dimkr/dohli

// this file is part of dohli.
//
// Copyright (c) 2020 Dima Krasner
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

package dns

import (
    "errors"
    "math"

    "golang.org/x/net/dns/dnsmessage"
)

// GetShortestTTL returns the lowest TTL among the answer records of a DNS
// response, or 0.
func GetShortestTTL(response []byte) int {
    var p dnsmessage.Parser

    if _, err := p.Start(response); err != nil {
        return 0
    }

    if err := p.SkipAllQuestions(); err != nil {
        return 0
    }

    var shortestTTL int
    first := true

    for {
        a, err := p.Answer()
        if errors.Is(err, dnsmessage.ErrSectionDone) {
            break
        }

        if err != nil {
            break
        }

        if a.Header.TTL >= math.MaxInt32 {
            continue
        }

        ttl := int(a.Header.TTL)
        if first || ttl < shortestTTL {
            shortestTTL = ttl
        }
        first = false
    }

    return shortestTTL
}

// ReplaceTTLInResponse sets the TTL of all answer records in a DNS response.
func ReplaceTTLInResponse(response []byte, TTL uint32) ([]byte, error) {
    var p dnsmessage.Parser

    header, err := p.Start(response)
    if err != nil {
        return nil, err
    }

    questions, err := p.AllQuestions()
    if err != nil {
        return nil, err
    }

    var additionals []dnsmessage.Resource

    additionals, err = p.AllAdditionals()
    if err != nil && !errors.Is(err, dnsmessage.ErrNotStarted) {
        return nil, err
    }
    var answers []dnsmessage.Resource

    for {
        answer, err := p.Answer()
        if errors.Is(err, dnsmessage.ErrSectionDone) {
            break
        }
        if err != nil {
            return nil, err
        }

        answer.Header.TTL = TTL

        answers = append(answers, answer)
    }

    msg := dnsmessage.Message{
        Header:      header,
        Questions:   questions,
        Answers:     answers,
        Additionals: additionals,
    }

    return msg.Pack()
}