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question about forwarding and sysfs entries

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  question about forwarding and sysfs entries wkevin <wkevils@gmail.com> - 2013-06-15 04:26 -0700
    Re: question about forwarding and sysfs entries Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2013-06-15 14:05 +0200
      Re: question about forwarding and sysfs entries Richard Kettlewell <rjk@greenend.org.uk> - 2013-06-15 16:14 +0100
        Re: question about forwarding and sysfs entries Pascal Hambourg <boite-a-spam@plouf.fr.eu.org> - 2013-06-22 14:40 +0200
    Re: question about forwarding and sysfs entries Pascal Hambourg <boite-a-spam@plouf.fr.eu.org> - 2013-06-22 14:42 +0200

#2213 — question about forwarding and sysfs entries

Hello,
As I know, in order that IPv4 traffic could be forwarded in a Linux machine, we should set  /proc/sys/net/ipv4/ip_forward to 1.

My question is:
there is also:
/proc/sys/net/ipv4/conf/all/forwarding
/proc/sys/net/ipv4/conf/default/forwarding
/proc/sys/net/ipv4/conf/eth0/forwarding
/proc/sys/net/ipv4/conf/eth1/forwarding
are they needed also in order to forward a packet which arrives 
on eth0 and should be transmitter from eth1 ? 

I would appreciate if someone can elaborate on it? 

regards,
Kevin

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#2214

Le 15/06/2013 13:26, wkevin a écrit :
> I would appreciate if someone can elaborate on it? 

From Documentation/networking/ip-sysctl.txt:

conf/default/*:
        Change the interface-specific default settings.

conf/all/*:
        Change all the interface-specific settings.

conf/interface/*:
        Change special settings per interface.

        The functional behaviour for certain settings is different
        depending on whether local forwarding is enabled or not.

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 - BOOLEAN
        Enable IP forwarding on this interface.

When you want to change a specific interface, use the conf/<interface>
one. Changing all will affect all interfaces (which seems logical). The
default one probably affects interfaces being created (?)

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#2216

Xavier Roche <xroche@free.fr.NOSPAM.invalid> writes:
> Le 15/06/2013 13:26, wkevin a écrit :
>> I would appreciate if someone can elaborate on it? 
>
> From Documentation/networking/ip-sysctl.txt:
>
> conf/default/*:
>         Change the interface-specific default settings.
>
> conf/all/*:
>         Change all the interface-specific settings.
>
> conf/interface/*:
>         Change special settings per interface.
>
>         The functional behaviour for certain settings is different
>         depending on whether local forwarding is enabled or not.
>
> 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 - BOOLEAN
>         Enable IP forwarding on this interface.
>
> When you want to change a specific interface, use the conf/<interface>
> one. Changing all will affect all interfaces (which seems logical). The
> default one probably affects interfaces being created (?)

I looked into the meaning of ‘all’ in another place back in March.  It
turns out to be a bit weird.  Here’s what I wrote at the time:

  I don’t think it has a single coherent meaning.  For instance:

   - Setting net.ipv4.conf.all.forwarding=1 sets
     net.ipv4.conf.<device>.forwarding for every device.

   - Setting net.ipv4.conf.all.accept_redirects=0 doesn’t disable it for
     any of the individual devices.

  This is done on a case-by-case basis for each sysctl (although most are
  like accept_redirects rather than forwarding).

  The value you get back is the last value you wrote to that sysctl, even
  if it’s now inconsistent with everything else.  For instance you can
  have:

      net.ipv4.conf.all.forwarding = 1
      net.ipv4.conf.default.forwarding = 0
      net.ipv4.conf.lo.forwarding = 0
      net.ipv4.conf.eth0.forwarding = 0
      net.ipv4.conf.br0.forwarding = 0
      net.ipv4.conf.vboxnet0.forwarding = 0
      net.ipv4.conf.pan0.forwarding = 0

  ...if you’ve set all.forwarding=1 and then set all the rest to 0.

  Moreover: these things are all just kernel variables, not behaviors as
  such; the way they get used depends on whether the relevant bit of
  kernel code reads the all version, the device version or both.  The
  documentation does reflect this point; for instance the accept_redirects
  section describes what combination of sysctl values will turn the
  behaviour on or off.  (I’ve not done an exhaustive check though.)

-- 
http://www.greenend.org.uk/rjk/

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#2219

Richard Kettlewell a écrit :
>
> I looked into the meaning of 'all' in another place back in March.  It
> turns out to be a bit weird.  Here's what I wrote at the time:
> 
>   I don't think it has a single coherent meaning.  For instance:
> 
>    - Setting net.ipv4.conf.all.forwarding=1 sets
>      net.ipv4.conf.<device>.forwarding for every device.
> 
>    - Setting net.ipv4.conf.all.accept_redirects=0 doesn't disable it for
>      any of the individual devices.
> 
>   This is done on a case-by-case basis for each sysctl (although most are
>   like accept_redirects rather than forwarding).

Indeed 'forwarding' is the special case. For most other parameters under
net.ipv4.conf, the operationnal value for each interface is a
combination of net.ipv4.conf.<interface>.<parameter> and
net.ipv4.conf.all.<parameter>. The operator may be AND, OR, MAX...
depending on each parameter. See the descriptions in ip-sysctl.txt.
AFAICS, it seems that most parameters under net.ipv6.conf.all other that
forwarding are just ignored.

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#2220

wkevin a écrit :

/proc/sys/net/ipv4/ip_forward

/proc/sys/ has nothing to do with sysfs (as mentionned in your subject).
sysfs is mounted on /sys.

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