^{Originally posted by jvfranklin July 27, 2024}
In src/dhcp.c make_message(), there's this code enforcing a minimum MTU:

else if (mtu < MTU_MIN) {
if (if_setmtu(ifp, MTU_MIN) == -1)

MTU_MIN is defined in src/dhcp-common.h:
#define IP_UDP_SIZE 28
#define MTU_MAX 1500 - IP_UDP_SIZE
#define MTU_MIN 576 + IP_UDP_SIZE

I work on an embedded Linux system that allows a minimum MTU on the link of 576. It appears from my initial testing that dhcpcd is able to send and receive DHCP messages without any issue if I change MTU_MIN=576. Our previous DHCP client was also able to handle this, as well. I don't see any fragmentation in the packet captures. The ethernet frames are less than 400 bytes, at least for my environment.

Is there a history on this code that requires MTU_MIN to be 604?

Also wondering about the MTU_MAX, though I haven't explored its effect in our product yet.