Question

A server with double interfaces.

1. One(eht0) is used for WAN which provides http/ssh services for internet users.

2. The other(eth1) is used to receive multicast data from intranet.

218.108.132.177 is public network gateway.

125.210.198.1 is private network gateway.

233.49.3.*/24 is multicast address.

10.0.11.*/24 is the source of multicast data.

When the route table is like below, ffmpeg can’t read the udp data from eth1, ffmpeg hung up:

rrca@rcasnap02:~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
218.108.132.176 *               255.255.255.252 U     0      0        0 eth0
125.210.198.0   *               255.255.255.240 U     0      0        0 eth1
default         218.108.132.177 0.0.0.0         UG    100    0        0 eth0
default         125.210.198.1   0.0.0.0         UG    100    0        0 eth1

or

rrca@rcasnap02:~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
218.108.132.176 *               255.255.255.252 U     0      0        0 eth0
125.210.198.0   *               255.255.255.240 U     0      0        0 eth1
default         218.108.132.177 0.0.0.0         UG    100    0        0 eth0
10.0.11.0       125.210.198.1   0.0.0.0         UG    100    0        0 eth1

or

rrca@rcasnap02:~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
218.108.132.176 *               255.255.255.252 U     0      0        0 eth0
125.210.198.0   *               255.255.255.240 U     0      0        0 eth1
default         218.108.132.177 0.0.0.0         UG    100    0        0 eth0
233.49.3.0      125.210.198.1   255.255.255.0   UG    100    0        0 eth1

I want to the ffmpeg work right,but now I think the two default route in route table disturb eachother, and I take a try, when the public gateway route is deleted, or the private gateway route is at the head of public default gateway route, ffmpeg works well, I think it read multicast from eth1.But the route table is not thus, ffmpeg can’t read data from eth1, I think it read data on eth0(which is not private network interface).

How to do ffmpeg works well with two interfaces at the same time?

Answer 1

You need to configure the correct route for multicast traffic. The kernel does an origin check on all incoming multicast traffic: if it arrives on a different interface from the one it would use to send such traffic it’s dropped.

Just set a route for multicast on your eth1 interface:

# route add -net 224.0.0.0/8 dev eth1

Alternatively, disable the origin checking:

# echo 0 > /proc/sys/net/ipv4/conf/eth1/rp_filter