libpcap: How to timeout pcap_loop() ?

B-Con · 12-15-2007, 05:29 PM

I'm using the libpcap library to sniff packets on an interface. (My target OS is Linux, but I don't think that helps here.)

I'm using pcap_loop() to capture packets, but pcap_loop() waits until a packet is captured before returning to the calling function. Is there any way to force it to only listen for a specified amount of time? I'd like to loop the main program while just checking each loop for new packets, rather than having to wait for a new packet to arrive before doing anything. I've looked around and am pretty sure there's no built-in feature for this (that works on Linux) so might there exist some type of work-around?

vart · 12-15-2007, 10:57 PM

probably you want to look something in inet like
http://www.tcpdump.org/lists/workers.../msg00178.html

Haven't read it throughoutly but seems to me they are talking about same type of problems

B-Con · 01-17-2008, 10:04 PM

From the link:

Quote:

If switching to pcap_dispatch() instead of pcap_loop() is not a bad option
for you, one other way of doing what you want is use select for timing
out. So you can access the pcap file descriptor and select on it for
either reads or timeouts. When select returns, if read fd_set is set, you
can call pcap_dispatch(), otherwise for timeout you can do your other
tasks.

What's this "select" he's talking about?

(BTW, sorry for taking so long to return, holidays+travel+start of school kept me busy. :P )

bithub · 01-18-2008, 12:57 AM

select() is a function which allows you to wait on I/O. It would allow you to check if there is data to be read on a socket, and then time out if there is nothing to receive.

12-15-2007, 05:29 PM	#1
B-Con Dogbert Join Date: Mar 2004 Posts: 25	libpcap: How to timeout pcap_loop() ? I'm using the libpcap library to sniff packets on an interface. (My target OS is Linux, but I don't think that helps here.) I'm using pcap_loop() to capture packets, but pcap_loop() waits until a packet is captured before returning to the calling function. Is there any way to force it to only listen for a specified amount of time? I'd like to loop the main program while just checking each loop for new packets, rather than having to wait for a new packet to arrive before doing anything. I've looked around and am pretty sure there's no built-in feature for this (that works on Linux) so might there exist some type of work-around? __________________ "Cryptographically secure linear feedback shift register based stream ciphers" -- a phrase that'll get any party started.
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12-15-2007, 10:57 PM	#2
vart CSharpener Join Date: Oct 2006 Posts: 5,242	probably you want to look something in inet like http://www.tcpdump.org/lists/workers.../msg00178.html Haven't read it throughoutly but seems to me they are talking about same type of problems __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is offline

01-18-2008, 12:57 AM	#4
bithub Registered User Join Date: Sep 2004 Location: California Posts: 2,805	select() is a function which allows you to wait on I/O. It would allow you to check if there is data to be read on a socket, and then time out if there is nothing to receive.
bithub is offline

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