UDP internal routing issue when using multiple NIC's

Question

I have a server that has two ethernet ports, each one running on a separate NIC (eth0 and eth1). I would like to connect eth0 to a separate machine that streams video over UDP (and no other traffic) while the other interface (eth1) is connected to the network gateway/router.

I don't care if the udp traffic port does not connect to the wider network.

Thank you!

EDIT: Since this question was put on hold, I would like to further clarify my system as per the comments below. My system consists of a machine, running linux, with two ethernet NIC's. I am receiving a UDP stream on one of those ports from a direct LAN-LAN connection to a security camera, and even though I've been able to read the packets coming in from the camera using tcpdump I have yet to be seeing anything coming through via gstreamer which I plan on using to display the video.

Both ports have a static IP address, configured as seen below:

eth0 Link encap:Ethernet HWaddr 5C:F8:21:34:80:F6 inet addr:192.168.1.233 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: fe80::5ef8:21ff:fe34:80f6%132688/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:1237 errors:0 dropped:0 overruns:0 frame:0 TX packets:90 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:113089 (110.4 KiB) TX bytes:14016 (13.6 KiB) Interrupt:99 eth1 Link encap:Ethernet HWaddr 5C:F8:21:34:80:F7 inet addr:192.168.1.234 Bcast:192.168.1.255 Mask:255.255.255.0 UP BROADCAST MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:0 (0.0 B) TX bytes:0 (0.0 B) lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1%132688/128 Scope:Host UP LOOPBACK RUNNING MTU:65536 Metric:1 RX packets:330 errors:0 dropped:0 overruns:0 frame:0 TX packets:330 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1 RX bytes:277171 (270.6 KiB) TX bytes:277171 (270.6 KiB) 

And the internal routing table:

Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth1 192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1 192.168.1.1 0.0.0.0 255.255.255.255 UH 0 0 0 eth1 

The camera is on address 192.168.1.239, and broadcasts to my server directly over UDP. (the destination MAC is hardcoded into the packets) It is broadcasting an MJPEG stream, and when I take the packets saved by tcpdump I can rebuild it into the MJPEG.

however, when I use gstreamer using the following command:

gst-launch-1.0 udpsrc multicast-iface=eth0, port=1234 ! filesink location=foo 

foo does not save any data. I've used this machine to save udp streams using gstreamer in the past, but not when both ports were in use. IE, the video was coming from another machine, through the router, to this server in question.

So, why can't these packets, which the server clearly sees and understands (it's not dropping them at least) not get to my gstreamer program?

Answer 1

Typically you'd assign your cross-connect its own /30 subnet, from another bit of RFC1918 (private) space. So you'd do something like this:

• 192.168.1.234/24 is your LAN on eth1
• 192.168.255.1/30 is this machine, on eth0 (the cross-connect)
• 192.168.255.2/30 is the video source, on its end of the cross-connect

If you then 192.168.255.1 and 192.168.255.2 as the IP addresses for your UDP connection, it'll flow over eth0.

It is possible to do this using the same subnet as the LAN (using /32 routes, policy routing, or bridge tricks), but all of those are more complicated and more likely to have issues with confusing random programs.

(Quick explanation of /x notation: It's a short way of specifying the netmask, by counting the number of 1s [binary!] from the left/most significant bit. So /30 means a netmask of 255.255.255.252.)