Florian Weimer writes: > * Seth David Schoen: > > > +On any locally-attached IP subnet, the lowest-numbered address and > > +highest-numbered address (e.g., the .0 and .255 addresses on a subnet > > +with netmask 255.255.255.0) are both designated as broadcast addresses. > > +These cannot usefully be assigned to an interface, and can only be > > +addressed with a socket on which the > > +.B SO_BROADCAST > > +option has been explicitly enabled. > > I think the broadcast address is actually settable to some degree, and > /31 subnets do not have them. I wouldn't be surprised if kernel > behavior also depended on the network device. Thanks! We're somewhat familiar with that because we are currently proposing to change this behavior. :-) Both things you noted are true: you can change broadcast addresses with ifconfig or ip (e.g. ip addr add x/y broadcast z dev d), and /31 subnets indeed don't have them. This is defined at https://github.com/torvalds/linux/blob/master/net/ipv4/fib_frontend.c#L1125 I'm not familiar with a way in which it depends on the network device other than the /31 issue. At least, this particular logic isn't affected by other aspects of the network device type. Could you suggest a way that these details could usefully be described here? The ability to change the broadcast address is very little-used, so I'm not sure many readers would benefit from details about it here, but I also wouldn't want to mislead them about that. Maybe just changing it to "are both designated, by default, as broadcast addresses"? Conceivably it could say something like On any locally-attached IP subnet that contains more than two IP addresses, the lowest-numbered address and highest-numbered address (e.g., the .0 and .255 addresses on a subnet with netmask 255.255.255.0) are both designated as broadcast addresses by default. These cannot usefully be assigned to an interface, and can only be addressed with a socket on which the .B SO_BROADCAST option has been explicitly enabled.
