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Properly parse alpn values in SVCB (#1363)
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* Modify the SVCBAlpn to properly parse/print

* Remove debug

* Change SVCB test from reflect to loop

* Refactor SVCB code to reduce indentation

* When stringifying SVCBAlpn, use strings.Builder for whole process

* Update comment in svcb.go

Co-authored-by: Miek Gieben <[email protected]>

* Describe why we use a specific size for the alpn buffer

Co-authored-by: Miek Gieben <[email protected]>
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@shane-ns1 @miekg
shane-ns1 and miekg authored May 10, 2022
1 parent 0d2c95b commit feda877
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Showing 3 changed files with 136 additions and 6 deletions.
8 changes: 6 additions & 2 deletions parse_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -1650,8 +1650,8 @@ func TestParseSVCB(t *testing.T) {
`example.com. SVCB 1 foo.example.com. ipv6hint="2001:db8::1,2001:db8::53:1"`: `example.com. 3600 IN SVCB 1 foo.example.com. ipv6hint="2001:db8::1,2001:db8::53:1"`,
`example.com. SVCB 1 example.com. ipv6hint="2001:db8::198.51.100.100"`: `example.com. 3600 IN SVCB 1 example.com. ipv6hint="2001:db8::c633:6464"`,
`example.com. SVCB 16 foo.example.org. alpn=h2,h3-19 mandatory=ipv4hint,alpn ipv4hint=192.0.2.1`: `example.com. 3600 IN SVCB 16 foo.example.org. alpn="h2,h3-19" mandatory="ipv4hint,alpn" ipv4hint="192.0.2.1"`,
`example.com. SVCB 16 foo.example.org. alpn="f\\\\oo\\,bar,h2"`: `example.com. 3600 IN SVCB 16 foo.example.org. alpn="f\\\\oo\\,bar,h2"`,
`example.com. SVCB 16 foo.example.org. alpn=f\\\092oo\092,bar,h2`: `example.com. 3600 IN SVCB 16 foo.example.org. alpn="f\\\092oo\092,bar,h2"`,
`example.com. SVCB 16 foo.example.org. alpn="f\\\\oo\\,bar,h2"`: `example.com. 3600 IN SVCB 16 foo.example.org. alpn="f\\\092oo\\\044bar,h2"`,
`example.com. SVCB 16 foo.example.org. alpn=f\\\092oo\092,bar,h2`: `example.com. 3600 IN SVCB 16 foo.example.org. alpn="f\\\092oo\\\044bar,h2"`,
// From draft-ietf-add-ddr-06
`_dns.example.net. SVCB 1 example.net. alpn=h2 dohpath=/dns-query{?dns}`: `_dns.example.net. 3600 IN SVCB 1 example.net. alpn="h2" dohpath="/dns-query{?dns}"`,
}
Expand Down Expand Up @@ -1704,6 +1704,10 @@ func TestParseBadSVCB(t *testing.T) {
`1 . ipv4hint=`, // empty ipv4
`1 . port=`, // empty port
`1 . echconfig=YUd`, // bad base64
`1 . alpn=h\`, // unterminated escape
`1 . alpn=h2\\.h3`, // comma-separated list with bad character
`1 . alpn=h2,,h3`, // empty protocol identifier
`1 . alpn=h3,`, // final protocol identifier empty
}
for _, o := range evils {
_, err := NewRR(header + o)
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92 changes: 88 additions & 4 deletions svcb.go
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Expand Up @@ -342,13 +342,57 @@ func (s *SVCBMandatory) copy() SVCBKeyValue {
// h.Hdr = dns.RR_Header{Name: ".", Rrtype: dns.TypeHTTPS, Class: dns.ClassINET}
// e := new(dns.SVCBAlpn)
// e.Alpn = []string{"h2", "http/1.1"}
// h.Value = append(o.Value, e)
// h.Value = append(h.Value, e)
type SVCBAlpn struct {
Alpn []string
}

func (*SVCBAlpn) Key() SVCBKey { return SVCB_ALPN }
func (s *SVCBAlpn) String() string { return strings.Join(s.Alpn, ",") }
func (*SVCBAlpn) Key() SVCBKey { return SVCB_ALPN }

func (s *SVCBAlpn) String() string {
// An ALPN value is a comma-separated list of values, each of which can be
// an arbitrary binary value. In order to allow parsing, the comma and
// backslash characters are themselves excaped.
//
// However, this escaping is done in addition to the normal escaping which
// happens in zone files, meaning that these values must be
// double-escaped. This looks terrible, so if you see a never-ending
// sequence of backslash in a zone file this may be why.
//
// https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-svcb-https-08#appendix-A.1
var str strings.Builder
for i, alpn := range s.Alpn {
// 4*len(alpn) is the worst case where we escape every character in the alpn as \123, plus 1 byte for the ',' separating the alpn from others
str.Grow(4*len(alpn) + 1)
if i > 0 {
str.WriteByte(',')
}
for j := 0; j < len(alpn); j++ {
e := alpn[j]
if ' ' > e || e > '~' {
str.WriteString(escapeByte(e))
continue
}
switch e {
// We escape a few characters which may confuse humans or parsers.
case '"', ';', ' ':
str.WriteByte('\\')
str.WriteByte(e)
// The comma and backslash characters themselves must be
// doubly-escaped. We use `\\` for the first backslash and
// the escaped numeric value for the other value. We especially
// don't want a comma in the output.
case ',':
str.WriteString(`\\\044`)
case '\\':
str.WriteString(`\\\092`)
default:
str.WriteByte(e)
}
}
}
return str.String()
}

func (s *SVCBAlpn) pack() ([]byte, error) {
// Liberally estimate the size of an alpn as 10 octets
Expand Down Expand Up @@ -383,7 +427,47 @@ func (s *SVCBAlpn) unpack(b []byte) error {
}

func (s *SVCBAlpn) parse(b string) error {
s.Alpn = strings.Split(b, ",")
if len(b) == 0 {
s.Alpn = []string{}
return nil
}

alpn := []string{}
a := []byte{}
for p := 0; p < len(b); {
c, q := nextByte(b, p)
if q == 0 {
return errors.New("dns: svcbalpn: unterminated escape")
}
p += q
// If we find a comma, we have finished reading an alpn.
if c == ',' {
if len(a) == 0 {
return errors.New("dns: svcbalpn: empty protocol identifier")
}
alpn = append(alpn, string(a))
a = []byte{}
continue
}
// If it's a backslash, we need to handle a comma-separated list.
if c == '\\' {
dc, dq := nextByte(b, p)
if dq == 0 {
return errors.New("dns: svcbalpn: unterminated escape decoding comma-separated list")
}
if dc != '\\' && dc != ',' {
return errors.New("dns: svcbalpn: bad escaped character decoding comma-separated list")
}
p += dq
c = dc
}
a = append(a, c)
}
// Add the final alpn.
if len(a) == 0 {
return errors.New("dns: svcbalpn: last protocol identifier empty")
}
s.Alpn = append(alpn, string(a))
return nil
}

Expand Down
42 changes: 42 additions & 0 deletions svcb_test.go
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Expand Up @@ -95,6 +95,48 @@ func TestDecodeBadSVCB(t *testing.T) {
}
}

func TestPresentationSVCBAlpn(t *testing.T) {
tests := map[string]string{
"h2": "h2",
"http": "http",
"\xfa": `\250`,
"some\"other,chars": `some\"other\\\044chars`,
}
for input, want := range tests {
e := new(SVCBAlpn)
e.Alpn = []string{input}
if e.String() != want {
t.Errorf("improper conversion with String(), wanted %v got %v", want, e.String())
}
}
}

func TestSVCBAlpn(t *testing.T) {
tests := map[string][]string{
`. 1 IN SVCB 10 one.test. alpn=h2`: {"h2"},
`. 2 IN SVCB 20 two.test. alpn=h2,h3-19`: {"h2", "h3-19"},
`. 3 IN SVCB 30 three.test. alpn="f\\\\oo\\,bar,h2"`: {`f\oo,bar`, "h2"},
`. 4 IN SVCB 40 four.test. alpn="part1,part2,part3\\,part4\\\\"`: {"part1", "part2", `part3,part4\`},
`. 5 IN SVCB 50 five.test. alpn=part1\,\p\a\r\t2\044part3\092,part4\092\\`: {"part1", "part2", `part3,part4\`},
}
for s, v := range tests {
rr, err := NewRR(s)
if err != nil {
t.Error("failed to parse RR: ", err)
continue
}
alpn := rr.(*SVCB).Value[0].(*SVCBAlpn).Alpn
if len(v) != len(alpn) {
t.Fatalf("parsing alpn failed, wanted %v got %v", v, alpn)
}
for i := range v {
if v[i] != alpn[i] {
t.Fatalf("parsing alpn failed, wanted %v got %v", v, alpn)
}
}
}
}

func TestCompareSVCB(t *testing.T) {
val1 := []SVCBKeyValue{
&SVCBPort{
Expand Down

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