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My recollection of SO_REUSEPORT behavior is that it will deliver a datagram to one listener, rather than all of them. The kernel will distribute datagrams fairl

My recollection of SO_REUSEPORT behavior is that it will deliver a datagram to one listener, rather than all of them. The kernel will distribute datagrams fairly between listeners over time, so none are starved. This makes it super easy to share load over multiple servers, but doesn't get the same datagram to multiple listeners. This seems to confirm: https://lwn.net/Articles/542629/ https://lwn.net/Articles/542629/ Maybe the exact behavior varies by platform. I've never used it on Windows.

squarefoot 5y ago
> Maybe the exact behavior varies by platform. Yes, that was the case. Also I probably confused SO_REUSEADDR, which in Windows is implemented quite differently, with SO_REUSEPORT. Anyway, I can confirm that on Windows (2000) I could open N programs whose receiving socket was bound on the same address:port and all of them would receive a copy of the transmitted datagram. If say I run 5 copies of the same agent, all of them would display the same data. Those were stand alone applications without any shared memory, so apparently the system made a copy of the buffer for each socket. I'm not sure however if I tested this particular behavior also under Unix, since I needed it only on the graphical interface, but on Windows it definitely worked.
haneefmubarak 5y ago
I haven't confirmed the behavior on BSD manually, but based on skimming https://stackoverflow.com/a/14388707/2334407 https://stackoverflow.com/a/14388707/2334407 and https://www.freebsd.org/cgi/man.cgi?query=setsockopt&sektion=2 https://www.freebsd.org/cgi/man.cgi?query=setsockopt&sektion... , I'm inclined to believe their implicit claim that the original behavior of SO_REUSEPORT is to duplicate everything to each listener. In fact, it looks like Linux copied the behavior of SO_REUSEPORT_LB and called it SO_REUSEPORT. Ahhh, the joys of "portability"...
- thaumasiotes 5y ago
  The freebsd page doesn't claim that it delivers everything to every listener. It says that's possible if multicast or broadcast is configured. That won't happen by default. > This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port.

squarefoot 5y ago

> Maybe the exact behavior varies by platform. Yes, that was the case. Also I probably confused SO_REUSEADDR, which in Windows is implemented quite differently, with SO_REUSEPORT. Anyway, I can confirm that on Windows (2000) I could open N programs whose receiving socket was bound on the same address:port and all of them would receive a copy of the transmitted datagram. If say I run 5 copies of the same agent, all of them would display the same data. Those were stand alone applications without any shared memory, so apparently the system made a copy of the buffer for each socket. I'm not sure however if I tested this particular behavior also under Unix, since I needed it only on the graphical interface, but on Windows it definitely worked.

haneefmubarak 5y ago

I haven't confirmed the behavior on BSD manually, but based on skimming https://stackoverflow.com/a/14388707/2334407 https://stackoverflow.com/a/14388707/2334407 and https://www.freebsd.org/cgi/man.cgi?query=setsockopt&sektion=2 https://www.freebsd.org/cgi/man.cgi?query=setsockopt&sektion... , I'm inclined to believe their implicit claim that the original behavior of SO_REUSEPORT is to duplicate everything to each listener. In fact, it looks like Linux copied the behavior of SO_REUSEPORT_LB and called it SO_REUSEPORT. Ahhh, the joys of "portability"...

thaumasiotes 5y ago
The freebsd page doesn't claim that it delivers everything to every listener. It says that's possible if multicast or broadcast is configured. That won't happen by default. > This option permits multiple instances of a program to each receive UDP/IP multicast or broadcast datagrams destined for the bound port.