net: use port 53 when checking for UDP routes

Using port 9 is weird and at least once triggered a suspicious
activity alert.

Fixes #67264

Change-Id: If4179f054829c175b9f3a51c3bc2a3ca4afa74b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/584416
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: ianlancetaylor

src/net/addrselect.go

@@ -42,7 +42,7 @@ func sortByRFC6724withSrcs(addrs []IPAddr, srcs []netip.Addr) {
 // number is irrelevant.
 func srcAddrs(addrs []IPAddr) []netip.Addr {
 	srcs := make([]netip.Addr, len(addrs))
-	dst := UDPAddr{Port: 9}
+	dst := UDPAddr{Port: 53}
 	for i := range addrs {
 		dst.IP = addrs[i].IP
 		dst.Zone = addrs[i].Zone