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Full Discussion: TCPdump
Special Forums IP Networking TCPdump Post 302951378 by Ultrix on Wednesday 5th of August 2015 05:14:19 PM
Old 08-05-2015
TCPdump

I've recently started learning to use TCPdump, and I find it pretty interesting. There's one thing I don't understand. When I tell it to capture packets on, say, the WiFi interface en1, it often captures packets sent or received by other hosts on the network. How can it do this? My understanding is that on a switched network, a host will only receive packets destined for that host. How is it that I can see packets destined for other hosts?
 

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PCAP_STATS(3PCAP)														 PCAP_STATS(3PCAP)

NAME
pcap_stats - get capture statistics SYNOPSIS
#include <pcap/pcap.h> int pcap_stats(pcap_t *p, struct pcap_stat *ps); DESCRIPTION
pcap_stats() fills in the struct pcap_stat pointed to by its second argument. The values represent packet statistics from the start of the run to the time of the call. pcap_stats() is supported only on live captures, not on ``savefiles''; no statistics are stored in ``savefiles'', so no statistics are available when reading from a ``savefile''. A struct pcap_stat has the following members: ps_recv number of packets received; ps_drop number of packets dropped because there was no room in the operating system's buffer when they arrived, because packets weren't being read fast enough; ps_ifdrop number of packets dropped by the network interface or its driver. The statistics do not behave the same way on all platforms. ps_recv might count packets whether they passed any filter set with pcap_set- filter(3PCAP) or not, or it might count only packets that pass the filter. It also might, or might not, count packets dropped because there was no room in the operating system's buffer when they arrived. ps_drop is not available on all platforms; it is zero on platforms where it's not available. If packet filtering is done in libpcap, rather than in the operating system, it would count packets that don't pass the filter. Both ps_recv and ps_drop might, or might not, count packets not yet read from the operating system and thus not yet seen by the application. ps_ifdrop might, or might not, be implemented; if it's zero, that might mean that no packets were dropped by the interface, or it might mean that the statistic is unavailable, so it should not be treated as an indication that the interface did not drop any packets. RETURN VALUE
pcap_stats() returns 0 on success and returns -1 if there is an error or if p doesn't support packet statistics. If -1 is returned, pcap_geterr() or pcap_perror() may be called with p as an argument to fetch or display the error text. SEE ALSO
pcap(3PCAP), pcap_geterr(3PCAP) 7 September 2009 PCAP_STATS(3PCAP)
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