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 0c5d187..7631392 100644 (file)
--- 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