4 ms·

So here's my understanding of the situation: when a DHCP server runs out of leases to assign, it will kill the oldest (in terms of use) lease. If the device tha

by jarek 15y ago

So here's my understanding of the situation: when a DHCP server runs out of leases to assign, it will kill the oldest (in terms of use) lease. If the device that held the lease is not on, for relevant values of "on", at the time, it will not be notified about this. The usual (perhaps standard, I'm not sure) process has the device confirming its DHCP lease when coming back on, and in this situation the device would be notified that it can't have the old lease as it's been repurposed, and the server will provide another lease (perhaps after killing some yet another lease). If this is an Apple device and it behaves in the manner described, after coming back "on" it will not consult DHCP, but rather it will reuse the lease it held previously. If the server has given that lease to another device, the Apple device will butt in, causing an IP conflict. Perversely, the Apple device will shortly discover this, actually do a DHCP request, and switch over with no indication to the user, leaving the other device to wonder why it had an IP conflict and how to handle it. Is this incorrect in any way?

tzs 15y ago
As far as I can see from the RFC and from Googling, there is no mechanism for a DHCP server to take back an IP address before the lease has expired, unless the client explicitly initiates relinquishing the lease. Once the address is given out, it is the client's for the term of the lease.
- saurik 15y ago
  Given that most routers are managing a class C network, and even then have allocated the first 100 addresses for internal use (starting the DHCP pool at .100), this would mean that it would be trivial to DoS any DHCP server in seconds by simply cycling MAC addresses and leasing all of their addresses. Obviously, this is not realistic, and no one would ever implement a DHCP server that allowed this to happen: it is going to keep track of some fixed number of leases (probably/hopefully as many IP addresses as it is managing) and, when it runs out, it will be forced to reclaim an address from another user. If anything, the fact that such a mechanism is not specified is simply going to make this "worse", as the specific implementation of how and when the reclamation happens is going to end up as "whatever makes sense to the guy working on that specific DHCP server implementation", and may constitute seemingly random behavior.
  - brigade 15y ago
    You don't even need to cycle MAC addresses; just send DHCPDECLINE messages for any address the server gives. DHCP requires that any address that elicits that response from a client MUST be marked unavailable, and provides no provision for it ever becoming available again. Ignoring the client is the only automatic mechanism DHCP provides for countering this. You have to remember that DHCP was written back when it was assumed that a competent systems administrator managed the server and could manually respond to running out of available addresses (which was considered a likely sign of misconfiguration), and didn't attempt to specify what to do in the case where it happens. Not giving out an address and notifying the systems administrator is what the DHCP spec recommends in this case. In fact, section 2.2 claims "The allocation mechanism (the collection of DHCP servers) guarantees not to reallocate that address within the requested time" so any reclamation of IP address on the part of the server is technically in violation of RFC 2131.
    - saurik 15y ago
      I'm confused... are you therefore suggesting that this is how people like Linksys should handle this problem? I fully realize that the spec was written in another time, but I don't think tzs did: he is actually citing the spec as a reason why these leases should stay around. When you go to a coffee shop, the router does not and probably should not play a chime when it has run out of DHCP leases, causing the barista to go around the room finding laptops that have been setup incorrectly... the router should, and almost certainly does, just "figure something out", likely by reclaiming addresses.
      - caf 15y ago
        There are sensible options available other than violating the leases. For example, for a highly-used access point, it would make sense to give out only very short leases - say, 10 minutes. That'd support a churn of 25 clients per minute if you're giving out addresses from a /24 (and there's no reason why you couldn't use a smaller netmask, down to /8). It'd also be sensible to ignore a single MAC address once it had more than a handful of outstanding leases. It doesn't help with a malicious client - but then neither does breaking leases. The malicious client can cause enough address flapping to make the network completely unuseable. You really can't protect a wifi network against a malicious client trying to DoS it - there's a myriad ways to do that apart from DHCP.
- sedev 15y ago
  That's where I'm at with this. What Jarek describes can happen, but that's a failure case, not the normal-operation case. And as portrayed in the article, the engineers at Apple have accounted for the failure case, so all's well. They simply have made the very reasonable assumption that the DHCP server will normally be operational, not in a crisis state, and that it will honor its contracts as specified in the RFC. How hard is this?
  - jarek 15y ago
    There is that principle of "conservative in what you send, liberal in what you accept" on which UNIX and half of the internet was built.
deskamess 15y ago
I think your description is correct. The problem is the impact of the false "ip conflict" that is caused by this optimization. A device that detects an ip conflict should not have to guess whether it is an ip conflict caused by an optimization on another device. As pointed out in other comments an ip conflict can cause other systems to drop existing connections. No one likes ip conflict notifications, or to have their connections dropped. Play nice. That's all.
- rlpb 15y ago
  The problem is that the DHCP server re-issued a valid lease to a second computer. Surely this is an issue with the DHCP server not playing nice, rather than Apple?
  - jarek 15y ago
    Do you feel there is a better, more tolerant, and less error prone course of action the DHCP server could take if it runs out of leases?
    - rlpb 15y ago
      If we are able to modify the DHCP server to do something sensible, why not modify default router settings to issue /16 addresses instead of /24? Or even 10/8? There's plenty of RFC1918 space to not run out. NAT state tables won't have to stretch that far as they can time out unused connections just like they do already. Would there be any unintended side-effects with doing this? If you don't like the NAT, go ahead and issue IPv6 :-)
  - hristov 15y ago
    Not really. The DHCP server may have many valid reasons for reissuing a technically valid lease, such as it being restarted and losing its table of valid leases. As I pointed out in a response above, the DHCP protocol makes it clear that devices should re-verify their addresses after waking up.
- tzs 15y ago
  This optimization does not cause IP address conflicts. If a DHCP server is not correctly keeping track of lease times and ensuring that addresses already assigned are not reused while the client still has a valid lease, you can get IP conflicts regardless of whether or not their are any Apple devices on the network.
rektide 15y ago
Please name one or more DHCP server which will knowingly violate a lease it has granted, via "kill the oldest" or any other means.

by jarek 15y ago

tzs 15y ago

As far as I can see from the RFC and from Googling, there is no mechanism for a DHCP server to take back an IP address before the lease has expired, unless the client explicitly initiates relinquishing the lease. Once the address is given out, it is the client's for the term of the lease.

saurik 15y ago
Given that most routers are managing a class C network, and even then have allocated the first 100 addresses for internal use (starting the DHCP pool at .100), this would mean that it would be trivial to DoS any DHCP server in seconds by simply cycling MAC addresses and leasing all of their addresses. Obviously, this is not realistic, and no one would ever implement a DHCP server that allowed this to happen: it is going to keep track of some fixed number of leases (probably/hopefully as many IP addresses as it is managing) and, when it runs out, it will be forced to reclaim an address from another user. If anything, the fact that such a mechanism is not specified is simply going to make this "worse", as the specific implementation of how and when the reclamation happens is going to end up as "whatever makes sense to the guy working on that specific DHCP server implementation", and may constitute seemingly random behavior.
- brigade 15y ago
  You don't even need to cycle MAC addresses; just send DHCPDECLINE messages for any address the server gives. DHCP requires that any address that elicits that response from a client MUST be marked unavailable, and provides no provision for it ever becoming available again. Ignoring the client is the only automatic mechanism DHCP provides for countering this. You have to remember that DHCP was written back when it was assumed that a competent systems administrator managed the server and could manually respond to running out of available addresses (which was considered a likely sign of misconfiguration), and didn't attempt to specify what to do in the case where it happens. Not giving out an address and notifying the systems administrator is what the DHCP spec recommends in this case. In fact, section 2.2 claims "The allocation mechanism (the collection of DHCP servers) guarantees not to reallocate that address within the requested time" so any reclamation of IP address on the part of the server is technically in violation of RFC 2131.
  - saurik 15y ago
    I'm confused... are you therefore suggesting that this is how people like Linksys should handle this problem? I fully realize that the spec was written in another time, but I don't think tzs did: he is actually citing the spec as a reason why these leases should stay around. When you go to a coffee shop, the router does not and probably should not play a chime when it has run out of DHCP leases, causing the barista to go around the room finding laptops that have been setup incorrectly... the router should, and almost certainly does, just "figure something out", likely by reclaiming addresses.
    - caf 15y ago
      There are sensible options available other than violating the leases. For example, for a highly-used access point, it would make sense to give out only very short leases - say, 10 minutes. That'd support a churn of 25 clients per minute if you're giving out addresses from a /24 (and there's no reason why you couldn't use a smaller netmask, down to /8). It'd also be sensible to ignore a single MAC address once it had more than a handful of outstanding leases. It doesn't help with a malicious client - but then neither does breaking leases. The malicious client can cause enough address flapping to make the network completely unuseable. You really can't protect a wifi network against a malicious client trying to DoS it - there's a myriad ways to do that apart from DHCP.
sedev 15y ago
That's where I'm at with this. What Jarek describes can happen, but that's a failure case, not the normal-operation case. And as portrayed in the article, the engineers at Apple have accounted for the failure case, so all's well. They simply have made the very reasonable assumption that the DHCP server will normally be operational, not in a crisis state, and that it will honor its contracts as specified in the RFC. How hard is this?
- jarek 15y ago
  There is that principle of "conservative in what you send, liberal in what you accept" on which UNIX and half of the internet was built.

deskamess 15y ago

I think your description is correct. The problem is the impact of the false "ip conflict" that is caused by this optimization. A device that detects an ip conflict should not have to guess whether it is an ip conflict caused by an optimization on another device. As pointed out in other comments an ip conflict can cause other systems to drop existing connections. No one likes ip conflict notifications, or to have their connections dropped. Play nice. That's all.

rlpb 15y ago
The problem is that the DHCP server re-issued a valid lease to a second computer. Surely this is an issue with the DHCP server not playing nice, rather than Apple?
- jarek 15y ago
  Do you feel there is a better, more tolerant, and less error prone course of action the DHCP server could take if it runs out of leases?
  - rlpb 15y ago
    If we are able to modify the DHCP server to do something sensible, why not modify default router settings to issue /16 addresses instead of /24? Or even 10/8? There's plenty of RFC1918 space to not run out. NAT state tables won't have to stretch that far as they can time out unused connections just like they do already. Would there be any unintended side-effects with doing this? If you don't like the NAT, go ahead and issue IPv6 :-)
- hristov 15y ago
  Not really. The DHCP server may have many valid reasons for reissuing a technically valid lease, such as it being restarted and losing its table of valid leases. As I pointed out in a response above, the DHCP protocol makes it clear that devices should re-verify their addresses after waking up.
tzs 15y ago
This optimization does not cause IP address conflicts. If a DHCP server is not correctly keeping track of lease times and ensuring that addresses already assigned are not reused while the client still has a valid lease, you can get IP conflicts regardless of whether or not their are any Apple devices on the network.

rektide 15y ago

Please name one or more DHCP server which will knowingly violate a lease it has granted, via "kill the oldest" or any other means.