sysctl arp_{ignore,announce} and /etc/network/interfaces

[andrew bezella]

hello -

we are (re-)implementing a linux virtual server load balancer and have a
gutsy node in place as one of the real servers.  as part of this, there
are some sysctl settings to be tweaked, and i am seeing an unexpected
(to me, at least) interaction with /etc/network/interfaces.

as a little more background, we are running with the lvs/dr
configuration(1).  to address the arp concerns(2) we are using the
arp_announce/arp_ignore sysctl fix(3) with the following entries
in /etc/sysctl.conf:
net.ipv4.conf.default.arp_ignore     = 1
net.ipv4.conf.default.arp_announce   = 2

the weirdness is that when an interface is set to come up automatically
("auto eth1" in /etc/network/interfaces) the default settings are not
applied to it.  monitoring the boot process, i can see that immediately
after the kernel variables are set from sysctl.conf
in /etc/rcS.d/S17procps.sh the values are:
/proc/sys/net/ipv4/conf/default/arp_announce = 2
/proc/sys/net/ipv4/conf/eth0/arp_announce = 2
/proc/sys/net/ipv4/conf/eth1/arp_announce = 0
/proc/sys/net/ipv4/conf/default/arp_ignore = 1
/proc/sys/net/ipv4/conf/eth0/arp_ignore = 1
/proc/sys/net/ipv4/conf/eth1/arp_ignore = 0

however, when the auto line is removed, the values are all set as
expected (i.e., default value applied to all interfaces).  i can then
manually `ifup` the interface and retain the settings.

i didn't have much success tracking down the "why" of this.  there are
easy alternative solutions (e.g., either listing the interfaces
explicitly in sysctl.conf or using "all" instead of "default) but i'm
curious whether this is a bug or if i'm just missing something.  any
thoughts?

tia!

	andy


(1) http://kb.linuxvirtualserver.org/wiki/LVS/DR
(2)
http://kb.linuxvirtualserver.org/wiki/ARP_Issues_in_LVS/DR_and_LVS/TUN_Clusters
(3)
http://kb.linuxvirtualserver.org/wiki/Using_arp_announce/arp_ignore_to_disable_ARP

-- 
andrew bezella <abezella at archive.org>
Internet Archive