Florian Weimer writes: > * Seth David Schoen: > > > 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 > > There is also the questions of netmask that aren't in the CIDR style > (so 255.255.0.255 instead of 255.255.255.0). Are they still > supported? Nope -- I tried setting such a mask with ifconfig and got EINVAL back from the kernel, while iproute2 doesn't even allow specifying a netmask as opposed to a prefix length. This EINVAL comes from here https://github.com/torvalds/linux/blob/4ee998b0ef8b6d7b1267cd4d953182224929abba/net/ipv4/devinet.c#L1214 where the bad_mask() macro is checked to see if the mask is valid. (I haven't reasoned about the way bad_mask() is implemented, but the purpose of the check seems to be whether the requested mask is CIDR-valid.) > > 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. > > Are there network devices that are neither point-to-point, nor do they > have broadcast support? We've talked about this question a bit in our project before. I don't believe so, but I can look into it further. It's logically conceivable that there could be such devices. > > 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"? > > My main concern is that the language should not imply that something > cannot happen if it actually can. Otherwise programmers will end up > using this guidance and create software that won't work in such > configurations. That makes sense. Are you OK with the "are both designated, by default, as broadcast addresses" or similar language? -- Seth David Schoen <schoen@xxxxxxxxxxx> | Qué empresa fácil no pensar http://www.loyalty.org/~schoen/ | en un tigre, reflexioné. | -- Borges, "El Zahir"
