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net/dns/resolver: clamp EDNS size

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This change (subject to some limitations) looks for the EDNS OPT record
in queries and responses, clamping the size field to fit within our DNS
receive buffer. If the size field is smaller than the DNS receive buffer
then it is left unchanged.

I think we will eventually need to transition to fully processing the
DNS queries to handle all situations, but this should cover the most
common case.

Mostly fixes #2066

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
This commit is contained in:
Adrian Dewhurst 2021-06-24 10:36:23 -04:00 committed by Adrian Dewhurst
parent c69d30cdd7
commit bcaae3e074
3 changed files with 288 additions and 82 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

54
net/dns/resolver/forwarder.go
View File

@ -88,6 +88,57 @@ func getTxID(packet []byte) txid {
return (txid(hash) << 32) | txid(dnsid)
}
// clampEDNSSize attempts to limit the maximum EDNS response size. This is not
// an exhaustive solution, instead only easy cases are currently handled in the
// interest of speed and reduced complexity. Only OPT records at the very end of
// the message with no option codes are addressed.
// TODO: handle more situations if we discover that they happen often
func clampEDNSSize(packet []byte, maxSize uint16) {
// optFixedBytes is the size of an OPT record with no option codes.
const optFixedBytes = 11
const edns0Version = 0
if len(packet) < headerBytes+optFixedBytes {
return
}
arCount := binary.BigEndian.Uint16(packet[10:12])
if arCount == 0 {
// OPT shows up in an AR, so there must be no OPT
return
}
opt := packet[len(packet)-optFixedBytes:]
if opt[0] != 0 {
// OPT NAME must be 0 (root domain)
return
}
if dns.Type(binary.BigEndian.Uint16(opt[1:3])) != dns.TypeOPT {
// Not an OPT record
return
}
requestedSize := binary.BigEndian.Uint16(opt[3:5])
// Ignore extended RCODE in opt[5]
if opt[6] != edns0Version {
// Be conservative and don't touch unknown versions.
return
}
// Ignore flags in opt[7:9]
if binary.BigEndian.Uint16(opt[10:12]) != 0 {
// RDLEN must be 0 (no variable length data). We're at the end of the
// packet so this should be 0 anyway)..
return
}
if requestedSize <= maxSize {
return
}
// Clamp the maximum size
binary.BigEndian.PutUint16(opt[3:5], maxSize)
}
type route struct {
Suffix dnsname.FQDN
Resolvers []netaddr.IPPort
@ -233,6 +284,8 @@ func (f *forwarder) send(ctx context.Context, txidOut txid, closeOnCtxDone *clos
// best we can do.
}
clampEDNSSize(out, maxResponseBytes)
return out, nil
}
@ -257,6 +310,7 @@ func (f *forwarder) forward(query packet) error {
}
txid := getTxID(query.bs)
clampEDNSSize(query.bs, maxResponseBytes)
resolvers := f.resolvers(domain)
if len(resolvers) == 0 {

23
net/dns/resolver/tsdns_server_test.go
View File

@ -68,7 +68,7 @@ func resolveToIP(ipv4, ipv6 netaddr.IP, ns string) dns.HandlerFunc {
// resolveToTXT returns a handler function which responds to queries of type TXT
// it receives with the strings in txts.
func resolveToTXT(txts []string) dns.HandlerFunc {
func resolveToTXT(txts []string, ednsMaxSize uint16) dns.HandlerFunc {
return func(w dns.ResponseWriter, req *dns.Msg) {
m := new(dns.Msg)
m.SetReply(req)
@ -93,6 +93,27 @@ func resolveToTXT(txts []string) dns.HandlerFunc {
}
m.Answer = append(m.Answer, ans)
queryInfo := &dns.TXT{
Hdr: dns.RR_Header{
Name: "query-info.test.",
Rrtype: dns.TypeTXT,
Class: dns.ClassINET,
},
}
if edns := req.IsEdns0(); edns == nil {
queryInfo.Txt = []string{"EDNS=false"}
} else {
queryInfo.Txt = []string{"EDNS=true", fmt.Sprintf("maxSize=%v", edns.UDPSize())}
}
m.Extra = append(m.Extra, queryInfo)
if ednsMaxSize > 0 {
m.SetEdns0(ednsMaxSize, false)
}
if err := w.WriteMsg(m); err != nil {
panic(err)
}

293
net/dns/resolver/tsdns_test.go
View File

@ -12,6 +12,8 @@ import (
"math/rand"
"net"
"runtime"
"strconv"
"strings"
"testing"
dns "golang.org/x/net/dns/dnsmessage"
@ -32,7 +34,9 @@ var dnsCfg = Config{
LocalDomains: []dnsname.FQDN{"ipn.dev."},
}
func dnspacket(domain dnsname.FQDN, tp dns.Type) []byte {
const noEdns = 0
func dnspacket(domain dnsname.FQDN, tp dns.Type, ednsSize uint16) []byte {
var dnsHeader dns.Header
question := dns.Question{
Name: dns.MustNewName(domain.WithTrailingDot()),
@ -41,19 +45,44 @@ func dnspacket(domain dnsname.FQDN, tp dns.Type) []byte {
}
builder := dns.NewBuilder(nil, dnsHeader)
builder.StartQuestions()
builder.Question(question)
if err := builder.StartQuestions(); err != nil {
panic(err)
}
if err := builder.Question(question); err != nil {
panic(err)
}
if ednsSize != noEdns {
if err := builder.StartAdditionals(); err != nil {
panic(err)
}
ednsHeader := dns.ResourceHeader{
Name: dns.MustNewName("."),
Type: dns.TypeOPT,
Class: dns.Class(ednsSize),
}
if err := builder.OPTResource(ednsHeader, dns.OPTResource{}); err != nil {
panic(err)
}
}
payload, _ := builder.Finish()
return payload
}
type dnsResponse struct {
ip netaddr.IP
txt []string
name dnsname.FQDN
rcode dns.RCode
truncated bool
ip netaddr.IP
txt []string
name dnsname.FQDN
rcode dns.RCode
truncated bool
requestEdns bool
requestEdnsSize uint16
responseEdns bool
responseEdnsSize uint16
}
func unpackResponse(payload []byte) (dnsResponse, error) {
@ -89,41 +118,98 @@ func unpackResponse(payload []byte) (dnsResponse, error) {
return response, err
}
ah, err := parser.AnswerHeader()
for {
ah, err := parser.AnswerHeader()
if err == dns.ErrSectionDone {
break
}
if err != nil {
return response, err
}
switch ah.Type {
case dns.TypeA:
res, err := parser.AResource()
if err != nil {
return response, err
}
response.ip = netaddr.IPv4(res.A[0], res.A[1], res.A[2], res.A[3])
case dns.TypeAAAA:
res, err := parser.AAAAResource()
if err != nil {
return response, err
}
response.ip = netaddr.IPv6Raw(res.AAAA)
case dns.TypeTXT:
res, err := parser.TXTResource()
if err != nil {
return response, err
}
response.txt = res.TXT
case dns.TypeNS:
res, err := parser.NSResource()
if err != nil {
return response, err
}
response.name, err = dnsname.ToFQDN(res.NS.String())
if err != nil {
return response, err
}
default:
return response, errors.New("type not in {A, AAAA, NS}")
}
}
err = parser.SkipAllAuthorities()
if err != nil {
return response, err
}
switch ah.Type {
case dns.TypeA:
res, err := parser.AResource()
for {
ah, err := parser.AdditionalHeader()
if err == dns.ErrSectionDone {
break
}
if err != nil {
return response, err
}
response.ip = netaddr.IPv4(res.A[0], res.A[1], res.A[2], res.A[3])
case dns.TypeAAAA:
res, err := parser.AAAAResource()
if err != nil {
return response, err
switch ah.Type {
case dns.TypeOPT:
_, err := parser.OPTResource()
if err != nil {
return response, err
}
response.responseEdns = true
response.responseEdnsSize = uint16(ah.Class)
case dns.TypeTXT:
res, err := parser.TXTResource()
if err != nil {
return response, err
}
switch ah.Name.String() {
case "query-info.test.":
for _, msg := range res.TXT {
s := strings.SplitN(msg, "=", 2)
if len(s) != 2 {
continue
}
switch s[0] {
case "EDNS":
response.requestEdns, err = strconv.ParseBool(s[1])
if err != nil {
return response, err
}
case "maxSize":
sz, err := strconv.ParseUint(s[1], 10, 16)
if err != nil {
return response, err
}
response.requestEdnsSize = uint16(sz)
}
}
}
}
response.ip = netaddr.IPv6Raw(res.AAAA)
case dns.TypeTXT:
res, err := parser.TXTResource()
if err != nil {
return response, err
}
response.txt = res.TXT
case dns.TypeNS:
res, err := parser.NSResource()
if err != nil {
return response, err
}
response.name, err = dnsname.ToFQDN(res.NS.String())
if err != nil {
return response, err
}
default:
return response, errors.New("type not in {A, AAAA, NS}")
}
return response, nil
@ -340,7 +426,7 @@ func TestDelegate(t *testing.T) {
// support these sizes of response without truncation because they are
// moderately common.
medTXT := generateTXT(1200, randSource)
largeTXT := generateTXT(4000, randSource)
largeTXT := generateTXT(3900, randSource)
// xlargeTXT is slightly above the maximum response size that we support,
// so there should be truncation.
@ -351,23 +437,20 @@ func TestDelegate(t *testing.T) {
// intend to handle responses this large, so there should be truncation.
hugeTXT := generateTXT(64000, randSource)
v4server := serveDNS(t, "127.0.0.1:0",
"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."),
records := []interface{}{
"test.site.",
resolveToIP(testipv4, testipv6, "dns.test.site."),
"nxdomain.site.", resolveToNXDOMAIN,
"small.txt.", resolveToTXT(smallTXT),
"med.txt.", resolveToTXT(medTXT),
"large.txt.", resolveToTXT(largeTXT),
"xlarge.txt.", resolveToTXT(xlargeTXT),
"huge.txt.", resolveToTXT(hugeTXT))
"small.txt.", resolveToTXT(smallTXT, noEdns),
"smalledns.txt.", resolveToTXT(smallTXT, 512),
"med.txt.", resolveToTXT(medTXT, 1500),
"large.txt.", resolveToTXT(largeTXT, maxResponseBytes),
"xlarge.txt.", resolveToTXT(xlargeTXT, 8000),
"huge.txt.", resolveToTXT(hugeTXT, 65527),
}
v4server := serveDNS(t, "127.0.0.1:0", records...)
defer v4server.Shutdown()
v6server := serveDNS(t, "[::1]:0",
"test.site.", resolveToIP(testipv4, testipv6, "dns.test.site."),
"nxdomain.site.", resolveToNXDOMAIN,
"small.txt.", resolveToTXT(smallTXT),
"med.txt.", resolveToTXT(medTXT),
"large.txt.", resolveToTXT(largeTXT),
"xlarge.txt.", resolveToTXT(xlargeTXT),
"huge.txt.", resolveToTXT(hugeTXT))
v6server := serveDNS(t, "[::1]:0", records...)
defer v6server.Shutdown()
r := newResolver(t)
@ -389,48 +472,84 @@ func TestDelegate(t *testing.T) {
}{
{
"ipv4",
dnspacket("test.site.", dns.TypeA),
dnspacket("test.site.", dns.TypeA, noEdns),
dnsResponse{ip: testipv4, rcode: dns.RCodeSuccess},
},
{
"ipv6",
dnspacket("test.site.", dns.TypeAAAA),
dnspacket("test.site.", dns.TypeAAAA, noEdns),
dnsResponse{ip: testipv6, rcode: dns.RCodeSuccess},
},
{
"ns",
dnspacket("test.site.", dns.TypeNS),
dnspacket("test.site.", dns.TypeNS, noEdns),
dnsResponse{name: "dns.test.site.", rcode: dns.RCodeSuccess},
},
{
"nxdomain",
dnspacket("nxdomain.site.", dns.TypeA),
dnspacket("nxdomain.site.", dns.TypeA, noEdns),
dnsResponse{rcode: dns.RCodeNameError},
},
{
"smalltxt",
dnspacket("small.txt.", dns.TypeTXT),
dnsResponse{txt: smallTXT, rcode: dns.RCodeSuccess},
dnspacket("small.txt.", dns.TypeTXT, 8000),
dnsResponse{txt: smallTXT, rcode: dns.RCodeSuccess, requestEdns: true, requestEdnsSize: maxResponseBytes},
},
{
"smalltxtedns",
dnspacket("smalledns.txt.", dns.TypeTXT, 512),
dnsResponse{
txt: smallTXT,
rcode: dns.RCodeSuccess,
requestEdns: true,
requestEdnsSize: 512,
responseEdns: true,
responseEdnsSize: 512,
},
},
{
"medtxt",
dnspacket("med.txt.", dns.TypeTXT),
dnsResponse{txt: medTXT, rcode: dns.RCodeSuccess},
dnspacket("med.txt.", dns.TypeTXT, 2000),
dnsResponse{
txt: medTXT,
rcode: dns.RCodeSuccess,
requestEdns: true,
requestEdnsSize: 2000,
responseEdns: true,
responseEdnsSize: 1500,
},
},
{
"largetxt",
dnspacket("large.txt.", dns.TypeTXT),
dnsResponse{txt: largeTXT, rcode: dns.RCodeSuccess},
dnspacket("large.txt.", dns.TypeTXT, maxResponseBytes),
dnsResponse{
txt: largeTXT,
rcode: dns.RCodeSuccess,
requestEdns: true,
requestEdnsSize: maxResponseBytes,
responseEdns: true,
responseEdnsSize: maxResponseBytes,
},
},
{
"xlargetxt",
dnspacket("xlarge.txt.", dns.TypeTXT),
dnsResponse{rcode: dns.RCodeSuccess, truncated: true},
dnspacket("xlarge.txt.", dns.TypeTXT, 8000),
dnsResponse{
rcode: dns.RCodeSuccess,
truncated: true,
// request/response EDNS fields will be unset because of
// they were truncated away
},
},
{
"hugetxt",
dnspacket("huge.txt.", dns.TypeTXT),
dnsResponse{rcode: dns.RCodeSuccess, truncated: true},
dnspacket("huge.txt.", dns.TypeTXT, 8000),
dnsResponse{
rcode: dns.RCodeSuccess,
truncated: true,
// request/response EDNS fields will be unset because of
// they were truncated away
},
},
}
@ -467,6 +586,18 @@ func TestDelegate(t *testing.T) {
}
}
}
if response.requestEdns != tt.response.requestEdns {
t.Errorf("requestEdns = %v; want %v", response.requestEdns, tt.response.requestEdns)
}
if response.requestEdnsSize != tt.response.requestEdnsSize {
t.Errorf("requestEdnsSize = %v; want %v", response.requestEdnsSize, tt.response.requestEdnsSize)
}
if response.responseEdns != tt.response.responseEdns {
t.Errorf("responseEdns = %v; want %v", response.requestEdns, tt.response.requestEdns)
}
if response.responseEdnsSize != tt.response.responseEdnsSize {
t.Errorf("responseEdnsSize = %v; want %v", response.responseEdnsSize, tt.response.responseEdnsSize)
}
})
}
}
@ -499,12 +630,12 @@ func TestDelegateSplitRoute(t *testing.T) {
}{
{
"general",
dnspacket("test.site.", dns.TypeA),
dnspacket("test.site.", dns.TypeA, noEdns),
dnsResponse{ip: testipv4, rcode: dns.RCodeSuccess},
},
{
"override",
dnspacket("test.other.", dns.TypeA),
dnspacket("test.other.", dns.TypeA, noEdns),
dnsResponse{ip: test4, rcode: dns.RCodeSuccess},
},
}
@ -561,7 +692,7 @@ func TestDelegateCollision(t *testing.T) {
// packets will have the same dns txid.
for _, p := range packets {
payload := dnspacket(p.qname, p.qtype)
payload := dnspacket(p.qname, p.qtype, noEdns)
err := r.EnqueueRequest(payload, p.addr)
if err != nil {
t.Error(err)
@ -764,15 +895,15 @@ func TestFull(t *testing.T) {
request []byte
response []byte
}{
{"all", dnspacket("test1.ipn.dev.", dns.TypeALL), allResponse},
{"ipv4", dnspacket("test1.ipn.dev.", dns.TypeA), ipv4Response},
{"ipv6", dnspacket("test2.ipn.dev.", dns.TypeAAAA), ipv6Response},
{"no-ipv6", dnspacket("test1.ipn.dev.", dns.TypeAAAA), emptyResponse},
{"upper", dnspacket("TEST1.IPN.DEV.", dns.TypeA), ipv4UppercaseResponse},
{"ptr4", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR), ptrResponse},
{"all", dnspacket("test1.ipn.dev.", dns.TypeALL, noEdns), allResponse},
{"ipv4", dnspacket("test1.ipn.dev.", dns.TypeA, noEdns), ipv4Response},
{"ipv6", dnspacket("test2.ipn.dev.", dns.TypeAAAA, noEdns), ipv6Response},
{"no-ipv6", dnspacket("test1.ipn.dev.", dns.TypeAAAA, noEdns), emptyResponse},
{"upper", dnspacket("TEST1.IPN.DEV.", dns.TypeA, noEdns), ipv4UppercaseResponse},
{"ptr4", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR, noEdns), ptrResponse},
{"ptr6", dnspacket("f.0.e.0.d.0.c.0.b.0.a.0.9.0.8.0.7.0.6.0.5.0.4.0.3.0.2.0.1.0.0.0.ip6.arpa.",
dns.TypePTR), ptrResponse6},
{"nxdomain", dnspacket("test3.ipn.dev.", dns.TypeA), nxdomainResponse},
dns.TypePTR, noEdns), ptrResponse6},
{"nxdomain", dnspacket("test3.ipn.dev.", dns.TypeA, noEdns), nxdomainResponse},
}
for _, tt := range tests {
@ -801,9 +932,9 @@ func TestAllocs(t *testing.T) {
want int
}{
// Name lowercasing and response slice created by dns.NewBuilder.
{"forward", dnspacket("test1.ipn.dev.", dns.TypeA), 2},
{"forward", dnspacket("test1.ipn.dev.", dns.TypeA, noEdns), 2},
// 3 extra allocs in rdnsNameToIPv4 and one in marshalPTRRecord (dns.NewName).
{"reverse", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR), 5},
{"reverse", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR, noEdns), 5},
}
for _, tt := range tests {
@ -857,9 +988,9 @@ func BenchmarkFull(b *testing.B) {
name string
request []byte
}{
{"forward", dnspacket("test1.ipn.dev.", dns.TypeA)},
{"reverse", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR)},
{"delegated", dnspacket("test.site.", dns.TypeA)},
{"forward", dnspacket("test1.ipn.dev.", dns.TypeA, noEdns)},
{"reverse", dnspacket("4.3.2.1.in-addr.arpa.", dns.TypePTR, noEdns)},
{"delegated", dnspacket("test.site.", dns.TypeA, noEdns)},
}
for _, tt := range tests {