Pieter de Boer wrote:
Is there any advantage/disadvantage in ARP implementation on FreeBSD
over that of Linux? Thanks.
I was unhappily surprised by this 'feature'. I find it pretty
counter-intuitive. I expect two interfaces to be seperated inside a
kernel, but Linux more or less binds them together. Incoming traffic
on the 'wrong' interface will gladly be accepted, too. This broke
things for me, because I didn't want to have that certain IP-address
accessible.
That said, this happens only when you have two interfaces connected to
the same subnet, which is a bit evil anyhow. It may be beneficial for
Linux to do things this way, perhaps for redundancy-purposes (two
interfaces, one IP-address, IP reachable over both interfaces, when
one fails, the other takes over.. no idea if that works out-of-the-box).
There is another side effect, which comes into view with certain
configurations behind load balancers. Foundry has an option (I believe
called "DSR" for Direct Server Return) which just fiddles with the MAC
address of the destination. Other companies load balancers will
probably have the same option, but I've no idea what they'll call it.
For the connection to be accepted, all servers which are expected to
answer for a particular load balanced IP address have to have that IP
configured on one of their interfaces, typically loopback. The host
sees that the connection is for one of its interfaces, accepts the
connection and life is happy. The return path from the host to the
originator bypasses the load balancer, and effectively halves the
traffic that the LB is having to process and do table lookups on, etc.
This obviously greatly increases the available capacity of the LB.
With a Linux box answering ARP as described above, it is possible that
the upstream router (or routers) COULD learn that the load balanced IP
actually belongs on one of the servers rather than the load balancer.
If that happens, your load balanced farm will quickly degrade and you'll
be scratching your head for hours to try and figure out whats going on.
Or the LB and the Linux box will get into an ARP war and random TCP
connections will get RSTs from the Linux box.
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