[release-branch.go1.16] net: increase maximum accepted DNS packet to 1232 bytes

The existing value of 512 bytes as is specified by RFC 1035.
However, the WSL resolver reportedly sends larger packets without
setting the truncation bit, which breaks using the Go resolver.
For 1.18 and backports, just increase the accepted packet size.
This is what GNU glibc does (they use 65536 bytes).

For 1.19 we plan to use EDNS to set the accepted packet size.
That will give us more time to test whether that causes any problems.

No test because I'm not sure how to write one and it wouldn't really
be useful anyhow.

For #6464
For #21160
For #44135
For #51127
For #51153
Fixes #51161

Change-Id: I0243f274a06e010ebb714e138a65386086aecf17
Reviewed-on: https://go-review.googlesource.com/c/go/+/386015
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 6e82ff83cfbef78aa60706c1a7167a31c30e7ef9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/386034
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

diff --git a/src/net/dnsclient_unix.go b/src/net/dnsclient_unix.go
index d7db0c8133ebc814eb2ed4d7c4a002b0e49eaa97..0aa9ad7b9332ba35ce87ee5a6e3bdaf22d7f4fb5 100644 (file)
--- a/src/net/dnsclient_unix.go
+++ b/src/net/dnsclient_unix.go
@@ -29,6 +29,10 @@ const (
        // to be used as a useTCP parameter to exchange
        useTCPOnly  = true
        useUDPOrTCP = false
+
+       // Maximum DNS packet size.
+       // Value taken from https://dnsflagday.net/2020/.
+       maxDNSPacketSize = 1232
 )
 
 var (
@@ -81,7 +85,7 @@ func dnsPacketRoundTrip(c Conn, id uint16, query dnsmessage.Question, b []byte)
                return dnsmessage.Parser{}, dnsmessage.Header{}, err
        }
 
-       b = make([]byte, 512) // see RFC 1035
+       b = make([]byte, maxDNSPacketSize)
        for {
                n, err := c.Read(b)
                if err != nil {

diff --git a/src/net/dnsclient_unix_test.go b/src/net/dnsclient_unix_test.go
index e7f7621a093c57aaaf6b0ea83434bd3afb7cbc5b..e8afbbe29e25f36eb6f9edeb2ee5c9bd7d9eee37 100644 (file)
--- a/src/net/dnsclient_unix_test.go
+++ b/src/net/dnsclient_unix_test.go
@@ -881,7 +881,7 @@ func (f *fakeDNSPacketConn) Close() error {
 func TestIgnoreDNSForgeries(t *testing.T) {
        c, s := Pipe()
        go func() {
-               b := make([]byte, 512)
+               b := make([]byte, maxDNSPacketSize)
                n, err := s.Read(b)
                if err != nil {
                        t.Error(err)