Add new option to dhclient to send DHCP requests on a certain interface

Bug #1026990 reported by Mike Harris
20
This bug affects 4 people
Affects Status Importance Assigned to Milestone
isc-dhcp (Ubuntu)
Triaged
Wishlist
Unassigned

Bug Description

We have an interesting situation with dhclient, multiple interfaces, and DHCP servers not on the same subnet as a network.

The setup:
- on a machine with two connected interfaces, eth0 and wlan0.
- the DHCP server for wlan0 is not on the same network segment as its clients.

When it's time to renew the lease for wlan0, dhclient will try to renew via unicast to the DHCP server. Since it's on a different segment, the kernels routing tables are used to decide what interface the packet is sent out over. This could result in a DHCP request for wlan0 getting sent out over eth0, which is undesirable.

The DHCP standard doesn't address this problem, so it's not really a bug, but it would be nice to resolve. This issue came up when we noticed errors on some switches.

To resolve this, we're thinking this might work:
a) Add a new flag to dhclient so that dhclient only sends IP packages via the interface the running instance is responsible for (ignoring the routing table).
b) Add an option to network-manager which passes this flag to dhclient

Revision history for this message
Launchpad Janitor (janitor) wrote :

Status changed to 'Confirmed' because the bug affects multiple users.

Changed in isc-dhcp (Ubuntu):
status: New → Confirmed
Revision history for this message
Stéphane Graber (stgraber) wrote :

Adding this will be rather tricky and I won't spend the time doing this myself.
However, from your description of it, couldn't you simply have the DHCP server provide an extra route to its own IP address (/32)?

This should ensure that the traffic to the DHCP server will go through the right interface.

(That's assuming the DHCP protocol lets you do that, but I vaguely remember there being options for that.)

Changed in isc-dhcp (Ubuntu):
importance: Undecided → Wishlist
status: Confirmed → Triaged
summary: - Add new option to dhclient to send DHCP requests on a cerrtain
- interface
+ Add new option to dhclient to send DHCP requests on a certain interface
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