[dhcp] Honor PXEBS_SKIP option in discovery control

It is permissible for a DHCP packet containing PXE options to specify
only "discovery control", instead of the more typical boot menu +
prompt options. This is the strategy used by older versions of
dnsmasq; by specifying the discovery control as PXEBS_SKIP, they cause
vendor PXE ROMs to ignore boot server discovery and just use the
filename and next-server options in the initial (Proxy)DHCP packet.
Modify gPXE to accept this behavior, to be more compatible with the
Intel firmware.

Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Tested-by: Kyle Kienapfel <kyle@shadowmage.org>

diff --git a/src/net/udp/dhcp.c b/src/net/udp/dhcp.c
index ce2c820..fa09741 100644 (file)
--- a/src/net/udp/dhcp.c
+++ b/src/net/udp/dhcp.c
@@ -401,6 +401,7 @@ static void dhcp_rx_offer ( struct dhcp_session *dhcp,
        int has_pxeclient;
        int pxeopts_len;
        int has_pxeopts;
+       uint8_t discovery_control;
        struct dhcp_offer *offer;
        int i;
 
@@ -439,9 +440,35 @@ static void dhcp_rx_offer ( struct dhcp_session *dhcp,
        has_pxeclient = ( ( vci_len >= ( int ) sizeof ( vci ) ) &&
                          ( strncmp ( "PXEClient", vci, sizeof (vci) ) == 0 ));
 
-       /* Identify presence of PXE-specific options */
+       /*
+        * Identify presence of PXE-specific options
+        *
+        * The Intel firmware appears to check for:
+        * - PXE_DISCOVERY_CONTROL exists and has bit 3 set, or
+        * - both PXE_BOOT_MENU and PXE_BOOT_MENU_PROMPT exist
+        *
+        * If DISCOVERY_CONTROL bit 3 is set, the firmware treats this
+        * packet like a "normal" non-PXE DHCP packet with respect to
+        * boot filename, except that it can come from ProxyDHCP. This
+        * is the scheme that dnsmasq uses in the simple case.
+        *
+        * Otherwise, if one of the boot menu / boot menu prompt
+        * options exists but not both, the firmware signals an
+        * error. If neither exists, the packet is not considered to
+        * contain DHCP options.
+        *
+        * In an effort to preserve semantics but be more flexible, we
+        * check only for bit 3 of DISCOVERY_CONTROL or the presence
+        * of BOOT_MENU. We don't care (yet) about the menu prompt.
+        */
        pxeopts_len = dhcppkt_fetch ( dhcppkt, DHCP_PXE_BOOT_MENU, NULL, 0 );
        has_pxeopts = ( pxeopts_len >= 0 );
+       if ( dhcppkt_fetch ( dhcppkt, DHCP_PXE_DISCOVERY_CONTROL,
+                            &discovery_control, 1 ) >= 0 ) {
+               /* Honor request to skip boot server discovery and use
+                  the provided filename */
+               has_pxeopts = has_pxeopts || ( discovery_control & PXEBS_SKIP );
+       }
        if ( has_pxeclient )
                DBGC ( dhcp, "%s", ( has_pxeopts ? " pxe" : " proxy" ) );