Is net.ipv6.conf.all.forwarding=1 equivalent to enabling forwarding for all individual interfaces?

Question

or it is the only way to enable forwarding for IPv6?

/proc/sys/net/ipv6/conf# grep '' */forwarding
all/forwarding:0
default/forwarding:1
eth0/forwarding:1
lo/forwarding:1
nat64/forwarding:1
tunl0/forwarding:1
veth_cm/forwarding:1
wifi0/forwarding:1
wlan0/forwarding:1

Does not route. I see packets in Wireshark, but they don't reach ip6tables.

/proc/sys/net/ipv6/conf# grep '' */forwarding
all/forwarding:1
default/forwarding:1
eth0/forwarding:1
lo/forwarding:1
nat64/forwarding:1
tunl0/forwarding:1
veth_cm/forwarding:1
wifi0/forwarding:1
wlan0/forwarding:1

Now network starts working.

/proc/sys/net/ipv6/conf# grep '' */forwarding
all/forwarding:1
default/forwarding:0
eth0/forwarding:0
lo/forwarding:0
nat64/forwarding:0
tunl0/forwarding:0
veth_cm/forwarding:0
wifi0/forwarding:0
wlan0/forwarding:0

Network still works. Are those individual per-interface switches even used? Can it be a Linux kernel bug (now running version 4.9.5).

Solution

Looks like indeed it is designed to work differently compared to IPv4's */forwarding and all/forwarding:

From https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt:

conf/all/forwarding - BOOLEAN
    Enable global IPv6 forwarding between all interfaces.

    IPv4 and IPv6 work differently here; e.g. netfilter must be used
    to control which interfaces may forward packets and which not.

    This also sets all interfaces' Host/Router setting
    'forwarding' to the specified value.  See below for details.

    This referred to as global forwarding.

forwarding - INTEGER
    Configure interface-specific Host/Router behaviour.

    Note: It is recommended to have the same setting on all
    interfaces; mixed router/host scenarios are rather uncommon.

    Possible values are:
        0 Forwarding disabled
        1 Forwarding enabled

    FALSE (0):

    By default, Host behaviour is assumed.  This means:

    1. IsRouter flag is not set in Neighbour Advertisements.
    2. If accept_ra is TRUE (default), transmit Router
       Solicitations.
    3. If accept_ra is TRUE (default), accept Router
       Advertisements (and do autoconfiguration).
    4. If accept_redirects is TRUE (default), accept Redirects.

    TRUE (1):

    If local forwarding is enabled, Router behaviour is assumed.
    This means exactly the reverse from the above:

    1. IsRouter flag is set in Neighbour Advertisements.
    2. Router Solicitations are not sent unless accept_ra is 2.
    3. Router Advertisements are ignored unless accept_ra is 2.
    4. Redirects are ignored.

    Default: 0 (disabled) if global forwarding is disabled (default),
         otherwise 1 (enabled).

So per-interface forwarding is only used for auxilary things like various flags and special packets behaviour, not for deciding whether to re-transmit packets or not.

Comments

[mleonard]

Yes, the Linux kernel behaviour is surprising here. I appreciate that you documented it here. +1

[Vi.]

I'd removed forwarding from per-interface config and named it is_router instead.