MTU keeps changing from 1500 to 576

I wanted to lock the MTU to 1500 for my Internet port so I made the following changes:
In rc.local I placed the command
ifconfig eth0 mtu 1500
In /etc/conf.d/dhcpcd  I added:
nohook mtu eth0
Note that the eth0 port is set up to use dhcp in rc.conf.
So it boots correctly, MTU is set to 1500, and Internet access is fine. Sometime during the day the MTU changes to 576.
Does anyone know what process is doing this? Is it something I can change?
Thanks in advance.
Mark

walterjwhite wrote:576 as that would dramatically slow down
MTU is a can of worms - I've played with it, to try to get my ADSL modem reliable, and now do:
ip route replace default via 192.168.1.1 mtu 1460
So now for me this works:
ping -s 3000 www.dslreports.com
576 is the *safest* value, AFAICT, because all IPV4 networks need to support it, as a minimum. And increasing the MTU is a trade-off between responsiveness and throughput, with the risk that MTU negotiation (via ICMP) can fail.
It would be interesting to know the rationale. Maybe it's a case of, "if you have to ask for the MTU, we'll answer with the safest MTU value."

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