Re: Incorrect routing to lo in spite of correct record in routing table

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 12 septembre 2011 à 23:14 +0600, Марк Коренберг a écrit :
> Suppose this (note netowrk masks):
> 
> [mmarkk@mmarkk-devel ~]# uname -a
> Linux mmarkk-devel 2.6.32-71.el6.x86_64 #1 SMP Wed Sep 1 01:33:01 EDT
> 2010 x86_64 x86_64 x86_64 GNU/Linux
> 
> [mmarkk@mmarkk-devel ~]# ip addr
> 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
>     inet 127.0.0.1/32 scope host lo
> 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
> state UP qlen 1000
>     inet 10.80.20.21/16 brd 10.80.255.255 scope global eth0
> 3: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN
>     inet 127.0.0.2/32 scope host dummy0
> 
> [mmarkk@mmarkk-devel ~]# ip route show
> 10.80.0.0/16 dev eth0  proto kernel  scope link  src 10.80.20.21
> unreachable 127.0.0.0/8 <---------------------- has been added by hand
> default via 10.80.90.40 dev eth0
> 
> [mmarkk@mmarkk-devel ~]# ip route show table local
> broadcast 10.80.0.0 dev eth0  proto kernel  scope link  src 10.80.20.21
> local 10.80.20.21 dev eth0  proto kernel  scope host  src 10.80.20.21
> local 127.0.0.2 dev dummy0  proto kernel  scope host  src 127.0.0.2
> broadcast 10.80.255.255 dev eth0  proto kernel  scope link  src 10.80.20.21
> local 127.0.0.1 dev lo  proto kernel  scope host  src 127.0.0.1
> 
> [mmarkk@mmarkk-devel ~]# ip route flush cache
> 
> [mmarkk@mmarkk-devel ~]# ip route get 127.0.0.3
> RTNETLINK answers: Network is unreachable <------------ good, that is OK
> 
> [mmarkk@mmarkk-devel ~]# ip route get 127.0.0.1
> local 127.0.0.1 dev lo  src 127.0.0.1  <--------- Good, that's OK too
>     cache <local>  mtu 16436 advmss 16396 hoplimit 64
> 
> [mmarkk@mmarkk-devel ~]# ip route get 127.0.0.2
> local 127.0.0.2 dev lo  src 127.0.0.2 <------------ HAY! this is not I
> want. Why device is "lo" (instead of dummy0) ?!
>     cache <local>  mtu 16436 advmss 16396 hoplimit 64
> 
> =========================
> The problem is that I want to do DNAT to localhost. I know, why DNAT
> to 127.0.0.1 (to interface "lo") does not work. I decide to do that
> using interface dummy0. It works as expected if I choose any address
> not in 127.0.0.0/8.
> 
> RFC 3330 said:
> -------------
> A datagram sent by a higher level protocol to an  address anywhere
> within this block should loop back inside the host. This is ordinarily
> implemented using only 127.0.0.1/32 for loopback, but no addresses
> within this block should ever appear on any network anywhere
> -------------
> So, my question: Why the kernel routes 127.0.0.2 to lo (instead of dummy0) ?
> 
> Well, If kernel is so smart to route any 127.0.0.0/8 to "lo", why it
> routes 127.0.0.3 to default gateway if I remove "unreachable
> 127.0.0.0/8" ?!
> 

Because you have 127.0.0.1/32 on lo, not 127.0.0.1/8

> I think there is some bug in kernel. What are you thinking about?
> 

I have no idea why you think there is a bug.

127.0.0.2 (dummy0) is a local address like 10.80.20.21 (eth0)

When you ping 10.80.20.21, the frame goes through loopback, not eth0.

# ip ro get 10.80.20.21
local 10.80.20.21 dev lo  src 10.80.20.21 
    cache <local> 

BTW, if you 'ifconfig lo down', ping to 10.80.20.21 doesnt work.