missing ip v6 link local route by Network-Manager

[Hadmut Danisch]

Hi,

I was analyzing, why all the Ubuntu 20.04 machines in my network can operate at full ethernet speed over IPv4, but some are significantly slower when using ipv6.

Short reason: Missing route by Network Manager

long reason description:

I'm using a FritzBox DOCSIS router (quit common in Germany), providing both IPv4 and IPv6 internet.

The IPv6 prefix is assigned by the provider only dynamically and changes from time to time. Since this makes internal firewalling difficult, I've configured the Router use a second IPv6 network range, a unique local address range (ULA), starting with fd..., standard practice for IPv6.

Thus the router advertises two ipv6 network prefixes, one official, which is dynamically assigned by the provider (2a02::...) and one ULA (fdfc:...), both a RA and for DHCP6 requests.

I found that the machines in my network deal differently with that.

Those Ubuntu 20.04 machines running Ubuntu Server and using netplan to configure interfaces, do things correctly. They assign an autogenerated IPv6-address for both ranges to the interfaces (lower 64 bit constructed from mac address) and two routes for each prefix, one interface/link local route and one to the router, e.g. (prefixes and addresses abbreviated for privacy reasons)

2a02:...::/64 dev eno1 proto ra metric 100 expires 6814sec pref medium
2a02:...::/64 via fe80::ROUTER dev eno1 proto ra metric 1024 expires 1414sec pref medium
fdfc:...::/64 dev eno1 proto ra metric 100 expires 6814sec pref medium
fdfc:...::/64 via fe80::ROUTER dev eno1 proto ra metric 1024 expires 1414sec pref medium
fe80::/64 dev eno1 proto kernel metric 256 pref medium
default via fe80::ROUTER dev eno1 proto ra metric 100 expires 1414sec mtu 1500 pref medium
default via fe80::ROUTER dev eno1 proto ra metric 1024 expires 1414sec pref medium

although the metric 1024 routes seem superfluous and useless, things work well, since the metric 100 routes (without via) allow the machine to directly communicate with each other, therefore fast (using the switch).

On the other hand, those desktop machines running Lubuntu 20.04, thus using Network-Manager to configure interfaces, do things wrong (i.e. routes are missing, there's only via-routes for both prefixes)

2a02:...::/64 via fe80::ROUTER dev enp4s0 proto ra metric 100 pref medium
fdfc:...::/64 via fe80::ROUTER dev enp4s0 proto ra metric 100 pref medium
fe80::/64 dev enp4s0 proto kernel metric 100 pref medium
default via fe80::ROUTER dev enp4s0 proto ra metric 100 pref medium

as you can see, there is no route allowing the machine to directly communicate with other hosts, except for the link local fe80 route.

Therefore, traffic between two machines in the LAN is not switched on the switch, but routed on the router, and thus significantly slower (although still working, that's why most people probably wouldn't notice).

I've tried several settings in the ipv6 tab of the Networks Editor (edit connections -> select wired connection -> IPv6 Settings), but neither could I find a man page describing the Automatic Methods, nor did I find any effect when changing them.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: network-manager 1.22.10-1ubuntu2.2
ProcVersionSignature: Ubuntu 5.4.0-77.86-generic 5.4.119
Uname: Linux 5.4.0-77-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: LXQt
Date: Sun Jul 18 00:59:09 2021
InstallationDate: Installed on 2020-06-12 (400 days ago)
InstallationMedia: Lubuntu 20.04 LTS "Focal Fossa" - Release amd64 (20200423)
RfKill:
 0: phy0: Wireless LAN
  Soft blocked: no
  Hard blocked: no
SourcePackage: network-manager
UpgradeStatus: No upgrade log present (probably fresh install)
nmcli-nm:
 RUNNING VERSION STATE STARTUP CONNECTIVITY NETWORKING WIFI-HW WIFI WWAN-HW WWAN
 running 1.22.10 connected started full enabled enabled enabled enabled enabled

[Hadmut Danisch (hadmut)]

Bug still exists in Lubuntu 21.10.