From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: problem with flowi structure
Date: Fri, 17 Sep 2010 11:08:53 +0200
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Cc: AIJAZ BAIG <aijazbaig1@gmail.com>,
	netfilter-devel <netfilter-devel@vger.kernel.org>,
	netdev@vger.kernel.org
To: Nicola Padovano <nicola.padovano@gmail.com>
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Le vendredi 17 septembre 2010 =C3=A0 10:46 +0200, Nicola Padovano a =C3=
=A9crit :
> Aijaz, great! Now it' clear. Thank you.
> It remains a problem: the meaning of
>=20
> fl.nl_u.ip4_u.saddr =3D 0
>=20
> But, first of all:
> - we know that a routing table doesn't contain the ip source
> - but the routing cache contains it.
>=20
> so my first question is: why routing cache has it and routing cache
> not? what it its usage?
> Then my second question: the meaning of fl.nl_u.ip4_u.saddr =3D 0
>=20
> thank you very much

# ip addr show dev eth1
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
qlen 1000 txqueues 4=20
    link/ether f4:ce:46:bb:32:d4 brd ff:ff:ff:ff:ff:ff
    inet 192.168.20.108/24 brd 192.168.20.255 scope global eth1
    inet6 fe80::f6ce:46ff:febb:32d4/64 scope link=20
       valid_lft forever preferred_lft forever
# ip addr add 192.168.20.67/24 dev eth1
# ip addr show dev eth1
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
qlen 1000 txqueues 4=20
    link/ether f4:ce:46:bb:32:d4 brd ff:ff:ff:ff:ff:ff
    inet 192.168.20.108/24 brd 192.168.20.255 scope global eth1
    inet 192.168.20.67/24 scope global secondary eth1
    inet6 fe80::f6ce:46ff:febb:32d4/64 scope link=20
       valid_lft forever preferred_lft forever
# ip ro
192.168.20.0/24 dev eth1  proto kernel  scope link  src 192.168.20.108=20
default via 192.168.20.254 dev eth1=20

# ip ro flush cache
# ip ro flush cache
# ip ro get 192.168.20.110
192.168.20.110 dev eth1  src 192.168.20.108=20
    cache  mtu 1500 advmss 1460 hoplimit 64
# ip ro get 192.168.20.110 from 192.168.20.67
192.168.20.110 from 192.168.20.67 dev eth1=20
    cache  mtu 1500 advmss 1460 hoplimit 64
# ip ro list cache
192.168.20.110 dev eth1  src 192.168.20.108=20
    cache  mtu 1500 advmss 1460 hoplimit 64
192.168.20.110 from 192.168.20.67 dev eth1=20
    cache  mtu 1500 advmss 1460 hoplimit 64

You can see two different routes in cache to reach 192.168.20.110,
one from "0.0.0.0" (we force src 192.168.20.108),
one from 192.168.20.67=20

With different keys (src addr, tos, fwmark, ...) we can have a lot of
different routes for apparently same destination