From: florian <florian@openbsd.org>
Date: Mon, 22 Mar 2021 16:28:25 +0000 (+0000)
Subject: BOOTP has a minimum packet length of 300 bytes. Since DHCP is
X-Git-Url: http://artulab.com/gitweb/?a=commitdiff_plain;h=2d2edb0e3ca50fe71a66c867079a162d6456111e;p=openbsd

BOOTP has a minimum packet length of 300 bytes. Since DHCP is
interoperable with BOOTP we should also send packets that have a
minimum size of 300.
I haven't seen a DHCP server that actually enforces this except the
one in vmd(8), but it doesn't cost us much and prevents hair pulling
later on when we find one in the wild.
OK deraadt
---

diff --git a/sbin/dhcpleased/frontend.c b/sbin/dhcpleased/frontend.c
index 0c5d187e4c5..76313923dc8 100644
--- a/sbin/dhcpleased/frontend.c
+++ b/sbin/dhcpleased/frontend.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: frontend.c,v 1.7 2021/03/19 07:43:27 florian Exp $	*/
+/*	$OpenBSD: frontend.c,v 1.8 2021/03/22 16:28:25 florian Exp $	*/
 
 /*
  * Copyright (c) 2017, 2021 Florian Obser <florian@openbsd.org>
@@ -57,6 +57,7 @@
 #include "checksum.h"
 
 #define	ROUTE_SOCKET_BUF_SIZE	16384
+#define	BOOTP_MIN_LEN		300	/* fixed bootp packet adds up to 300 */
 
 struct bpf_ev {
 	struct event		 ev;
@@ -783,6 +784,7 @@ build_packet(uint8_t message_type, uint32_t xid, struct ether_addr *hw_address,
 	static uint8_t	 dhcp_server_identifier[] = {DHO_DHCP_SERVER_IDENTIFIER,
 		4, 0, 0, 0, 0};
 	struct dhcp_hdr	*hdr;
+	ssize_t		 len;
 	uint8_t		*p;
 	char		*c;
 
@@ -834,7 +836,13 @@ build_packet(uint8_t message_type, uint32_t xid, struct ether_addr *hw_address,
 	*p = DHO_END;
 	p += 1;
 
-	return (p - dhcp_packet);
+	len = p - dhcp_packet;
+
+	/* dhcp_packet is initialized with DHO_PADs */
+	if (len < BOOTP_MIN_LEN)
+		len = BOOTP_MIN_LEN;
+
+	return (len);
 }
 
 void