enhanced config to select network interface by name.

conf.go

@@ -38,6 +38,7 @@ type Configuration struct {
 	LogAllPrinterAttrs bool           // Get *all* printer attrs, for logging
 	ColorConsole       bool           // Enable ANSI colors on console
 	Quirks             QuirksDb       // Quirks data base
+	Interface          int            // index of the interface to use
 }
 
 // Conf contains a global instance of program configuration
@@ -55,6 +56,7 @@ var Conf = Configuration{
 	LogMaxBackupFiles:  5,
 	LogAllPrinterAttrs: false,
 	ColorConsole:       true,
+	Interface:          -1,
 }
 
 // ConfLoad loads the program configuration
@@ -113,7 +115,7 @@ func confLoadInternal(path string) error {
 			case confMatchName(rec.Key, "dns-sd"):
 				err = rec.LoadNamedBool(&Conf.DNSSdEnable, "disable", "enable")
 			case confMatchName(rec.Key, "interface"):
-				err = rec.LoadNamedBool(&Conf.LoopbackOnly, "all", "loopback")
+				err = rec.LoadInetInterface(&Conf.Interface)
 			case confMatchName(rec.Key, "ipv6"):
 				err = rec.LoadNamedBool(&Conf.IPV6Enable, "disable", "enable")
 			}

dnssd_avahi.go

@@ -1,5 +1,4 @@
 //go:build linux || freebsd
-// +build linux freebsd
 
 /* ipp-usb - HTTP reverse proxy, backed by IPP-over-USB connection to device
  *
@@ -76,10 +75,10 @@ func newDnssdSysdep(log *Logger, instance string,
 	}
 
 	// Obtain index of loopback interface
-	loopback, err := Loopback()
-	if err != nil {
-		goto ERROR // Very unlikely to happen
-	}
+	// loopback, err := Loopback()
+	// if err != nil {
+	// 	goto ERROR // Very unlikely to happen
+	// }
 
 	// Obtain AvahiPoll
 	poll, err = avahiGetPoll()
@@ -124,14 +123,7 @@ func newDnssdSysdep(log *Logger, instance string,
 	avahiEgroupMap[sysdep.egroup] = sysdep
 
 	// Compute iface and proto, adjust fqdn
-	iface = C.AVAHI_IF_UNSPEC
-	if Conf.LoopbackOnly {
-		iface = loopback
-		old := sysdep.fqdn
-		sysdep.fqdn = "localhost"
-		sysdep.log.Debug(' ', "DNS-SD: FQDN: %q->%q", old, sysdep.fqdn)
-	}
-
+	iface = Conf.Interface
 	proto = C.AVAHI_PROTO_UNSPEC
 	if !Conf.IPV6Enable {
 		proto = C.AVAHI_PROTO_INET
@@ -156,16 +148,10 @@ func newDnssdSysdep(log *Logger, instance string,
 			cInstance = C.CString(instance)
 		}
 
-		// Handle loopback-only mode
-		ifaceInUse := iface
-		if svc.Loopback {
-			ifaceInUse = loopback
-		}
-
 		// Register service type
 		rc = C.avahi_entry_group_add_service_strlst(
 			sysdep.egroup,
-			C.AvahiIfIndex(ifaceInUse),
+			C.AvahiIfIndex(iface),
 			C.AvahiProtocol(proto),
 			0,
 			cInstance,
@@ -187,7 +173,7 @@ func newDnssdSysdep(log *Logger, instance string,
 			cSubtype := C.CString(subtype)
 			rc = C.avahi_entry_group_add_service_subtype(
 				sysdep.egroup,
-				C.AvahiIfIndex(ifaceInUse),
+				C.AvahiIfIndex(iface),
 				C.AvahiProtocol(proto),
 				0,
 				cInstance,

inet_interface.go

@@ -0,0 +1,51 @@
+//go:build linux || freebsd
+
+/* ipp-usb - HTTP reverse proxy, backed by IPP-over-USB connection to device
+ *
+ * Copyright (C) 2020 and up by Alexander Pevzner (pzz@apevzner.com)
+ * See LICENSE for license terms and conditions
+ *
+ * INET interface index discovery
+ */
+
+package main
+
+// #cgo pkg-config: avahi-client
+//
+// #include <stdlib.h>
+// #include <avahi-client/publish.h>
+// #include <avahi-common/error.h>
+// #include <avahi-common/thread-watch.h>
+// #include <avahi-common/watch.h>
+import "C"
+
+import (
+	"errors"
+	"fmt"
+	"net"
+)
+
+// InetInterface returns index of named interface
+func InetInterface(name string) (int, error) {
+	switch name {
+	case "all":
+		return C.AVAHI_IF_UNSPEC, nil
+	case "lo":
+	case "loopback":
+		return Loopback()
+	default:
+		break
+	}
+
+	interfaces, err := net.Interfaces()
+	if err == nil {
+		for _, iface := range interfaces {
+			if iface.Name == name {
+				return iface.Index, nil
+			}
+		}
+		err = errors.New("not found")
+	}
+
+	return 0, fmt.Errorf("Inet interface discovery: %s", err)
+}

inifile.go

@@ -418,6 +418,16 @@ func (rec *IniRecord) LoadIPPort(out *int) error {
 	return nil
 }
 
+func (rec *IniRecord) LoadInetInterface(out *int) error {
+	iFaceIndex, err := InetInterface(rec.Value)
+	if err == nil {
+		Conf.LoopbackOnly = rec.Value == "lo" || rec.Value == "loopback"
+		*out = iFaceIndex
+		return err
+	}
+	return rec.errBadValue(" %s not found", rec.Value)
+}
+
 // LoadBool loads boolean value
 // The destination remains untouched in a case of an error
 func (rec *IniRecord) LoadBool(out *bool) error {

ipp-usb.conf

@@ -13,7 +13,8 @@
   # printer to the local network. This way you can share your printer
   # with other computers in the network, as well as with iOS and Android
   # devices.
-  interface = loopback # all | loopback
+  # Name of the interface (eg. eth0 or lo) or all or loopback
+  interface = loopback # all | loopback | eth0 | vlan.1
 
   # Enable or disable IPv6
   ipv6 = enable        # enable | disable

systemd-udev/ipp-usb.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=Daemon for IPP over USB printer support
 Documentation=man:ipp-usb(8)
-After=cups.service avahi-daemon.service
+After=cups.service avahi-daemon.service network-online.target
 Wants=avahi-daemon.service
 
 [Service]