I have instrumented ipconfig, and determined that the ultimate source of the
problem
is that, for the case of multiple interfaces, ipconfig has a dependency on the
kernel's probe order of the network interfaces.
For whatever reason, the -31 kernel probes the network devices in one
order (e.g., ens3 then ens4), and the -57 kernel in the other order
(ens4 first then ens3).
The probe order of network devices (and PCI devices in general) is
explicitly not defined, and so this is not a bug in the kernel itself;
ipconfig is failing due to its dependency on a specific enumeration
order.
The issue in ipconfig is that it is using a single packet socket to
attempt to multiplex packet traffic on multiple interfaces. Presuming
that ens3 will answer DHCP and ens4 will not, for the case that works,
the order ends up being something like:
send DHCP request on ens3
send DHCP request on ens4
[ system gets DHCP response via ens3 ]
try to receive DHCP reply sent by peer for ens3; this matches, and all is happy
For the case that it fails, the sequence is roughly:
send DHCP request on ens4
send DHCP request on ens3
[ system gets DHCP response via ens3 ]
try to receive DHCP reply sent by peer for ens4; the reply is actually for
ens3, so ipconfig
throws it away (as the XID, et al, don't match what is expected for the ens4
DHCP request).
This repeats until ipconfig gives up.
As I said above, the issue is that ipconfig is trying to multiplex
traffic for two interfaces on one packet socket. This is fine for
sending, but for receiving on an unbound packet socket, there is no way
to receive a packet sent to a specific interface. Packets are delivered
to recvfrom/recvmsg in the order received.
I note that ipconfig sets sll.sll_ifindex on the msghdr provided to
recvfrom and recvmsg system calls; perhaps the author believed that this
limits received packets to only packets received on that ifindex. This
is not the case, and the sll_ifindex passed to recvfrom/recvmsg is
ignored.
I'm looking into whether or not there is an simple fix for this that
will let ipconfig function without major rework to utilize one packet
socket per interface.
** Tags removed: kernel-key
** Package changed: linux (Ubuntu) => klibc (Ubuntu)
** Changed in: klibc (Ubuntu)
Status: Triaged => Confirmed
** Changed in: klibc (Ubuntu)
Assignee: (unassigned) => Jay Vosburgh (jvosburgh)
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https://bugs.launchpad.net/bugs/1652348
Title:
initrd dhcp fails / ignores valid response
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