On 5/6/21 8:46 AM, Belanger, Martin wrote:

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On 5/6/21 8:05 AM, Hannes Reinecke wrote:

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On 5/5/21 4:31 PM, Belanger, Martin wrote:

[ .. ]

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1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state

UNKNOWN

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group default qlen 1000
      link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
      inet 100.0.0.100/24 scope global lo
         valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc

fq_codel

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state UP group default qlen 1000
      link/ether 08:00:27:21:65:ec brd ff:ff:ff:ff:ff:ff
      inet 100.0.0.100/24 scope global enp0s3
         valid_lft forever preferred_lft forever
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc

fq_codel

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state UP group default qlen 1000
      link/ether 08:00:27:4f:95:5c brd ff:ff:ff:ff:ff:ff
      inet 100.0.0.100/24 scope global enp0s8
         valid_lft forever preferred_lft forever

The above is a VM that I configured with the same IP address
(100.0.0.100) on all interfaces. Doing a reverse lookup to identify
the unique interface associated with 100.0.0.100 would simply not
work here. And this is why the option host_iface is required. I
understand that the above config does not represent a standard host
system, but I'm using this to prove a point: "we can never know how a
user will configure their system and the above configuration is
perfectly fine by Linux".

... and messing up any switch MAC address caching when doing so. I
guess the network admin will come down hard on you if you try that on
a production system.
And I sincerely question whether this is a valid use-case; I'm already
getting grief from our network admins if I dare to put two network
interfaces from the same machine in the same network.

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The current TCP implementation for host_traddr uses
bind()-before-connect(). This is a common construct to set the source
IP address on the socket before connecting. This has no effect on how
Linux will select the interface for the connection. That's because
Linux uses the Weak End System model as described in RFC1122 [2].
Setting the source address on a connection is a common requirement
that linux-nvme needs to support. In fact, specifying the Source IP
address is a mandatory FedGov requirement (e.g. connection to a
RADIUS/TACACS+ server). Consider the following configuration.

$ ip addr list dev enp0s8
3: enp0s8: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc

fq_codel

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state UP group default qlen 1000
      link/ether 08:00:27:4f:95:5c brd ff:ff:ff:ff:ff:ff
      inet 192.168.56.101/24 brd 192.168.56.255 scope global dynamic
noprefixroute enp0s8
         valid_lft 426sec preferred_lft 426sec
      inet 192.168.56.102/24 scope global secondary enp0s8
         valid_lft forever preferred_lft forever
      inet 192.168.56.103/24 scope global secondary enp0s8
         valid_lft forever preferred_lft forever
      inet 192.168.56.104/24 scope global secondary enp0s8
         valid_lft forever preferred_lft forever

Here we can see that several addresses are associated with interface
enp0s8. By default, Linux will select the default IP address,
192.168.56.101, as the source address when connecting over interface
enp0s8. Some users, however, want the ability to specify a different
address (e.g.,
192.168.56.103) to be used as the source address.
The option host_traddr can be used as-is to perform this function (I
tested it).

No disagreement here.

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In conclusion, I believe that for TCP we need 2 options. One that can
be used to specify an interface. And one that can be used to set the
source address. And users should be allowed to use one or the other,
or both, or none.
Of course, the documentation for host_traddr will need some
clarification. It should state that when used for TCP connection,
this option only sets the source address. And the documentation for
host_iface should say that this option only applies to TCP
connections.

I'm with James Smart here. I do fail to see the need for 'host_iface'
_without_ 'host_traddr'; especially for IPv6 where several addresses
are standard just specifying 'host_iface' simply is not enough, and
one has to specify 'host_traddr' additionally.

So 'host_iface' should be contingent on 'host_traddr', meaning we can
just expand the syntax of 'host_traddr'.
One easy possibility would be to add ',nobind' to the host_traddr
syntax which would indicate that we should _not_ bind to the
underlying interface; I do think that binding to the respective
interface should be the default.

A-ha. Just spoke to our network folks, and they clarified the usage of binding
to an IP address vs binding to a network interface.
Apparently, binding to a source IP address does just that, setting the source
IP address of the outgoing packet. That packet will _still_ be subjected to the
normal routing table, as the routing table is just influenced by the
_destination_ IP address.
So if we want to have it routed via a specific interface (and thereby
influencing the routing table) we need to bind it to that interface.

The only valid scenario our network folks could come up with where we do
_not_ want to bind to an interface is for asymmetric flows, ie in cases where
the outgoing flow is routed to one interface and the incoming flow is arriving
on another interface. But even they admitted that it's not a common
scenario, and probably will be killed by anti-spoofing software running on
the core switches ...

But if we want to support _that_ then clearly binding to a specific interface
doesn't work.

So I would vote for making binding to the network interface holding the IP
address the default, and add an option ',nobind' to host_traddr to skip it.

Cheers,

Hannes
--
Dr. Hannes Reinecke		        Kernel Storage Architect
hare@suse.de			               +49 911 74053 688
SUSE Software Solutions Germany GmbH, 90409 Nürnberg
GF: F. Imendörffer, HRB 36809 (AG Nürnberg)

Hi Hannes,

If the only concern here is the addition of yet another option (--host-iface), then may I suggest a simpler approach. What I'm proposing adheres to RFC4007 [1], which defines a way to specify an interface by using the '%' delimiter between the Destination IP address and the Interface. In fact, "ping" uses this approach [2]. With ping, one can force the connection to go a specific interface like this:

ping <dest-ip-addr>%<interface>

Ping only supports this syntax for IPv6 no?

Extending this approach to nvme-cli we arrive to something like this:

nvme discover --traddr 100.64.29.2%enp0s8 --host-traddr 192.168.56.102 ....

We already support this for IPv6, we can do that also for IPv4, but this
syntax may not be trivially expected for ipv4?

This tells nvme to connect to 100.64.29.2 on interface enp0s8. We make no change to the --host-traddr option. It continues to be used to specify the Source IP address only (for the rare cases where users want to specify a Source Address other than the default). With this, the interface is specified by name and not by its associated address. This is not only more intuitive, but, as I stated before, eliminates the problem caused by mapping the same IP address to multiple interfaces (not to mention that doing a reverse lookup on an IP address to find the interface is extra work that we don’t need to do in kernel space).

Maybe we do something like ping -I for host_traddr, from ping man pages:

-I interface
            interface is either an address, an interface name or a VRF 
name. If interface is an address, it sets source address to specified 
interface address. If interface is an
            interface name, it sets source interface to specified 
interface. If interface is a VRF name, each packet is routed using the 
corresponding routing table; in this case, the -I
            option can be repeated to specify a source address. NOTE: 
For IPv6, when doing ping to a link-local scope address, link 
specification (by the '%'-notation in destination, or
            by this option) can be used but it is no longer required.


Without the repetition though, unless we need to support two interfaces
that share the same multiple addresses in the same subnet, which sounds
completely crazy to me...

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