http://spf.pobox.com

 proc spf {domain} {
    set tok [dns::resolve $domain -type SPF] 
    dns::wait $tok
    if {[dns::status $tok] eq "ok"} {
        eval [linsert [dns::result $tok] 0 array set d]
        set result [string range $d(rdata) 1 end] ;# first byte is the string length.
        set code   ok
    } else {
        set result [dns::error $tok]
        set code   error
    }
    dns::cleanup $tok
    return -code $code $result
 }

 package require spf
 dns::configure -nameserver <nameserver>
 spf::spf 192.168.0.4 aol.com    --> ?
 spf::spf www.ox.ac.uk ox.ac.uk  --> +

 # spf.tcl - Copyright (C) 2004 Pat Thoyts <[email protected]>
 #
 #                         Sender Policy Framework
 #
 #    http://spf.pobox.com/
 #    http://www.ietf.org/internet-drafts/draft-mengwong-spf-01.txt
 #
 # Domains using SPF:
 #   pobox.org      - mx, a, ptr
 #   oxford.ac.uk   - include
 #   gnu.org        - ip4
 #   aol.com        - ip4, ptr
 #   altavista.com  - exists,  multiple TXT replies.
 #   oreilly.com    - mx, ptr, include (invalid domain)
 #   motleyfool.com - include (looping includes)
 #
 # -------------------------------------------------------------------------
 # See the file "license.terms" for information on usage and redistribution
 # of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 # -------------------------------------------------------------------------
 #
 # $Id: 11495,v 1.9 2004-06-08 06:01:00 jcw Exp $
 
 package require Tcl 8.2;                # tcl minimum version
 package require dns;                    # tcllib 1.3
 package require logger;                 # tcllib 1.3
 
 namespace eval spf {
     variable version 1.0.0
     variable rcsid {$Id: 11495,v 1.9 2004-06-08 06:01:00 jcw Exp $}
 
     namespace export spf
 
     variable log
     if {![info exists log]} { 
         set log [logger::init spf]
         ${log}::setlevel warn
         proc ${log}::stdoutcmd {level text} {
             variable service
             puts "\[[clock format [clock seconds] -format {%H:%M:%S}]\
                 $service $level\] $text"
         }
     }
 }
 
 # -------------------------------------------------------------------------
 # sender : ip address of sender
 # target : domain name to test against.
 #
 proc ::spf::spf {sender target} {
     variable log
 
     if {[catch {SPF $target} spf]} {
         ${log}::debug "error fetching SPF record: $spf"
         return none
     }
     
     return [Spf $sender $target $spf]
 }
 
 proc ::spf::Spf {sender target spf} {
     variable log
     if {![regexp {^v=spf(\d)\s+} $spf -> version]} {
         return none
     }
 
     ${log}::debug "$spf"
 
     if {$version != 1} {
         return -code error "version mismatch: we only understand SPF 1\
             this domain has provided version \"$version\""
     }
 
     if {![is_ip4_addr $sender]} {
         set ips [A $sender]
         set sender [lindex $ips 0]
     }
     
     set result ?
     set seen_domains $target
     set explanation {denied}
 
     set directives [lrange [split $spf { }] 1 end]
     foreach directive $directives {
         set prefix [string range $directive 0 0]
         if {$prefix eq "+" || $prefix eq "-" 
             || $prefix eq "?" || $prefix eq "~"} {
             set directive [string range $directive 1 end]
         } else {
             set prefix "+"
         }
 
         set cmd [string tolower [lindex [split $directive {:/=}] 0]]
         set param [string range $directive [string length $cmd] end]
 
         if {[info command ::spf::_$cmd] == {}} {
             # 6.1 Unrecognised directives terminate processing
             #     but unknown modifiers are ignored.
             if {[string match "=*" $param]} {
                 continue
             } else {
                 set result unknown
                 break
             }
         } else {
             if {[catch {::spf::_$cmd $sender $target $param} res]} {
                 if {$res eq "none" || $res eq "error" || $res eq "unknown"} {
                     return $res
                 }
                 return -code error "error in \"$cmd\": $res"
             }
             if {$res} { set result $prefix }
         }
         
         ${log}::debug "$prefix $cmd\($param) -> $result"
         if {$result eq "+"} break
     }
     
     return $result
 }
 
 proc ::spf::loglevel {level} {
     variable log
     ${log}::setlevel $level
 }
 
 # get a guaranteed unique and non-present token id.
 proc ::spf::create_token {} {
     variable uid
     set id [incr uid]
     while {[info exists [set token [namespace current]::$id]]} {
         set id [incr uid]
     }
     return $token
 }
 
 # -------------------------------------------------------------------------
 #
 #                      SPF MECHANISM HANDLERS
 #
 # -------------------------------------------------------------------------
 
 # 4.1: The "all" mechanism is a test that always matches.  It is used as the
 #      rightmost mechanism in an SPF record to provide an explicit default
 #
 proc ::spf::_all {sender target param} {
     return 1
 }
 
 # 4.2: The "include" mechanism triggers a recursive SPF query.
 #
 proc ::spf::_include {sender target param} {
     variable log
     upvar seen_domains Seen
 
     if {[string range $param 0 0] ne ":"} {
         return -code error "dubious parameters for \"include\""
     }
     set r ?
     set domain [string range $param 1 end]
     if {[lsearch $Seen $domain] == -1} {
         lappend Seen $domain
         if {[catch {set r [spf $sender $domain]}]} {
             return -code error error
         }
         if {$r eq "none" || $r eq "unknown"} {
             return -code error $r
         }
     }
     return [expr {$r eq "+"}]
 }
 
 # 4.4: This mechanism matches if the <sending-host> is one of the
 #      <target-name>'s IP addresses.
 #
 proc ::spf::_a {sender target param} {
     variable log
     foreach {domain bits} [splitspec [string trimleft $param :]] {}
     if {$domain == {}} {
         set domain $target
     }
     set dips [A $domain]
     set ip [ipmask $sender $bits]
     foreach dip $dips {
         ${log}::debug "  compare: ${sender}/${bits} with ${dip}/${bits}"
         set dp [ipmask $dip $bits]
         if {$ip == $dp} {
             return 1
         }
     }
     return 0
 }
 
 # 4.5: This mechanism matches if the <sending-host> is one of the MX hosts
 #      for a domain name.
 #
 proc ::spf::_mx {sender target param} {
     variable log
     foreach {domain bits} [splitspec [string trimleft $param :]] {}
     if {$domain eq ""} {
         set domain $target
     }
     ${log}::debug "  fetching MX for $domain"
     set mxs [MX $domain]
 
     set ip [ipmask $sender $bits]
     foreach mx $mxs {
         set mx [lindex $mx 1]
         set mxips [A $mx]
         foreach mxip $mxips {
             ${log}::debug "  compare: ${sender}/${bits} with ${mxip}/${bits}"
             set mp [ipmask $mxip $bits]
             if {$ip == $mp} {
                 return 1
             }
         }
     }
     return 0
 }
 
 # 4.6: This mechanism tests if the <sending-host>'s name is within a
 #      particular domain.
 #
 proc ::spf::_ptr {sender target param} {
     variable log
     set validnames {}
     if {[catch { set names [PTR $sender] } msg]} {
         ${log}::debug "  \"$sender\" $msg"
         return 0
     }
     foreach name $names {
         set ips [A $name]
         foreach ip $ips {
             if {$ip eq $sender} {
                 lappend validnames $name
                 continue
             }
         }
     }
 
     ${log}::debug "  validnames: $validnames"
     set domain [string trimleft $param :]
     if {$domain == {}} {
         set domain $target
     }
     foreach name $validnames {
         if {[string match "*$domain" $name]} {
             return 1
         }
     }
 
     return 0
 }
 
 # 4.7: These mechanisms test if the <sending-host> falls into a given IP
 #      network.
 #
 proc ::spf::_ip4 {sender target param} {
     variable log
     foreach {network bits} [splitspec [string range $param 1 end]] {}
     set net [ipmask $network $bits]
     set ipx [ipmask $sender $bits]
     ${log}::debug "  compare ${sender}/${bits} to ${network}/${bits}"
     if {$ipx == $net} {
         return 1
     }
     return 0
 }
 
 # 4.7: These mechanisms test if the <sending-host> falls into a given IP
 #      network.
 #
 proc ::spf::_ip6 {sender target param} {
     variable log
     ${log}::warn "ip6 address handling not implemented."
     return 0
 }
 
 # 4.8: This mechanism is used to construct an arbitrary host name that is
 #      used for a DNS A record query.  It allows for complicated schemes
 #      involving arbitrary parts of the mail envelope to determine what is
 #      legal.
 #
 proc ::spf::_exists {sender target param} {
     variable log
     set param [string range $param 1 end]
     ${log}::warn "exists mechanism handling not implemented."
     return 0
 }
 
 # 5.1: Redirected query
 #
 proc ::spf::_redirect {sender target param} {
     variable log
     set domain [string range $param 1 end]
     ${log}::debug ">> redirect to $domain"
     set r [spf $sender $target $domain]
     return -code return $r
 }
 
 # 5.2: Explanation
 #
 proc ::spf::_exp {sender target param} {
     variable log
     set domain [string range $param 1 end]
     set exp [TXT $domain]
     set exp [Expand $exp]
 }
 
 # 5.3: Sender accreditation
 #
 proc ::spf::_accredit {sender target param} {
     variable log
     ${log}::debug "  accredit modifier ignored"
     return 0
 }
 
 
 # 7: Macro expansion
 #
 proc ::spf::Expand {txt} {
     variable log
     ${log}::warn "macro expansion not implemented"
     return 0
 }
 
 # -------------------------------------------------------------------------
 #
 # DNS helper procedures.
 #
 # -------------------------------------------------------------------------
 
 proc ::spf::Resolve {domain type resultproc} {
     if {[info command $resultproc] == {}} {
         return -code error "invalid arg: \"$resultproc\" must be a command"
     }
     set tok [dns::resolve $domain -type $type]
     dns::wait $tok
     if {[dns::status $tok] eq "ok"} {
         set result [$resultproc $tok]
         set code   ok
     } else {
         set result [dns::error $tok]
         set code   error
     }
     dns::cleanup $tok
     return -code $code $result
 }
 
 proc ::spf::SPF {domain} {
     set r [Resolve $domain SPF ::dns::result]
     set txt ""
     foreach res $r {
         set ndx [lsearch $res rdata]
         incr ndx
         if {$ndx != 0} {
             append txt [string range [lindex $res $ndx] 0 end]
         }
     }
     return $txt
 }
 
 proc ::spf::TXT {domain} {
     set r [Resolve $domain TXT ::dns::result]
     set txt ""
     foreach res $r {
         set ndx [lsearch $res rdata]
         incr ndx
         if {$ndx != 0} {
             append txt [string range [lindex $res $ndx] 0 end]
         }
     }
     return $txt
 }
 
 proc ::spf::A {name} {
     return [Resolve $name A ::dns::address]
 }
 
 proc ::spf::PTR {addr} {
     return [Resolve $addr A ::dns::name]
 }
 
 proc ::spf::MX {domain} {
     set r [Resolve $domain MX ::dns::name]
     return [lsort -index 0 $r]
 }
 
 # -------------------------------------------------------------------------
 
 # FIX ME - Factor these helpers out into an IPv4 address module or something.
 
 proc ::spf::ip2x {ip {validate 0}} {
     set octets [split $ip .]
     if {[llength $octets] != 4} {
         set octets [lrange [concat $octets 0 0 0] 0 3]
     }
     if {$validate} {
         foreach oct $octets {
             if {$oct < 0 || $oct > 255} {
                 return -code error "invalid ip address"
             }
         }
     }
     binary scan [binary format c4 $octets] H8 x
     return 0x$x
 }
 
 proc ::spf::ipmask {ip {bits {}}} {
     if {[string length $bits] < 1} { set bits 32 }
     set ipx [ip2x $ip]
     if {[string is integer $bits]} {
         set mask [expr {(0xFFFFFFFF << (32 - $bits)) & 0xFFFFFFFF}]
     } else {
         set mask [ip2x $bits]
     }
     return [format 0x%08x [expr {$ipx & $mask}]]
 }
     
 proc ::spf::is_ip4_addr {ip} {
     if {[catch {ip2x $ip true}]} {
         return 0
     }
     return 1
 }
 
 proc ::spf::splitspec {spec} {
     set bits 32
     set domain $spec
     set slash [string last / $spec]
     if {$slash != -1} {
         incr slash -1
         set domain [string range $spec 0 $slash]
         incr slash 2
         set bits [string range $spec $slash end]
     }
     return [list $domain $bits]
 }
     
 # -------------------------------------------------------------------------
 
 package provide spf $::spf::version
 
 # -------------------------------------------------------------------------
 # Local Variables:
 #   indent-tabs-mode: nil
 # End:

 proc ::spf::_redirect {sender target param} {
     variable log
     set domain [string range $param 1 end]
     ${log}::debug ">> redirect to $domain"
     set r [spf $sender $domain]
     return [expr {$r eq "+"}]
 }

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