net: add Sys field to Interface for retrieving platform-specific information

[hickeng]

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

go version go1.7 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/vagrant/vicsmb"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build018341538=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?

package main

import (
    "fmt"
    "net"
    "os"
    "os/exec"
)

func main() {

    alias := "testalias"

    ipalias := exec.Command("/sbin/ip", "link", "set", "dev", "lo", "alias", alias)
    _, err := ipalias.CombinedOutput()
    if err != nil {
        fmt.Printf("failed to invoke /sbin/ip to set alias on loopback interface: %s", err)
        os.Exit(1)
    }

    intf, err := net.InterfaceByName(alias)
    if intf == nil {
        fmt.Printf("failed to locate interface by alias %s: %s\n", alias, err)
        os.Exit(1)
    }

    fmt.Println("success!!")
}

What did you expect to see?

Aliasing link lo
success!!

What did you see instead?

Aliasing link lo
failed to locate interface by alias testalias: route ip+net: no such network interface

#  ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    alias testalias

If InterfaceByName doens't check aliases on systems that support them then interface aliases become problematic. If you have to know that the name is an aliases it defeats the point.

[robpike]

Can you please create a reproducing case for this that does not involve an external repository?

[hickeng]

@robpike updated to invoke /sbin/ip instead of setting alias inline.

[mikioh]

Any good use case of IFLA_IFALIAS doesn't come to my mind.

# ip link set alias lo rack/chassis/slot/port-convention

# ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    alias rack/chassis/slot/port-convention

# ip link show rack/chassis/slot/port-convention
Device "rack/chassis/slot/port-convention" does not exist.

# ip addr add 1.2.3.4/24 dev rack/chassis/slot/port-convention
Device "rack/chassis/slot/port-convention" does not exist.

# ip route add 10.0.0.0/16 nexthop dev rack/chassis/slot/port-convention
Cannot find device "rack/chassis/slot/port-convention"

Besides current API doesn't expose such platform-specific information for consistency across platforms; for example, it never displays information related to route-labels, similar to IFLA_IFALIAS of Linux, on OpenBSD. I'm fine to have x/net/netlink package for some Linux-specific use case as we have x/net/route for BSD variants, but I'm not keen to add support for IFLA_IFALIAS into net package.

[hickeng]

@mikioh I'll admit the internet's not been much help in explaining an intended use of alias outside of https://tools.ietf.org/html/rfc2233 - however I'm wanting to use it for exactly that case (although not formally an NMA):

ifAlias, provides a location in which a network management application can store a non-volatile interface-naming value of its own choice

That the ip command semantics don't abide by the dictionary definition of alias is a different conversation, although consistency can be an extremely compelling argument.

[mikioh]

For pure management plane or hybrid control/management plane stuff, how about this?

• Add Sys field to the existing Interface struct to expose platform-specific information the same as os.FileInfo, os.ProcAttr
• Network interface identification API never retrieves aliases, descriptions or other opaque strings as its key to avoid unnecessary confusion for interworking w/ de facto or de jule protocols that transport interface names

[bradfitz]

@mikioh, and I assume the Sys field would be of type interface{}? What's the concrete type of the net.Interface.Sys value you'd put in there for, say, Linux?

[quentinmit]

Are interface aliases in the same namespace as interface names? Can I add the alias lo to my eth0 interface or does that fail?

[mikioh]

On Linux IFLA_IFALIAS is an attribute that represents an opaque string member in netdevice structure. It doesn't collaborate with any control plane process.

Can I add the alias lo to my eth0

Yes.

[mikioh]

I'd like to see Sys []interface{} rather than Sys interface{} to separate at least miscellaneous data and statistical information. On Linux I guess that type InterfaceData { Alias string /* interface alias */; Type int /* platform-dependent interface type */} is enough as a first step. Anyway I'd like to know how @hickeng feels about it.