DNS resolver - test implemention - not prod ready

DNS_resolver

func handlePacket(packetData: Data,syntheticIp: String) {
        if(syntheticIp == "" || syntheticIp.isEmpty){
            os_log(.default, log: self.log, "*****No synthetic ip found")
            return
        }
    
        let udpPayload : Data = packetData[28..<packetData.count]
        do {
//to deserialize we used https://github.com/Bouke/DNS
            let dnsQuery = try Message(deserialize: udpPayload)
            os_log(.default, log: self.log, "________inside dnsQuery:===>>responseData=>> %{public}@",dnsQuery.debugDescription)
            var dnsResponse = Message(id: dnsQuery.id, type: .response)
            // Add the questions from the query to the response
            dnsResponse.questions = dnsQuery.questions

            // Add the answers to the response
            let answer = HostRecord<IPv4>(name: dnsQuery.questions.first?.name ?? "", ttl: 3600, ip: IPv4(syntheticIp)!)
            dnsResponse.answers.append(answer)

            let responseData = try dnsResponse.serialize()
            updateDnsDataPacket(packetData: packetData, syntheticIp: syntheticIp, queryResponseData: responseData)

        } catch {
            os_log(.default, log: self.log, "inside handlePacket:dnsQuery=error %{public}@",error as CVarArg)
        }
        
    }
    
    
    func updateDnsDataPacket(packetData: Data,syntheticIp: String, queryResponseData: Data){
        // Get a mutable copy of the packet
        let mutablePacket = NSMutableData(data: packetData)
        //convert ipv4 ip address to UInt32
        let newDestinationIP = ipv4StringToUInt32(syntheticIp)
        if(newDestinationIP == nil){
            os_log(.default, log: self.log, "syntheticIp to UInt32 failed")
            return;
        }
        
        // Update the destination IP address
        let newDestinationIPBytes = withUnsafeBytes(of: newDestinationIP!.bigEndian) { Data($0) }
        newDestinationIPBytes.withUnsafeBytes { (pointer: UnsafeRawBufferPointer) -> Void in
            mutablePacket.replaceBytes(in: NSRange(location: 16, length: 4), withBytes: pointer.baseAddress!)
        }
        let udpLength = queryResponseData.count + 8 //update the udp length [constructed response data length + 8(udp header default length)]
        let udpLengthHexString = String(format: "%04X", udpLength)

        
        let ipV4Length = udpLength+20 //update the ipv4 length [udp data length + 20(ipv4 header default length)]
        let ipV4LengthHexString = String(format: "%04X", ipV4Length)
        
        var updatedDataPacket = mutablePacket as Data
        var updatedHexString = updatedDataPacket.hexEncodedString()


        //update ipv4 length
        let ipv4StartIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 4) //byte offset 2-3 is for ipv4 length
        let ipv4EndIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 8)
        updatedHexString.replaceSubrange(ipv4StartIndex..<ipv4EndIndex, with: ipV4LengthHexString)

        //update udp length
        let udpStartIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 48) //byte offset 24-25 is for udp length
        let udpEndIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 52)
        updatedHexString.replaceSubrange(udpStartIndex..<udpEndIndex, with: udpLengthHexString)

        //update udp response
        let udpResponseStartIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 56) //byte offset 28-END is for udp query data
        let udpResponseEndIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: updatedHexString.count)
        updatedHexString.replaceSubrange(udpResponseStartIndex..<udpResponseEndIndex, with: queryResponseData.hexEncodedString())

        os_log(.default, log: self.log, "updatedHexString: %{public}@",updatedHexString)

        let rawDataPacket = Data(fromHexEncodedString: updatedHexString)!
        let udpPacket : Data = rawDataPacket[20..<rawDataPacket.count]// your UDP packet as Data
        os_log(.default, log: self.log, "updatedHexString: udpPacket : %{public}@",udpPacket.hexEncodedString())

//        let byteArray = [UInt8](udpPacket)
//        let checksum = checksum(bytes: byteArray)
//        os_log(.default, log: self.log, "updatedHexString checksum: %{public}@",checksum.description)
        
        
        
        let ipv4CheckSum = calculateIPv4HeaderChecksum(packetData: rawDataPacket)
        let ipV4CheckSumString = String(format: "%04X", ipv4CheckSum)


        //update ipv4 checksum
        let ipv4CheckSumStartIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 20) //byte offset 10-11 is for ipv4 checksum
        let ipv4CheckSumEndIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 24)
        updatedHexString.replaceSubrange(ipv4CheckSumStartIndex..<ipv4CheckSumEndIndex, with: ipV4CheckSumString)
        
        //update udp checksum
        let udpCheckSumStartIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 52) //byte offset 26-27 is for udp checksum
        let udpCheckSumEndIndex = updatedHexString.index(updatedHexString.startIndex, offsetBy: 56)
        updatedHexString.replaceSubrange(udpCheckSumStartIndex..<udpCheckSumEndIndex, with: "0000")
    
        
//        let checksum = calculateUDPChecksum(packetData: rawDataPacket)
//        os_log(.default, log: self.log, "updatedHexString checksum: %{public}@",checksum)

        
        //finally update the mutable packet with the update hex string
        updatedDataPacket = Data(fromHexEncodedString: updatedHexString)!
        os_log(.default, log: self.log, "updatedHexString: befire sending %{public}@",updatedDataPacket.hexEncodedString())

        //write packet to server
        let proto = protocolNumber(for: updatedDataPacket)
        let success = self.packetFlow.writePackets([updatedDataPacket], withProtocols: [proto])
        os_log(.default, log: self.log, "packetFlow.writePackets: %{public}@",success.description)
    }
    
    

Does it work? where is the method calculateIPv4HeaderChecksum