Re: When does Linux drop UDP packets?

[david@lang.hm]

On Thu, 4 Jun 2009, Steven Rostedt wrote:

> On Thu, Jun 04, 2009 at 04:53:47PM +0200, Philipp Reh wrote:
>> Dear list,
>>
>> I have the following setting in which a client that resides on the same
>> physical network as a server wants to receive any UDP packet that
>> arrives on any of its interfaces sent by that server.
>>
>> The code sets the broadcast flag, calls bind to INADDR_ANY and
>> uses recvfrom from there on.
>>
>> Let's say the server resides in the subnet 192.168.6.255 and the
>> client in 192.168.3.255. The server uses its real IP as the packet's
>> sender ip (192.168.6.5).
>
> You don't say what the client IP is. Let's assume that it is 192.168.3.1
> for simplicity.
>
>>
>> Now the first problem I've encountered is the following:
>> If the client removes its default route and doesn't have any route
>> pointing into the subnet the server is in, the packets get discarded
>> (still tcpdump sees them).
>>
>
> Are you saying that the server sent to 192.168.3.1 with source ip of
> 192.168.6.5 and the client did not see it?
>
>> The second problem is that if the server uses the broadcast address as
>> its sender address (255.255.255.255), the packets get always discarded
>> (again, tcpdump sees them).
>
> Again, what was the destination IP address?
>
>> Now if the server fakes its sender address to be in the client's subnet,
>> every packet arrives again.
>
> So the only thing you change is the sender address?
>
> What tools are you using to read the packets, and how do you know it is
> dropped?


I have seen the same thing. I have syslog servers on one subnet without a 
default route. If I configure a server on another subnet to send it logs I 
can see the packets with tcpdump, but syslogd will not record them.

If I configure a route on the recieving box that makes it think that it 
can get to the sender (note that the route can be completely bogus, 
pointing at a wrong or non-existing gateway) the kernel is happy and the 
packets show up to syslogd

the systems I am running do _not_ have selinux on them.

I have seen this as far back as 2.6.12 so it's not a recent change.

if you need examples with IP addresses

box 1
IP 10.1.1.2

router
IP 10.1.1.1
IP 192.168.1.1

box 2
IP 192.168.1.2

If I configure box 2 to have a route to box1, but do not configure box 1 
to have any route (including not having a default route) that would get it 
to a 192.168.1.x subnet tcpdump on box 1 will show the syslog packets, but 
syslog (and any non-pcap tool) will not see the packets)

if I configure a route on box 1 to have a default route of 10.1.1.3 (which 
does not exist, so cannot possibly route packets anywhere) then everything 
works.

>> So my real question is:
>> When does Linux discard packets and how can I prevent it from doing
>> that?

for this problem, set a default route that points at a non-existing 
gateway and I believe that your problem will go away.

David Lang