iptables MARK + ip rule fwmark on locally generated packets

iptables MARK + ip rule fwmark on locally generated packets

[Fredrik Ax]

Hi guys,

I'm a pretty experienced Linux / network developer and administrator,
but I can't get my head around this one.

The long story is that I have a box used as router/fw/proxy running
Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
interfaces (eth2,eth3,eth4) on the same external subnet. One of the
interfaces is used for doing masquerading of other
subnets. Masquerading (not snat) is chosen because the interfaces are
on dhcp, and I don't want to have to rewrite the fw rules each time I
get a new addr ... already have enough with dhclient-hooks for fixing
the routing tables dns-updates, etc ;-) What I basically want to do is
make the proxy's request to go out the same ifc as the masqueraded
packets getting a src addr of s41.s42.s43.s44. Other locally generated
packets should get a src addr s21.s22.s23.s24.

To accomplish this I'm using iptables to mark all, to port 80, locally
generated tcp packets:

% iptables -t mangle -vnL OUTPUT
Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
 pkts bytes target     prot opt in     out     source               destination         
 1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 

I have verified that the iptables rule marks them fine enough.

Then the ip rule with prio 99 below should then catch them and route
according to table eth4 below. That rule however does, for some reason
not match those packets, instead they are routed according to table
eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
disable that rule they are routed according the the prio 300 rule
(getting src addr s31.s32.s33.s34).

prompt% ip rule
0:	from all lookup local 
1:	from all lookup main 
99:	from all fwmark 0x4 lookup eth4 
100:	from 10.116.254.0/26 lookup eth4 
100:	from 10.116.255.34 lookup eth3 
100:	from 10.116.255.64/26 lookup eth4 
200:	from all lookup eth2 
300:	from all lookup eth3 
400:	from all lookup eth4 
32767:	from all lookup default 

prompt% ip route show table eth2
broadcast b1.b2.b3.b4 dev eth2  scope link  src s21.s22.s23.s24 
broadcast n1.n2.n3.n4 dev eth2  scope link  src s21.s22.s23.s24 
n1.n2.n3.n4/m dev eth2  scope link  src s21.s22.s23.s24 
default via g1.g2.g3.g4 dev eth2  src s21.s22.s23.s24 

prompt% ip route show table eth3
broadcast b1.b2.b3.b4 dev eth3  scope link  src s31.s32.s33.s34 
broadcast n1.n2.n3.n4 dev eth3  scope link  src s31.s32.s33.s34 
n1.n2.n3.n4/m dev eth3  scope link  src s31.s32.s33.s34 
default via g1.g2.g3.g4 dev eth3  src s31.s32.s33.s34 

prompt% ip route show table eth4 
broadcast b1.b2.b3.b4 dev eth4  scope link  src s41.s42.s43.s44 
broadcast n1.n2.n3.n4 dev eth4  scope link  src s41.s42.s43.s44 
n1.n2.n3.n4/m dev eth4  scope link  src s41.s42.s43.s44 
default via g1.g2.g3.g4 dev eth4  src s41.s42.s43.s44 


What am I doing wrong here?

TIA
/frax


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Re: iptables MARK + ip rule fwmark on locally generated packets

[Patrick McHardy]

Fredrik Ax wrote:
> Hi guys,
> 
> I'm a pretty experienced Linux / network developer and administrator,
> but I can't get my head around this one.
> 
> The long story is that I have a box used as router/fw/proxy running
> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> interfaces is used for doing masquerading of other
> subnets. Masquerading (not snat) is chosen because the interfaces are
> on dhcp, and I don't want to have to rewrite the fw rules each time I
> get a new addr ... already have enough with dhclient-hooks for fixing
> the routing tables dns-updates, etc ;-) What I basically want to do is
> make the proxy's request to go out the same ifc as the masqueraded
> packets getting a src addr of s41.s42.s43.s44. Other locally generated
> packets should get a src addr s21.s22.s23.s24.
> 
> To accomplish this I'm using iptables to mark all, to port 80, locally
> generated tcp packets:
> 
> % iptables -t mangle -vnL OUTPUT
> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
>  pkts bytes target     prot opt in     out     source               destination         
>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
> 
> I have verified that the iptables rule marks them fine enough.
> 
> Then the ip rule with prio 99 below should then catch them and route
> according to table eth4 below. That rule however does, for some reason
> not match those packets, instead they are routed according to table
> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
> disable that rule they are routed according the the prio 300 rule
> (getting src addr s31.s32.s33.s34).
> 
> ...
> 
> 
> What am I doing wrong here?

Source address selection happens before the packet is even generated,
so iptables marking in OUTPUT can't affect it.

Re: iptables MARK + ip rule fwmark on locally generated packets

[Fredrik Ax]

On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:

> Fredrik Ax wrote:
> > Hi guys,
> > 
> > I'm a pretty experienced Linux / network developer and administrator,
> > but I can't get my head around this one.
> > 
> > The long story is that I have a box used as router/fw/proxy running
> > Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> > interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> > interfaces is used for doing masquerading of other
> > subnets. Masquerading (not snat) is chosen because the interfaces are
> > on dhcp, and I don't want to have to rewrite the fw rules each time I
> > get a new addr ... already have enough with dhclient-hooks for fixing
> > the routing tables dns-updates, etc ;-) What I basically want to do is
> > make the proxy's request to go out the same ifc as the masqueraded
> > packets getting a src addr of s41.s42.s43.s44. Other locally generated
> > packets should get a src addr s21.s22.s23.s24.
> > 
> > To accomplish this I'm using iptables to mark all, to port 80, locally
> > generated tcp packets:
> > 
> > % iptables -t mangle -vnL OUTPUT
> > Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
> >  pkts bytes target     prot opt in     out     source               destination         
> >  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
> > 
> > I have verified that the iptables rule marks them fine enough.
> > 
> > Then the ip rule with prio 99 below should then catch them and route
> > according to table eth4 below. That rule however does, for some reason
> > not match those packets, instead they are routed according to table
> > eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
> > disable that rule they are routed according the the prio 300 rule
> > (getting src addr s31.s32.s33.s34).
> > 
> > ...
> > 
> > 
> > What am I doing wrong here?
> 
> Source address selection happens before the packet is even generated,
> so iptables marking in OUTPUT can't affect it.

So, to accomplish this I would have to oute it through a dummy
interface to make iptables able to mark it before it goes out?

TIA,
/frax

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Re: iptables MARK + ip rule fwmark on locally generated packets

[Patrick McHardy]

Fredrik Ax wrote:
> On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:
> 
>> Fredrik Ax wrote:
>>> Hi guys,
>>>
>>> I'm a pretty experienced Linux / network developer and administrator,
>>> but I can't get my head around this one.
>>>
>>> The long story is that I have a box used as router/fw/proxy running
>>> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
>>> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
>>> interfaces is used for doing masquerading of other
>>> subnets. Masquerading (not snat) is chosen because the interfaces are
>>> on dhcp, and I don't want to have to rewrite the fw rules each time I
>>> get a new addr ... already have enough with dhclient-hooks for fixing
>>> the routing tables dns-updates, etc ;-) What I basically want to do is
>>> make the proxy's request to go out the same ifc as the masqueraded
>>> packets getting a src addr of s41.s42.s43.s44. Other locally generated
>>> packets should get a src addr s21.s22.s23.s24.
>>>
>>> To accomplish this I'm using iptables to mark all, to port 80, locally
>>> generated tcp packets:
>>>
>>> % iptables -t mangle -vnL OUTPUT
>>> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
>>>  pkts bytes target     prot opt in     out     source               destination         
>>>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
>>>
>>> I have verified that the iptables rule marks them fine enough.
>>>
>>> Then the ip rule with prio 99 below should then catch them and route
>>> according to table eth4 below. That rule however does, for some reason
>>> not match those packets, instead they are routed according to table
>>> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
>>> disable that rule they are routed according the the prio 300 rule
>>> (getting src addr s31.s32.s33.s34).
>>>
>>> ...
>>>
>>>
>>> What am I doing wrong here?
>> Source address selection happens before the packet is even generated,
>> so iptables marking in OUTPUT can't affect it.
> 
> So, to accomplish this I would have to oute it through a dummy
> interface to make iptables able to mark it before it goes out?

You need some criteria for your routing rules that is available
when the socket is routed. That's everything but the packet mark.
Using a seperate device will work.

For ethernet, the macvlan device might be a good choice if you
don't mind using different MAC addresses for each IP.

Re: iptables MARK + ip rule fwmark on locally generated packets

[Fredrik Ax]

On Fri, Jan 22, 2010 at 11:53:45AM +0100, Patrick McHardy wrote:

> Fredrik Ax wrote:
> > On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:
> > 
> >> Fredrik Ax wrote:
> >>> Hi guys,
> >>>
> >>> I'm a pretty experienced Linux / network developer and administrator,
> >>> but I can't get my head around this one.
> >>>
> >>> The long story is that I have a box used as router/fw/proxy running
> >>> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> >>> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> >>> interfaces is used for doing masquerading of other
> >>> subnets. Masquerading (not snat) is chosen because the interfaces are
> >>> on dhcp, and I don't want to have to rewrite the fw rules each time I
> >>> get a new addr ... already have enough with dhclient-hooks for fixing
> >>> the routing tables dns-updates, etc ;-) What I basically want to do is
> >>> make the proxy's request to go out the same ifc as the masqueraded
> >>> packets getting a src addr of s41.s42.s43.s44. Other locally generated
> >>> packets should get a src addr s21.s22.s23.s24.
> >>>
> >>> To accomplish this I'm using iptables to mark all, to port 80, locally
> >>> generated tcp packets:
> >>>
> >>> % iptables -t mangle -vnL OUTPUT
> >>> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
> >>>  pkts bytes target     prot opt in     out     source               destination         
> >>>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
> >>>
> >>> I have verified that the iptables rule marks them fine enough.
> >>>
> >>> Then the ip rule with prio 99 below should then catch them and route
> >>> according to table eth4 below. That rule however does, for some reason
> >>> not match those packets, instead they are routed according to table
> >>> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
> >>> disable that rule they are routed according the the prio 300 rule
> >>> (getting src addr s31.s32.s33.s34).
> >>>
> >>> ...
> >>>
> >>>
> >>> What am I doing wrong here?
> >> Source address selection happens before the packet is even generated,
> >> so iptables marking in OUTPUT can't affect it.
> > 
> > So, to accomplish this I would have to oute it through a dummy
> > interface to make iptables able to mark it before it goes out?
> 
> You need some criteria for your routing rules that is available
> when the socket is routed. That's everything but the packet mark.
> Using a seperate device will work.
> 
> For ethernet, the macvlan device might be a good choice if you
> don't mind using different MAC addresses for each IP.

Thanks, I'll have a look at it ... 

Just one more question, the host is actually run as a domU on XEN and
all of the eth2-4 interfaces are on a in dom0 created bridge, bridging
in a vlan where the tagged traffic is on a blanace-rr bond-device.

Would it create any problems creating a macvlan device on top of this?

Thanks AGAIN,
/frax



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SOLVED: iptables MARK + ip rule fwmark on locally generated packets

[Fredrik Ax]

On Fri, Jan 22, 2010 at 12:12:57PM +0100, Fredrik Ax wrote:

> On Fri, Jan 22, 2010 at 11:53:45AM +0100, Patrick McHardy wrote:
> 
> > Fredrik Ax wrote:
> > > On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:
> > > 
> > >> Fredrik Ax wrote:
> > >>> Hi guys,
> > >>>
> > >>> I'm a pretty experienced Linux / network developer and administrator,
> > >>> but I can't get my head around this one.
> > >>>
> > >>> The long story is that I have a box used as router/fw/proxy running
> > >>> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> > >>> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> > >>> interfaces is used for doing masquerading of other
> > >>> subnets. Masquerading (not snat) is chosen because the interfaces are
> > >>> on dhcp, and I don't want to have to rewrite the fw rules each time I
> > >>> get a new addr ... already have enough with dhclient-hooks for fixing
> > >>> the routing tables dns-updates, etc ;-) What I basically want to do is
> > >>> make the proxy's request to go out the same ifc as the masqueraded
> > >>> packets getting a src addr of s41.s42.s43.s44. Other locally generated
> > >>> packets should get a src addr s21.s22.s23.s24.
> > >>>
> > >>> To accomplish this I'm using iptables to mark all, to port 80, locally
> > >>> generated tcp packets:
> > >>>
> > >>> % iptables -t mangle -vnL OUTPUT
> > >>> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
> > >>>  pkts bytes target     prot opt in     out     source               destination         
> > >>>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
> > >>>
> > >>> I have verified that the iptables rule marks them fine enough.
> > >>>
> > >>> Then the ip rule with prio 99 below should then catch them and route
> > >>> according to table eth4 below. That rule however does, for some reason
> > >>> not match those packets, instead they are routed according to table
> > >>> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
> > >>> disable that rule they are routed according the the prio 300 rule
> > >>> (getting src addr s31.s32.s33.s34).
> > >>>
> > >>> ...
> > >>>
> > >>>
> > >>> What am I doing wrong here?
> > >> Source address selection happens before the packet is even generated,
> > >> so iptables marking in OUTPUT can't affect it.
> > > 
> > > So, to accomplish this I would have to oute it through a dummy
> > > interface to make iptables able to mark it before it goes out?
> > 
> > You need some criteria for your routing rules that is available
> > when the socket is routed. That's everything but the packet mark.
> > Using a seperate device will work.
> > 
> > For ethernet, the macvlan device might be a good choice if you
> > don't mind using different MAC addresses for each IP.
> 
> Thanks, I'll have a look at it ... 
> 
> Just one more question, the host is actually run as a domU on XEN and
> all of the eth2-4 interfaces are on a in dom0 created bridge, bridging
> in a vlan where the tagged traffic is on a blanace-rr bond-device.
> 
> Would it create any problems creating a macvlan device on top of this?

Never mind the above question, I found, i.m.h.o. a simpler, solution to it.

Since the packets are actually marked correctly they get routed out
through eth4, just with the source address as if it was to be routed
outh through eth2, I therefore simply masquerade those packets in the
nat postrouting.

Many thanks for your good answers!
/frax

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Re: iptables MARK + ip rule fwmark on locally generated packets

[Patrick McHardy]

Fredrik Ax wrote:
> On Fri, Jan 22, 2010 at 11:53:45AM +0100, Patrick McHardy wrote:
>>> So, to accomplish this I would have to oute it through a dummy
>>> interface to make iptables able to mark it before it goes out?
>> You need some criteria for your routing rules that is available
>> when the socket is routed. That's everything but the packet mark.
>> Using a seperate device will work.
>>
>> For ethernet, the macvlan device might be a good choice if you
>> don't mind using different MAC addresses for each IP.
> 
> Thanks, I'll have a look at it ... 
> 
> Just one more question, the host is actually run as a domU on XEN and
> all of the eth2-4 interfaces are on a in dom0 created bridge, bridging
> in a vlan where the tagged traffic is on a blanace-rr bond-device.
> 
> Would it create any problems creating a macvlan device on top of this?

No, that should be fine.

Re: iptables MARK + ip rule fwmark on locally generated packets

[Mart Frauenlob]

On 22.01.2010 11:53, netfilter-owner@vger.kernel.org wrote:
> Fredrik Ax wrote:
>> On Fri, Jan 22, 2010 at 11:09:43AM +0100, Patrick McHardy wrote:
>>
>>> Fredrik Ax wrote:
>>>> Hi guys,
>>>>
>>>> I'm a pretty experienced Linux / network developer and administrator,
>>>> but I can't get my head around this one.
>>>>
>>>> The long story is that I have a box used as router/fw/proxy running
>>>> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
>>>> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
>>>> interfaces is used for doing masquerading of other
>>>> subnets. Masquerading (not snat) is chosen because the interfaces are
>>>> on dhcp, and I don't want to have to rewrite the fw rules each time I
>>>> get a new addr ... already have enough with dhclient-hooks for fixing
>>>> the routing tables dns-updates, etc ;-) What I basically want to do is
>>>> make the proxy's request to go out the same ifc as the masqueraded
>>>> packets getting a src addr of s41.s42.s43.s44. Other locally generated
>>>> packets should get a src addr s21.s22.s23.s24.
>>>>
>>>> To accomplish this I'm using iptables to mark all, to port 80, locally
>>>> generated tcp packets:
>>>>
>>>> % iptables -t mangle -vnL OUTPUT
>>>> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
>>>>  pkts bytes target     prot opt in     out     source               destination         
>>>>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
>>>>
>>>> I have verified that the iptables rule marks them fine enough.
>>>>
>>>> Then the ip rule with prio 99 below should then catch them and route
>>>> according to table eth4 below. That rule however does, for some reason
>>>> not match those packets, instead they are routed according to table
>>>> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
>>>> disable that rule they are routed according the the prio 300 rule
>>>> (getting src addr s31.s32.s33.s34).
>>>>
>>>> ...
>>>>
>>>>
>>>> What am I doing wrong here?
>>> Source address selection happens before the packet is even generated,
>>> so iptables marking in OUTPUT can't affect it.
>>
>> So, to accomplish this I would have to oute it through a dummy
>> interface to make iptables able to mark it before it goes out?
> 
> You need some criteria for your routing rules that is available
> when the socket is routed. That's everything but the packet mark.
> Using a seperate device will work.
> 
> For ethernet, the macvlan device might be a good choice if you
> don't mind using different MAC addresses for each IP.

sorry to jump in, but when i look at the nf packet flow picture from here:
http://jengelh.medozas.de/images/nf-packet-flow.png

it says in 'output path':
mangle OUTPUT --> reroute check --> nat OUTPUT

what does the 'reroute check' mean, if not consulting the ip rules ->
read the fwmark -> use the according routing table?

what am i missing?

thanks for help

best regards

Mart

Re: iptables MARK + ip rule fwmark on locally generated packets

[Fredrik Ax]

On Fri, Jan 22, 2010 at 11:07:55AM +0100, Fredrik Ax wrote:

> Hi guys,
> 
> I'm a pretty experienced Linux / network developer and administrator,
> but I can't get my head around this one.
> 
> The long story is that I have a box used as router/fw/proxy running
> Debian Squeeze with a customized 2.6.32 x86_64 kernel having three
> interfaces (eth2,eth3,eth4) on the same external subnet. One of the
> interfaces is used for doing masquerading of other
> subnets. Masquerading (not snat) is chosen because the interfaces are
> on dhcp, and I don't want to have to rewrite the fw rules each time I
> get a new addr ... already have enough with dhclient-hooks for fixing
> the routing tables dns-updates, etc ;-) What I basically want to do is
> make the proxy's request to go out the same ifc as the masqueraded
> packets getting a src addr of s41.s42.s43.s44. Other locally generated
> packets should get a src addr s21.s22.s23.s24.
> 
> To accomplish this I'm using iptables to mark all, to port 80, locally
> generated tcp packets:
> 
> % iptables -t mangle -vnL OUTPUT
> Chain OUTPUT (policy ACCEPT 3234 packets, 2254K bytes)
>  pkts bytes target     prot opt in     out     source               destination         
>  1114  181K MARK       tcp  --  *      *       0.0.0.0/0            0.0.0.0/0           tcp dpt:80 MARK set 0x4 
> 
> I have verified that the iptables rule marks them fine enough.
> 
> Then the ip rule with prio 99 below should then catch them and route
> according to table eth4 below. That rule however does, for some reason
> not match those packets, instead they are routed according to table
> eth2 below (prio 200 rule), getting src addr s21.s22.s23.s24.  If I
> disable that rule they are routed according the the prio 300 rule
> (getting src addr s31.s32.s33.s34).
> 
> prompt% ip rule
> 0:	from all lookup local 
> 1:	from all lookup main 
> 99:	from all fwmark 0x4 lookup eth4 
> 100:	from 10.116.254.0/26 lookup eth4 
> 100:	from 10.116.255.34 lookup eth3 
> 100:	from 10.116.255.64/26 lookup eth4 
> 200:	from all lookup eth2 
> 300:	from all lookup eth3 
> 400:	from all lookup eth4 
> 32767:	from all lookup default 
> 
> prompt% ip route show table eth2
> broadcast b1.b2.b3.b4 dev eth2  scope link  src s21.s22.s23.s24 
> broadcast n1.n2.n3.n4 dev eth2  scope link  src s21.s22.s23.s24 
> n1.n2.n3.n4/m dev eth2  scope link  src s21.s22.s23.s24 
> default via g1.g2.g3.g4 dev eth2  src s21.s22.s23.s24 
> 
> prompt% ip route show table eth3
> broadcast b1.b2.b3.b4 dev eth3  scope link  src s31.s32.s33.s34 
> broadcast n1.n2.n3.n4 dev eth3  scope link  src s31.s32.s33.s34 
> n1.n2.n3.n4/m dev eth3  scope link  src s31.s32.s33.s34 
> default via g1.g2.g3.g4 dev eth3  src s31.s32.s33.s34 
> 
> prompt% ip route show table eth4 
> broadcast b1.b2.b3.b4 dev eth4  scope link  src s41.s42.s43.s44 
> broadcast n1.n2.n3.n4 dev eth4  scope link  src s41.s42.s43.s44 
> n1.n2.n3.n4/m dev eth4  scope link  src s41.s42.s43.s44 
> default via g1.g2.g3.g4 dev eth4  src s41.s42.s43.s44 

You might also want to know that the local routes for eth2-4 are removed in the local table,
and that the main table holds no default routes.

> 
> 
> What am I doing wrong here?
> 
> TIA
> /frax
> 

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