x/net: expose LOWER_UP flag in NICs

[x1unix]

What version of Go are you using (go version)?

$ go version 1.15.4

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""

GOARCH="amd64"

GOBIN=""

GOCACHE="/home/x1unix/.cache/go-build"

GOENV="/home/x1unix/.config/go/env"

GOEXE=""

GOFLAGS=""

GOHOSTARCH="amd64"

GOHOSTOS="linux"

GOINSECURE=""

GOMODCACHE="/home/x1unix/go/pkg/mod"

GONOPROXY=""

GONOSUMDB=""

GOOS="linux"

GOPATH="/home/x1unix/go"

GOPRIVATE=""

GOPROXY="https://proxy.golang.org,direct"

GOROOT="/usr/lib/go"

GOSUMDB="off"

GOTMPDIR=""

GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"

GCCGO="gccgo"

AR="ar"

CC="gcc"

CXX="g++"

CGO_ENABLED="1"

GOMOD=""

CGO_CFLAGS="-g -O2"

CGO_CPPFLAGS="-w"

CGO_CXXFLAGS="-g -O2"

CGO_FFLAGS="-g -O2"

CGO_LDFLAGS="-g -O2"

PKG_CONFIG="pkg-config"

GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build478690309=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Hello. I want to be able to detect if network interface is physical or virtual.
In Linux, there is a special LOWER_UP flag that indicates that NIC is physical, not virtual.

Currently, net package exposes only a few flags, but not LOWER_UP flag:

const (
	FlagUp           Flags = 1 << iota // interface is up
	FlagBroadcast                      // interface supports broadcast access capability
	FlagLoopback                       // interface is a loopback interface
	FlagPointToPoint                   // interface belongs to a point-to-point link
	FlagMulticast                      // interface supports multicast access capability
)

LOWER_UP is described as IFF_LOWER_UP at include/uapi/linux/if.h

What did you expect to see?

Can you add please LowerUp flag to net package?

What did you see instead?

[networkimprov]

cc @odeke-em @ianlancetaylor

[cagedmantis]

/cc @bradfitz @ianlancetaylor

[GanShun]

Hi, the Uroot team also has an ip implementation at https://github.com/u-root/u-root/tree/master/cmds/core/ip which at this point uses net.Flags. This means we're restricted to displaying only the 5 flags currently supported. While we could read the raw flags directly and do all the parsing ourselves, it seems like it would be much nicer to have the official net package support it.

[ianlancetaylor]

I'm not clear if this should be in the net package or the x/net package.

I don't see any difficulty to adding it to the x/net package.

[prattmic]

The existing type is in the net package, so I think any additions would have to go there: https://cs.opensource.google/go/go/+/master:src/net/interface.go;l=30-46;drc=58e381b0b22352dda355f6d95fa101b773766c72?ss=go

Interface could also potentially have a RawFlags method that returns the underlying OS-specific flags, though I'm not sure if that would be useful to folks, or if having the OS-neutral flags is the main benefit here.

[x1unix]

@prattmic is it possible to introduce a flag with OS-neutral name but with the same purposes?
Windows also allows to check if NIC is virtual or physical, so this solution can be cross-platform.

[prattmic]

@x1unix I'm not really the best person to answer that question, as I'm not particularly involved in net. That sounds reasonable to me (net.Flags is in fact an OS-neutral type). I don't think it would work for @GanShun's use case, which IIUC, depends on net.Flags.String() returning the Linux name, but perhaps that is just out of scope.