[dns] Support DNS search lists

Update the DNS resolver to support DNS search lists (as provided by
DHCP option 119, DHCPv6 option 24, or NDP option 31).

Add validation code to ensure that parsing of DNS packets does not
overrun the input, get stuck in infinite loops, or (worse) write
beyond the end of allocated buffers.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/udp/dhcp.c

@@ -86,6 +86,7 @@ static uint8_t dhcp_request_options_data[] = {
 		      DHCP_LOG_SERVERS, DHCP_HOST_NAME, DHCP_DOMAIN_NAME,
 		      DHCP_ROOT_PATH, DHCP_VENDOR_ENCAP, DHCP_VENDOR_CLASS_ID,
 		      DHCP_TFTP_SERVER_NAME, DHCP_BOOTFILE_NAME,
+		      DHCP_DOMAIN_SEARCH,
 		      128, 129, 130, 131, 132, 133, 134, 135, /* for PXE */
 		      DHCP_EB_ENCAP, DHCP_ISCSI_INITIATOR_IQN ),
 	DHCP_END

src/net/udp/dhcpv6.c

@@ -979,3 +979,12 @@ const struct setting filename6_setting __setting ( SETTING_BOOT, filename ) = {
 	.type = &setting_type_string,
 	.scope = &ipv6_scope,
 };
+
+/** DNS search list setting */
+const struct setting dnssl6_setting __setting ( SETTING_IP_EXTRA, dnssl ) = {
+	.name = "dnssl",
+	.description = "DNS search list",
+	.tag = DHCPV6_DOMAIN_LIST,
+	.type = &setting_type_dnssl,
+	.scope = &ipv6_scope,
+};