[dhcp] Add generic facility for using cached network settings

When a DHCP session is started (using autoboot or a command-line `dhcp
net0'), check whether the new setting use-cached (DHCP option 175.178)
is TRUE; if so, skip DHCP and rely on currently registered
settings. This lets one combine a static IP with autoboot.

Before checking the use-cached setting, call a weak
get_cached_dhcpack() hook that can be implemented by particular builds
of gPXE supporting some fashion of retrieving a cached DHCPACK packet.
If one is available, it is registered as an options source, and then
either that packet's option 175.178 or the user's prior manual
use-cached setting can allow skipping duplicate DHCP.

Using cached packets is not the default because DHCP servers are often
configured to give gPXE different options than they give a vendor PXE
client; in order to break the infinite loop of PXE chaining, one would
need to load a gPXE with an embedded image that does something more
than autoboot.

Signed-off-by: Marty Connor <mdc@etherboot.org>

1 files changed, 21 insertions, 1 deletions

diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index aba90002..0edd0ad8 100644
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -110,6 +110,14 @@ struct setting user_class_setting __setting = {
 	.type = &setting_type_string,
 };
 
+/** Use cached network settings */
+struct setting use_cached_setting __setting = {
+	.name = "use-cached",
+	.description = "Use cached network settings",
+	.tag = DHCP_EB_USE_CACHED,
+	.type = &setting_type_uint8,
+};
+
 /**
  * Name a DHCP packet type
  *
@@ -1237,16 +1245,28 @@ static struct sockaddr dhcp_peer = {
  *
  * @v job		Job control interface
  * @v netdev		Network device
- * @ret rc		Return status code
+ * @ret rc		Return status code, or positive if cached
  *
  * Starts DHCP on the specified network device.  If successful, the
  * DHCPACK (and ProxyDHCPACK, if applicable) will be registered as
  * option sources.
+ *
+ * On a return of 0, a background job has been started to perform the
+ * DHCP request. Any nonzero return means the job has not been
+ * started; a positive return value indicates the success condition of
+ * having fetched the appropriate data from cached information.
  */
 int start_dhcp ( struct job_interface *job, struct net_device *netdev ) {
 	struct dhcp_session *dhcp;
 	int rc;
 
+	/* Check for cached DHCP information */
+	get_cached_dhcpack();
+	if ( fetch_uintz_setting ( NULL, &use_cached_setting ) ) {
+		DBG ( "DHCP using cached network settings\n" );
+		return 1;
+	}
+
 	/* Allocate and initialise structure */
 	dhcp = zalloc ( sizeof ( *dhcp ) );
 	if ( ! dhcp )