ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Jeroen van Ingen]

Hi,

We're having an issue on our Linux PPTP servers. After the first PPTP
client is connected, locally generated broadcast packets go out the ppp0
interface while the routing rules should select eth0.

Some details were already mentioned on the linux-kernel list, see eg
http://lkml.indiana.edu/hypermail/linux/kernel/1111.2/00290.html for
reference.

Finally we were able to narrow it down to one specific commit:
e066008b38ca9ace1b6de8dbbac8ed460640791d ("ipv4: Fix __ip_dev_find() to
use ifa_local instead of ifa_address."). With all recent kernels (tested
up to 3.2.0rc3) we observe this issue and it's solved by reverting this
single patch.

This is the first time we've had to debug a kernel issue. Any advice on
how to proceed would be very welcome. If it's not possible to have this
patch reverted in the kernel, hopefully someone can explain how to work
around this behavior.


Regards,

Jeroen van Ingen
ICT Service Centre
University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Julian Anastasov]

	Hello,

On Tue, 29 Nov 2011, Jeroen van Ingen wrote:

> Hi,
> 
> We're having an issue on our Linux PPTP servers. After the first PPTP
> client is connected, locally generated broadcast packets go out the ppp0
> interface while the routing rules should select eth0.
> 
> Some details were already mentioned on the linux-kernel list, see eg
> http://lkml.indiana.edu/hypermail/linux/kernel/1111.2/00290.html for
> reference.
> 
> Finally we were able to narrow it down to one specific commit:
> e066008b38ca9ace1b6de8dbbac8ed460640791d ("ipv4: Fix __ip_dev_find() to
> use ifa_local instead of ifa_address."). With all recent kernels (tested
> up to 3.2.0rc3) we observe this issue and it's solved by reverting this
> single patch.
> 
> This is the first time we've had to debug a kernel issue. Any advice on
> how to proceed would be very welcome. If it's not possible to have this
> patch reverted in the kernel, hopefully someone can explain how to work
> around this behavior.

	__ip_dev_find can cause problem if same IP is added on many
interfaces because it uses hash table implemented with hlist.
Old versions used only routing lookup and the routing returns
the first created local route, i.e. the first device where this
IP was added is returned.

	And now it is risky to use __ip_dev_find in
ip_route_output_slow when:

- saddr is provided
- desired oif is 0
- daddr is multicast/lbcast

	We select oif by ignoring route ordering. May be some
ppp device wins here because it has this saddr added last but
is first in hlist.

	What is the case after first client is connected, can
you show output from:

ip addr show
ip route list table local

	If the above is true may be we have to find a
way to return the first device where the IP is added.
May be this is the main rule that is used when one adds
same IP on many interfaces.

	May be the solution is to convert inet_addr_lst
from hlist to normal list, so that we can append new
addresses at tail and __ip_dev_find to find the first
device where IP was added.

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[David Miller]

From: Julian Anastasov <ja@ssi.bg>
Date: Wed, 30 Nov 2011 01:06:46 +0200 (EET)

> 	May be the solution is to convert inet_addr_lst
> from hlist to normal list, so that we can append new
> addresses at tail and __ip_dev_find to find the first
> device where IP was added.

Sure, but do we really want to guarentee this behavior forever?

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Julian Anastasov]

	Hello,

On Tue, 29 Nov 2011, David Miller wrote:

> From: Julian Anastasov <ja@ssi.bg>
> Date: Wed, 30 Nov 2011 01:06:46 +0200 (EET)
> 
> > 	May be the solution is to convert inet_addr_lst
> > from hlist to normal list, so that we can append new
> > addresses at tail and __ip_dev_find to find the first
> > device where IP was added.
> 
> Sure, but do we really want to guarentee this behavior forever?

	I remember for such issue with ipsec%d interfaces
years ago. May be the PPP servers should be configured
to use another local IP for PPP devices because broadcasts
and multicasts may need their own local address - they can use
addresses to refer to output devices.

	Not sure what else we can do, now we have to waste
1-2KB for this to work before someone recreates the eth
addresses and changes the ordering.

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Jeroen van Ingen]

Hi Julian, David,

> > > 	May be the solution is to convert inet_addr_lst
> > > from hlist to normal list, so that we can append new
> > > addresses at tail and __ip_dev_find to find the first
> > > device where IP was added.
> > 
> > Sure, but do we really want to guarentee this behavior forever?
> 
> 	I remember for such issue with ipsec%d interfaces
> years ago. May be the PPP servers should be configured
> to use another local IP for PPP devices because broadcasts
> and multicasts may need their own local address - they can use
> addresses to refer to output devices.
> 
> 	Not sure what else we can do, now we have to waste
> 1-2KB for this to work before someone recreates the eth
> addresses and changes the ordering.

FYI: we successfully tested two scenarios that provide a workaround with
the current kernel versions:

1) Explicitly configuring Radius to use one of the secondary IPs as
source for the DHCP broadcast. Since the IP we chose is only bound to
eth0, this broadcast goes out the correct interface. Other
system-generated broadcast would probably still go out the wrong
interface, but at least it allows us to accept more than one PPTP
client.

2) Adding an option to the "pppd" config, so the ppp-devices it creates
do not use the primary IP from eth0 but rather one of the secondary
addresses. This way we don't have to modify any other software that
might generate a broadcast.


While the second option provides a workable solution for us, we're still
under the impression that this change in behavior might cause a problem
for other users and/or configurations as well. We'll leave the rest of
the considerations to the real experts, while remaining curious about
the outcome :)

Thanks again for your assistance.


Regards,

Jeroen van Ingen
ICT Service Centre
University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Julian Anastasov]

	Hello,

On Thu, 1 Dec 2011, Jeroen van Ingen wrote:

> FYI: we successfully tested two scenarios that provide a workaround with
> the current kernel versions:
> 
> 1) Explicitly configuring Radius to use one of the secondary IPs as
> source for the DHCP broadcast. Since the IP we chose is only bound to
> eth0, this broadcast goes out the correct interface. Other
> system-generated broadcast would probably still go out the wrong
> interface, but at least it allows us to accept more than one PPTP
> client.
> 
> 2) Adding an option to the "pppd" config, so the ppp-devices it creates
> do not use the primary IP from eth0 but rather one of the secondary
> addresses. This way we don't have to modify any other software that
> might generate a broadcast.
> 
> 
> While the second option provides a workable solution for us, we're still
> under the impression that this change in behavior might cause a problem
> for other users and/or configurations as well. We'll leave the rest of
> the considerations to the real experts, while remaining curious about
> the outcome :)

	Yes, thanks for confirming that this is the actual
problem! I hope David will find the best way to restore
this feature.

> Thanks again for your assistance.
> 
> 
> Regards,
> 
> Jeroen van Ingen
> ICT Service Centre
> University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: ipv4: broadcast sometimes leaves wrong interface (since commit e066008b38ca9ace1b6de8dbbac8ed460640791d)

[Jeroen van Ingen]

Hi Julian,

Thanks for your assistance, it's very much appreciated.

> 	__ip_dev_find can cause problem if same IP is added on many
> interfaces because it uses hash table implemented with hlist.
> Old versions used only routing lookup and the routing returns
> the first created local route, i.e. the first device where this
> IP was added is returned.

Right, and all ppp interfaces get the same IP as the interface that the
clients use to connect to, which in our case is the "main" IP for the
server.

> 	And now it is risky to use __ip_dev_find in
> ip_route_output_slow when:
> 
> - saddr is provided
> - desired oif is 0
> - daddr is multicast/lbcast
> 
> 	We select oif by ignoring route ordering. May be some
> ppp device wins here because it has this saddr added last but
> is first in hlist.
> 
> 	What is the case after first client is connected, can
> you show output from:
> 
> ip addr show

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP
qlen 1000
    link/ether 00:08:02:91:c9:1f brd ff:ff:ff:ff:ff:ff
    inet 130.89.254.233/27 brd 130.89.254.255 scope global eth0
    inet 130.89.254.234/27 brd 130.89.254.255 scope global secondary
eth0:1
    inet 130.89.254.235/27 brd 130.89.254.255 scope global secondary
eth0:2
    inet 130.89.254.236/27 brd 130.89.254.255 scope global secondary
eth0:3
    inet6 fe80::208:2ff:fe91:c91f/64 scope link
       valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc mq
state UP qlen 1000
    link/ether 00:1e:0b:76:83:d6 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::21e:bff:fe76:83d6/64 scope link
       valid_lft forever preferred_lft forever
4: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master br0 state UP qlen 100
    link/ether 42:a7:29:fa:de:4a brd ff:ff:ff:ff:ff:ff
    inet6 fe80::40a7:29ff:fefa:de4a/64 scope link
       valid_lft forever preferred_lft forever
5: tap1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master br0 state UP qlen 100
    link/ether 3e:1c:6f:5f:a2:a0 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::3c1c:6fff:fe5f:a2a0/64 scope link
       valid_lft forever preferred_lft forever
6: tap2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master br0 state UP qlen 100
    link/ether 6a:17:16:92:93:35 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::6817:16ff:fe92:9335/64 scope link
       valid_lft forever preferred_lft forever
7: tap3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master br0 state UP qlen 100
    link/ether ce:99:a3:e6:32:cd brd ff:ff:ff:ff:ff:ff
    inet6 fe80::cc99:a3ff:fee6:32cd/64 scope link
       valid_lft forever preferred_lft forever
8: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
UP
    link/ether 00:1e:0b:76:83:d6 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::21e:bff:fe76:83d6/64 scope link
       valid_lft forever preferred_lft forever
9: eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 1000
    link/ether 00:1e:0b:76:83:d4 brd ff:ff:ff:ff:ff:ff
10: eth1.183@eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
noqueue master br0 state UP
    link/ether 00:1e:0b:76:83:d6 brd ff:ff:ff:ff:ff:ff
    inet6 fe80::21e:bff:fe76:83d6/64 scope link
       valid_lft forever preferred_lft forever
11: eth1.184@eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
noqueue state UP
    link/ether 00:1e:0b:76:83:d6 brd ff:ff:ff:ff:ff:ff
    inet 130.89.96.8/21 brd 130.89.103.255 scope global eth1.184
    inet6 fe80::21e:bff:fe76:83d6/64 scope link
       valid_lft forever preferred_lft forever
14: ppp0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1400 qdisc
pfifo_fast state UNKNOWN qlen 3
    link/ppp
    inet 130.89.254.233 peer 130.89.100.119/32 scope global ppp0


> ip route list table local

broadcast 127.0.0.0 dev lo  proto kernel  scope link  src 127.0.0.1
local 127.0.0.0/8 dev lo  proto kernel  scope host  src 127.0.0.1
local 127.0.0.1 dev lo  proto kernel  scope host  src 127.0.0.1
broadcast 127.255.255.255 dev lo  proto kernel  scope link  src
127.0.0.1
broadcast 130.89.96.0 dev eth1.184  proto kernel  scope link  src
130.89.96.8
local 130.89.96.8 dev eth1.184  proto kernel  scope host  src
130.89.96.8
broadcast 130.89.103.255 dev eth1.184  proto kernel  scope link  src
130.89.96.8
broadcast 130.89.254.224 dev eth0  proto kernel  scope link  src
130.89.254.233
local 130.89.254.233 dev eth0  proto kernel  scope host  src
130.89.254.233
local 130.89.254.233 dev ppp0  proto kernel  scope host  src
130.89.254.233
local 130.89.254.234 dev eth0  proto kernel  scope host  src
130.89.254.233
local 130.89.254.235 dev eth0  proto kernel  scope host  src
130.89.254.233
local 130.89.254.236 dev eth0  proto kernel  scope host  src
130.89.254.233
broadcast 130.89.254.255 dev eth0  proto kernel  scope link  src
130.89.254.233


> 	If the above is true may be we have to find a
> way to return the first device where the IP is added.
> May be this is the main rule that is used when one adds
> same IP on many interfaces.
> 
> 	May be the solution is to convert inet_addr_lst
> from hlist to normal list, so that we can append new
> addresses at tail and __ip_dev_find to find the first
> device where IP was added.

Perhaps the solution in our case is indeed to have all PPTP clients
connect to a secondary IP on the server; then all PPP interfaces still
share the same address, but it probably won't be the address that our
server selects as source for broadcast traffic (DHCP or otherwise). We
already have a couple of secondaries on the interface in question (as
you can see in "ip addr show"). We'll try this tomorrow, it's also in
line with your later suggestion in reply to David.

Another workaround for us might be to tell Radius to use a different
source IP for the DHCP address allocation when a client connects. Just
tested with a second client, this seems to work.

I'll let you know the results tomorrow.


Regards,

Jeroen van Ingen
ICT Service Centre
University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands