Re: Question about 'via', 'src' and 'nexthop'

Linux Advanced Routing and Traffic Control

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 




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/
--
To unsubscribe from this list: send the line "unsubscribe lartc" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html




[Index of Archives]     [LARTC Home Page]     [Netfilter]     [Netfilter Development]     [Network Development]     [Bugtraq]     [GCC Help]     [Yosemite News]     [Linux Kernel]     [Fedora Users]
  Powered by Linux