reason for dropped packets?

Hello everyone,

I am using a linux box as a router. I have enabled packet
forwarding in my Linux machine (kernel 2.6.16-13) via the

echo 1 > /proc/sys/net/ipv4/ip_forward command.


Packet forwarding over the various interfaces is working fine.


I am sending anonymized packets (using a tool called Click) which
generates addresses from the entire IPv4 address space to this
machine. My routing table (which is about 167K entries long) is set up
to forward all the incoming packets to one of two interfaces eth0 and
eth1 (there are a total of 4 interfaces in the box). A couple of other
boxes at the other end of these interfaces are collecting these
packets and couting them.

I keep observing a certain amount of packets being dropped by the
router. The number of dropped packets vary each time I run my test
script. Some are being dropped by the incoming NIC itself (as observed
by the output of ifconfig <interface_name>), but these are very few
compared to the ones being dropped by the router itself.

Nothing is turning up in /var/log/messages and I have set the
printk verbosity to 8 using the
echo 8 > /proc/sys/kernel/printk command.

Two questions:
---------------------
How do I figure out why these packets are being dropped?
How do I count the number of packets being dropped?


Just an aside:
When I send non-anonymized packets to this machine (ie.
by using the IP addresses of the subnets attached to the router) no
packets are being lost at all even for much higher packets per second
injection rate. Also, the switches are not losing packets because I
ran the tests using short cross-over cables also.


Thank You.

Azeem Khan
Bombay, India.

Hello,

Azeem a écrit :
>
> I am sending anonymized packets (using a tool called Click) which
> generates addresses from the entire IPv4 address space to this
> machine. [...]
> I keep observing a certain amount of packets being dropped by the
> router. The number of dropped packets vary each time I run my test
> script. Some are being dropped by the incoming NIC itself (as observed
> by the output of ifconfig <interface_name>), but these are very few
> compared to the ones being dropped by the router itself.
[...]
> How do I figure out why these packets are being dropped?
> How do I count the number of packets being dropped?
>
> Just an aside:
> When I send non-anonymized packets to this machine (ie.
> by using the IP addresses of the subnets attached to the router) no
> packets are being lost

Check /proc/sys/net/ipv4/conf/*/rp_filter which controls source address
validation by reversed path.

Hello Pascal,

Thank you for responding. The rp_filter was set to 1 for all my
interfaces. I set them all to 0. It did not help. I still lost a lot
of packets.

Then I enabled the log_martians capability and I noticed a bunch
of invalid addresses being logged by the system. The addresses were
those whose first octet was zero '0'.

However, the number of messages/packets logged is far fewer than
the number of packets being dropped.


Therefore 3 questions:
1. I assume, martian logging is rate limited somehow?
1.1 If so, how can I at least get a count
of the martian packets?
1.2 Can I modify kernel code somewhere to
get this count?


Thank you.

Azeem Khan
Bombay, India.

On Apr 26, 5:51 pm, Pascal Hambourg <boite-a-s...@plouf.fr.eu.org>
wrote:
> Hello,
>
> Azeem a écrit :
>
>
>
>
>
> > I am sending anonymized packets (using a tool called Click) which
> > generates addresses from the entire IPv4 address space to this
> > machine. [...]
> > I keep observing a certain amount of packets being dropped by the
> > router. The number of dropped packets vary each time I run my test
> > script. Some are being dropped by the incoming NIC itself (as observed
> > by the output of ifconfig <interface_name>), but these are very few
> > compared to the ones being dropped by the router itself.
> [...]
> > How do I figure out why these packets are being dropped?
> > How do I count the number of packets being dropped?
>
> > Just an aside:
> > When I send non-anonymized packets to this machine (ie.
> > by using the IP addresses of the subnets attached to the router) no
> > packets are being lost
>
> Check /proc/sys/net/ipv4/conf/*/rp_filter which controls source address
> validation by reversed path.