Modify /etc/resolv.conf when connecting/disconnecting

The `podman network connect` and `podman network disconnect`
commands give containers access to different networks than the
ones they were created with; these networks can also have DNS
servers associated with them. Until now, however, we did not
modify resolv.conf as network membership changed.

With this PR, `podman network connect` will add any new
nameservers supported by the new network to the container's
/etc/resolv.conf, and `podman network disconnect` command will do
the opposite, removing the network's nameservers from
`/etc/resolv.conf`.

Fixes #9603

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/networking_linux.go

@@ -1170,6 +1170,7 @@ func (c *Container) NetworkDisconnect(nameOrID, netName string, force bool) erro
 	}
 
 	// update network status if container is running
+	oldStatus, statusExist := networkStatus[netName]
 	delete(networkStatus, netName)
 	c.state.NetworkStatus = networkStatus
 	err = c.save()
@@ -1180,8 +1181,26 @@ func (c *Container) NetworkDisconnect(nameOrID, netName string, force bool) erro
 	// Reload ports when there are still connected networks, maybe we removed the network interface with the child ip.
 	// Reloading without connected networks does not make sense, so we can skip this step.
 	if rootless.IsRootless() && len(networkStatus) > 0 {
-		return c.reloadRootlessRLKPortMapping()
+		if err := c.reloadRootlessRLKPortMapping(); err != nil {
+			return err
+		}
 	}
+
+	// Update resolv.conf if required
+	if statusExist {
+		stringIPs := make([]string, 0, len(oldStatus.DNSServerIPs))
+		for _, ip := range oldStatus.DNSServerIPs {
+			stringIPs = append(stringIPs, ip.String())
+		}
+		if len(stringIPs) == 0 {
+			return nil
+		}
+		logrus.Debugf("Removing DNS Servers %v from resolv.conf", stringIPs)
+		if err := c.removeNameserver(stringIPs); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }
 
@@ -1263,11 +1282,36 @@ func (c *Container) NetworkConnect(nameOrID, netName string, netOpts types.PerNe
 	if err != nil {
 		return err
 	}
+
 	// The first network needs a port reload to set the correct child ip for the rootlessport process.
 	// Adding a second network does not require a port reload because the child ip is still valid.
 	if rootless.IsRootless() && len(networks) == 0 {
-		return c.reloadRootlessRLKPortMapping()
+		if err := c.reloadRootlessRLKPortMapping(); err != nil {
+			return err
+		}
 	}
+
+	ipv6, err := c.checkForIPv6(networkStatus)
+	if err != nil {
+		return err
+	}
+
+	// Update resolv.conf if required
+	stringIPs := make([]string, 0, len(results[netName].DNSServerIPs))
+	for _, ip := range results[netName].DNSServerIPs {
+		if (ip.To4() == nil) && !ipv6 {
+			continue
+		}
+		stringIPs = append(stringIPs, ip.String())
+	}
+	if len(stringIPs) == 0 {
+		return nil
+	}
+	logrus.Debugf("Adding DNS Servers %v to resolv.conf", stringIPs)
+	if err := c.addNameserver(stringIPs); err != nil {
+		return err
+	}
+
 	return nil
 }