[Xorp-users] Re: xorp fails to detect multicast traffic arriving over a GRE tunnel in Linux.

Aaron agibson@confabulator.net
Sun, 01 May 2005 15:36:31 -0500


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Interesting, I had a similar issue with my DSL Modem (Speedstream 5100b)
in pppoe-on-device mode with both gif and tun devices.

pppoe-on-device mode is when the 5100b terminates the pppoe connection
and hands the public IP address to the computer (in this case, my
FreeBSD router) via DHCP. This seems to break gif/tun functionality.

if I terminate pppoe on the FreeBSD router (pass through the 5100b) all
worked perfectly, but put a greater cpu load on the pc than I cared for
at the time.

I wonder if there is some way around this.

- --Aaron


Pavlin Radoslavov wrote:
>>I found the fix for this. :-)
> 
> 
> Great!
> I was in the process of tracing the problem, but I just found that
> my DSL modem/NAT/firewall at home doesn't like GRE tunnels...
> 
> 
>>On Linux one has to do:
>>echo 0 > /proc/sys/net/ipv4/conf/all/rp_filter
>>
>>The reverse path filter was dropping all the multicast packets coming
>>over the tunnel.
>>
>>Maybe this could be added to a FAQ or something like that.
> 
> 
> Actually, it is already in ERRATA and the user manual (the PIM-SM
> section) :)
> 
> Thanks,
> Pavlin
> 
> 
>>James
>>
>>On 5/1/05, James Courtier-Dutton <james.dutton@gmail.com> wrote:
>>
>>>I have tried to get xorp to route multicast traffic using PIM-SM, but
>>>it has failed.
>>>I have 2 hosts A and B with a GRE tunnel over the internet between them.
>>>The tunnel name is called off0.
>>>The multicast traffic is sourced on the ethernet interface of xorp A.
>>>On xorp B I have the following:
>>>In the xorp_rtrmgr window I see:
>>>[ 2005/05/01 18:55:39 TRACE xorp_pimsm4 PIM ] RX PIM_HELLO from
>>>192.168.10.1 to 224.0.0.13 on vif off0
>>>
>>>But on xorp B:
>>>tcpdump -i off0
>>>tcpdump: WARNING: arptype 778 not supported by libpcap - falling back
>>>to cooked socket
>>>tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
>>>listening on off0, link-type LINUX_SLL (Linux cooked), capture size 96 bytes
>>>18:55:38.495167 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 77
>>>18:55:38.571164 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 67
>>>18:55:38.653146 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 111
>>>18:55:38.728131 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 50
>>>18:55:38.810118 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 114
>>>18:55:38.886102 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 78
>>>18:55:38.967095 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 93
>>>18:55:39.028094 IP 192.168.10.1 > PIM-ROUTERS.MCAST.NET: pim v2 Hello
>>>(Hold-time 1m45s) (LAN-Prune-Delay: T-bit=0 lan-delay=500ms
>>>override-interval=2500ms) (DR-Priority: 1) (Genid: 0x248bf4be)
>>>18:55:39.041087 IP 158.234.90.20.1119 > 233.64.38.50.2000: UDP, length: 96
>>>
>>>So, the strange thing is that xorp is seeing the pim v2 hello, but
>>>just not seeing the multicast stream 233.64.38.50:2000. Does anyone
>>>have any idea's why.
>>>
>>>To help me track this down, could someone please point me to the
>>>section of code that opens the socket for the multicast stream, and
>>>the section of code that receives multicast stream packets.
>>>
>>>At the moment, I have concluded that the Linux module ip_gre has a
>>>bug, whereby xorp does not think it is multicast capable.
>>>
>>>On xorp A (home0 is the GRE tunnel):
>>>ip mroute show
>>>(158.234.90.23, 233.64.38.50)    Iif: eth1       Oifs: home0
>>>(158.234.90.20, 233.64.38.50)    Iif: eth1       Oifs: home0
>>>
>>>On xorp B (off0 is the GRE tunnel)
>>>ip mroute show
>>>
>>>i.e. empty.
>>>
>>>I am hoping for some sort of route between off0 and eth0, as the PC
>>>requesting this multicast group is on eth0 at site B.
>>>
>>>Can anyone help?
>>>
>>
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>>Xorp-users@xorp.org
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> 
> 
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