Re: [PATCH] RFC: fall back to IPv4-LL in case of DHCP failure

[Pavel Simerda <psimerda redhat com>]

> In some cases, it is useful to have a static configuration which
> succeeds
> to configure a network interface regardless of whether a DHCP server
> is available or not. One such use-case is an embedded system with
> factory default settings, which would fall back to IPv4-LL when
> connected directly to the notebook of a field engineer.

Please also compare to IPv6-LL. The IPv6 way has the advantage of
allways having a link-local address managed by the kernel. The
only thing that is needed is to have the interface 'up'.

It doesn't interfere with global address configuration and it works
without timeouts.

Currently, the device with link-local IPv6 address is not considered
configured (when mode=auto) but it's usable for LL communication
unless NM sets the interface 'down'.

> With this patch, when DHCP reports a timeout, the connection attempt
> is restart with phase 3 of autoipd-based IPv4 Link-Local
> configuration.
> For now this behaviour is hard-coded, but I'd make it configurable
> if this approach is acceptable.
> 
> For configuration, a new IPv4 method could be added (either a new
> 'Automatic with fall back' or 'DHCP only', making enabled fall back
> the default in 'Automatic'). Alternatively an option could be added
> to the IPv4 configuration (i.e. 'll-fall-back=true').

The problem with this approach is that for example a lame wireless
connection will succeed to setup IPv4LL and networking will be
broken even though another wireless connection would succeed fully.

Cheers,

Pavel


> Best regards,
> Jan Luebbe
> 
> ---
>  src/nm-device.c |   28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/src/nm-device.c b/src/nm-device.c
> index 6eaa94d..886d5bc 100644
> --- a/src/nm-device.c
> +++ b/src/nm-device.c
> @@ -204,9 +204,10 @@ typedef struct {
>  	DBusGProxyCall    *fw_call;
>  
>  	/* avahi-autoipd stuff */
> -	GPid    aipd_pid;
> -	guint   aipd_watch;
> -	guint   aipd_timeout;
> +	GPid     aipd_pid;
> +	guint    aipd_watch;
> +	guint    aipd_timeout;
> +	gboolean aipd_failed;
>  
>  	/* IP6 configuration info */
>  	NMIP6Config *  ip6_config;
> @@ -1262,6 +1263,8 @@ aipd_cleanup (NMDevice *self)
>  	}
>  
>  	aipd_timeout_remove (self);
> +
> +	priv->aipd_failed = FALSE;
>  }
>  
>  static NMIP4Config *
> @@ -1331,7 +1334,8 @@ nm_device_handle_autoip4_event (NMDevice *self,
>  	if (s_ip4)
>  		method = nm_setting_ip4_config_get_method (s_ip4);
>  
> -	if (g_strcmp0 (method, NM_SETTING_IP4_CONFIG_METHOD_LINK_LOCAL) !=
> 0)
> +	if (   g_strcmp0 (method, NM_SETTING_IP4_CONFIG_METHOD_LINK_LOCAL)
> != 0
> +	    && g_strcmp0 (method, NM_SETTING_IP4_CONFIG_METHOD_AUTO) != 0)
>  		return;
>  
>  	iface = nm_device_get_iface (self);
> @@ -1423,6 +1427,7 @@ aipd_timeout_cb (gpointer user_data)
>  		nm_log_info (LOGD_AUTOIP4, "(%s): avahi-autoipd timed out.",
>  		nm_device_get_iface (self));
>  		priv->aipd_timeout = 0;
>  		aipd_cleanup (self);
> +		priv->aipd_failed = TRUE;
>  
>  		if (priv->ip4_state == IP_CONF)
>  			nm_device_activate_schedule_ip4_config_timeout (self);
> @@ -2499,6 +2504,21 @@
> nm_device_activate_schedule_stage3_ip_config_start (NMDevice *self)
>  static NMActStageReturn
>  real_act_stage4_ip4_config_timeout (NMDevice *self,
>  NMDeviceStateReason *reason)
>  {
> +	NMDevicePrivate *priv = NM_DEVICE_GET_PRIVATE (self);
> +	NMConnection *connection;
> +	NMSettingIP4Config *s_ip4;
> +	const char *method;
> +
> +	connection = nm_device_get_connection (self);
> +	g_assert (connection);
> +
> +	s_ip4 = nm_connection_get_setting_ip4_config (connection);
> +	g_assert (s_ip4);
> +
> +	method = nm_setting_ip4_config_get_method (s_ip4);
> +	if (   strcmp (method, NM_SETTING_IP4_CONFIG_METHOD_AUTO) == 0
> +	    && !priv->aipd_failed)
> +		return aipd_start (self, reason);
>  	if (nm_device_ip_config_should_fail (self, FALSE)) {
>  		*reason = NM_DEVICE_STATE_REASON_IP_CONFIG_UNAVAILABLE;
>  		return NM_ACT_STAGE_RETURN_FAILURE;
> --
> 1.7.10
> 
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