Add host.containers.internal entry into container's etc/hosts

This change adds the entry `host.containers.internal` to the `/etc/hosts`
file within a new containers filesystem. The ip address is determined by
the containers networking configuration and points to the gateway address
for the containers networking namespace.

Closes #5651

Signed-off-by: Baron Lenardson <lenardson.baron@gmail.com>

4 files changed, 169 insertions, 45 deletions

diff --git a/libpod/container.go b/libpod/container.go
index c49d8feeb..bad91d54f 100644
--- a/libpod/container.go
+++ b/libpod/container.go
@@ -126,6 +126,8 @@ type Container struct {
 
 	// This is true if a container is restored from a checkpoint.
 	restoreFromCheckpoint bool
+
+	slirp4netnsSubnet *net.IPNet
 }
 
 // ContainerState contains the current state of the container
diff --git a/libpod/container_internal_linux.go b/libpod/container_internal_linux.go
index 7d57e8965..17b894ce0 100644
--- a/libpod/container_internal_linux.go
+++ b/libpod/container_internal_linux.go
@@ -1358,6 +1358,34 @@ func (c *Container) restore(ctx context.Context, options ContainerCheckpointOpti
 	return c.save()
 }
 
+// Retrieves a container's "root" net namespace container dependency.
+func (c *Container) getRootNetNsDepCtr() (depCtr *Container, err error) {
+	containersVisited := map[string]int{c.config.ID: 1}
+	nextCtr := c.config.NetNsCtr
+	for nextCtr != "" {
+		// Make sure we aren't in a loop
+		if _, visited := containersVisited[nextCtr]; visited {
+			return nil, errors.New("loop encountered while determining net namespace container")
+		}
+		containersVisited[nextCtr] = 1
+
+		depCtr, err = c.runtime.state.Container(nextCtr)
+		if err != nil {
+			return nil, errors.Wrapf(err, "error fetching dependency %s of container %s", c.config.NetNsCtr, c.ID())
+		}
+		// This should never happen without an error
+		if depCtr == nil {
+			break
+		}
+		nextCtr = depCtr.config.NetNsCtr
+	}
+
+	if depCtr == nil {
+		return nil, errors.New("unexpected error depCtr is nil without reported error from runtime state")
+	}
+	return depCtr, nil
+}
+
 // Make standard bind mounts to include in the container
 func (c *Container) makeBindMounts() error {
 	if err := os.Chown(c.state.RunDir, c.RootUID(), c.RootGID()); err != nil {
@@ -1396,24 +1424,9 @@ func (c *Container) makeBindMounts() error {
 			// We want /etc/resolv.conf and /etc/hosts from the
 			// other container. Unless we're not creating both of
 			// them.
-			var (
-				depCtr  *Container
-				nextCtr string
-			)
-
-			// I don't like infinite loops, but I don't think there's
-			// a serious risk of looping dependencies - too many
-			// protections against that elsewhere.
-			nextCtr = c.config.NetNsCtr
-			for {
-				depCtr, err = c.runtime.state.Container(nextCtr)
-				if err != nil {
-					return errors.Wrapf(err, "error fetching dependency %s of container %s", c.config.NetNsCtr, c.ID())
-				}
-				nextCtr = depCtr.config.NetNsCtr
-				if nextCtr == "" {
-					break
-				}
+			depCtr, err := c.getRootNetNsDepCtr()
+			if err != nil {
+				return errors.Wrapf(err, "error fetching network namespace dependency container for container %s", c.ID())
 			}
 
 			// We need that container's bind mounts
@@ -1698,7 +1711,12 @@ func (c *Container) generateResolvConf() (string, error) {
 		nameservers = resolvconf.GetNameservers(resolv.Content)
 		// slirp4netns has a built in DNS server.
 		if c.config.NetMode.IsSlirp4netns() {
-			nameservers = append([]string{slirp4netnsDNS}, nameservers...)
+			slirp4netnsDNS, err := GetSlirp4netnsDNS(c.slirp4netnsSubnet)
+			if err != nil {
+				logrus.Warn("failed to determine Slirp4netns DNS: ", err.Error())
+			} else {
+				nameservers = append([]string{slirp4netnsDNS.String()}, nameservers...)
+			}
 		}
 	}
 
@@ -1779,7 +1797,12 @@ func (c *Container) getHosts() string {
 	if c.Hostname() != "" {
 		if c.config.NetMode.IsSlirp4netns() {
 			// When using slirp4netns, the interface gets a static IP
-			hosts += fmt.Sprintf("# used by slirp4netns\n%s\t%s %s\n", slirp4netnsIP, c.Hostname(), c.config.Name)
+			slirp4netnsIP, err := GetSlirp4netnsGateway(c.slirp4netnsSubnet)
+			if err != nil {
+				logrus.Warn("failed to determine slirp4netnsIP: ", err.Error())
+			} else {
+				hosts += fmt.Sprintf("# used by slirp4netns\n%s\t%s %s\n", slirp4netnsIP.String(), c.Hostname(), c.config.Name)
+			}
 		} else {
 			hasNetNS := false
 			netNone := false
@@ -1802,6 +1825,36 @@ func (c *Container) getHosts() string {
 			}
 		}
 	}
+
+	// Add gateway entry
+	var depCtr *Container
+	if c.config.NetNsCtr != "" {
+		// ignoring the error because there isn't anything to do
+		depCtr, _ = c.getRootNetNsDepCtr()
+	} else if len(c.state.NetworkStatus) != 0 {
+		depCtr = c
+	} else {
+		depCtr = nil
+	}
+
+	if depCtr != nil {
+		for _, pluginResultsRaw := range depCtr.state.NetworkStatus {
+			pluginResult, _ := cnitypes.GetResult(pluginResultsRaw)
+			for _, ip := range pluginResult.IPs {
+				hosts += fmt.Sprintf("%s host.containers.internal\n", ip.Gateway)
+			}
+		}
+	} else if c.config.NetMode.IsSlirp4netns() {
+		gatewayIP, err := GetSlirp4netnsGateway(c.slirp4netnsSubnet)
+		if err != nil {
+			logrus.Warn("failed to determine gatewayIP: ", err.Error())
+		} else {
+			hosts += fmt.Sprintf("%s host.containers.internal\n", gatewayIP.String())
+		}
+	} else {
+		logrus.Debug("network configuration does not support host.containers.internal address")
+	}
+
 	return hosts
 }
 
diff --git a/libpod/networking_linux.go b/libpod/networking_linux.go
index cfed5a1f2..1e763dac5 100644
--- a/libpod/networking_linux.go
+++ b/libpod/networking_linux.go
@@ -37,16 +37,12 @@ import (
 )
 
 const (
-	// slirp4netnsIP is the IP used by slirp4netns to configure the tap device
-	// inside the network namespace.
-	slirp4netnsIP = "10.0.2.100"
-
-	// slirp4netnsDNS is the IP for the built-in DNS server in the slirp network
-	slirp4netnsDNS = "10.0.2.3"
-
 	// slirp4netnsMTU the default MTU override
 	slirp4netnsMTU = 65520
 
+	// default slirp4ns subnet
+	defaultSlirp4netnsSubnet = "10.0.2.0/24"
+
 	// rootlessCNINSName is the file name for the rootless network namespace bind mount
 	rootlessCNINSName = "rootless-cni-ns"
 )
@@ -360,15 +356,20 @@ func (r *Runtime) GetRootlessCNINetNs(new bool) (*RootlessCNI, error) {
 				}
 
 				// build a new resolv.conf file which uses the slirp4netns dns server address
-				resolveIP := slirp4netnsDNS
+				resolveIP, err := GetSlirp4netnsDNS(nil)
+				if err != nil {
+					return nil, errors.Wrap(err, "failed to determine default slirp4netns DNS address")
+				}
+
 				if netOptions.cidr != "" {
 					_, cidr, err := net.ParseCIDR(netOptions.cidr)
 					if err != nil {
 						return nil, errors.Wrap(err, "failed to parse slirp4netns cidr")
 					}
-					// the slirp dns ip is always the third ip in the subnet
-					cidr.IP[len(cidr.IP)-1] = cidr.IP[len(cidr.IP)-1] + 3
-					resolveIP = cidr.IP.String()
+					resolveIP, err = GetSlirp4netnsDNS(cidr)
+					if err != nil {
+						return nil, errors.Wrapf(err, "failed to determine slirp4netns DNS address from cidr: %s", cidr.String())
+					}
 				}
 				conf, err := resolvconf.Get()
 				if err != nil {
@@ -377,7 +378,7 @@ func (r *Runtime) GetRootlessCNINetNs(new bool) (*RootlessCNI, error) {
 				searchDomains := resolvconf.GetSearchDomains(conf.Content)
 				dnsOptions := resolvconf.GetOptions(conf.Content)
 
-				_, err = resolvconf.Build(filepath.Join(cniDir, "resolv.conf"), []string{resolveIP}, searchDomains, dnsOptions)
+				_, err = resolvconf.Build(filepath.Join(cniDir, "resolv.conf"), []string{resolveIP.String()}, searchDomains, dnsOptions)
 				if err != nil {
 					return nil, errors.Wrap(err, "failed to create rootless cni resolv.conf")
 				}
@@ -577,7 +578,7 @@ func (r *Runtime) setupRootlessNetNS(ctr *Container) error {
 		// set up port forwarder for CNI-in-slirp4netns
 		netnsPath := ctr.state.NetNS.Path()
 		// TODO: support slirp4netns port forwarder as well
-		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath, "")
+		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath)
 	}
 	return nil
 }
diff --git a/libpod/networking_slirp4netns.go b/libpod/networking_slirp4netns.go
index c46dc6972..74d390d29 100644
--- a/libpod/networking_slirp4netns.go
+++ b/libpod/networking_slirp4netns.go
@@ -308,15 +308,89 @@ func (r *Runtime) setupSlirp4netns(ctr *Container) error {
 		return err
 	}
 
+	// Set a default slirp subnet. Parsing a string with the net helper is easier than building the struct myself
+	_, ctr.slirp4netnsSubnet, _ = net.ParseCIDR(defaultSlirp4netnsSubnet)
+
+	// Set slirp4netnsSubnet addresses now that we are pretty sure the command executed
+	if netOptions.cidr != "" {
+		ipv4, ipv4network, err := net.ParseCIDR(netOptions.cidr)
+		if err != nil || ipv4.To4() == nil {
+			return errors.Errorf("invalid cidr %q", netOptions.cidr)
+		}
+		ctr.slirp4netnsSubnet = ipv4network
+	}
+
 	if havePortMapping {
 		if netOptions.isSlirpHostForward {
 			return r.setupRootlessPortMappingViaSlirp(ctr, cmd, apiSocket)
 		}
-		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath, netOptions.cidr)
+		return r.setupRootlessPortMappingViaRLK(ctr, netnsPath)
 	}
+
 	return nil
 }
 
+// Get expected slirp ipv4 address based on subnet. If subnet is null use default subnet
+// Reference: https://github.com/rootless-containers/slirp4netns/blob/master/slirp4netns.1.md#description
+func GetSlirp4netnsIP(subnet *net.IPNet) (*net.IP, error) {
+	_, slirpSubnet, _ := net.ParseCIDR(defaultSlirp4netnsSubnet)
+	if subnet != nil {
+		slirpSubnet = subnet
+	}
+	expectedIP, err := addToIP(slirpSubnet, uint32(100))
+	if err != nil {
+		return nil, errors.Wrapf(err, "error calculating expected ip for slirp4netns")
+	}
+	return expectedIP, nil
+}
+
+// Get expected slirp Gateway ipv4 address based on subnet
+// Reference: https://github.com/rootless-containers/slirp4netns/blob/master/slirp4netns.1.md#description
+func GetSlirp4netnsGateway(subnet *net.IPNet) (*net.IP, error) {
+	_, slirpSubnet, _ := net.ParseCIDR(defaultSlirp4netnsSubnet)
+	if subnet != nil {
+		slirpSubnet = subnet
+	}
+	expectedGatewayIP, err := addToIP(slirpSubnet, uint32(2))
+	if err != nil {
+		return nil, errors.Wrapf(err, "error calculating expected gateway ip for slirp4netns")
+	}
+	return expectedGatewayIP, nil
+}
+
+// Get expected slirp DNS ipv4 address based on subnet
+// Reference: https://github.com/rootless-containers/slirp4netns/blob/master/slirp4netns.1.md#description
+func GetSlirp4netnsDNS(subnet *net.IPNet) (*net.IP, error) {
+	_, slirpSubnet, _ := net.ParseCIDR(defaultSlirp4netnsSubnet)
+	if subnet != nil {
+		slirpSubnet = subnet
+	}
+	expectedDNSIP, err := addToIP(slirpSubnet, uint32(3))
+	if err != nil {
+		return nil, errors.Wrapf(err, "error calculating expected dns ip for slirp4netns")
+	}
+	return expectedDNSIP, nil
+}
+
+// Helper function to calculate slirp ip address offsets
+// Adapted from: https://github.com/signalsciences/ipv4/blob/master/int.go#L12-L24
+func addToIP(subnet *net.IPNet, offset uint32) (*net.IP, error) {
+	// I have no idea why I have to do this, but if I don't ip is 0
+	ipFixed := subnet.IP.To4()
+
+	ipInteger := uint32(ipFixed[3]) | uint32(ipFixed[2])<<8 | uint32(ipFixed[1])<<16 | uint32(ipFixed[0])<<24
+	ipNewRaw := ipInteger + offset
+	// Avoid overflows
+	if ipNewRaw < ipInteger {
+		return nil, errors.Errorf("integer overflow while calculating ip address offset, %s + %d", ipFixed, offset)
+	}
+	ipNew := net.IPv4(byte(ipNewRaw>>24), byte(ipNewRaw>>16&0xFF), byte(ipNewRaw>>8)&0xFF, byte(ipNewRaw&0xFF))
+	if !subnet.Contains(ipNew) {
+		return nil, errors.Errorf("calculated ip address %s is not within given subnet %s", ipNew.String(), subnet.String())
+	}
+	return &ipNew, nil
+}
+
 func waitForSync(syncR *os.File, cmd *exec.Cmd, logFile io.ReadSeeker, timeout time.Duration) error {
 	prog := filepath.Base(cmd.Path)
 	if len(cmd.Args) > 0 {
@@ -363,7 +437,7 @@ func waitForSync(syncR *os.File, cmd *exec.Cmd, logFile io.ReadSeeker, timeout t
 	return nil
 }
 
-func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath, slirp4CIDR string) error {
+func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath string) error {
 	syncR, syncW, err := os.Pipe()
 	if err != nil {
 		return errors.Wrapf(err, "failed to open pipe")
@@ -390,17 +464,11 @@ func (r *Runtime) setupRootlessPortMappingViaRLK(ctr *Container, netnsPath, slir
 		}
 	}
 
-	childIP := slirp4netnsIP
-	// set the correct childIP when a custom cidr is set
-	if slirp4CIDR != "" {
-		_, cidr, err := net.ParseCIDR(slirp4CIDR)
-		if err != nil {
-			return errors.Wrap(err, "failed to parse slirp4netns cidr")
-		}
-		// the slirp container ip is always the hundredth ip in the subnet
-		cidr.IP[len(cidr.IP)-1] = cidr.IP[len(cidr.IP)-1] + 100
-		childIP = cidr.IP.String()
+	slirp4netnsIP, err := GetSlirp4netnsIP(ctr.slirp4netnsSubnet)
+	if err != nil {
+		return errors.Wrapf(err, "failed to get slirp4ns ip")
 	}
+	childIP := slirp4netnsIP.String()
 outer:
 	for _, r := range ctr.state.NetworkStatus {
 		for _, i := range r.IPs {