On Tuesday, 02/09/2016 at 04:16 GMT, Robert J Brenneman
<bren...@gmail.com> wrote:
> Multi homed linux seems to consider any MAC address as good as any other
> when responding to an ARP. The default seems to assume you have every
> ethernet port plugged to the same logical network. If you run tcpdump on
> all interfaces for both machines, I bet you see the target of the telnet
> request sometimes responding to the ARP out the wrong interface with the
> wrong MAC address.
>
> Try this - on both your Linux x86 and Linux s390x systems, set this
> variable in /etc/sysctl.conf
> net.ipv4.conf.all.arp_announce = 1
>
> and make it effective with 'sysctl -p /etc/sysctl.conf'
It's called "ARP Flux" and it's a nasty business. IMO, all distros should
ship with
net.ipv4.conf.all.arp_announce = 1
net.ipv4.conf.all.arp_ignore = 1
net.ipv4.conf.all.arp_notify = 1
net.ipv4.conf.all.arp_filter = 1
Details below. I can't imagine how the kernel maintainers allowed such
changes to ever be made in the first place, and then why the distros
didn't override it back into sanity.
Alan Altmark
Senior Managing z/VM and Linux Consultant
Lab Services System z Delivery Practice
IBM Systems & Technology Group
ibm.com/systems/services/labservices
office: 607.429.3323
mobile; 607.321.7556
alan_altm...@us.ibm.com
IBM Endicott
arp_filter - BOOLEAN
0 - (default) The kernel can respond to arp requests with
addresses
from other interfaces. This may seem wrong but it usually
makes
sense, because it increases the chance of successful
communication.
IP addresses are owned by the complete host on Linux, not
by
particular interfaces [say, what?]. Only for more complex
setups like load-
balancing, does this behaviour cause problems. [I am SO
laughing right now.]
1 - Allows you to have multiple network interfaces on the
same
subnet, and have the ARPs for each interface be answered
based on whether or not the kernel would route a packet
from
the ARP'd IP out that interface (therefore you must use
source
based routing for this to work). In other words it allows
control
of which cards (usually 1) will respond to an arp
request.
arp_filter for the interface will be enabled if at least
one of
conf/{all,interface}/arp_filter is set to TRUE,
it will be disabled otherwise
arp_announce - INTEGER
Define different restriction levels for announcing the
local
source IP address from IP packets in ARP requests sent on
interface:
0 - (default) Use any local address, configured on any
interface
1 - Try to avoid local addresses that are not in the
target's
subnet for this interface. This mode is useful when
target
hosts reachable via this interface require the source IP
address in ARP requests to be part of their logical
network
configured on the receiving interface. When we generate
the
request we will check all our subnets that include the
target IP and will preserve the source address if it is
from
such subnet. If there is no such subnet we select source
address according to the rules for level 2.
2 - Always use the best local address for this target.
In this mode we ignore the source address in the IP
packet
and try to select local address that we prefer for talks
with
the target host. Such local address is selected by
looking
for primary IP addresses on all our subnets on the
outgoing
interface that include the target IP address. If no
suitable
local address is found we select the first local address
we have on the outgoing interface or on all other
interfaces,
with the hope we will receive reply for our request and
even sometimes no matter the source IP address we
announce.
The max value from conf/{all,interface}/arp_announce is
used.
Increasing the restriction level gives more chance for
receiving answer from the resolved target while
decreasing
the level announces more valid sender's information.
arp_ignore - INTEGER
Define different modes for sending replies in response to
received ARP requests that resolve local target IP
addresses:
0 - (default): reply for any local target IP address,
configured
on any interface
1 - reply only if the target IP address is local address
configured on the incoming interface
2 - reply only if the target IP address is local address
configured on the incoming interface and both with the
sender's IP address are part from same subnet on this
interface
3 - do not reply for local addresses configured with
scope host,
only resolutions for global and link addresses are
replied
4-7 - reserved
8 - do not reply for all local addresses
The max value from conf/{all,interface}/arp_ignore is
used
when ARP request is received on the {interface}
arp_notify - BOOLEAN
Define mode for notification of address and device
changes.
0 - (default): do nothing
1 - Generate gratuitous arp requests when device is
brought up
or hardware address changes.
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