Every physical link faces outside, up or down

The filter accepts what does not arrive on a link the machine names as
outward, and the host named only links carrying a default route. An
unplugged wired port was left unfiltered for whenever it was plugged in
(novox/hq issue 197). A link backed by a physical device is now named
whether or not it is up.

internal/outward/links.go

@@ -29,17 +29,32 @@ import (
 // routing table without one.
 const ProcNet = "/proc/net"
 
-// Links are the interfaces carrying a default route, for both address families, sorted and without
-// repeats.
+// SysClassNet is where the kernel lists the machine's network interfaces, one directory each. A
+// parameter for the same reason.
+const SysClassNet = "/sys/class/net"
+
+// Links are the interfaces carrying a default route, for both address families, and every interface
+// backed by a physical device, sorted and without repeats.
+//
+// **A physical link faces outside whether or not it is up** (novox/hq issue 197). The filter accepts
+// whatever did not arrive on a link named here, so a link left out of this list is not filtered at
+// all. A cable unplugged when the machine last reported carries no default route, and was left out:
+// plugged in, everything arriving on it was accepted until the next report and the next push — and a
+// second physical link that never carries the default route was never filtered. A physical device is
+// read from the kernel's own list, where it has a `device` entry; a bridge, a veth, the tunnel and the
+// loopback have none, and stay what they are, this machine's own.
 //
 // A machine may have more than one: a laptop with a cable and a radio has two, and both face
 // outside. A machine with none — no route off itself — returns nothing, and the mesh refuses to
 // compose a filter for it rather than writing a rule around a link with no name, which would be a
 // rule set that does not load and a machine filtering nothing while its unit reports success.
-func Links(procNet string) ([]string, error) {
+func Links(procNet, sysClassNet string) ([]string, error) {
 	if procNet == "" {
 		procNet = ProcNet
 	}
+	if sysClassNet == "" {
+		sysClassNet = SysClassNet
+	}
 	seen := map[string]bool{}
 
 	four, err := defaultsV4(filepath.Join(procNet, "route"))
@@ -50,7 +65,11 @@ func Links(procNet string) ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
-	for _, name := range append(four, six...) {
+	devices, err := physical(sysClassNet)
+	if err != nil {
+		return nil, err
+	}
+	for _, name := range append(append(four, six...), devices...) {
 		if name != "" && name != "lo" {
 			seen[name] = true
 		}
@@ -64,6 +83,25 @@ func Links(procNet string) ([]string, error) {
 	return out, nil
 }
 
+// physical is every interface the kernel lists with a device behind it. A list that is not there is
+// not an error — a machine without sysfs mounted reports what its routing table says, as before.
+func physical(sysClassNet string) ([]string, error) {
+	entries, err := os.ReadDir(sysClassNet)
+	if os.IsNotExist(err) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	var out []string
+	for _, e := range entries {
+		if _, err := os.Stat(filepath.Join(sysClassNet, e.Name(), "device")); err == nil {
+			out = append(out, e.Name())
+		}
+	}
+	return out, nil
+}
+
 // defaultsV4 reads /proc/net/route, whose columns are
 //
 //	Iface Destination Gateway Flags RefCnt Use Metric Mask ...