macOS doesn't honor DHCP option interface-mtu

Originator:pepi.zawodsky
Number:rdar://40579212 Date Originated:26-May-2018 04:19 PM
Status:Open Resolved:
Product:macOS + SDK Product Version:10.12.6
Classification:Performance Reproducible:Always
 
Summary:
When a network, configured via DHCP, sets option interface-mtu to tell a client, that it must lower its MTU to being able to use the network, this option is ignored by macOS, resulting in inability to use the network at usable performance or at all.

Steps to Reproduce:
Use ISC DHCPd
set this option required by your network. (For example certain wireless or tunnels uplinks require a lower MTU)
"option interface-mtu 1340;"

Expected Results:
The network stack must honour this option and lower its MTU for the respective link.

Actual Results:
macOS ignores the option and uses the default MTU of 1500 bytes resulting in a failure to use the network.

Version:
10.12.6

Notes:
Given that macOS and iOS share a lot of networking code it is very likely that iOS is affected by the same bug.

Comments


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