DHCP servers in lan

Question: Why not setup a central DHCP server (separate from the access points) for wired and wireless clients? I guess I am curious if you have any wired clients? If so - how are they getting their addresses? Can you use the same server you are using for DNS?

There is only 1 hard wired client- with fixed IP. DNS is provided via ISP and local wireless clients seem to discover dns provided they can either connect to DHCP server direct or are given fixed IP .

Seem to discover? Uh, this isn't something to be vague about.

I'd much rather have centralized DHCP. One added benefit to this is being able to better diagnose networking problems using DHCP leases. The machines boot themselves as DHCP but get the same IP address every time. This way if a device is acting up it should be easy to find via logs and such. That and you can easily see any new devices as they'll show up as new leases, not reservations.

There's another point to consider, if you've got fifty machines then you've got the chance that all routers may, at some point, have to issue leases to every one of them. Just how large are the subnets?

Feh, the headaches of managing multiple routers aren't worth it. Turn it off on them and use a centralized one with leases instead.

Thank you but that is what I am trying to do . If you re-read my earlier post, I indicate that the client pc's cannot obtain an IP other than ones which are near the only router rigged up for DHCP. Or is there something I must do to enable discovery of the DHCP server for client machines ?

Thank you but that is what I am trying to do . If you re-read my earlier post, I indicate that the client pc's cannot obtain an IP other than ones which are near the only router rigged up for DHCP. Or is there something I must do to enable discovery of the DHCP server for client machines ?

Are all the AP's actually AP's or are they wireless routers acting as AP's? The latter is fine but you have to plug into a LAN port not it's WAN port. (sorry if you know this but something doesn't sound right) Also are all AP's using the same SSID and encryption info? The only thing that should be different about each AP is the IP address and wireless channel (make sure they are not all on the same channel.)

Your network should look like this

Internet Modem to WAN of Internet Router/AP1 with DHCP enabled, from it's LAN port 1-4 (if applicable) to each of the 4 other AP's setup with static addresses outside of the DHCP range of Router/AP1. With this setup all wireless devices that associate with any of the AP's will receive a DHCP lease from the internet router and all non lan traffic with be routed to the internet via it's default gateway.

I would setup like this router/AP - 192.168.1.254, DHCP 192.168.1.100-199 AP1-4 - 192.169.1.201-204.

Adair

Something may be wrong here... I Have all the routers set up with different SSID. Something like AccessPoint1... APKichen..APAccounts etc... they are all using NO encryption till I get it working correctly. Some of them are using CH6 and some CH11. About half each. Does the SSID hold a clue here ?

"-keevill-" hath wroth:

Bad idea.

I think I was the one that suggested you do that. I'm too lazy to lookup the previous thread and supply a reference.

Nope. That's not normal. It should work. I have several systems, including my palatial office, setup that way. However, I don't recall the details. Where you can go wrong is:

1. If you have all your access points or routers setup as routers. That won't work as the DHCP broadcast has to go from the LAN->WAN interface.
2. If your unspecified hardware has a problem passing broadcasts to or from the LAN to the wireless interfaces. Note that DCHP is done with broadcasts and do not require an IP address to function.
3. You have some device between the access point LAN ports that you didn't mention (such as an overly smart managed switch, VLAN, VPN, or router).

Easy. Download a DHCP/BOOTP query tool:

Fire it up on your LAN/WLAN and see if it can find a DHCP server. Type anything you want into the "device identifier" field. No need to assign an IP address. In fact, you can intentionally assign a totally wrong IP address to the client and it should still work as it's all running on the MAC layer. I just tried it with my static IP assigned workstation and it works fine. Here's the results: option PKT:Opcode=2 option PKT:HType=1 option PKT:HLen=2 option PKT:Hops=0 option DHCP message type=5 option PKT:Flags=32768 option PKT:Seconds=0 option PKT:XID=41 option PKT:SName= option PKT:Boot file= option PKT:CIAddr=0.0.0.0 option PKT:YIAddr=192.168.1.100 option PKT:SIAddr=192.168.1.1 option PKT:GIAddr=0.0.0.0 option PKT:Magic cookie=99.130.83.99 option Subnet mask=255.255.255.0 option Gateways=192.168.1.1 option Domain name servers=192.168.1.1 option Broadcast address=192.168.1.255 option Server identifier=192.168.1.1 option DHCP address lease time=7200 option DHCP renewal time=3600 option DHCP rebinding time=6300 option PKT:CHAddr=00-00

The test to run is very simple. Try the DHCP query first from locally connected via CAT5 laptop. First try plugging it into the unspecified device acting as a router and DHCP server. That should work and give you a clue as to what to expect. Next, plug in to one of the routers you have setup as an access point (with DHCP server disabled). That should also work. Note that we have done NOTHING that involves wireless at this time. Try it with your other access points. If the access point does not have a built in ethernet switch, borrow a cheapo ethernet switch, and install it between the backhaul cable and the access point.

One you have determined that it works via the wired CAT5 network, try it via wireless. There's a real possibility that it will not work because of problems passing broadcasts between wired and wireless. Look for settings such as "AP isolation" or "client isolation" that will cause such problems. If you experience this problem, please disclose the exact hardware, hardware version, firmware version of the access point so I can raise hell with the manufactory.

Jeff Liebermann hath wroth:

A bit more on using the above DHCP query tool:

1. Do NOT check the box for "use BOOTP protocol".

1. Windoze, in its infinite wisdom, caches DHCP requests. You can sorta tell by how long it takes to return results. If it takes about

2-3 seconds, it's a new lookup. If it responds instantly, then it's coming from the local cache.

1. I couldn't figure out how to clear the DHCP cache, so I found it easier to just convince the lookup tool to do a new lookup. To do that: - Punch the "Reset" button to clear the display - Change the "Device Identifier" to something unique. If you watch the "your address" box, it will return a different IP address (at least it does that with my static IP system).

very detailed course of action Jeff - I will attempt this tomorrow when I am in office and post back. You did not say if the SSID naming of all the routers should be the same or different.Mine are all different as I say. The hardware is all SMC with one Belkin thrown in. 4 are modem / routers (with modem function not used of course) and the belkin is a router only.

"-keevill-" hath wroth:

Thanks. I'm rather interested in the results.

It's easier to have them all different for testing. However, it really doesn't matter for testing DHCP. With different SSID's, you can control which access point you connect with. With all the same SSID's, seamless roaming is easier. My experience is that if the manufacturer does not advertise roaming abilities, it's not going to work.

Ummmm.... model numbers, hardware versions, and firmware versions? Take inventory.

I am sorry to report a total failure of that test. I cannot get any response from that test tool no matter whether I use hard wired connection or wireless. Connecting my laptop to any of the access points whether they have DHCP enabled or not produces a 'no responses received' message from the tool. I can ping any station on the network from the notebook and an IPCONFIG/all shows an ip address 192.168.0.83 which has been given my by the only router with DHCP configured. The router IP is correctly shown as 192.168.0.247 which is also the DG. If I move away to another access point on which DCHP has NOT been configured to run, then I cannot connect. All this has been done from my notebook.

"-keevill-" hath wroth:

Weird. It has always worked on everything I've tried except for a Windoze Server 2003 with all the sandbox security features enabled. It also doesn't work with DHCP AUTH (RFC-3118).

I did some tinkering and produced "No response received" if I unchecked the "Request Broacast Response" box. Please check this box. Also, uncheck the "Use BOOTP protocol" box.

Sounds correct. If you can get a DHCP assigned IP address, the query tool should work. There's no way you can get a valid DHCP assigned IP address and also not have the query tool work. They go together. Does your router have a logging feature, which might show some errors? Syslog?

Did you do the aformentioned failed test via wired or wireless?

Can I presume that the DHCP server is your unspecified model Belkin router? Could you disclose the model number and firmware version? Also the SMC access points.

I don't know what to tell you. This may be an indication of why using a single DHCP server wasn't working for you. It *SHOULD* work, but apparently doesn't. Do you have a home system or another router available for tinkering? If so, then plug your computer into the router and try the DHCP query tool again.

However, that doesn't really solve the original problem. If the query tool doesn't work on your LAN/WLAN, then DHCP is not going to work through multiple bridges (access points).

At this point, I usually drag out the protocol analyzer (Wireshark or Ethereal), sniff the traffic, spend an hour decoding the junk, but usually can identify the problem. If you feel ambitious, download Wireshark:

and sniff some exchanges (using ipconfig /renew to initiate the exchange). Compare with what the query tool does. I should mention that if you've never done this before, there's going to be a rather major learning curve.

Methinks we're onto the cause of the problem. I guess(tm) is that it's pointing to the unspecified model Belkin router, if that's the DHCP server.

Interesting as people have said dhcp is a broadcast protocol ? which i think i swhere the problem is

Do your ap's have an entry for ip helper addresses ?

In Cisco and other routers/acess points (DDWRT for example) you can set a helper address so that things like DHCP can work.

can you post the ranges of ip adresses your using for each ap (they are all in the same network right ?)

of the top of my head you could get a linksys router put ddwrt on it and use that as a central dhcp server as long as all the access points where on the same network or you could get a second hand cisco 2500 or 2600 and use that to hand out ip addresses

The second hand cisco route would alow you to have each ap have its own network if you desired - just set up mutiple ip adresses on the ethernet port.

currently I have disabled DHCP on all but the unit which connects directly to the ISP - a non-wireless SMC7901bra ADSL modem / router and 1 other router - SMC2904WBRA2 . The former runs on the range 192.168.0.100-199 and the latter

192.168.0.81-99. Only wireless client pcs which are within range of the device which has the DHCP enabled can connect to the network. Other machines I must give fixed IP when they connect to routers in their range where DHCP is not enabled.