DHCP clients by default send a fair chunk of data which can identify you to the local DHCP server. In return they provide you with a stable IP address and configuration parameters.
At a bare minimum, the hardware address of the interface is sent- this is required to work.
So, how to solve this dilema of wanting total anonymity? The answer is to randomise the hardware address. This will happen when the carrier is down OR dhcpcd starts with the interface down. Then, dhcpcd will use this random hardware address to set a DUID LL which will be used inplace of any saved DUID and set the IAID to all zeros. This combo is used by DHCP and DHCPv6 to identify a lease. As this is randomised each time the carrier comes up you get a different IP address!
Try not to use this on an unstable link as it could drain the DHCP server of addresses :(
But we can’t stop there!
dhcpcd also sends some identifying options as well! For example, this is sent in the vendor class identifier:
dhcpcd-8.99.0:NetBSD-9.99.17:amd64:x86_64
It does not identify you or the device in anyway, but it does say what software is being used on which hardware. This could be used by DHCP servers to hand out a specific image to download and boot from TFTP for network boot clients.
Now, there are a gazzillion and one DHCP options out there- we don’t know what you’ve configured.
So dhcpcd will mask all of them when anonymous mode is activated, unless they are essential for enabling dhcpcd to work correctly on the network.
But wait! What if you really want to leak something? Like say your on a corporate network that uses DHCP security and still want to remain anonymous?
Well you can! Any request or option after the anonymous option in dhcpcd.conf is turned on.
So the placing of the anonymous directive is important, unlike other dhcpcd options. So far this is the only implementation of RFC 7844 which does this :)
This is NOT enabled by default because most people want stable addresses AND a flappy link could drain addresses as disussed earlier.