[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, 6 insertions, 1 deletions

diff --git a/src/usr/dhcpmgmt.c b/src/usr/dhcpmgmt.c
index aa969856..f82a3bb8 100644
--- a/src/usr/dhcpmgmt.c
+++ b/src/usr/dhcpmgmt.c
@@ -55,8 +55,13 @@ int dhcp ( struct net_device *netdev ) {
 	printf ( "DHCP (%s ", netdev->name );
 	while ( hlen-- )
 		printf ( "%02x%c", *(chaddr++), ( hlen ? ':' : ')' ) );
-	if ( ( rc = start_dhcp ( &monojob, netdev ) ) == 0 )
+
+	if ( ( rc = start_dhcp ( &monojob, netdev ) ) == 0 ) {
 		rc = monojob_wait ( "" );
+	} else if ( rc > 0 ) {
+		printf ( " using cached\n" );
+		rc = 0;
+	}
 
 	return rc;
 }