Re: Failover route

[Grant Taylor <gtaylor@xxxxxxxxxxxxxxxxxx>]

On 3/16/19 10:22 AM, Erik Auerswald wrote:
> Hi,

Hi,

> As a crude method I have used a cron job to ping the gateways and add 
> or remove (default) routes based on ping results in the past.  You could 
> use iproute2's 'ip link' to check interface status instead (or first).

Is the following true:

 · The NIC link is staying up.
 · The IP is remaining assigned to the interface.
 · You are conditionally adding / changing / removing the default 
gateway that is the far side of the link.
 · You have some sort of ECMP and / or PBR in place to work with 
multiple default gateways.

I found issues with relying on link status in the following scenario:

+-------+     +------------+     /   +-----+   === Ethernet
| Linux +=====+ SDSL Modem +----/----+ ISP |   --- SDSL
+-------+     +------------+   /     +-----+    /  Break

Linux will think that the link is up, because it is up to the SDSL 
modem.  Yet there is no end-to-end connection between Linux and the ISP 
because there is a break in the SDSL connection to between the SDSL 
modem and the ISP.

I have fought this very scenario.  The same could be true if you use an 
Ethernet switch as a media converter going between copper and fiber.

The problem is that Linux sees the link as always being up, despite 
there being a break further down the line.

> You may be able to subscribe to Netlink messages to get informed about 
> interface state changes. At least for a prototype it might suffice to 
> use iproute2's 'ip monitor' to receive Netlink messages, but I have not 
> yet tried this.

I like the idea and the ingenuity.  Unfortunately it suffers from the 
underlying problem above.

> Routing daemons are supposed to do that, so using one with static default 
> routes might give you the intended results.

I don't know that I've heard that routing daemons are /supposed/ /to/ 
rely on local link state.  I think that many do for various reasons. 
But I've never heard of any such mandate before.

To me, monitoring for loss of a link is mostly an optimization for the 
routing daemon to know that a link is unusable in short order and can 
remove the associated routes.  I'm fairly certain that the Linux kernel 
will do the same when link goes down, and remove routes that were using 
that link.

Sure, there are some link-state protocols that may want to know this 
information as part of the protocol's operation.  As such the routing 
daemon that implements said protocol would want to know too.



--
Grant. . . .
unix || die

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