Bad DHCP-option: interface-mtu 576

Discussion created by dennis369 on Jul 19, 2018

I am subscribed to Internet 150 unlimited.

My hitron modem is in bridged mode

I have a PC running FreeBSD acting as my Internet router / firewall providing DHCP to my LAN clients and doing NAT etc.

Recently I began experiencing intermittent/slow Internet access on all my LAN clients.


I finally isolated the cause as being a bad DHCP option being offered to my router's WAN interface by Shaw's DHCP server.


Specifically the DHCP option is: interface-mtu 576

The option is being offered by DHCP server:


This forced my router's WAN interface to change it's MTU from the default 1500 to 576.

This cause very poor Internet performance with lots of fragmentation and dropped packets.

I presume this option must be in error as all Ethernet networks use a MTU of 1500 by default.


I have resolved the issue and restored normal and expected Internet service by superseding this DHCP option on my router by adding this to dhclient.conf

supersede interface-mtu 1500;

My system is Freebsd but I believe this applies to Linux systems as well.


I can only assume that the majority of customers that do not bridge their modems and instead use the hitron for routing are not having connectivity and fragmentation issues? I have no way of knowing if/how the hitron works around the issue.


The purpose of this post is in the hope that you will forward this information to the team responsible for network infrastructure and/or DHCP servers of your Internet service, as they will definitely understand the wide-reaching implications and customer impact of this bad DHCP option.


Thank you in advance for addressing this issue.


Kind regards,