[Bug 1874661] Re: networkd sometimes doesn't set mtu received by dhcp

[Christoph Ziebuhr]

After the weekend, I observed that the original MTU gets set on every second renew:
Apr 23 09:51:00 ct-prod systemd-networkd[1737]: ens4: Saved original MTU: 1420
Apr 23 09:51:00 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1460
Apr 23 21:50:59 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1420
Apr 24 09:50:59 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1460
Apr 24 21:50:58 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1420
Apr 25 09:50:59 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1460
Apr 25 21:50:59 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1420
Apr 26 09:51:00 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1460
Apr 26 21:50:59 ct-prod systemd-networkd[1737]: ens4: Setting MTU: 1420

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Title:
  networkd sometimes doesn't set mtu received by dhcp

Status in systemd package in Ubuntu:
  Confirmed

Bug description:
  I'm running ubuntu-minimal-1910-eoan on google compute engine with systemd 242-7ubuntu3.7.
  GCE has mtu 1460, set by dhcp. After having networking issues, I figured out that the mtu on the interface was sometimes incorrectly set to 1500.
  I manually changed it to 1420 to have a working system again and enabled debugging for systemd-networkd.

  Log (filtered for ens4, some columns removed for better readability):
  09:51:00 ens4: New device has no master, continuing without
  09:51:00 ens4: Flags change: +UP +LOWER_UP +RUNNING +MULTICAST +BROADCAST
  09:51:00 ens4: Link 2 added
  09:51:00 ens4: udev initialized link
  09:51:00 ens4: Saved original MTU: 1420
  09:51:00 ens4: Adding address: fe80::4001:aff:fea4:2/64 (valid forever)
  09:51:00 ens4: Gained IPv6LL
  09:51:00 ens4: Adding address: 10.164.0.2/32 (valid for 15h 15s)
  09:51:00 ens4: Adding route: dst: ff00::/8, src: n/a, gw: n/a, prefsrc: n/a
  09:51:00 ens4: Adding route: dst: fe80::4001:aff:fea4:2/128, src: n/a, gw: n/a, prefsrc: n/a
  09:51:00 ens4: Adding route: dst: fe80::/64, src: n/a, gw: n/a, prefsrc: n/a
  09:51:00 ens4: Adding route: dst: 10.164.0.2/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: Adding route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: Adding route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  09:51:00 ens4: Link state is up-to-date
  09:51:00 ens4: found matching network '/run/systemd/network/10-netplan-ens4.network'
  09:51:00 Setting '/proc/sys/net/ipv6/conf/ens4/proxy_ndp' to '0'
  09:51:00 Setting '/proc/sys/net/ipv6/conf/ens4/use_tempaddr' to '0'
  09:51:00 Setting '/proc/sys/net/ipv6/conf/ens4/accept_ra' to '0'
  09:51:00 ens4: Started LLDP.
  09:51:00 ens4: Setting address genmode for link
  09:51:00 ens4: Setting address genmode done.
  09:51:00 ens4: Removing address 10.164.0.2
  09:51:00 ens4: Acquiring DHCPv4 lease
  09:51:00 ens4: Discovering IPv6 routers
  09:51:00 ens4: Removing address: 10.164.0.2/32 (valid for 15h 15s)
  09:51:00 ens4: Removing route: dst: 10.164.0.2/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: DHCPv4 address 10.164.0.2/32 via 10.164.0.1
  09:51:00 ens4: Setting MTU: 1460
  09:51:00 Setting '/proc/sys/net/ipv6/conf/ens4/disable_ipv6' to '0'
  09:51:00 ens4: IPv6 successfully enabled
  09:51:00 ens4: Setting MTU done.
  09:51:00 ens4: Updating address: 10.164.0.2/32 (valid for 1d)
  09:51:00 ens4: Adding route: dst: 10.164.0.2/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: Configuring route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: Configuring route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  09:51:00 ens4: Updating route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  09:51:00 ens4: Updating route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  09:51:00 ens4: Configured
  21:50:59 ens4: DHCP lease lost
  21:50:59 ens4: Removing address 10.164.0.2
  21:50:59 ens4: Setting MTU: 1420
  21:50:59 Setting '/proc/sys/net/ipv6/conf/ens4/disable_ipv6' to '0'
  21:50:59 ens4: IPv6 successfully enabled
  21:50:59 ens4: DHCPv4 address 10.164.0.2/32 via 10.164.0.1
  21:50:59 ens4: Removing route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  21:50:59 ens4: Removing route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  21:50:59 ens4: Removing address: 10.164.0.2/32 (valid for 12h 1s)
  21:50:59 ens4: Removing route: dst: 10.164.0.2/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  21:50:59 ens4: Setting MTU done.
  21:50:59 ens4: Adding address: 10.164.0.2/32 (valid for 1d)
  21:50:59 ens4: Adding route: dst: 10.164.0.2/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  21:50:59 ens4: Configuring route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  21:50:59 ens4: Configuring route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  21:50:59 ens4: Updating route: dst: 10.164.0.1/32, src: n/a, gw: n/a, prefsrc: 10.164.0.2
  21:50:59 ens4: Updating route: dst: n/a, src: n/a, gw: 10.164.0.1, prefsrc: 10.164.0.2
  21:50:59 ens4: Configured

  On dhcp renew (09:51:00), mtu got properly changed to 1460.
  On next dhcp renew (21:50:59) it got changed back to previous value 1420.
  In all cases, /run/systemd/netif/leases/2 correctly contained MTU=1460.

  There have been some changes to mtu handling in the past, I assume
  this is a regression, potentially from
  https://github.com/systemd/systemd/issues/12552 or
  https://github.com/systemd/systemd/issues/6593.

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