feat: a run on a host without an IPv6 resolver gets no IPv6 nameserver

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: karo <karolin.kostial@gmail.com>

Author: The127

internal/builder/network.go

@@ -8,24 +8,37 @@ import (
 // mac is the builder's card's, 52:54:00 as QEMU's own and then mis.
 const mac = "52:54:00:6d:69:73"
 
+// Resolving is which address families the host resolves names in. QEMU
+// forwards a query only to a resolver of the query's own family.
+type Resolving struct {
+	IPv4 bool
+	IPv6 bool
+}
+
 // Network is the builder VM's card on QEMU's user network and the network a
 // run gets through it, from one set of numbers so the two agree.
-func Network() (qemu.Card, protocol.Network) {
+func Network(resolving Resolving) (qemu.Card, protocol.Network) {
 	ipv4 := qemu.Family{Prefix: "10.0.2.0/24", Gateway: "10.0.2.2", Nameserver: "10.0.2.3"}
 	ipv6 := qemu.Family{Prefix: "fd6d:6973:6f00::/64", Gateway: "fd6d:6973:6f00::2", Nameserver: "fd6d:6973:6f00::3"}
 
 	card := qemu.Card{MAC: mac, IPv4: ipv4, IPv6: ipv6}
 	network := protocol.Network{
 		Card: mac,
-		IPv4: run(ipv4, "10.0.2.15/24"),
-		IPv6: run(ipv6, "fd6d:6973:6f00::15/64"),
+		IPv4: run(ipv4, "10.0.2.15/24", true),
+		IPv6: run(ipv6, "fd6d:6973:6f00::15/64", resolving.IPv6),
 	}
 
 	return card, network
 }
 
 // run is a run's side of one family of the card's network, with the address
-// given.
-func run(family qemu.Family, address string) protocol.Family {
-	return protocol.Family{Address: address, Gateway: family.Gateway, Nameserver: family.Nameserver}
+// given. It gets the card's nameserver only where the host resolves, one
+// that forwards nowhere would cost every lookup a timeout.
+func run(family qemu.Family, address string, resolves bool) protocol.Family {
+	run := protocol.Family{Address: address, Gateway: family.Gateway}
+	if resolves {
+		run.Nameserver = family.Nameserver
+	}
+
+	return run
 }

internal/builder/network_test.go

@@ -12,7 +12,7 @@ import (
 
 func TestTheRunsLookForTheMACOfTheBuildersCard(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	require.NotEmpty(t, card.MAC)
@@ -21,7 +21,7 @@ func TestTheRunsLookForTheMACOfTheBuildersCard(t *testing.T) {
 
 func TestARunGetsAnIPv4AddressOfItsOwnOnTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv4.Prefix)
@@ -37,7 +37,7 @@ func TestARunGetsAnIPv4AddressOfItsOwnOnTheCardsNetwork(t *testing.T) {
 
 func TestARunGoesOutThroughTheIPv4GatewayOfTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv4.Prefix)
@@ -50,7 +50,7 @@ func TestARunGoesOutThroughTheIPv4GatewayOfTheCardsNetwork(t *testing.T) {
 
 func TestARunResolvesThroughTheIPv4NameserverOfTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv4.Prefix)
@@ -63,7 +63,7 @@ func TestARunResolvesThroughTheIPv4NameserverOfTheCardsNetwork(t *testing.T) {
 
 func TestARunGetsAnIPv6AddressOfItsOwnOnTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv6.Prefix)
@@ -79,7 +79,7 @@ func TestARunGetsAnIPv6AddressOfItsOwnOnTheCardsNetwork(t *testing.T) {
 
 func TestARunGoesOutThroughTheIPv6GatewayOfTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv6.Prefix)
@@ -92,7 +92,7 @@ func TestARunGoesOutThroughTheIPv6GatewayOfTheCardsNetwork(t *testing.T) {
 
 func TestARunResolvesThroughTheIPv6NameserverOfTheCardsNetwork(t *testing.T) {
 	// act
-	card, network := builder.Network()
+	card, network := builder.Network(builder.Resolving{IPv4: true, IPv6: true})
 
 	// assert
 	prefix, err := netip.ParsePrefix(card.IPv6.Prefix)
@@ -102,3 +102,13 @@ func TestARunResolvesThroughTheIPv6NameserverOfTheCardsNetwork(t *testing.T) {
 	assert.True(t, prefix.Contains(nameserver))
 	assert.Equal(t, card.IPv6.Nameserver, network.IPv6.Nameserver)
 }
+
+func TestARunOnAHostThatResolvesOnlyOverIPv4GetsNoIPv6Nameserver(t *testing.T) {
+	// act
+	card, network := builder.Network(builder.Resolving{IPv4: true})
+
+	// assert
+	assert.NotEmpty(t, card.IPv6.Nameserver)
+	assert.Empty(t, network.IPv6.Nameserver)
+	assert.Equal(t, card.IPv4.Nameserver, network.IPv4.Nameserver)
+}