net: Setting LocalAddr to v4 address in net.Dialer not effective if v6 available

[bong0]

1. What version of Go are you using (go version)?
   go version go1.5.3 linux/amd64
2. What operating system and processor architecture are you using (go env)?

GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/bongo/dev/go"
GORACE=""
GOROOT="/usr/lib/golang"
GOTOOLDIR="/usr/lib/golang/pkg/tool/linux_amd64"
GO15VENDOREXPERIMENT=""
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"

1. What did you do?
   I ran the following code: http://play.golang.org/p/sujhjFG8W2

and did a tcpdump in parallel looking for packages containing my local ipv4 that should be used for connecting:

sudo tcpdump -n ip host 192.168.0.99
[sudo] password for bongo: 
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on wlp3s0, link-type EN10MB (Ethernet), capture size 262144 bytes

1. What did you expect to see?
   • There should be packages showing up in tcpdump
   • the "actual ip used" line should show a IPv4 address and not IPv6
2. What did you see instead?
   • "actual ip used" line shows my public v6 address as used for connection
   • there are no packages showing up using the forced LocalAddr as source
3. What do you wish?
   I wish that
   • forcing of LocalAddr is effective
   • it's possible to assign multiple LocalAddrs (one for v4 one for v6) since I want to use a forced address other than the default one also if a host is available on v6 (dual stack/fallback)
   • the design of http.Transport and on a lower level net.Dialer is not in a way that forcing v4 or v6 only connectivity is possible (at least I tried hard to find a way that's exposed and did not manage to), this should be made possible
4. What do you suspect to be the reason for this behaviour?
   I believe that somewhere in the source of the net/http library a hardcoded "tcp" automatically chooses IPv6 for connecting though it definitely shouldn't...

[bong0]

I quickly tried wrapping the Dial function as described at https://groups.google.com/d/msg/golang-nuts/_VgvTySj25o/-TtM2jYR-0EJ into a Function that sets "tcp4" forcibly but it still has no effect on the outcome.
Full modified source at: http://play.golang.org/p/DhlAe3pJt2

           Dial: (func(network, addr string) (net.Conn, error) {
                    return (&net.Dialer{
                            Timeout:        3 * time.Second,
                            LocalAddr:      nil,
                            DualStack:      false,
                    }).Dial("tcp4", addr)
            }),

local ip to connect 192.168.0.99
actual ip used:  2a02:[redacted]

[bong0]

I discovered this may be related to the used http client since when constructing from a http.Client v4 is used.
Closing this issue for now, until I verified that there is still something not working for which the library is not responsible.

[bong0]

Okay, I consider this issue as closed and open up another one for the part of assigning multiple LocalAddrs for DualStack.

This version of pester fixes it: sethgrid/pester#6