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Fix missed DHCP responses caused by pcap timeout race #1

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Fix missed DHCP responses caused by pcap timeout race #1

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27 changes: 26 additions & 1 deletion src/dhcp_probe.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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)
{
Expand Down Expand Up @@ -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) {
Expand Down Expand Up @@ -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;
}