Use shorter pcap buffer timeout for response capture

dhcp-probe uses response_wait_time as the total window for listening
for DHCP/BOOTP replies, enforced by alarm() and pcap_breakloop().

The timeout passed to pcap_open_live(), however, is a packet buffer
timeout.  When both values are the same, replies can remain buffered
inside libpcap until dhcp_probe's alarm fires.  In that case
pcap_dispatch() can return due to pcap_breakloop() before the buffered
reply is delivered to process_response(), causing the alert path to be
missed. This is an undesired race condition resulting in the dhcp-probe
tool to fail to detect DHCP servers nearly 100 percent of the time (at
least on certian networks).

Use a shorter pcap buffer timeout derived from response_wait_time so
pcap_dispatch() wakes periodically and processes captured replies before
the overall response window expires.

Change-Id: If4b9d6076447fcecdbe069277325d067dfb0fd55

Author: Konur

src/dhcp_probe.c

@@ -32,6 +32,9 @@ static const char copyright[] = "Copyright 2000-2021, The Trustees of Princeton
 static const char contact[] = "networking at princeton dot edu";
 #endif
 
+#define PCAP_PACKET_BUFFER_TIMEOUT_DIVISOR 20
+#define PCAP_PACKET_BUFFER_TIMEOUT_MIN_MS 1
+
 /* initialize options to defaults */
 int debug = 0;
 int dont_fork = 0;
@@ -75,6 +78,9 @@ struct ether_addr my_eaddr;
 int use_8021q = 0;
 int vlan_id = 0;
 
+/* Forward Declarations */
+static int get_pcap_packet_buffer_timeout_ms(void);
+
 int 
 main(int argc, char **argv)
 {
@@ -517,7 +523,7 @@ main(int argc, char **argv)
 			pcap_open_retries = PCAP_OPEN_LIVE_RETRY_MAX;
 			while (pcap_open_retries--) {
 				pcap_errbuf[0] = '\0'; /* so we can tell if a warning was produced on success */
-				if ((pd = pcap_open_live(ifname, snaplen, promiscuous, GetResponse_wait_time(), pcap_errbuf)) != NULL) {
+				if ((pd = pcap_open_live(ifname, snaplen, promiscuous, get_pcap_packet_buffer_timeout_ms(), pcap_errbuf)) != NULL) {
 					break; /* success */
 				} else { /* failure */
 					if (pcap_open_retries == 0) {
@@ -1297,3 +1303,22 @@ usage(void)
 	return;
 }
 
+/*
+ * pcap_open_live()'s timeout is a packet buffer timeout, not the
+ * total amount of time dhcp_probe should wait for responses.  Keep
+ * the pcap buffer timeout shorter than response_wait_time so
+ * pcap_dispatch() periodically returns and processes any buffered
+ * replies before our SIGALRM-based response window expires.
+ */
+static int 
+get_pcap_packet_buffer_timeout_ms(void)
+{
+	int pcap_timeout;
+
+	pcap_timeout = GetResponse_wait_time() / PCAP_PACKET_BUFFER_TIMEOUT_DIVISOR;
+
+	if (pcap_timeout < PCAP_PACKET_BUFFER_TIMEOUT_MIN_MS)
+		pcap_timeout = PCAP_PACKET_BUFFER_TIMEOUT_MIN_MS;
+
+	return pcap_timeout;
+}