Arch Linux

Description:

Both DHCPv4 and IPv6 router advertisements announce a MTU of 2274. The dhcpcd daemon sets this MTU correctly, but restores the default value of 1500 for an unknown reason later, when the first router advertisement is observed. (BTW, why does dhcpcd care about IPv6 router advertisements...? Perhaps router advertisements and DHCP (both v4 and v6) should not be mixed together this way.)

When I set a MTU of 2274 manually on the client before connecting, the "restored" value is then 2274 again, so everything seems to work fine. However, such a configuration would not reflect possible future changes of MTU on the WiFi AP.

The buggy hook script that restores the MTU when dhcpcd is running (instead of restoring it only when dhcpcd terminates) is called dhcpcd-hooks/10-mtu.

Additional info:
* package version(s)
5.5.4-1
* config and/or log files etc.
Mar 8 23:17:10 octopus dhcpcd[10126]: version 5.5.4 starting
Mar 8 23:17:10 octopus dhcpcd[10126]: wlan0: sending IPv6 Router Solicitation
Mar 8 23:17:10 octopus dhcpcd[10126]: wlan0: broadcasting for a lease
Mar 8 23:17:10 octopus dhcpcd[10126]: wlan0: offered 10.0.1.16 from 10.0.1.1
Mar 8 23:17:10 octopus dhcpcd[10126]: wlan0: ignoring offer of 10.0.1.16 from 10.0.1.1
Mar 8 23:17:11 octopus dhcpcd[10126]: wlan0: acknowledged 10.0.1.16 from 10.0.1.1
Mar 8 23:17:11 octopus dhcpcd[10126]: wlan0: checking for 10.0.1.16
Mar 8 23:17:14 octopus dhcpcd[10126]: wlan0: Router Advertisement from fe80::e6ce:8fff:fe54:d565
Mar 8 23:17:14 octopus dhcpcd[10126]: forked to background, child pid 10157
Mar 8 23:17:16 octopus dhcpcd[10157]: wlan0: leased 10.0.1.16 for 3600 seconds
Mar 8 23:17:16 octopus dhcpcd[10171]: wlan0: wlan0: MTU set to 2274
Mar 8 23:17:46 octopus dhcpcd[10157]: wlan0: Router Advertisement from fe80::e6ce:8fff:fe54:d565
Mar 8 23:17:46 octopus dhcpcd[10202]: wlan0: wlan0: MTU restored to 1500
Mar 8 23:18:25 octopus dhcpcd[10157]: wlan0: Router Advertisement from fe80::e6ce:8fff:fe54:d565
Mar 8 23:19:01 octopus dhcpcd[10157]: wlan0: Router Advertisement from fe80::e6ce:8fff:fe54:d565

Steps to reproduce:
Set up a WiFi AP capable to achieve a MTU larger than the default of 1500. Let the dhcpd and radvd announce this MTU. (The v6 dhcpd cannot announce MTU, only the v4 dhcpd can do so. MTU for IPv6 is annouced through radvd.) Then the log file above is what you will get on the client. The MTU value is only honored for a couple of seconds, restored back later for no obvious reason.