net: ignore edns0 option in resolv.conf

We use EDNS(0) by default. No need to fall back to netdns=cgo if we
see a explicit request for EDNS(0) in resolv.conf.

For #51153

Change-Id: I135363112e3de43ce877aad45aba71d1448068b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/415579
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: ianlancetaylor

src/net/dnsconfig_unix.go

@@ -109,6 +109,9 @@ func dnsReadConfig(filename string) *dnsConfig {
 					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
 					// https://man.openbsd.org/resolv.conf.5
 					conf.useTCP = true
+				case s == "edns0":
+					// We use EDNS by default.
+					// Ignore this option.
 				default:
 					conf.unknownOpt = true
 				}