Networking

DHCP

If you've used Linux, you've probably heard of Dynamic Host Configuration Protocol (DHCP). Join us for this Guild Meeting as Vincent Danen explains what DHCP is all about.

TechProGuild held an online chat on Oct. 12, 2000 in which Vincent Danen discussed DHCP. Here's the edited transcript from that chat.

Note: TechProGuild edits Guild Meeting transcripts for clarity.

MODERATOR: For tonight, Vincent Danen is here to talk about DHCP.

SPEAKER_VDANEN: Definitely!

MODERATOR: I thank you for your patience, all.

SPEAKER_VDANEN: No problem. Thanks, Mike.

MODERATOR: I'll have a few announcements to make along the way, but don't want to hold up progress, so take it away, Mr. Danen!

WELL: Hi.

WELL: Okay.

What is the concept?
WELL: Let us start with the concept first, how DHCP works.

SPEAKER_VDANEN: Sure. We can start with concepts. =) DHCP, or Dynamic Host Configuration Protocol, is a means to provide networked computers with information about themselves and their environment.

WELL: Carry on.

SPEAKER_VDANEN: It is typically used by "semi-permanent" networking types like dynamic ADSL or cable Internet connections to provide the client computers (yours) with information like hostname, IP address, gateway address, DNS server addresses, mail, news, proxy information, and so forth.

WELL: Is it only available for NT and Linux server only?

SPEAKER_VDANEN: DHCP has uses outside of just the ISP. However, I use DHCP within my own network to provide my computers information about themselves, such as their static IP address, gateway, DNS information, and so forth.

SPEAKER_VDANEN: The DHCP server is available for Linux and NT, yes. I imagine it's available for Win2000 Server, but I am not a Windows person at all, so I don't know for sure. There are DHCP servers for other UNIX platforms, as well as Open/FreeBSD. I even think there is one for OS/2. It's a widespread protocol.

WELL: As I know IP address is not permanent.

WELL: By using DHCP.

MODERATOR: Welcome, andy_davis, welcome harold966. Good to see you again.

WELL: How about me?

ANDY_DAVIS: Hi, moderator.

SPEAKER_VDANEN: You can assign permanent IP addresses to servers using DHCP. I do this here. You can assign a specific address to a computer based on their Ethernet MAC address.

MODERATOR: And of course, well. Well-come. Get it?

HAROLD966: Thank you. Sorry for being late.

MODERATOR: That's okay. So was I. ;-) Had trouble logging on.

SPEAKER_VDANEN: Good evening folks!

Therefore I am
ANDY_DAVIS: I thought dynamic meant "not static." Is this how the cable company assigns IPs to their clients then, with the MAC?

WELL: Yes, but we can set it permanent, correct?

SPEAKER_VDANEN: Yes. Think of it in context, however. DHCP provides information to hosts that ask for it. For example, I turn my computer on, when the network comes up, it probes for a DHCP server and asks it, basically, "Who am I?" At this point, the server gives it an address, which can be a specified range of addresses, or a specific address based on the presented Ethernet MAC address.

SPEAKER_VDANEN: Exactly! You can make certain computers static and the rest dynamic... absolutely.

ANDY_DAVIS: You clarified a lot!

WELL: Another question, how to change our IP address?

SPEAKER_VDANEN: Do you mean change your address from within the DHCP server, or as the client computer?

HAROLD966: Can you fake DHCP in an in-house network? Let’s say you have 3 computers in your house with no Internet access.

WELL: Both.

SPEAKER_VDANEN: Well, from a client's standpoint, if you wanted to change your IP address, you could do so by giving it a static IP address and disabling DHCP. The results are anyone's guess. You cannot change the DHCP server's behavior by changing your client computer's address.

ANDY_DAVIS: Off the subject for a second, your dynamic Web site articles are great!

SPEAKER_VDANEN: From the server standpoint, yes, you would just edit your configuration file and specify the new IP address (if you were specifying it statically based on the MAC address), or you could specify a new range of addresses.

WELL: No way to change the IP address range for DHCP server?

WELL: Okay.

SPEAKER_VDANEN: Harold, most definitely. I use DHCP to assign static IP addresses to my network and this static IP address differs from the IP address my ISP provides me.

SPEAKER_VDANEN: Well, yes, you can change the IP address range by configuring the server.

SPEAKER_VDANEN: Andy: thank you! You'll like what's coming up. As you may have noticed, it's an ongoing series that is getting (almost) ridiculously long. =)

Change of address
ANDY_DAVIS: Is there a limit to how many ranges the server can be configured with?

WELL: I think static IP better than dynamic, since you cannot trace the IP address for a particular PC in the network.

WELL: Yes, it should be a limit, cannot change to the address, which causes conflict.

SPEAKER_VDANEN: I'm honestly not sure. I've never tried to have it deal with more than one IP address range. Obviously they must be kept on the private Class A, B, or C segments.

ANDY_DAVIS: What I mean is, if you have a server with dual NICs, can it provide DHCP for each segment?

SPEAKER_VDANEN: Okay, so you want eth0 to be 192.168.x.x while eth1 is 172.x.x.x? For that, I think you would need two separate DHCP servers (but I could be wrong, like I said, I've never tried it).

WELL: Another question, how [are] we going to change the IP address if it is connecting through proxy server?

SPEAKER_VDANEN: Your proxy would have to be configured for specific services. I think the DHCP protocol supercedes the proxy (i.e., make your proxy for web/ftp/mail/etc. and not for something like DHCP) (I believe most proxies work this way, but I'm not 100% on this as proxies tend to act differently between Linux and Windows and I don't personally use proxy servers).

WELL: Okay, my PC IP is 128.1.2.xx, but when surfing the Web, the IP is remaining permanent (e.g., 128.1.9.1, instead of my PC address).

SPEAKER_VDANEN: Well, this is due to IP masquerading, I would imagine. The proxy server (if you use one) must do IP masquerading, or your gateway does. This has nothing to do with the DHCP.

MODERATOR: Greetings, mishma. Welcome!

SPEAKER_VDANEN: Welcome!

Clarification
SPEAKER_VDANEN: How many here are looking to use their own DHCP server?

ANDY_DAVIS: I'll be looking into using my own within the next month.

SPEAKER_VDANEN: Andy, it's worth it. I had to get a little tricky to make mine work because each of my computers has an Internet-accessible static IP address (gotta love IP aliasing), but it works very well despite that (I just had to alias the static IP address from my provider to eth0:0 and use DHCP on eth0 itself and it works great).

ANDY_DAVIS: Got it. More clarity.

SPEAKER_VDANEN: You need more clarity?

FREDMERTZ: I'm having some trouble understanding what DHCP does. Despite the speaker's excellent explanations.

WELL: Is it that DHCP can be used in internal network only?

MODERATOR: We have 25 minutes left. Just thought you should know. ;-)

SPEAKER_VDANEN: DHCP is basically a protocol that is used to inform client computers of networking information for themselves, such as their IP address, their gateway address, their DNS server addresses, etc. It can also be used to provide information like the SMTP/POP3 IP addresses, Web addresses, and so forth.

ANDY_DAVIS: No, I meant that you had just provided more clarity.

SPEAKER_VDANEN: When the client computer boots and is set to dynamic address or DHCP, it probes for a DHCP server to provide that information to it automatically.

SPEAKER_vdanen, ahh, that's good. =)

FREDMERTZ: Oh I get it. That's right. If you don't assign static IPs, the DHCP server assigns them based on a range you give them.

ANDY_DAVIS: Think of it as plug-and-play in a way. An admin has the network config ready.

SPEAKER_VDANEN: Well, no. DHCP can be used for external (Internet) networks as well. Most ISPs use DHCP for cable Internet and dynamic ADSL. However, you can only hand out an IP address range that you control/have purchased.

That is the question
WELL: Can we install or uninstall this DHCP function?

SPEAKER_VDANEN: Exactly!

FREDMERTZ: But what about this IP aliasing? Is this done through DHCP?

SPEAKER_VDANEN: Well, sure. Turn off dynamic or DHCP addressing on your client computer. Give it a static address. It may not be able to talk to anyone, but you can certainly do it.

ANDY_DAVIS: For a PC as soon as it's connected to the network, [it] saves support having to having to type in all that stuff.

WELL: What do you mean cannot talk to anyone?

FREDMERTZ: Does the DHCP do the name resolution as well? Or is that separate?

SPEAKER_VDANEN: Exactly. Which is why some people like using DHCP in networks that provide only static IP addresses based on MAC addresses. You just have to configure things once and if anything needs to be changed (like the IP address of the DNS server), it only has to be done in one place.

SPEAKER_VDANEN: No, DHCP just provides information. A DNS server is what provides name resolution.

SPEAKER_VDANEN: Two separate things.

FREDMERTZ: Right. Thanks. How do the client machines know what machine is the DHCP server?

ANDY_DAVIS: DNS, a topic for a heavy guild meeting. :-)

SPEAKER_VDANEN: They do a broadcast probe. I'm not sure how to explain it easily.

SPEAKER_VDANEN: Ain't that the truth! DNS is not an easy beast to wrap your mind around if you want to set up your own server.=)

FREDMERTZ: That makes sense. I think we had a problem once with two DHCP servers accidentally handing out the same IP addresses. How can you make sure that doesn't happen? Is it a common problem, or was it just a one time goof.

WELL: We can still see the PC by clicking Network Neighborhood.

SPEAKER_VDANEN: Fred: I think that's a one-time goof. You specify what range of IP addresses the server is able to hand out and they really shouldn't cross over with what another server may be handing out. =)

SPEAKER_VDANEN: Well, I'm not sure I follow you.

WELL: So slow, only my messages posted out.

WELL: That’s why you can't follow.

WELL: And some of my posting is being blocked.

SPEAKER_VDANEN: Some of your messages are being blocked going to the chat room?

WELL: Yes.

MODERATOR: Try to stay on the topic well. I'm the chat room great blocking power.

WELL: Too strict.

FREDMERTZ: So anyway, does DHCP set the leases for machines, as well?

WELL: Okay, repeat the question, what do you mean by cannot talk to anyone if using static IP address?

MODERATOR: 10 minutes, BTW.

MODERATOR: Good discussion so far. Gotta couple of quick things. One is that as usual, someone will win the chat tonight and be eligible for our monthly prize.

In my opinion
WELL: Is it MAC address also provided by DHCP server?

MODERATOR: The prize this month is two TechRepublic branded gadgets: a wall clock and a GPS. Yep, that's right, a GPS.

MODERATOR: Finally, I must tell you that Oct. 19, 2000 will be the last Guild Meeting chat. Instead, we're going to move to a neat feature, threaded discussions. We think members will like that a lot.

MODERATOR: Okay, that's all for now. Sorry to interrupt.

WELL: Having a good discussion better than having a good prize.

MODERATOR: Very true, well.

WELL: Why stop live chat?

MODERATOR: Basically, it doesn't seem to be a feature enough members care to participate in. On the other hand, it's a lot of administration. We hope the threaded discussions will be a better replacement.

WELL: But forum is too bad, not interactive.

MODERATOR: True, well. Forums are not as spur of the moment.

MODERATOR: Okay, sorry to keep you folks from discussing DHCP. Back to the sheep as they say in France.

HAROLD966: That’s true, Well. I am with you.

WELL: This one is better, something like ICQ.

FREDMERTZ: I like the discussions, too.

WELL: I tried posted questions for more than a year, nobody answered my questions.

ANDY_DAVIS: Maybe we'll move back to Guild Meeting chats when we all have wireless devices. :-)

FREDMERTZ: I mean the chats, but I also like threaded discussions.

WELL: That what I have by using the TechRepublic forum.

WELL: Too bad

HAROLD966: Maybe most of the people don’t come because they see it [as] too advanced. Why not make a meeting for let’s say beginners?

ANDY_DAVIS: Is this based on a survey or stats?

MODERATOR: It's a good idea, harold966, and I'll pass it along to the powers that be. (though if you think I’m strict...)

MODERATOR: Stats? Sure, the kind of stats where there are only 4 people in this meeting.

WELL: If we must act fast, then the live chat is very useful.

MODERATOR: Thank you. I enjoy moderating them as well as hosting them, on occasion.

Winding down
ANDY_DAVIS: So, anything more about DHCP tonight?

WELL: It is because the same topic is not interesting.

MODERATOR: Vincent, you still there, waiting patiently?

MODERATOR: While Vincent prepares his summary, ahem. Well, you've certainly been chatty tonight. So, I'd like to declare you the winner for tonight's chat.

MODERATOR: Congratulations!

WELL: Yes, because all this time, we don’t have a chance to talk so much.

ANDY_DAVIS: So maybe it's not numbers but quality/chatty content?

FREDMERTZ: Congratulations, Well!

WELL: Thank you everyone.

ANDY_DAVIS: Cheers, Well.

MODERATOR: andy_davis - I'd have to concur that the content is valuable, though the page views for transcripts are not high either.

WELL: Yes, every one should cheers.

MODERATOR: Well, send your real live name, address, and the alias you used to mjackman@techrepublic.com

HAROLD966: Congrats, Well. Bye everyone.

WELL: Is TechRepublic going to maintain this chat room?

MODERATOR: Bye hardold966. Thanks for stopping by.

WELL: How?

MODERATOR: By e-mail to mjackman@techrepublic.com

MODERATOR: That's it for tonight. Please stop by again next week for our final round of chats. Take care all.

ANDY_DAVIS: Bye all. Thanks, Vincent and MODERATOR, mjackman.

WELL: What I mean is how is it regarding my suggestion?

MODERATOR: What suggestion was that?

MODERATOR: Greetings, danenv. Sorry you missed all the fun!

WELL: I think this should e-mail to all the users so they will come and join in.

DANENV: Sorry folks. I had some major difficulty. =(

DANENV: This is vdanen at a different computer. =(

MODERATOR: Oh well, we're just wrapping up. Any final words, Vincent? And thanks again, you did a terrific job.

MODERATOR: Well, it's a good suggestion, but I'm afraid the decision has already been made.

WELL: How is it?

MODERATOR: Vincent, thanks again for your patient explanations on a tough subject.

WELL: Just have a poll, let the users to decide whether to maintain this chat room or not.

DANENV: Just an apology for having a very upsetting Netscape freeze my entire computer and hopefully everyone learned a little something tonight. If you want more information on DHCP, please read the article I wrote on the subject. It should be a little enlightening I hope!=)

MODERATOR: I'm sure it will be.

MODERATOR: Sorry, Well but feel free to write to tpgedit@techrepublic.com and express your opinion.

DANENV: Thanks again everyone! A pleasure as usual!

MODERATOR: Thanks for coming.– Good night all!

WELL: Speaker, are you there?

DANENV: Yes, I am here.

WELL: Thank you, see you next week

WELL: Err, you change your name?

DANENV: Yes, there was a slight problem that has been fixed now.

WELL: I think it is running out of time, you still haven’t answered my questions.

MODERATOR: Going to go now. Sorry Well, time is up. I have to close the room now. Have a good evening.

DANENV: Please feel free to e-mail me personally if you have any further questions: vdanen@andrakesoft.com

WELL: You too.

DANENV: Sorry vdanen@mandrakesoft.com (typing too fast).

WELL: Good night, everyone.

About

Vincent Danen works on the Red Hat Security Response Team and lives in Canada. He has been writing about and developing on Linux for over 10 years and is a veteran Mac user.

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