Fix very hard to reach DoS attack vector, which would involve more than

8 billion network packets.  Mixture of many many malformed and proper
packets could result in a division by zero.

ok krw@

diff --git a/usr.sbin/dhcpd/packet.c b/usr.sbin/dhcpd/packet.c
index 30efac8..24520fc 100644 (file)
--- a/usr.sbin/dhcpd/packet.c
+++ b/usr.sbin/dhcpd/packet.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: packet.c,v 1.6 2013/12/05 21:03:40 krw Exp $  */
+/*     $OpenBSD: packet.c,v 1.7 2014/07/26 07:48:49 tobias Exp $       */
 
 /* Packet assembly code, originally contributed by Archie Cobbs. */
 
@@ -180,7 +180,7 @@ decode_udp_ip_header(struct interface_info *interface, unsigned char *buf,
        ip_packets_seen++;
        if (wrapsum(checksum(buf + bufix, ip_len, 0)) != 0) {
                ip_packets_bad_checksum++;
-               if (ip_packets_seen > 4 &&
+               if (ip_packets_seen > 4 && ip_packets_bad_checksum != 0 &&
                    (ip_packets_seen / ip_packets_bad_checksum) < 2) {
                        note("%d bad IP checksums seen in %d packets",
                            ip_packets_bad_checksum, ip_packets_seen);
@@ -206,6 +206,7 @@ decode_udp_ip_header(struct interface_info *interface, unsigned char *buf,
        if ((len < 0) || (len + data > buf + bufix + buflen)) {
                udp_packets_length_overflow++;
                if (udp_packets_length_checked > 4 &&
+                   udp_packets_length_overflow != 0 &&
                    (udp_packets_length_checked /
                    udp_packets_length_overflow) < 2) {
                        note("%d udp packets in %d too long - dropped",
@@ -230,7 +231,7 @@ decode_udp_ip_header(struct interface_info *interface, unsigned char *buf,
        udp_packets_seen++;
        if (usum && usum != sum) {
                udp_packets_bad_checksum++;
-               if (udp_packets_seen > 4 &&
+               if (udp_packets_seen > 4 && udp_packets_bad_checksum != 0 &&
                    (udp_packets_seen / udp_packets_bad_checksum) < 2) {
                        note("%d bad udp checksums in %d packets",
                            udp_packets_bad_checksum, udp_packets_seen);