Messages in this thread | | | Date | Thu, 4 Jun 2009 10:57:56 -0700 (PDT) | From | david@lang ... | Subject | Re: When does Linux drop UDP packets? |
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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
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