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Some DHCP server implementations only send the important requested DHCP options in the final BOOTP reply (DHCPACK). One example is systemd-networkd. However, RFC2131, in section 4.3.1 states: > The server MUST return to the client: > [...] > o Parameters requested by the client, according to the following > rules: > > -- IF the server has been explicitly configured with a default > value for the parameter, the server MUST include that value > in an appropriate option in the 'option' field, ELSE I've reported the issue here: https://github.com/systemd/systemd/issues/27471 Linux PNP DHCP client implementation only takes into account the DNS servers received in the first BOOTP reply (DHCPOFFER). This usually isn't an issue as servers are required to put the same values in the DHCPOFFER and DHCPACK. However, RFC2131, in section 4.3.2 states: > Any configuration parameters in the DHCPACK message SHOULD NOT > conflict with those in the earlier DHCPOFFER message to which the > client is responding. The client SHOULD use the parameters in the > DHCPACK message for configuration. When making Linux PNP DHCP client (cmdline ip=dhcp) interact with systemd-networkd DHCP server, an interesting "protocol misunderstanding" happens: Because DNS servers were only specified in the DHCPACK and not in the DHCPOFFER, Linux will not catch the correct DNS servers: in the first BOOTP reply (DHCPOFFER), it sees that there is no DNS, and sets as fallback the IP of the DHCP server itself. When the second BOOTP reply comes (DHCPACK), it's already too late: the kernel will not overwrite the fallback setting it has set previously. This patch makes the kernel overwrite its DNS fallback by DNS servers specified in the DHCPACK if any. Signed-off-by: Martin Wetterwald <martin@wetterwald.eu> Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.