Tcpdump dropping packets

Chuck Swiger cswiger at
Thu Jun 8 02:44:54 UTC 2006

Paul Schmehl wrote:
> I'm fiddling around with ntop, but, after an initial packet capture, it 
> doesn't capture any more traffic.  It claims that libpcap is dropping 
> all the packets.
> If I run tcpdump like this:
> tcpdump -i <interface>
> I get this:
> 15 packets captured
> 51104 packets received by filter
> 50288 packets dropped by kernel
> If I run tcpdump like this:
> tcpdump -i <interface> -w filename
> I get this:
> 65235 packets captured
> 65489 packets received by filter
> 0 packets dropped by kernel
> Is there a sysctl tweak that can at least reduce the packet loss?  Is 
> there a setting in ntop that I'm missing?

tcpdump can write to a file for decoding later much more efficiently than it 
can deal with live processing, DNS lookups, etc.  You can help matters out 
slightly by increasing the underlying PCAP/BPF buffer size or by filtering out 
all but the traffic you want to see.

Check sysctl debug.bpf_bufsize, but also do a search on this because there may 
be a patch needed for PCAP in order for buffers larger than 32K to actually 
work. [1]

> If I send tcpdump to a file, can ntop read the file continuously?  Or 
> will it only read it one time?

Dunno.  I recall that ntop-1 was much more useful and stable than the current 
ntop seems to be...


[1]: Or has that been fixed?

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