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package rewrite
import (
"regexp"
"strconv"
"strings"
"github.com/miekg/dns"
)
// ResponseRule contains a rule to rewrite a response with.
type ResponseRule struct {
Active bool
Type string
Pattern *regexp.Regexp
Replacement string
TTL uint32
}
// ResponseReverter reverses the operations done on the question section of a packet.
// This is need because the client will otherwise disregards the response, i.e.
// dig will complain with ';; Question section mismatch: got example.org/HINFO/IN'
type ResponseReverter struct {
dns.ResponseWriter
originalQuestion dns.Question
ResponseRewrite bool
ResponseRules []ResponseRule
}
// NewResponseReverter returns a pointer to a new ResponseReverter.
func NewResponseReverter(w dns.ResponseWriter, r *dns.Msg) *ResponseReverter {
return &ResponseReverter{
ResponseWriter: w,
originalQuestion: r.Question[0],
}
}
// WriteMsg records the status code and calls the underlying ResponseWriter's WriteMsg method.
func (r *ResponseReverter) WriteMsg(res1 *dns.Msg) error {
// Deep copy 'res' as to not (e.g). rewrite a message that's also stored in the cache.
res := res1.Copy()
res.Question[0] = r.originalQuestion
if r.ResponseRewrite {
for _, rr := range res.Ns {
rewriteResourceRecord(res, rr, r)
}
for _, rr := range res.Answer {
rewriteResourceRecord(res, rr, r)
}
for _, rr := range res.Extra {
rewriteResourceRecord(res, rr, r)
}
}
return r.ResponseWriter.WriteMsg(res)
}
func rewriteResourceRecord(res *dns.Msg, rr dns.RR, r *ResponseReverter) {
var (
isNameRewritten bool
isTTLRewritten bool
isValueRewritten bool
name = rr.Header().Name
ttl = rr.Header().Ttl
value string
)
for _, rule := range r.ResponseRules {
if rule.Type == "" {
rule.Type = "name"
}
switch rule.Type {
case "name":
rewriteString(rule, &name, &isNameRewritten)
case "value":
value = getRecordValueForRewrite(rr)
if value != "" {
rewriteString(rule, &value, &isValueRewritten)
}
case "ttl":
ttl = rule.TTL
isTTLRewritten = true
}
}
if isNameRewritten {
rr.Header().Name = name
}
if isTTLRewritten {
rr.Header().Ttl = ttl
}
if isValueRewritten {
setRewrittenRecordValue(rr, value)
}
}
func getRecordValueForRewrite(rr dns.RR) (name string) {
switch rr.Header().Rrtype {
case dns.TypeSRV:
return rr.(*dns.SRV).Target
case dns.TypeMX:
return rr.(*dns.MX).Mx
case dns.TypeCNAME:
return rr.(*dns.CNAME).Target
case dns.TypeNS:
return rr.(*dns.NS).Ns
case dns.TypeDNAME:
return rr.(*dns.DNAME).Target
case dns.TypeNAPTR:
return rr.(*dns.NAPTR).Replacement
case dns.TypeSOA:
return rr.(*dns.SOA).Ns
default:
return ""
}
}
func setRewrittenRecordValue(rr dns.RR, value string) {
switch rr.Header().Rrtype {
case dns.TypeSRV:
rr.(*dns.SRV).Target = value
case dns.TypeMX:
rr.(*dns.MX).Mx = value
case dns.TypeCNAME:
rr.(*dns.CNAME).Target = value
case dns.TypeNS:
rr.(*dns.NS).Ns = value
case dns.TypeDNAME:
rr.(*dns.DNAME).Target = value
case dns.TypeNAPTR:
rr.(*dns.NAPTR).Replacement = value
case dns.TypeSOA:
rr.(*dns.SOA).Ns = value
}
}
func rewriteString(rule ResponseRule, str *string, isStringRewritten *bool) {
regexGroups := rule.Pattern.FindStringSubmatch(*str)
if len(regexGroups) == 0 {
return
}
s := rule.Replacement
for groupIndex, groupValue := range regexGroups {
groupIndexStr := "{" + strconv.Itoa(groupIndex) + "}"
s = strings.Replace(s, groupIndexStr, groupValue, -1)
}
*isStringRewritten = true
*str = s
}
// Write is a wrapper that records the size of the message that gets written.
func (r *ResponseReverter) Write(buf []byte) (int, error) {
n, err := r.ResponseWriter.Write(buf)
return n, err
}
|
