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net: Resolve*([::]) gives IPv4 zero, not IPv6 #20911

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heschi opened this issue Jul 5, 2017 · 3 comments
Closed

net: Resolve*([::]) gives IPv4 zero, not IPv6 #20911

heschi opened this issue Jul 5, 2017 · 3 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.9

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@heschi
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heschi commented Jul 5, 2017

https://go-review.googlesource.com/45088 changed the behavior such that ResolveUDPAddr("[::]") followed by DialUDP on the result dials IPv4 localhost, not IPv6 localhost, which then fails when IPv4 is not configured. This is because ResolveUDPAddr unconditionally prefers IPv4 addresses when they're present in the addrList returned by internetAddrList. Looking at the code, I expect ResolveTCPAddr will have the same problem.

package main
import (
	"net"
	"fmt"
	"os"
	"strings"
)

func main() {
	addr, err := net.ResolveUDPAddr("udp", "[::]:9999")
	if err != nil {
		panic(err)
	}
	conn, err := net.DialUDP("udp", nil, addr)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s (%#v) %s (%#v)\n", addr, addr, conn.RemoteAddr(), conn.RemoteAddr())
	if strings.Contains(conn.RemoteAddr().String(), "127.0.0.1") {
		fmt.Printf("Fail\n")
		os.Exit(1)
	}
}

At d8a7990:
[::]:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}, Port:9999, Zone:""}) [::1]:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1}, Port:9999, Zone:""})

At 78cf0e5:

0.0.0.0:9999 (&net.UDPAddr{IP:net.IP{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xff, 0xff, 0x0, 0x0, 0x0, 0x0}, Port:9999, Zone:""}) 127.0.0.1:9999 (&net.UDPAddr{IP:net.IP{0x7f, 0x0, 0x0, 0x1}, Port:9999, Zone:""})

cc @bradfitz
@bradfitz bradfitz added this to the Go1.9 milestone Jul 5, 2017
@bradfitz bradfitz added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 5, 2017
@bradfitz bradfitz self-assigned this Jul 5, 2017
@bradfitz
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bradfitz commented Jul 5, 2017

This is because ResolveUDPAddr unconditionally prefers IPv4 addresses when they're present in the addrList returned by internetAddrList.

Wow, okay.

Thanks for finding that. That's certainly not documented at https://golang.org/pkg/net/#ResolveUDPAddr

This is causing breakages somewhere I assume?

@heschi
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heschi commented Jul 5, 2017

That's just my read of the code, of course, but it seems pretty clear:
return addrs.first(isIPv4).(*UDPAddr), nil
where "isIPv4" is the predicate used to pick the preferred address from the list.

Yeah, a few internal breakages.

@bradfitz bradfitz added the NeedsFix The path to resolution is known, but the work has not been done. label Jul 6, 2017
@gopherbot gopherbot removed the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jul 6, 2017
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CL https://golang.org/cl/47554 mentions this issue.

@golang golang locked and limited conversation to collaborators Jul 6, 2018
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