DNS packet header class
The Net::DNS::Header class represents the header portion of a DNS packet. An Header object is created whenever a new packet is parsed or as user request.
header = Net::DNS::Header.new # ;; id = 18123 # ;; qr = 0 opCode: 0 aa = 0 tc = 0 rd = 1 # ;; ra = 0 ad = 0 cd = 0 rcode = 0 # ;; qdCount = 1 anCount = 0 nsCount = 0 arCount = 0 header.format # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 18123 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # |0| 0 |0|0|1|0|0| 0 | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 1 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # packet is an instance of Net::DNS::Packet header = packet.header puts "Answer is #{header.auth? ? '' : 'non'} authoritative"
A lot of methods were written to keep a compatibility layer with the Perl version of the library, as long as methods name which are more or less the same.
Creates a new Net::DNS::Header object with the desired values, which can be specified as an Hash argument. When called without arguments, defaults are used. If a data string is passed, values are taken from parsing the string.
Examples:
# Create a new Net::DNS::Header object header = Net::DNS::Header.new # Create a new Net::DNS::Header object passing values header = Net::DNS::Header.new(:opCode => 1, :rd => 0) # Create a new Net::DNS::Header object with binary data header = Net::DNS::Header.new(data)
Default values are:
:id => auto generated :qr => 0 # Query response flag :aa => 0 # Authoritative answer flag :tc => 0 # Truncated packet flag :ra => 0 # Recursiond available flag :rCode => 0 # Response code (status of the query) :opCode => 0 # Operational code (purpose of the query) :cd => 0 # Checking disable flag :ad => 0 # Only relevant in DNSSEC context :rd => 1 # Recursion desired flag :qdCount => 1 # Number of questions in the dns packet :anCount => 0 # Number of answer RRs in the dns packet :nsCount => 0 # Number of authoritative RRs in the dns packet :arCount => 0 # Number of additional RRs in the dns packet
See also each option for a detailed explanation of usage.
# File lib/net/dns/header.rb, line 214 def initialize(arg = {}) if arg.kind_of? Hash new_from_hash(arg) else raise ArgumentError, "Wrong argument class `#{arg.class}'" end end
Creates a new Net::DNS::Header object from binary data, which is passed as a string object as argument. The configurations parameters are taken from parsing the string.
Example:
# Create a new Net::DNS::Header object with binary data header = Net::DNS::Header.new(data) header.auth? #=> "true" if it comes from authoritative name server
# File lib/net/dns/header.rb, line 234 def self.parse(arg) if arg.kind_of? String o = allocate o.send(:new_from_binary, arg) o else raise ArgumentError, "Wrong argument class `#{arg.class}'" end end
Set the aa flag (authoritative answer) to either true or false. You can also use 0 or 1.
This flag indicates whether a DNS answer packet contains authoritative data, meaning that is was generated by a nameserver authoritative for the domain of the question.
Must only be set to true in DNS answer packets.
# File lib/net/dns/header.rb, line 430 def aa=(val) case val when true @aa = 1 when false @aa = 0 when 0,1 @aa = val else raise ArgumentError, ":aa must be true(or 1) or false(or 0)" end end
Set the ad flag to either true ot false. You can also use 0 or 1.
The AD bit is only set on answers where signatures have been cryptographically verified or the server is authoritative for the data and is allowed to set the bit by policy.
# File lib/net/dns/header.rb, line 585 def ad=(val) case val when true @ad = 1 when false @ad = 0 when 0,1 @ad = val else raise ArgumentError, ":ad must be true(or 1) or false(or 0)" end end
Sets the number of RRs in an answer section
# File lib/net/dns/header.rb, line 639 def anCount=(val) if (0..65535).include? val @anCount = val else raise WrongCountError, "Wrong number of count (#{val}), must be 0-65535" end end
Sets the number of RRs in an addictional section
# File lib/net/dns/header.rb, line 659 def arCount=(val) if (0..65535).include? val @arCount = val else raise WrongCountError, "Wrong number of count: `#{val}' must be 0-65535" end end
Checks whether the response is authoritative
if header.auth? puts "Response is authoritative" else puts "Answer is NOT authoritative" end
# File lib/net/dns/header.rb, line 417 def auth? @aa == 1 end
Set the cd flag (checking disabled) to either true ot false. You can also use 0 or 1.
# File lib/net/dns/header.rb, line 557 def cd=(val) case val when true @cd = 1 when false @cd = 0 when 0,1 @cd = val else raise ArgumentError, ":cd must be true(or 1) or false(or 0)" end end
Checks whether checking is enabled or disabled.
Checking is enabled by default.
# File lib/net/dns/header.rb, line 550 def checking? @cd == 0 end
Returns the header data in binary format, appropriate for use in a DNS query packet.
hdata = header.data puts "Header is #{hdata.size} bytes"
# File lib/net/dns/header.rb, line 323 def data arr = [] arr.push(@id) arr.push((@qr<<7)|(@opCode<<3)|(@aa<<2)|(@tc<<1)|@rd) arr.push((@ra<<7)|(@ad<<5)|(@cd<<4)|@rCode.code) arr.push(@qdCount) arr.push(@anCount) arr.push(@nsCount) arr.push(@arCount) arr.pack("n C2 n4") end
Checks for errors in the DNS packet
unless header.error? puts "No errors in DNS answer packet" end
# File lib/net/dns/header.rb, line 616 def error? @rCode.code > 0 end
The Net::DNS::Header#format method prints out the header in a special ascii representation of data, in a way similar to those often found on RFCs.
p Net::DNS::Header.new.format # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 18123 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # |0| 0 |0|0|1|0|0| 0 | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 1 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ # | 0 | # +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This can be very usefull for didactical purpouses :)
# File lib/net/dns/header.rb, line 297 def format del = ("+-" * 16) + "+\n" len = del.length str = del + "|" + @id.to_s.center(len-3) + "|\n" str += del + "|" + @qr.to_s str += "|" + @opCode.to_s.center(7) str += "|" + @aa.to_s str += "|" + @tc.to_s str += "|" + @rd.to_s str += "|" + @ra.to_s str += "|" + @ad.to_s str += "|" + @cd.to_s.center(3) str += "|" + @rCode.to_s.center(7) + "|\n" str += del + "|" + @qdCount.to_s.center(len-3) + "|\n" str += del + "|" + @anCount.to_s.center(len-3) + "|\n" str += del + "|" + @nsCount.to_s.center(len-3) + "|\n" str += del + "|" + @arCount.to_s.center(len-3) + "|\n" + del str end
Set the ID for the current header. Useful when performing security tests.
# File lib/net/dns/header.rb, line 338 def id=(val) if (0..65535).include? val @id = val else raise ArgumentError, "ID `#{val}' out of range" end end
Inspect method, prints out all the options and relative values.
p Net::DNS::Header.new # ;; id = 18123 # ;; qr = 0 opCode: 0 aa = 0 tc = 0 rd = 1 # ;; ra = 0 ad = 0 cd = 0 rcode = 0 # ;; qdCount = 1 anCount = 0 nsCount = 0 arCount = 0
This method will maybe be changed in the future to a more pretty way of display output.
# File lib/net/dns/header.rb, line 255 def inspect ";; id = #@id\n" + if false # @opCode == "UPDATE" #do stuff else ";; qr = #@qr\t" + "opCode: #{opCode_str}\t" + "aa = #@aa\t" + "tc = #@tc\t" + "rd = #@rd\n" + ";; ra = #@ra\t" + "ad = #@ad\t" + "cd = #@cd\t" + "rcode = #{@rCode.type}\n" + ";; qdCount = #@qdCount\t"+ "anCount = #@anCount\t"+ "nsCount = #@nsCount\t"+ "arCount = #@arCount\n" end end
Sets the number of RRs in an authority section
# File lib/net/dns/header.rb, line 649 def nsCount=(val) if (0..65535).include? val @nsCount = val else raise WrongCountError, "Wrong number of count (#{val}), must be 0-65535" end end
Set the opCode variable to a new value. This fields indicates the type of the question present in the DNS packet; val can be one of the values QUERY, IQUERY or STATUS.
Example:
include Net::DNS header = Header.new header.opCode = Header::STATUS
# File lib/net/dns/header.rb, line 401 def opCode=(val) if (0..2).include? val @opCode = val else raise WrongOpcodeError, "Wrong opCode value (#{val}), must be QUERY, IQUERY or STATUS" end end
Returns a string representation of the opCode
puts "Packet is a #{header.opCode_str}" #=> Packet is a QUERY
# File lib/net/dns/header.rb, line 383 def opCode_str OPARR[@opCode] end
Sets the number of entries in a question section
# File lib/net/dns/header.rb, line 629 def qdCount=(val) if (0..65535).include? val @qdCount = val else raise WrongCountError, "Wrong number of count (#{val}), must be 0-65535" end end
Set the qr query response flag to be either true or false. You can also use the values 0 and 1. This flag indicates if the DNS packet contains a query or an answer, so it should be set to true in DNS answer packets. If qr is true, the packet is a response.
# File lib/net/dns/header.rb, line 358 def qr=(val) case val when true @qr = 1 when false @qr = 0 when 0,1 @qr = val else raise ArgumentError, ":qr must be true(or 1) or false(or 0)" end end
Checks whether the header is a query (qr bit set to 0)
# File lib/net/dns/header.rb, line 348 def query? @qr == 0 end
Set the rCode value. This should only be done in DNS answer packets.
# File lib/net/dns/header.rb, line 623 def rCode=(val) @rCode = RCode.new(val) end
Returns an error array for the header response code, or nil if no error is generated.
error, cause = header.rCode_str puts "Error #{error} cause by: #{cause}" if error #=> Error ForErr caused by: The name server #=> was unable to interpret the query
# File lib/net/dns/header.rb, line 606 def rCode_str return rCode.type, rCode.explanation end
Checks whether recursion is available. This flag is usually set by nameservers to indicate that they support recursive-type queries.
# File lib/net/dns/header.rb, line 524 def r_available? @ra == 1 end
Set the ra flag (recursion available) to either true or false. You can also use 0 and 1.
This flag must only be set in DNS answer packets.
# File lib/net/dns/header.rb, line 533 def ra=(val) case val when true @ra = 1 when false @ra = 0 when 0,1 @ra = val else raise ArgumentError, ":ra must be true(or 1) or false(or 0)" end end
Alias for Header#recursive= to keep compatibility with the Perl version.
# File lib/net/dns/header.rb, line 516 def rd=(val) self.recursive = val end
Sets the recursion desidered bit. Remember that recursion query support is optional.
header.recursive = true hdata = header.data # suitable for sending
Consult RFC1034 and RFC1035 for a detailed explanation of how recursion works.
# File lib/net/dns/header.rb, line 498 def recursive=(val) case val when true @rd = 1 when false @rd = 0 when 1 @rd = 1 when 0 @rd = 0 else raise WrongRecursiveError, "Wrong value (#{val}), please specify true (1) or false (0)" end end
Checks whether the packet has a recursion bit set, meaning that recursion is desired
# File lib/net/dns/header.rb, line 484 def recursive? @rd == 1 end
Checks whether the header is a response (qr bit set to 1)
# File lib/net/dns/header.rb, line 374 def response? @qr == 1 end
Set the tc flag (truncated packet) to either true ot false. You can also use 0 or 1.
The truncated flag is used in response packets to indicate that the amount of data to be trasmitted exceedes the maximum allowed by the protocol in use, tipically UDP, and that the data present in the packet has been truncated. A different protocol (such has TCP) need to be used to retrieve full data.
Must only be set in DNS answer packets.
# File lib/net/dns/header.rb, line 468 def tc=(val) case val when true @tc = 1 when false @tc = 0 when 0,1 @tc = val else raise ArgumentError, ":tc must be true(or 1) or false(or 0)" end end
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