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[MultiNIC] Propagate SkipDefaultRoutes to CNI and fix windows endpoint default route behavior #4213

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[MultiNIC] Propagate SkipDefaultRoutes to CNI and fix windows endpoint default route behavior #4213

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1 change: 1 addition & 0 deletions cns/restserver/ipam.go
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Original file line number Diff line number Diff line change
Expand Up @@ -158,6 +158,7 @@ func (service *HTTPRestService) requestIPConfigHandlerHelperStandalone(ctx conte
NICType: resp[i].NetworkInterfaceInfo.NICType,
NetworkContainerPrimaryIPConfig: resp[i].IPConfiguration,
NetworkContainerID: resp[i].NetworkContainerID,
SkipDefaultRoutes: resp[i].SkipDefaultRoutes,
}
podIPInfoList = append(podIPInfoList, podIPInfo)
if resp[i].AllowHostToNCCommunication || resp[i].AllowNCToHostCommunication {
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11 changes: 11 additions & 0 deletions network/endpoint_windows.go
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Expand Up @@ -346,6 +346,17 @@ func (nw *network) configureHcnEndpoint(epInfo *EndpointInfo) (*hcn.HostComputeE
hcnEndpoint.Routes = append(hcnEndpoint.Routes, hcnRoute)
}

if epInfo.SkipDefaultRoutes && len(hcnEndpoint.Routes) == 0 {
logger.Info("Adding dummy default route for SkipDefaultRoutes=true",
zap.String("endpoint", infraEpName),
zap.String("nicType", string(epInfo.NICType)))

hcnEndpoint.Routes = append(hcnEndpoint.Routes, hcn.Route{
NextHop: "0.0.0.0", // Dummy gateway
DestinationPrefix: "0.0.0.0/0", // Default route
})
}
Comment on lines +349 to +358
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Copilot AI Feb 3, 2026

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The condition len(hcnEndpoint.Routes) == 0 may not cover all multi-NIC scenarios where Windows might add implicit default routes. According to the PR description, this dummy route is added to prevent Windows from implicitly adding a default route when SkipDefaultRoutes is true. However, if epInfo.Routes contains non-default routes (such as specific subnet routes from networkConfig.Routes or networkConfig.CnetAddressSpace), this dummy route won't be added, and Windows might still implicitly add a default route.

Consider whether the condition should be:

  • if epInfo.SkipDefaultRoutes && !hasDefaultRoute(hcnEndpoint.Routes) - to explicitly check if a default route already exists rather than checking if the route list is empty
  • Or clarify in a comment why this workaround is only needed when there are no routes at all

Additionally, this new behavior lacks test coverage. Consider adding a test case in endpoint_windows_test.go that verifies the dummy route is added when SkipDefaultRoutes is true and no routes are configured.

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for _, ipAddress := range epInfo.IPAddresses {
prefixLength, _ := ipAddress.Mask.Size()
ipConfiguration := hcn.IpConfig{
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