Wrong scope used with unbound and multiple interfaces

[GrantRoot1970]

The wrong scope may be used when the host has multiple interfaces and the container is set to unbound (INSTANCE_IP: 0.0.0.0 and INSTANCE_LISTEN: 0.0.0.0).

This is because the Instance.IP is used to determine the scope rather than using the IP of the interface that received the request (see the following patch). This bug may be present in other functions, but I did not review any others.

PATCH

--- scopes.go.bug       2024-01-30 05:49:52.000000000 -0500
+++ scopes.go   2024-01-30 18:59:15.000000000 -0500
@@ -99,6 +99,21 @@
        // To prioritise requests from a DHCP relay being matched correctly, give their subnet
        // match a 1 bit more priority
        const dhcpRelayBias = 1
+       // Use the instance ip unless the the interface is not bound
+       ip := extconfig.Get().Instance.IP
+       if req.oob != nil {
+               if ief, err := net.InterfaceByIndex(req.oob.IfIndex); err == nil {
+                       if addrs, err := ief.Addrs(); err == nil {
+                               for _, addr := range addrs {
+                                       if ipv4Addr := addr.(*net.IPNet).IP.To4(); ipv4Addr != nil {
+                                               ip = ipv4Addr.String()
+                                               req.log.Debug("Unbound interface found", zap.String("ifname",  ief.Name), zap.String("ip",  ip))
+                                               break
+                                       }
+                               }
+                       }
+               }
+       }
        for _, scope := range r.scopes {
                // Check based on gateway IP (highest priority)
                gatewayMatchBits := scope.match(req.GatewayIPAddr)
@@ -106,12 +121,12 @@
                        req.log.Debug("selected scope based on cidr match (gateway IP)", zap.String("scope", scope.Name))
                        match = scope
                        longestBits = gatewayMatchBits + dhcpRelayBias
                // Handle local broadcast, check with the instance's listening IP
                // Only consider local scopes if we don't have a match already
-               localMatchBits := scope.match(net.ParseIP(extconfig.Get().Instance.IP))
+               localMatchBits := scope.match(net.ParseIP(ip))
                if localMatchBits > -1 && localMatchBits > longestBits {
-                       req.log.Debug("selected scope based on cidr match (instance IP)", zap.String("scope", scope.Name))
+                       req.log.Debug("selected scope based on cidr match (instance/interface IP)", zap.String("scope", scope.Name))
                        match = scope
                        longestBits = localMatchBits
                }

[BeryJu]

@GrantRoot1970 thanks a lot for reporting these issues and providing fixes too!