From 67c8e0eff7a166944254e66371e3485a682c302b Mon Sep 17 00:00:00 2001 From: Ben Nemec Date: Fri, 8 Jul 2022 16:59:27 -0500 Subject: [PATCH] Host IP and Interface Selection As OpenShift is deployed in increasingly complex networking environments, we have gotten many requests for more control over which interface is used for the primary node IP. We provided a basic mechanism for this with [KUBELET_NODEIP_HINT](https://github.com/openshift/machine-config-operator/pull/2888) but as users have started to exercise that some significant limitations have come to light. --- .../network/ip-interface-selection.md | 300 ++++++++++++++++++ 1 file changed, 300 insertions(+) create mode 100644 enhancements/network/ip-interface-selection.md diff --git a/enhancements/network/ip-interface-selection.md b/enhancements/network/ip-interface-selection.md new file mode 100644 index 0000000000..8f3466a9db --- /dev/null +++ b/enhancements/network/ip-interface-selection.md @@ -0,0 +1,300 @@ +--- +title: ip-interface-selection +authors: + - "@cybertron" +reviewers: + - "@jcaamano" + - "@tsorya" + - "@flaper87" +approvers: + - "@danwinship" + - "@trozet" +api-approvers: + - "None" +creation-date: 2022-07-07 +last-updated: 2022-09-06 +tracking-link: + - https://issues.redhat.com/browse/OPNET-134 +see-also: + - https://github.com/openshift/baremetal-runtimecfg/issues/119 +replaces: +superseded-by: +--- + +# Host IP and Interface Selection + +## Summary + +As OpenShift is deployed in increasingly complex networking environments, we +have gotten many requests for more control over which interface is used for +the primary node IP. We provided a basic mechanism for this with +[KUBELET_NODEIP_HINT](https://github.com/openshift/machine-config-operator/pull/2888) +but as users have started to exercise that some significant limitations have +come to light. + +## Motivation + +Some users want to have a great deal of control over how their network traffic +is routed. Because we use the default route for interface and IP selection, +in some cases they are not able to route traffic the way they want. + +### User Stories + +As a deployer, I want my cluster traffic to stay on an isolated network with +no external gateway. External traffic will travel on a different interface +that is managed by a service like MetalLB. + +### Goals + +Ensure that all host networked services on a node have consistent interface +and IP selection. + +### Non-Goals + +Support for platforms that do not use the nodeip-configuration service today. + +Complete support for multiple nics, with full control over what traffic +gets routed where. However, this work should be able to serve as the basis +for a broader multi-nic feature so we should avoid designs that would limit +future work in this area. + +## Proposal + +The following is a possibly incomplete list of places that we do IP/interface +selection. Ideally these should all use a single mechanism to do that so they +are all consistent with each other. + +- Node IP (Kubelet and CRIO) +- configure-ovs +- resolv-prepender +- Keepalived +- Etcd + +In some cases (resolv-prepender) it may not matter if the IP selected is +consistent with the other cases, but Node IP, configure-ovs, and Keepalived all +need to match because their functionality depends on it. I'm less familiar with +the requirements for Etcd, but it seems likely that should match as well. In +general, it seems best if all IP selection logic comes from one place, whether +it's strictly required or not. + +### Workflow Description + +At deployment time the cluster administrator will include a manifest that sets +KUBELET_NODEIP_HINT appropriately. The nodeip-configuration service (which +will now be set as a dependency for all other services that need IP/interface +selection) will use that value to determine the desired IP and interface for +all services on the node. It will write the results of this selection to a +well-known location which the other services will consume. This way, we don't +need to duplicate the selection logic to multiple places. It will happen once +and be reused as necessary. + +#### Example + +- resolv-prepender has to run before any other node ip selection can take + place. Without resolv.conf populated the nodeip-configuration service cannot + pull the runtimecfg image. + - Note: This is not relevant for UPI deployments. +- nodeip-configuration runs and selects one or more IPs. It writes them to + the Kubelet and CRIO configuration files (this is the existing behavior). +- nodeip-configuration also writes the following files (new behavior): + - /run/nodeip-configuration/primary-ip + - /run/nodeip-configuration/ipv4 + - /run/nodeip-configuration/ipv6 +- When configure-ovs runs, it looks for the primary-ip file and bridges the + interface associated with that IP. It may also look at the ipv4 and ipv6 + addresses to determine which IP versions should be present on the bridge. + If the nodeip-configuration files are not found, the logic will remain as + it is today. +- When keepalived runs it will read the IP from the primary-ip file and use + the associated interface for VRRP traffic. + - Note: When [dual stack VIP support](https://github.com/openshift/enhancements/blob/master/enhancements/network/on-prem-dual-stack-vips.md) + is implemented it will need to look at the appropriate ipvX file for each + VIP. Until then, it should always use the primary IP. + +#### Variation [optional] + +Moved discussion of KUBELET_NODEIP_HINT to open questions. + +### API Extensions + +NA + +### Implementation Details/Notes/Constraints [optional] + +Currently configure-ovs runs before nodeip-configuration. In this design we +would need to reverse that. There are currently no dependencies between the +two services that would prevent such a change. + +As noted above, we want to make sure we don't do anything that would further +complicate the implementation of a higher level multi-nic feature. The current +design should not be a problem for that. For example, if at some point we add +a feature allowing deployers to specify that they want cluster traffic on +eth0, external traffic on eth1, and storage traffic on eth2, that feature +would simply need to appropriately populate the KUBELET_NODEIP_HINT file that +would be directly created by the deployer in the current design. By providing +a common interface to configure host-networked services, this should actually +simplify any such future enhancements. + +Initially, nodeip-configuration as also going to write an interface file so +services would not have to figure out the correct interface to use for a given +IP. However, this is problematic for use cases like OVNKubernetes because on +initial boot the IP will be on the interface itself, while on subsequent boots +the address will be on br-ex. Because the IP may move after +nodeip-configuration runs, we don't want to persist the interface. + +### Risks and Mitigations + +- There is some risk to changing the order of critical system services like + nodeip-configuration and configure-ovs. This will not affect deployments + that do not use OVNKubernetes as their CNI, but since we intend that to be + the default going forward it is a significant concern. + + We intend to mitigate this risk by first merging the order change without + any additional changes included in this design. This way, if any races + between services are found once the change is being tested more broadly + it will be easy to revert. + + We will also test the ordering change as much as possible before merging + it, but it's unlikely we can exercise it to the same degree that running + across hundreds of CI jobs per day will. + +- Currently all host services are expected to listen on the same IP and + interface. If at some point in the future we need host services listening + on multiple different interfaces, this may not work. However, because we + are centralizing all IP selection logic in nodeip-configuration, it should + be possible to extend that to handle multiple interfaces if necessary. + +### Drawbacks + +This design only considers host networking, and it's likely that in the future +we will want a broader feature that provides an interface to configure pod +traffic routing as well. However, if/when such a feature is implemented it +should be able to use the same configuration interface for host services +that deployers would use directly after this is implemented. + +Additionally, there are already ways to [implement traffic steering for pod +networking.](https://youtu.be/EpbUWwjadYM) We may at some point want to +integrate them more closely, but host networking is currently a much bigger +pain point and worth addressing on its own. + +## Design Details + +### Open Questions [optional] + +- Currently this only applies to UPI clusters deployed using platform None. + Do we need something similar for IPI? + - Answer: Yes. In IPI deployments the VIP is always used for node ip + selection. In an environment where the VIP subnet is isolated this could + result in the node IP and the interface selected by configure-ovs not + matching. + +- In UPI deployments it is also possible to set the Node IP by manually writing + configuration files for Kubelet and CRIO. Trying to look for all possible + ways a user may have configured those services seems complex and error-prone. + Can we just require them to use this mechanism if they want custom IP + selection? + - Answer: We probably don't need an answer before implementing this. The + changes described here shouldn't affect any existing deployments, it will + only allow some deployments that are currently having issues the the ip + selection behavior to be successful. It's probably reasonable to require + anyone who is struggling with this to use our interface though. + +- The interface file written by nodeip-configuration will no longer be valid + once configure-ovs moves the IP to br-ex. Should we re-run + nodeip-configuration at that point to update it, should be leave the + interface file alone, or should we just not write the interface at all and + use the IP address(es) exclusively for interface selection? + - Answer: Just don't persist the interface name. As long the IP used to get + the interface is consistent between services we should get the behavior + we need. Also, this option is more flexible for future features, like the + ability to run dual stack clusters with ipv4 and ipv6 on different + interfaces. + +- We may want to rename KUBELET_NODEIP_HINT to reflect the fact that it will + now affect more than just Kubelet. Is just NODEIP_HINT acceptable? How do we + handle backward compatibility with the KUBELET version of the name to avoid + breaking existing users of that functionality? I'm not sure how much logic + we can inject in the systemd service. + - Answer: NODEIP_HINT should be fine. Even the existing KUBELET_NODEIP_HINT + is taken from a [file named nodeip-configuration](https://github.com/openshift/machine-config-operator/blob/3cf95882fa8e967d9dca1495a5dcde659d75ba46/templates/common/_base/units/nodeip-configuration.service.yaml#L39) + We can maintain compatibility using bash variable magic, e.g. + `${NODEIP_HINT:-$KUBELET_NODEIP_HINT}`. + +### Test Plan + +In general this will be covered by existing e2e tests. However, we should +expand coverage of the nodeip-configuration and configure-ovs components as +they have proven to have a lot of edge cases. + +At this point I'm not sure exactly what a more-targeted set of tests for +host networking would look like though. We likely cannot stand up the large +variety of networking environments needed to run e2e tests with all of the +possible architectures, so we need something more like a unit or functional +test. + +Given that this work will not make the testing situation any worse, it's +possible we could defer the testing improvements to followup work. + +### Graduation Criteria + +NA + +#### Dev Preview -> Tech Preview + +NA + +#### Tech Preview -> GA + +NA + +#### Removing a deprecated feature + +NA + +### Upgrade / Downgrade Strategy + +NA + +### Version Skew Strategy + +From version to version the selection process must remain consistent in order +to avoid IPs and interfaces changing. As a result, version skew should not be +a problem. + +### Operational Aspects of API Extensions + +NA + +#### Failure Modes + +NA + +#### Support Procedures + +This should not drastically affect support. It will still be possible to +determine what address/interface a service is using by looking at its config +and/or logs. The only change will be where that comes from and that the new +files can also be consulted to check the behavior of nodeip-configuration. + +## Implementation History + +KUBELET_NODEIP_HINT was implemented in 4.11 and backported to older releases +to improve the user experience where they needed to override the default logic +in nodeip-configuration. + +## Alternatives + +* Leave things basically as they are, but teach services like configure-ovs and + keepalived to understand KUBELET_NODEIP_HINT so they all select the same IP + and interface. This would result in some level of duplicated logic and likely + lead to more problems down the road. + +* Instead of making the ip selection logic smarter, document a mechanism for + users to be explicit about the desired network layout. In theory this is + already possible, but it only helps with configure-ovs. It's still possible + other services may select incorrect ips. This is also a rather complex way + to support specifying an interface for configure-ovs. + +## Infrastructure Needed [optional] + +NA