-modifier
redirect         = "redirect" "=" domain-spec
explanation      = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
ip4-cidr-length  = "/" 1*DIGIT
ip6-cidr-length  = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network      = qnum "." qnum "." qnum "." qnum
qnum             = DIGIT                 ; 0-9
                   / %x31-39 DIGIT       ; 10-99
                   / "1" 2DIGIT          ; 100-199
                   / "2" %x30-34 DIGIT   ; 200-249
                   / "25" %x30-35        ; 250-255
          ; 伝統的なドット区切り4分節表記 例 192.0.2.0ip6-network
      　　　　　　= <as per [RFC 3513], section 2.2>
          ; 例 2001:DB8::CD30
domain-spec      = macro-string domain-end
domain-end       = ( "." toplabel [ "." ] ) / macro-expand
toplabel         = ( *alphanum ALPHA *alphanum ) /
                   ( 1*alphanum "-" *( alphanum / "-" ) alphanum )
                   ; TLD制限を追加したLDH規則
                   ; （［RFC3696］の2章を参照のこと）
alphanum         = ALPHA / DIGIT
explain-string   = *( macro-string / SP )
macro-string     = *( macro-expand / macro-literal )
macro-expand     = ( "%{" macro-letter transformers *delimiter "}" )
                   / "%%" / "%_" / "%-"
macro-literal    = %x21-24 / %x26-7E
                   ; “%”以外の可視文字macro-letter
　　　　　　　　　= "s" / "l" / "o" / "d" / "i" / "p" / "h" /
                   "c" / "r" / "t"
transformers     = *DIGIT [ "r" ]
delimiter        = "." / "-" / "+" / "," / "/" / "_" / "="
name             = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header-field     = "Received-SPF:" [CFWS] result FWS [comment FWS]
                   [ key-value-list ] CRLF

result           = "Pass" / "Fail" / "SoftFail" / "Neutral" /
                   "None" / "TempError" / "PermError"
key-value-list   = key-value-pair *( ";" [CFWS] key-value-pair )
                   [";"]
key-value-pair   = key [CFWS] "=" ( dot-atom / quoted-string )
key              = "client-ip" / "envelope-from" / "helo" /
                   "problem" / "receiver" / "identity" /
                    mechanism / "x-" name / name
identity         = "mailfrom"   ; "MAIL FROM"アイデンティティ用
                   / "helo"     ; "HELO"アイデンティティ用
                   / name       ; その他のアイデンティティ
dot-atom         = <unquoted word as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様の非引用語）
quoted-string    = <quoted string as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様の引用文字列）
comment          = <comment string as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様のコメント文字列）
CFWS             = <comment or folding white space as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様のコメントまたは多重空白）
FWS              = <folding white space as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様の多重空白）
CRLF             = <standard end-of-line token as per [RFC2822]>　
　　　　　　　　　　（［RFC2822］仕様の標準文末トークン）

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