nvme-tcp: allow selecting the network interface for connections
In our application, we need a way to force TCP connections to go out a specific IP interface instead of letting Linux select the interface based on the routing tables. Add the 'host-iface' option to allow specifying the interface to use. When the option host-iface is specified, the driver uses the specified interface to set the option SO_BINDTODEVICE on the TCP socket before connecting. This new option is needed in addtion to the existing host-traddr for the following reasons: Specifying an IP interface by its associated IP address is less intuitive than specifying the actual interface name and, in some cases, simply doesn't work. That's because the association between interfaces and IP addresses is not predictable. IP addresses can be changed or can change by themselves over time (e.g. DHCP). Interface names are predictable [1] and will persist over time. Consider the following configuration. 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state ... 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 ... 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 ... 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 does 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 users will configure their systems". By te way, The above configuration is perfectly fine by Linux. The current TCP implementation for host_traddr performs a bind()-before-connect(). This is a common construct to set the source IP address on a TCP socket before connecting. This has no effect on how Linux selects the interface for the connection. That's because Linux uses the Weak End System model as described in RFC1122 [2]. On the other hand, setting the Source IP Address has benefits and should be supported by linux-nvme. In fact, setting 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 ... 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 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 always selects 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 source address (e.g., 192.168.56.102, 192.168.56.103, ...). The option host_traddr can be used as-is to perform this function. In conclusion, I believe that we need 2 options for TCP connections. One that can be used to specify an interface (host-iface). And one that can be used to set the source address (host-traddr). 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 is only available for TCP connections. References: [1] https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/ [2] https://tools.ietf.org/html/rfc1122 Tested both IPv4 and IPv6 connections. Signed-off-by:Martin Belanger <martin.belanger@dell.com> Reviewed-by:
Sagi Grimberg <sagi@grimberg.me> Reviewed-by:
Hannes Reinecke <hare@suse.de> Signed-off-by:
Christoph Hellwig <hch@lst.de>
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e21e0243
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