In the previous installment on UDP broadcasting and service discovery, the basics of both were explored, including an implementation in the form of NyanSD and its protocol. Contained in the comment section was a very good demonstration of why one of the most exciting aspects of software development is the opportunity to share your latest creations with other people. This being the ability to get solid feedback on all the points – including any potential boneheaded omissions – that you really should address, whether intentional or accidental.
The most pertinent point raised was definitely that of broadcast addresses and IPv4 subnets, with the latter topic especially being something that the sysadmins at the office would talk about all the time, but which us software developers were always happy to ignore as something that didn’t concern us. Turns out the joke was on me and everyone else – like our esteemed readers – who thought that they could escape the fascinating world of subnets, as today we’ll take an in-depth look at what subnets are and how they are relevant to the world of UDP network discovery.
I somewhat alluded in the first article to the topic of ‘which broadcast address to use’ as being somewhat of a rough topic to figure out, which is clearly why I just stuck to a blatantly ‘works for me’ /24 subnet that usually will work on networks, until it does not.
Subnet And Conquer
The short version of ‘what is a subnet’ is to point at the subnet mask that we have been mostly mindlessly mashing into networking configuration dialogs along with the IPv4 address for many decades now. Usually this takes the form of 255.255.255.0, which is just the human-readable version of the actual bitmask. Here the loopback interface already tends to use 255.0.0.0 as its netmask, which is a detail that tends to be easy to gloss over as this is just one of those local OS things.
Putting netmasks in the crudest and simplest terms, they are a bitmask that is used to identify how an IPv4 pool of addresses is split up by defining which bits of the 32-bit IPv4 address identify a subnet. Normally we call the trailing part of an IPv4 address (the .123) the host identifier, with the preceding section the network identifier.
By masking part of this host ID and using it to create a subnet identifier, we can then use this for additional routing, just at the cost of a reduced number of possible host IDs within that subnet.
As an example, the common 255.255.255.0 mask identifies the first 24 bits (3 bytes) of the 32-bit (4-byte) IPv4 address, hence the mask being referred to as /24. With this mask, the remaining host ID bits allow for 256 hosts, of which two are not used for hosts: the first (e.g. 192.168.0.0) and last (e.g. 192.168.0.255) in the range. The last host ID in the range forms the broadcast address for that subnet.
This is why, for a /24 subnet, you can generally get away with just slapping a .255 on the end of an interface’s address, but also why for other subnet configurations it’s likely to explode violently.
To get briefly back to the loopback’s /8 style netmask, this means a single subnet with a maximum of 16,777,214 hosts, which ought to be sufficient for local system networking shenanigans. Its opposite extreme would be the /31 style netmask, which with just two potential host IDs is practically useless.
IPv6 subnetting is similar, but due to the much larger address pool and differences in the protocol this is a whole other kettle of fish that is as likely to send a network administrator’s heart racing in excitement as it is to make the average software developer run away screaming. This can be a fun topic for another day, perhaps.
This overview of IPv4 subnetting also skips over details like the different classes of IPv4 subnets beyond the Class A type here, but those are happily left to sysadmins and kin for now.
Sub-casting
In order to thus obtain the broadcast address for a given network interface you need to know two things: the IPv4 address and its associated netmask. From this you can then tell three things: the subnet ID, the broadcast address in that subnet, and the current host ID. Of these we only really care about the the second item.
Although you can obtain the broadcast address yourself by applying the netmask to the address, the OS’s APIs tend to happily give you the precomputed broadcast address. If that’s not your style or not an option, a manual procedure is to:
- Determine the number of host ID bits using the netmask.
- Set all bits to
1in these bits to get the highest possible host ID. - Use this value along with the original masked (i.e. network ID) bits to obtain the broadcast address.
If we thus start with a 192.168.0.0/24 network, we end up with 192.168.0.255, while for a 192.168.0.0/26 network with just six bits available the maximum value is 64, ergo we get 192.168.0.63, since we start counting at 0.
With this we can now broadcast UDP packets on any interface without any (major) worries.
Local Broadcast Address
A small glitch in the whole above story is that there’s actually another broadcast address, one which is always the same for each interface and can be considered to make the whole preceding explanation completely irrelevant. This being the local, or limited, broadcast address, which is either the best thing since sliced bread or the worst sin ever committed in the history of IP networking, depending on whom you ask.
This cheat code takes the form of the address 255.255.255.255 and if you send a packet on a UDP socket to it, you’ll get happy UDP responses from any service that is listening on the specified port. This raises the point of why you’d not just use this broadcast address on all interface, rather than bother with all the earlier described nonsense.
The only major difference between this local broadcast address and the earlier described directed broadcast address is that the latter can also used to target a foreign network, instead of just the local network. This makes it a very attractive option if you just want to query the local network with UDP broadcast packets.
As for why you’d not want to use a local broadcast address, I couldn’t really find any references or citations on why this would be the case. Both would appear to be perfectly valid approaches to broadcasting, each with its own pros and cons.
Bugs
One final topic was my mistaken hardcoding of a /24 style broadcast address in NyanSD. Here reader ziew helpfully pointed me towards the Poco::Net::NetworkInterface::broadcastAddress() function, which seemed perfect. Unfortunately Poco’s implementation at least on Windows 10 appears to be rather broken.
After getting only 0.0.0.0 as broadcast address from this function, I had a bit of a look at what was happening, including checking what I got as subnet mask both for the default index parameter and for the next index. Across two different Windows 10 installations and both GCC in MSYS2 as well as MSVC 2017/2022 with various versions of Poco the returned values were… interesting enough to file a bug report on the Poco issue tracker.
Clearly this isn’t going to be fixed just yet, but on the bright side the horrific atrocity that I committed by hardcoding a /24 broadcast address will still work on basically every home LAN out there that NymphCast is likely to be used on.
Maybe I could just switch to a local broadcast address and that’d be even better. Feel free to torch down this idea in the comments, just be sure to provide solid reasoning and cite your sources.
A Complex Topic
Writing out the above pretty much clarifies I think why past me got a bit overwhelmed when trying to ‘just do a UDP broadcast thing’. Even just scratching the surface of IPv4 subnets and not even venturing into IPv6 territory makes one already feel a bit antsy.
Certainly, one could totally argue that anything other than a /24 network is unlikely to be encountered outside of certain government and business networks with either very specific needs, very enthusiastic sysadmins, or both, but it’s always better to design software with such real-life scenarios in mind.
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This articles is written by : Nermeen Nabil Khear Abdelmalak
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