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tcpdump: strange logging?


 
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# 1  
Old 02-22-2008
tcpdump: strange logging?

Hi,

I want to secure my ntp servers by setting restriction rules. Before doing so I want to monitor the traffic going through the ntp port of the servers to be able to change in time the configuration of "the unwanted clients" so that they do not "loose" the correct time. To do this I am using tcpdump. Mostly is working really fine, just I record 2 types of strange connections which I do not understand, maybe I did not understand how deeply work tcpdump, and I am asking your help to understand them.

the tcp command I am executing is like:

Code:
tcpdump -n -i interface port 123

say sss.sss.sss.sss is the IP address of the machine where I am running the tcpdump (the ntp server), and ccc.ccc.ccc.<number> is the IP of another machine. Note: we have a 10. network, some machine produce a 192.168 addresses but they are not routed. Note: we are speaking about a big company i.e. tons of firewalls, routers etc lie within the servers.

A normal ntp query connection looks like:

Code:
19:33:37.764379 IP ccc.ccc.ccc.001.123 > sss.sss.sss.sss.123: NTPv4, Client, length 48
19:33:37.764421 IP sss.sss.sss.sss.123 > ccc.ccc.ccc.001.123: NTPv4, Server, length 48

remote client requests time (I line Client), the local server answers (II line Server): fine.

Now the strange connection Nr1:

Code:
19:33:38.801105 IP 10.ccc.ccc.002.123 > 10.ccc.ccc.003.123: NTPv4, Server, length 48

Note:
1) it is a single line, not a pair Client, Server
2) the "source" of the packages is not the local host (the machine where I am running the tcpdump)
3) the "destination" of the packages is not the local host the machine where I am running the tcpdump)

where does this package come from?

Now the strange connection Nr2:

Code:
19:33:38.801105 IP 10.ccc.ccc.004.123 > 10.ccc.ccc.005.3121: NTPv4, Server, length 48

Note:
1) same as before but the "destination" port is not 123

where does this package come from?

Now the strange connection Nr3:

Code:
08:00:04.904630 IP 192.168.ccc.006.123 > sss.sss.sss.sss.123: NTPv3, Client, length 48
08:00:04.904646 IP sss.sss.sss.sss.123 > 192.168.ccc.006.123: NTPv3, Server, length 48

Note:
1) Now the structure remote client asks local server answers is respected
2) but the remote client has an IP which is not routed at all??!?!

How can tcpdump get those packages?

So my questions are:
1) How to interpret those packages records? (i.e. do I miss something in the understanding of tcpdump?)
2) Can I ignore those "strange records" in the configuration of my secure ntp server?

Thanks a lot.

Last edited by one71; 02-22-2008 at 06:23 AM..
# 2  
Old 02-25-2008
I have partially understood the strange connections popping up.

tcpdump sets the network interface in promiscuous mode. This means that:

Quote:
In computing, promiscuous mode is a configuration of a network card that makes the card pass all traffic it receives to the central processing unit rather than just packets addressed to it
Still I do not understand the "strange connection Nr3", any idea?
Thanks
# 3  
Old 02-26-2008
To be honest: i have no idea.

Having said this, here is a suspicion: as long as you can arp someone you can have a connection. it doesn't matter that layer 3 wouldn't work when your layer 2 already handles the connection.

Have you looked in your ARP-cache which MAC adress this strange adress maps to?

bakunin

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