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Falsehoods Programmers Believe About LANs
- ronsor 28d ago> There is one DHCP server. There'd better be or your network is probably hosed.
- wmf 28d agoIn theory it's fine to have multiple DHCP servers as long as they don't step on each other. But in reality a bunch of the things mentioned in this post are ways for the network to be hosed. Unfortunately, enough LANs are semi-hosed that it's worth working around known problems.
- Hikikomori 28d agoThat works mostly, you'll have problems if you have duplicate IP addresses though.
- kryogen1c 28d agoWindows supports dhcp failover, where multiple dhcp servers can exist on the same subnet.
- briandw 28d agoYou don’t have to have one. My first LAN didn’t. I had no idea what I was doing and picked 17.0.0.0 addresses for local ones and I couldn’t connect to Apples website.
- smkelly 28d agohttps://datatracker.ietf.org/doc/html/draft-ietf-dhc-failover-12 https://datatracker.ietf.org/doc/html/draft-ietf-dhc-failove...
- pezezin 28d agoAt work I manage a network with two DHCP servers. One is a dnsmasq that assigns the IP address, hostname, and basic network parameters to all network devices, and the other is a LTSP server (https://ltsp.org/ https://ltsp.org/) that provides network boot images only for certain devices that require it. It might not look pretty, but it works really well.
- 0xcde4c3db 28d agoWhat really matters is that there's a coherent policy for assigning addresses, which is absolutely compatible with having multiple DHCP servers on the same subnet (e.g. each server configured to hand out non-overlapping IP ranges).
- rcxdude 28d agoIt's less about the IP ranges they're giving out not overlapping and more about the requests that they respond to not overlapping (or at least that they don't contradict one another). The classic 'someone plugged a consumer wifi router into the office/university network' problem often is made worse because the consumer wifi router gives out different addresses.
- Sharlin 28d agoWell, consumer Windowses (XP Home? It’s been a while) used to hose networks because they were "helpfully" running their own DHCP server.
- StilesCrisis 28d ago>Two hosts on the same subnet can reach each other without leaving the LAN. Barring "very buggy software," when is this false?
- Waterluvian 28d agoYeah… I feel like “falsehoods X believe about Y” need to omit anything a reasonable X would consider to be a bug. Edit: fair point. Maybe not.
- basilikum 28d agoVPNs
- alanwreath 28d agoI read that like the meme: <guy with messy hair>Aliens</guy with messy hair>
- dipierro 28d agoPort- or Client-isolation on Ethernet switches and Wi-Fi access points respectively. Often seen in corporate environments and (properly configured) guest networks.
- elaida73 28d agoThis feels like a bastardization of the already questionable "Falsehoods ___ believe about ___" format.
- rcxdude 28d agoHow so? It's pretty within the format IMO.
- elaida73 28d agoMaybe I'm pulling at a fake distinction here but the progenitor blog posts of this format that I'm thinking of were incorrect models of constructs that almost everyone uses in their everyday life constantly ( names, dates, time etc ) Whereas I find it kinda unlikely that someone both knows what a DHCP server is, and holds a belief that there is only ever exactly one on their network?
- rcxdude 28d agoHmmm, In my experience programmers are often not particularly good at networking, and this can result in all kinds of bad assumptions (or awkward workarounds for things that could be much better solved at the network level).
- milch 27d agoMaybe I don't remember the names, dates, and others correctly but I don't think they had things like > Wi-Fi and Ethernet clients are equivalent peers on the LAN. > Wi-Fi and Ethernet clients aren’t equivalent peers on the LAN. To be honest I don't know what this is supposed to be telling me. They can or can't be on the same VLAN? They could have different or the same QoL? If we hammer traffic on both at the same time bad things may or may not happen? > Only IPv4 is running on my LAN. > Only IP is running on my LAN. > Okay, only IP, IPX/SPX, Phase IV, AppleTalk, VINES, NetBEUI, or SNA are running on my LAN. > Seriously? Practically I'm not sure how this affects someone writing a networked application. Sure, IPv4/IPv6, but if I get a bug report in the year 2026 that my application is broken because it doesn't work on the AppleTalk network, it gets a WONTFIX. Sure, maybe if I'm writing a low-level network driver or maintaining a mainframe application these are things I have to care about. I think that may be the difference between the other posts and this post - non-experts in Onomastics and Chronology deal with names and dates all the time, hence resulting in issues like government websites not accepting Ng as a last name because it is less than 3 characters or not allowing entering February 29th as a birthday.
- amelius 28d agoFalsehood People Believe about Programmers: Programmers think the models they use are perfect.
- judge2020 28d agoMostly programmers think a system will act predictably given the same inputs. Most of computing doesn't work without that assumption.
- sublinear 28d agoYou're conflating "predictable" with "reproducible". "Predictable" assumes there is an observer making a guess, but we all live by the knowledge that our assumptions may be wrong. We care less about that, and more about whether the system behaves consistently. Consistent behavior is either a bug or a feature depending on what was expected. The system just is, man, and that's what we need. "Nobody sits like this rock sits. You rock, rock. The rock just sits and is. You show us how to just sit here and that's what we need."
- wat10000 28d agoI’d say this one is frequently true, and is one of the things that distinguishes senior from junior programmers.
- orthogonal_cube 28d agoProgrammers certainly believe that, but they quickly learn not to.
- mitxela 28d ago* there is something called a LAN * two people who say they are on the same LAN agree on what the LAN is
- unethical_ban 28d agoI get the idea, but this isn't necessarily useful as advice and toys with definitions. It's ESPN levels of conversation-baiting. Lots of assumptions can be reasonably made about residential LANs without being true of enterprise LANs/DMZs. If a residential LAN is RFC1918 and has no NAT, then it is still a LAN... without internet access. Then the "A & !A" fallacy of "MAC addresses can’t be changed. MAC addresses can be changed." Just kind of a mess. Would be more interesting to explore the edge cases rather than throw out half-truths and 0.05% instances. I like the GNOME 2006 era though.
- aduwah 28d agoRead it as chain of thoughts not as a list
- mitxela 28d agoBoth A and !A can be fallacies if sometimes A. The biggest fallacy here, though, and not stated, is that "LAN" is a meaningful concept.
- unethical_ban 28d agoThe loosest definition might be "isolated network or leaf node of the internet" if one considers a non-natted DMZ to be a lan. If one thinks all the assumptions in the article are bogus, then I agree there's no definition of the term. >99% of the time, being natted and/or having private address space is the definition.
- tonetheman 28d ago[dead]
- deleted 28d ago[deleted]
- LoganDark 28d ago> My LAN uses IP addresses. I've never heard of a LAN without IP addresses. Is that a thing? I'd assume that no IP addresses means you can't even use TCP/IP at all, right? Is there something else you'd use instead?
- solraph 28d agoIPX/SPX is not IP based.
- LoganDark 28d agoCan that still be used from modern operating systems? Maybe only Linux?
- solraph 28d agoI suspect recentish versions of Windows still have the driver available, just not installed by default. However the article is about falsehoods programmers believe about LANs. If you are writing some kind of driver and assume all traffic you receive is IP traffic and don't verify the header, you are going to have a bad time.
- peanut-walrus 28d agoQuite common to run unnumbered IS-IS backbones. Whether this can rightfully be considered a "LAN" is up for debate.
- mitxela 28d agoYou can send packets over Ethernet, without IP. That's what people did before IP was invented and took over most networked communication. It's less convenient, because you can't reuse the IP stack, so almost nobody does.
- ButlerianJihad 28d agoEthernet uses frames, not packets. A falsehood that programmers believe about networking is that everything is a packet. In fact, the "PDU" or protocol data unit for each OSI layer is defined and named differently. So Ethernet communicates with frames. You can simply send frames across a purely Ethernet network. Or, you can take a PDU from an upper layer, encapsulate it as Ethernet frame, and send it across. Most LAN protocols are flexible like that. https://en.wikipedia.org/wiki/Frame_(networking) https://en.wikipedia.org/wiki/Frame_(networking) https://en.wikipedia.org/wiki/Network_packet https://en.wikipedia.org/wiki/Network_packet https://en.wikipedia.org/wiki/Protocol_data_unit#Internet_protocol_suite https://en.wikipedia.org/wiki/Protocol_data_unit#Internet_pr... And TFA provides a list of upper-layer protocols that may be encountered on a LAN today: "IP, IPX/SPX, Phase IV, AppleTalk, VINES, NetBEUI, or SNA". NetBIOS and NetBEUI are particularly interesting here, because NetBIOS Frames did use IEEE 802.2 LLC, while NBT uses TCP/IP for NetBIOS transport. https://en.wikipedia.org/wiki/NetBIOS https://en.wikipedia.org/wiki/NetBIOS
- suprjami 28d agoThis just lists every possible configuration permutation. Nothing insightful here.
- rayoleary 28d agoNothing burger of a post
- AnonC 28d agoSome of these seem a bit obscure. I wish the author had given detailed examples or linked to pages with details on the points.
- ofrzeta 27d agoNot sure what a "LAN" is but it happened to me at work that I thought .0 is not a valid host address until I learned about Layer-3 only networks. So also no ARP. I guess this is still a LAN although it's probably an "Enterprise LAN".
- allarm 19d ago> Layer-3 only huh? 1.2.3.0/23 is a valid host address, it doesn't have to be Layer-3 only or whatever. it's just basic subnetting.
- jyounker 27d agoI love the number of people who are commenting about how the article is wrong, and then finding out that they are actually wrong.