Start and stop a bash script based on network activity?

creativetweak · 05-04-2008, 05:51 AM

I have a wget script that I use to queue large downloads. Currently I use cron to shedule it to run late at night and kill it in the morning so I still have all of my limited bandwidth available during the day.

What I'm wanting to do is remove the cron entries and have the script start with the machine. Then I need somthing to stop the script when any of the other machines on the networks send any interesting packets (ie http, ftp, etc)

Any ideas?

unSpawn · 05-04-2008, 06:26 AM

Wouldn't it be easier to shape bandwidth? It can be done so the class wget is in starves when other prio streams are necessary.

creativetweak · 05-04-2008, 06:33 AM

From memory shaping dowstream is painfull and wastefull as there is no way to slow it short of dropping packets and hoping the sender gets the idea and reduces its tcp window.

If I'm way off let me know...

seraphim172 · 05-04-2008, 07:10 AM

Leave cron settings as they are, put your conditional branches into the script that is called by cron.

The definition of "interesting packets (ie http, ftp, etc)" on a network is a nice, but not very helpful term. If you are doing a gigabyte download with wget and you open your browser for a Google search, what do you want to happen? Abort wget even if only 2 bytes are missing from the full download, or keep it running even if you only had 2 bytes downloaded so far.

Setting '--limit-rate' for wget will only apply to the next download. Wget will certainly retry downloading later. I don't know if it can resume exiting ones and how reliable that is.

I would rather make the script to look for a certain value in a prefs file to see if a next wget download can begin. The prefs file can be a text file with "run" or "pause" as the content, written by hand or by a webpage script.

Not very sophisticated, but quite simple. That's what I would do.

Linux Archive

creativetweak · 05-04-2008, 08:06 AM

If I had a gigabyte+ download running and someone fires up a browser the download should be dropped instantly and hope that wget can resume downloading.

Im trying to throw together a bash script at the moment that will pipe interesting packets (with the help of some tcpdump filters) into a loop that keeps track of the last interesting packet. when the time since the last interesting packet is greater then some threshold (say 5 mins) then start the wget script. If there has been a packet in the last 5 mins then make sure the script is stopped.

creativetweak · 05-04-2008, 08:33 AM

Here it is so far... Not perfect... there seems to be some buffering happening somehwere and I dont like relying on saving the lastpacket time to disk... But it does serve as an example of what im looking for if anyone knows of anything similar already out there...

Code:

idletime=5		# how long between interesting packets before starting?
date +%s > lastpacket
status="stopped"

# 'interesting ports'.. only port 80 for now, also need to filter out traffic
# to/from localhost.
tcpdump port 80 | while read line; do
	date +%s > lastpacket
done&

while [ 1 ]; do
	sleep 1
	lastime=`cat lastpacket`
	curtime=`date +%s`
	elapsed=`expr $curtime - $lastime`

	if [ $elapsed -gt $idletime ] && [ $status = "stopped" ] ; then
		echo "Start script"
		status="running"
	fi

	if [ $elapsed -lt $idletime ] && [ $status = "running" ] ; then
		echo "Stop script"
		status="stopped"
	fi
done

konsolebox · 05-04-2008, 07:04 PM

i suggest you do elif instead:

Code:

	if [ $elapsed -gt $idletime ] && [ $status = "stopped" ] ; then
		echo "Start script"
		status="running"
	elif [ $elapsed -lt $idletime ] && [ $status = "running" ] ; then
		echo "Stop script"
		status="stopped"
	fi

and in while:

Code:

while :; do ...; done

creativetweak · 05-04-2008, 07:46 PM

Cheers, Couldnt remember the else if bash syntax... Im assuming : evaluates to true?

konsolebox · 05-04-2008, 10:28 PM

it's not its real purpose to return true but it always does return true.