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DHCP discovery ingnored by servers

I asked a question about the DHCP protocol ("Which part of the protocol do I not understand") and was given a comment indicating that at least one of the close voters hadn't read the question, along with the explanation that this group is for "professionally managed networks in a business environment."

Well, I professionally manage a network in a business environment, and I have a question. It appears to be similar to other DHCP questions accepted by this group:

Why do DHCP servers lease IP address based on Option-61 (Client-ID)?

Can/Should a DHCP server send a DHCPDISCOVER message?

Random DHCP clients do not accept Cisco Catalyst DHCP server's offer

The network is under my control, I have paid support for some of my network hardware (but not for my test environment). I have (multiple) DHCP servers: I have a production problem (clients not registering).

I understand that I may have to provide additional information. I understand that if nobody here knows the answer, I will have to go elsewhere.

But this is farcical: I'm to go to server fault because network engineering won't answer questions about network engineering?

If you don't want to read network engineering questions, the least you can do is to add meaningful close reasons and avoid unhelpful comments

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  • On the What topics can I ask about here? page, in the Off-Topic section, it says, "protocols above L4 in the OSI model," and that would include DHCP, which rides on top of UDP (the layer-4 protocol). DHCP is also concerned with host configuration, which is off-topic. We support configuring on-topic network devices, e.g. routers, that may have a DHCP server that needs to be configured, but the protocol itself is generally off-topic.
    – Ron Maupin Mod
    Commented Jul 8, 2018 at 2:12
  • I'm not clear: you support configuring DHCP on routers, but not on hosts? I'm not familiar with that distinction. In any case, I have a general problem, not limited to one server or even one network: If no one here understands DHCP, I'm in the wrong place. I'm sorry that I was mislead by the presence of DHCP questions and answers, and I can see why you would want to close instead of just letting the question die.
    – david
    Commented Jul 9, 2018 at 7:51
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    We understand DHCP, and we participate on sites where it is on-topic. We support the configuration of the built-in DHCP server for the on-topic devices. Host and server configurations are off-topic, but network device (routers, switches, etc.) configurations (for which the manufacturer offers optional, paid support) are on-topic. Protocols above OSI layer-4 are off-topic. It is spelled out in the page I linked in my above comment.
    – Ron Maupin Mod
    Commented Jul 9, 2018 at 10:52
  • FWIW, I just posted a related Q/A in meta
    – Mike Pennington
    Commented Jul 9, 2018 at 13:12

3 Answers 3

Reset to default
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Having had time to look at the question, I may have closed it as unclear what you are asking.

There is conflicting information and the information you provide seems to indicate key pieces of information are lacking. Some examples:

  • Changes in terminology - you mention both DISCOVER and REQUEST packets in your post. This indicates you likely either don't understand what is actually going on or you are leaving out key pieces of information that you may not feel are relevant.
  • You mention DISCOVER packets are being sourced from 198.254.xxx.xxx. This indicates your client already has what it believes to be an assigned IP address, so typically this would be a REQUEST and not a DISCOVER, however this could indicate that your client has multiple interfaces and is in fact sending the DISCOVER out the wrong interface.

We can not answer "Which part of the protocol do I not understand?" when we can't understand what is actually happening in your network. You need to provide more (accurate) information so we understand what is going on before we can begin to tell you which part of your understanding is incorrect (or even if your understanding is right or wrong in the first place).

Some things that would help:

  • Configs and software version from the Cisco 800 router
  • The actual log entries you mention in your post
  • Packet captures (if you have or can collect)
  • Debug output from the DHCP server process on the Cisco 800 router
  • Vendor/model and version of software running on the LwIP device in question

However, don't limit yourself to just this information if you gathered other information in the troubleshooting. Even if you don't think it is relevant yourself, include it as often users will omit details they think are irrelevant when they very well could be.

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  • Thank you for the helpful suggestions. I need to go back to the logs to verify the request/ discovery. Post configs :) after being told we don't do config :) I wasn't sure what format to post logs and captures, so I originally thought I would wait for comment: do you have guidelines I've missed? FWIW, IMHO: no answer at all is always an answer. I was surprised by the close without reasonable comment, and the presence of unreasonable comment. Ultimately, most network problems are solvable by changing configuration, and I expect this will turn out to be the same.
    – david
    Commented Jul 9, 2018 at 7:10
  • @david regarding “do you have guidelines for what I missed”, please read my response where I requested that you add more information to the question. The hyperlink makes this quite clear
    – Mike Pennington
    Commented Jul 9, 2018 at 11:54
  • @david, we don't do configuration of most server resources on this site as it was determined there is a community (SF) previously established which already did this well. You could also ask about the underlying protocol there, but IME the typical network pro tends to understand it better than the typical server admin (with quite a few exceptions). As for formats for logs and captures, generally text (using preformatted text marking preferrably) works well, and any standard packet capture format. Remember, closing on SE sites isn't permanent, it is part of the process of improving questions.
    – YLearn Mod
    Commented Jul 9, 2018 at 19:39
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I think there is a problem with your DHCP server(s) configuration, which is off-topic on this forum. It's not a matter of there being a question or problem with the protocol itself.

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  • When I understand that part of the absence of network traffic that I've misunderstood, I'm sure that I will be able to fix it by changing the configuration of the network devices. Looking at the site information and at other questions and answers, it wasn't clear that network problems were off-topic in this forum. Perhaps you'll get a chance to clear that up.
    – david
    Commented Jul 9, 2018 at 8:05
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    @david Perhaps if you give up the "I don't understand the protocol" bit and pay more attention to your server configuration, You will likely find the solution to your problem. I haven't seen your network configuration, of course, but unless you're doing something really weird, it's not going to interfere with DHCP packets. If your server receives the Discovery packet, but doesn't respond, I don't think the problem is in your understanding of the protocol.
    – Ron Trunk
    Commented Jul 9, 2018 at 11:54
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was given a comment indicating that at least one of the close voters

Heh, well I was one of those close voters.

Today, we only support networking appliances with paid support. OpenDHCP doesn't have paid support AFAICT.

hadn't read the question

I read your question.

You edited the question to be on-topic after we closed it. Now I have another question: Did you really try this and have the same problem on Cisco IOS as you did with OpenDHCP?

Perhaps you don't realize that your question has a detailed edit history.

Closed 29 Jun 2018 Post hoc CISCO SYSTEM edit

So when you asked the question, you claimed it was an OpenDHCP problem.

You did the right thing by coming to meta; please try to be a bit more fair to us.

The help center makes it clear that today, an OpenDHCP freeware appliance is off-topic.

enter image description here

However, that's beside the point now; you said this also happens running dhcp services on Cisco IOS.

If you want the question reopened, please add more information to the question. At a minimum, it would help to have a tshark capture illustrating the problem, and a copy of your Cisco IOS dhcp configuration.

If you add more on-topic information, we can reopen your question.

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  • I DID NOT CLAIM IT WAS AN OPENDHCP QESTION i claimed that it was a protocol question._ Actually, AS I CLAIMED IN THE ORIGINAL POST, I have worked with several different servers (you may not be aware, there is a detailed edit history). I originally chose a server I thought would be easy for readers to verify: I though perhaps you didn't like it, so I edited the post: Of course I edited the post! Is there a rule that when requesting a reopen, "Post most not be improved" ??? Cripes. "ADD MORE CONTENT" could have been an original comment. It wasn't.
    – david
    Commented Jul 9, 2018 at 7:12
  • Regardless of your uppercase ranting, the people closing the question could not ignore the fact that you were using an unsupported OpenDHCP server. With respect to your "protocol question": We have lately had a lot of closure of "protocol questions above OSI Layer4", I created a meta discussion to (hopefully) clarify the situation.
    – Mike Pennington
    Commented Jul 9, 2018 at 22:48

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