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Message-ID: <alpine.DEB.1.10.0906041148210.7953@asgard> Date: Thu, 4 Jun 2009 11:49:20 -0700 (PDT) From: david@...g.hm To: Eric Dumazet <dada1@...mosbay.com> cc: Steven Rostedt <rostedt@...dmis.org>, Philipp Reh <sefi@...-f-i.de>, linux-kernel@...r.kernel.org Subject: Re: When does Linux drop UDP packets? On Thu, 4 Jun 2009, Eric Dumazet wrote: > david@...g.hm a ?crit : >> 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. > > I guess you need to change rp_filter settings > > Documentation/networking/ip-sysctl.txt > > rp_filter - INTEGER > > 0 - No source validation. > 1 - Strict mode as defined in RFC3704 Strict Reverse Path > Each incoming packet is tested against the FIB and if the interface > is not the best reverse path the packet check will fail. > By default failed packets are discarded. > 2 - Loose mode as defined in RFC3704 Loose Reverse Path > Each incoming packet's source address is also tested against the FIB > and if the source address is not reachable via any interface > the packet check will fail. > > Current recommended practice in RFC3704 is to enable strict mode > to prevent IP spoofing from DDos attacks. If using asymmetric routing > or other complicated routing, then loose mode is recommended. > > conf/all/rp_filter must also be set to non-zero to do source validation > on the interface > > Default value is 0. Note that some distributions enable it > in startup scripts. thanks, On my Debian based systems I have commented this out of the sysctl.conf file, but apparently there is some other script that is setting it anyway. I'll have to track down what it's doing. David Lang > >> >>>> 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. > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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