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dhcpcd -U <interface> only works with dhcpcd daemon running #286

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holmanb opened this issue Jan 19, 2024 · 0 comments
Open

dhcpcd -U <interface> only works with dhcpcd daemon running #286

holmanb opened this issue Jan 19, 2024 · 0 comments

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@holmanb
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holmanb commented Jan 19, 2024 •
edited

works as expected:

# dhcpcd --dumplease -4 eth0
reason=REBOOT
interface=eth0
protocol=dhcp
ip_address=10.0.0.4
subnet_cidr=24
broadcast_address=10.0.0.255
network_number=10.0.0.0
server_name=IAD201121002027
subnet_mask=255.255.255.0
classless_static_routes=0.0.0.0/0 10.0.0.1 168.63.129.16/32 10.0.0.1 169.254.169.254/32 10.0.0.1
routers=10.0.0.1
domain_name_servers=168.63.129.16
domain_name=v1n0blf1qpvufijtbcm2fvnk3c.bx.internal.cloudapp.net
dhcp_lease_time=4294967295
dhcp_message_type=5
dhcp_server_identifier=168.63.129.16
dhcp_renewal_time=4294967295
dhcp_rebinding_time=4294967295

without the daemon, this surprisingly fails

root@SRU-worked-azure:/home/ubuntu/dhcpcd-10.0.6# pkill -9 dhcpcd
root@SRU-worked-azure:/home/ubuntu/dhcpcd-10.0.6# dhcpcd --dumplease -4 eth0
dhcpcd is not running

Inspecting the code, it appears that if no data is read from the input buffer, the manager daemon is always contacted. This means that parsing via stdin (which is broken) is the only way to run --dumplease without contacting the manager daemon.
holmanb added a commit to holmanb/dhcpcd that referenced this issue Jan 20, 2024
@holmanb
Workaround for broken stdin logic
799d002 
Previous code used the non-existance of data in the input buffer to
determine whether to attempt parsing from stdin. When data is not
immediately available in the input buffer, that logic caused the code
to attempt to contact the manager daemon, which also fails. This means
that `dhcpcd -U <interface>` will sometimes fail:

 # dhcpcd --dumplease --ipv4only < /var/lib/dhcpcd/<iface>.lease
 dhcpcd is not running

This behavior is a race condition possibly exacerbated by slow storage
media or virtualization. The following reproducer makes this behavior
testable on any platform:

 # (sleep 0.1; sudo cat /var/lib/dhcpcd/<iface>.lease) | dhcpcd --dumplease -4

Note: Running dhcpcd -U <iface> explicitly requires a running manager
daemon (see upstream issue NetworkConfiguration#286). Therefore, parsing via stdin is the
only way to read leases without running the manager daemon. This is an
important feature for users that get dhcp leases using --oneshot.

Fixes NetworkConfiguration#286
holmanb added a commit to holmanb/dhcpcd that referenced this issue Jan 20, 2024
@holmanb
Workaround for broken stdin logic
25e7a3d 
Previous code used the non-existance of data in the input buffer to
determine whether to attempt parsing from stdin. This is racy, so
`dhcpcd -U < /path/to/lease` will sometimes fail:

  # dhcpcd --dumplease --ipv4only < /var/lib/dhcpcd/<iface>.lease
  dhcpcd is not running

 The fix uses the (non-)existance of an interface argument to indicate
 that dhcpcd should wait for data to arrive on stdin.

Fixes NetworkConfiguration#286
@holmanb holmanb mentioned this issue Jan 20, 2024
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