Greetings Leroy,
I have slightly rearranged your questions.
Can someone elaborate on the use of src? The statements I have
seen ("the source address to prefer when sending to the
destinations covered by the route prefix.") seem vague. When
should 'src' be used and why?
The 'src' parameter offers a hint. Frankly, this hint is probably
not used very often, but here are the circumstances under which it
may be useful. You may have a machine with many IPs, connected to
many local networks. Let's suppose that you want all connections
to destinations in network A (192.0.2.0/24) to use the local address
192.0.2.17. If so, then you could:
ip route add 192.0.2.0/24 dev eth0 scope link src 192.0.2.17
If an application just asks the kernel for a socket (and does not
explicitly specify the source address), then the 'src' parameter is
the hint to suggest that the originating IP for the connection will
be 192.0.2.17. That's all.
Here are some descriptions of how source address selection works
under Linux. (Yes, this is a bit old, but, as far as I know, it
remains correct.)
http://linux-ip.net/gl/ip-cref/node155.html
http://linux-ip.net/html/linux-ip.html#routing-saddr-selection
I'm getting the impression that 'via' is only used when the IP
address is not local to the device, is this correct or is there
more to it?
The 'via' parameter is always 'via ADDRESS'. The address should be
directly reachable on some layer 2 by the system. So, in the case
of the above example, let's assume that your routing table looks
like this:
default via 192.0.2.1 dev eth0 proto static
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.17
The second route tells the Linux system that it is directly
connected to 192.0.2.0/24 with the layer 2 device/interface known as
'eth0'. The default route says to send any other packets through
192.0.2.1, which (of course) must itself be reachable--it is because
of the direct connection to 192.0.2.0/24 on device/interface eth0.
It seems that 'nexthop' is used for load balancing and possibly
redundancy, are there any other situations or reasons?
As you surmised, 'nexthop' is used when you have multiple routers
offering different routing paths out of a network. You can create a
route with only a single 'nexthop' entry, but this will devolve to
the simplest case. Here's an example of what I mean:
# ip route add 198.51.100.0/24 nexthop via 192.0.2.1 weight 1
# ip route show
default via 192.0.2.1 dev eth0 proto static
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.17
198.51.100.0/24 via 192.0.2.1 dev eth0
OK, so let's remove that route with only one 'nexthop' phrase and
add a route with two 'nexthop' phrases:
# ip route del 198.51.100.0/24 nexthop via 192.0.2.1 weight 1
# ip route add 198.51.100.0/24 nexthop via 192.0.2.1 weight 1 nexthop via 192.0.2.2 weight 1
# ip route show
default via 192.0.2.1 dev eth0 proto static
192.0.2.0/24 dev eth0 proto kernel scope link src 192.0.2.17
198.51.100.0/24
nexthop via 192.0.2.1 dev eth0 weight 1
nexthop via 192.0.2.2 dev eth0 weight 1
I am, personally, unfamiliar with the kernel's decisions on
balancing flows using multipath routes. I have seen admonitions to
be wary of the possible packet reordering (which may or may not be a
big deal to any individual application).
Good luck!
-Martin
--
Martin A. Brown
http://linux-ip.net/
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