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SuperHub 3 scrambling DHCP allocations

g4wsz
On our wavelength

For some time (months) I've been noticing misbehaviour from the SuperHub3 DHCP server.

I'm talking about normal Dynamic address allocation, not address reservation.

It's my understanding that DHCP servers are supposed to keep track of IP addresses issued to MAC addresses such that when a client reboots, its MAC address is looked up in a table (cache) and if a matching MAC address is found an attempt is made to issue the same IP address as previously offered.

This is what has happened for many years with VM hubs, including my current SH3 until a few months ago (I am hazy about exactly when problems started).  From that time on up until now I am finding that the SH3 DHCP server is offering the wrong IP address - usually one previously used by another client (IE *not* randomly chosen from the whole range available).

I even have one case in which a client's IP address was changed (by DHCP) while TCP connections using the original source address were in use by running processes - rendering the process non-functional.

I suspect that a bug has been introduced in the cache lookup process where the wrong table entry is being returned by the MAC address search - eg an out-by-one error?

Has this been reported and is there a fix available already please?

12 REPLIES 12

John_GS
Forum Team
Forum Team

Hi @g4wsz 

Thanks for posting and welcome back to the community.

I am not gonna lie - I am not sure what you mean here. I do wish to help you get this sorted if I can though, so please can you explain a bit more?

Kind regards,

John_GS
Forum Team


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Client62
Legend

From that time on up until now I am finding that the SH3 DHCP server is offering the wrong IP address - usually one previously used by another client (IE *not* randomly chosen from the whole range available).

The Hub 3 or any DHCP server do re-allocate a free IP once the DHCP lease has expired - the lease time is 86,400 Seconds (1 day) by default for the Hub 3, hence an IP unused for over a day will be free to be reused in a new DHCP request.


I even have one case in which a client's IP address was changed (by DHCP) while TCP connections using the original source address were in use by running processes - rendering the process non-functional.

MAC Address Randomisation would cause this to happen. If mid session, the host computer changed MAC and placed a new DHCP request, any DHCP server would see that as a new host and allocate a new IP from the available list.

Adduxi
Very Insightful Person
Very Insightful Person

As pointed out above all DHCP IP's have a lease time, and in the case of the Hub 3, it's one day.  However clients will make a request in half that time.  If the client lease is not requested, then it is not renewed and put back in the pool for reuse.  If it is important to have the same IP, then either use Static, or Reserved DHCP, or increase the lease time.

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g4wsz
On our wavelength

I don't know where to start with this answer - it's about a change in behaviour of our SuperHub3 that happened/started a few weeks/months ago.  Because of the sudden change, I believe a bug has been introduced into the SuperHub3 firmware.   A friend nearby also noticed it happen them, so I don't *think* a replacement SuperHub3 will help.  I would suggest that the problem should be passed on to someone who understands DHCP from the point of view of the SuperHub3's internal firmware.

Is there any way I can provide more/better information to you?

Thanks,

Steve

g4wsz
On our wavelength

All noted, thanks but these devices are left running 24/7, as is the SuperHub - the only exceptions being reboots or power cuts of course.

I don't think my devices have any MAC randomisation enabled (except perhaps for an iPhone).

I suspect that the SuperHub DHCP service may not be correcting retaining state across reboots/updates.

g4wsz
On our wavelength

As replied to another answer, the devices concerned are running 24/7 and can be observed to be issuing DHCP Requests on a regular basis.  So my point is that DHCP allocations are being muddled up between my clients within the "half-life" of their leases: I believe that that shouldn't happen.

I do agree that static addresses are the proper way to go, with DHCP reservations being a poor-man's alternative - but that assumes that the SuperHub's current firmware will keep the reservations unmangled between reboots!  From that latter point of view I am hoping that VM will look into the DHCP code and see what changed recently.

Client62
Legend

Our Hub 3 is in Router mode we are on Software Version : 9.1.2208.100.  DHCP lease time is 1 day.

We do not see any issues with the functioning of the DHCP server.

Our HP Workstation is DHCP client is has used 192.168.0.150 for over 12 months.

Unexpected IP changes would be seen in a local area network that has more than 1 DHCP server enabled.

If the IP is so important, just add the IP / MAC to the HUB 3 DHCP Reserved list and the problem will be resolved.



legacy1
Alessandro Volta

@g4wsz wrote:

As replied to another answer, the devices concerned are running 24/7 and can be observed to be issuing DHCP Requests on a regular basis.  So my point is that DHCP allocations are being muddled up between my clients within the "half-life" of their leases: I believe that that shouldn't happen.


Would be a bigger problem if this happened to the WAN but yes can be a problem LAN side.

A chuck of code may have been removed for the DHCP server in router mode so that the hub gives out any IP on DHCP request so test what happen when you unplug replug a device in if the IP changes.

thinking about the change could be like DHCP request comes in for renew but the hub checks ARP and even though the MAC matches the hub just goes nope new IP 

VM may do something to fix this but it be years to tell who does the firmware what needs to be changed and why easy fix is have the lease time set to many years or what ever the max is on the hub.

Or just use modem mode and get your own router

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Roger_Gooner
Alessandro Volta

The hub requests an IP address and gets one from the DHCP server. What problem do you have?

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Hub 5, TP-Link TL-SG108S 8-port gigabit switch, 360
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