[RndTbl] Enabling Linux failover routing

[Sean A. Walberg]

On Wed, 11 May 2005, John Lange wrote:

> What do you base this on? Google turns up plenty of references to the
> gc_timeout method of failover but none are specific on what they mean by
> "when a route dies".

Poke around net/ipv4/route.c.  The gc_timeout parameter is a garbage 
collection timer for clearing out the routing table.  The only place the 
routing table timers seem to be played with are in that file... Every so 
often the value is decremented, and if it goes down to zero it's wiped 
out.

http://mailman.ds9a.nl/pipermail/lartc/2002q4/005296.html

seems to be a good explanation of the various garbage collection 
parameters. 


> I can't think of any reason why the kernel would care if the route timed
> out because the link was down or because the route was down for some
> other reason.

It's not if the kernel cares, it's how the kernel knows.  The only thing 
specified in a route is the prefix and the next hop.

route add default gw 1.1.1.1
route add default gw 2.2.2.2 metric 10

Given no exchange of routing information between the peers, the only thing 
the kernel has to go in is the link status of the next hop interfaces, 
which is also in the routing table.

For instance, unplug your cable modem from the coax side.  Check your 
routing table, the 0.0.0.0 route should still have the "U" flag, meaning 
the route is active.

A more concrete example (edited slightly for brevity)

Here look at 172.16.20/24, out interface vmnet8.

[root at bob root]# netstat -rn
Kernel IP routing table
Destination     Gateway     Genmask         Flags   MSS Window  irtt Iface
172.16.20.0     0.0.0.0     255.255.255.0   U         0 0          0 vmnet8
192.168.1.0     0.0.0.0     255.255.255.0   U         0 0          0 eth0
0.0.0.0         192.168.1.1 0.0.0.0         UG        0 0          0 eth0

The U flag means the route is up and viable.  I'll shut down int vmnet8, 
and watch the route go away.

[root at bob root]# ifconfig vmnet8 down; route
Kernel IP routing table
Destination     Gateway     Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *           255.255.255.0   U     0      0        0 eth0
default         poochie.ertw0.0.0.0         UG    0      0        0 eth0

Route's gone, because the next hop is gone.  Bring back up the interface.

[root at bob root]# ifconfig vmnet8 up; route
Kernel IP routing table
Destination     Gateway     Genmask         Flags Metric Ref    Use Iface
172.16.20.0     *           255.255.255.0   U     0      0        0 vmnet8
192.168.1.0     *           255.255.255.0   U     0      0        0 eth0
default         poochie.ertw0.0.0.0         UG    0      0        0 eth0

Route's back.

It all comes down to "How CAN the kernel know a route is down".  The only 
information it has is the outgoing interface status.

That said, Bill brought up the idea of ARP tables.  This has two problems:

1 - If the link is idle for a while, the arp cache will clear, and be seen 
as a failure
2 - There are lots of layer 2 networks that don't use ARP.  The ethernet 
code is separate from the ipv4 code.

In addition, this is a common problems on hardware routers.  Even a box 
that's specifically designed to be a router can't do it without resorting 
to ICMP tests and policy routing. 

Sean


-- 
Sean A. Walberg <sean at ertw.com>                    http://www.ertw.com