Sorry, Eoan is EOL in just a few weeks, and I don't think this will make
it. please upgrade to Focal if still affected by this bug.
** Changed in: systemd (Ubuntu Eoan)
Status: New => Won't Fix
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https://bugs.launchpad.net/bugs/1874661
Title:
networkd sometimes doesn't set mtu received by dhcp
Status in systemd package in Ubuntu:
Fix Released
Status in systemd source package in Eoan:
Won't Fix
Status in systemd source package in Focal:
Fix Released
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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