On 06/08/2011 10:47 PM, NeilBrown wrote:
On Wed, 8 Jun 2011 10:39:08 -0700 greearb@xxxxxxxxxxxxxxx wrote:
From: Ben Greear<greearb@xxxxxxxxxxxxxxx>
This lets one specify the source IP address for
sockets, allowing users to leverage routing rules
on multi-homed systems.
I gotta say I think this is rather horrible.....
And you haven't even seen the kernel side yet ;)
As I understand it, the problem is bindresvport.
It binds to a port number before making a connection, so the local address
that is bound to is the 'default' rather than the best one to reach the given
target. And in some network configs this can be bad, because e.g. the target
may not be able to reply to that 'default' address.
So you want to be able to specify the local endpoint fully when you bind, so
you require/allow the user to specify the local endpoint.
Wouldn't it be soooo much nicer if the tools could just figure out the
'correct' local endpoint and just use that? Obviously "yes" but maybe that
isn't straight forward. Have you looked into that at all?
Not nicer in all cases.
Sometimes a user knows best, and may want to use non-obvious
routing setups, such as rules that send pkts to different gateways
if they are from different source addresses.
Worst case (which may be so incredibly bad it isn't worth considering) is
that we could extract the routing table from the kernel and "figure it out".
There are potentially multiple routing tables.
But I suspect there is an easier way... What if you create a UDP socket,
'connect' to some arbitrary port on the target machine, and then use
getsockname to get the local endpoint address of that socket. That wouldn't
generate any network traffic, but should give you the preferred local
endpoint for talking to that peer??
For the in-kernel code I wouldn't accept a trick like that, but there is
presumably some way to find the preferred local endpoint for some address
more directly ... certainly worth asking on net-dev if we cannot figure one
out.
So I'm thinking: yes, there is a real need, but I think there must be a
better solution.
What think you?
I think I really do want to specify the local IP address. There
are lots of other tools that allow this (ping, ssh, cifs, etc),
and there is a good reason for allowing it, primarily when used
in conjunction with routing rules that are based on source
IP.
Thanks,
Ben
--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc http://www.candelatech.com
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