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Antrea OVS Pipeline

Terminology

  • Node Route Controller: the K8s controller which is part of the Antrea Agent and watches for updates to Nodes. When a Node is added, it updates the local networking configuration (e.g. configure the tunnel to the new Node). When a Node is deleted, it performs the necessary clean-ups.
  • peer Node: this is how we refer to other Nodes in the cluster, to which the local Node is connected through a Geneve, VXLAN, GRE, or STT tunnel.
  • Global Virtual MAC: a virtual MAC address which is used as the destination MAC for all tunnelled traffic across all Nodes. This simplifies networking by enabling all Nodes to use this MAC address instead of the actual MAC address of the appropriate remote gateway. This enables each vSwitch to act as a "proxy" for the local gateway when receiving tunnelled traffic and directly take care of the packet forwarding. At the moment, we use an hard-coded value of aa:bb:cc:dd:ee:ff.
  • Antrea-native Policies: Antrea ClusterNetworkPolicy and Antrea NetworkPolicy CRDs, as documented here.
  • normal action: OpenFlow defines this action to submit a packet to "the traditional non-OpenFlow pipeline of the switch". That is, if a flow uses this action, then the packets in the flow go through the switch in the same way that they would if OpenFlow was not configured on the switch. Antrea uses this action to process ARP traffic as a regular learning L2 switch would.
  • table-miss flow entry: a "catch-all" entry in a OpenFlow table, which is used if no other flow is matched. If the table-miss flow entry does not exist, by default packets unmatched by flow entries are dropped (discarded).
  • conjunctive match fields: an efficient way in OVS to implement conjunctive matches, that is a match for which we have multiple fields, each one with a set of acceptable values. See OVS fields for more information.
  • conntrack: a connection tracking module that can be used by OVS to match on the state of a TCP, UDP, ICMP, etc., connection. See the OVS Conntrack tutorial for more information.
  • dmac table: a traditional L2 switch has a "dmac" table which maps learned destination MAC address to the appropriate egress port. It is often the same physical table as the "smac" table (which matches on the source MAC address and initiate MAC learning if the address is unknown).
  • group action: an action which is used to process forwarding decisions on multiple OVS ports. Examples include: load-balancing, multicast, and active/standby. See OVS group action for more information.
  • IN_PORT action: an action to output the packet to the port on which it was received. This is the only standard way to output the packet to the input port.
  • session affinity: a load balancer feature that always selects the same backend Pod for connections from a particular client. For a K8s Service, session affinity can be enabled by setting service.spec.sessionAffinity to ClientIP (default is None). See K8s Service for more information about session affinity.

This document currently makes the following assumptions:

  • Antrea is used in encap mode (an overlay network is created between all Nodes)
  • All the Nodes are Linux Nodes
  • IPv6 is disabled
  • AntreaProxy is enabled
  • AntreaPolicy is enabled

Dumping the Flows

This guide includes a representative flow dump for every table in the pipeline, in order to illustrate the function of each table. If you have a cluster running Antrea, you can dump the flows for a given Node as follows:

kubectl exec -n kube-system <ANTREA_AGENT_POD_NAME> -c antrea-ovs -- ovs-ofctl dump-flows <BRIDGE_NAME> [--no-stats] [--names]

where <ANTREA_AGENT_POD_NAME> is the name of the Antrea Agent Pod running on that Node and <BRIDGE_NAME> is the name of the bridge created by Antrea (br-int by default).

Registers

We use 2 32-bit OVS registers to carry information throughout the pipeline:

  • reg0 (NXM_NX_REG0):
    • bits [0..3] are used to store the traffic source (from tunnel: 0, from local gateway: 1, from local Pod: 2). It is set in ClassifierTable.
    • bit 16 is used to indicate whether the destination MAC address of a packet is "known", i.e. corresponds to an entry in L2ForwardingCalcTable, which is essentially a "dmac" table.
    • bit 18 is used to indicate whether the packet should be output to the port on which it was received. It is consumed in L2ForwardingOutTable to output the packet with action IN_PORT.
    • bit 19 is used to indicate whether the destination and source MACs of the packet should be rewritten in l3ForwardingTable. The bit is set for packets received from the tunnel port in ClassifierTable. The destination MAC of such packets is the Global Virtual MAC and should be rewritten to the destination port's MAC before output to the port. When such a packet is destined to a Pod, its source MAC should be rewritten to the local gateway port's MAC too.
  • reg1 (NXM_NX_REG1): it is used to store the egress OF port for the packet. It is set in DNATTable for traffic destined to Services and in L2ForwardingCalcTable otherwise. It is consumed in L2ForwardingOutTable to output each packet to the correct port.
  • reg3 (NXM_NX_REG3): it is used to store selected Service Endpoint IPv4 address in OVS group entry. It is consumed in EndpointDNATTable.
  • reg4 (NXM_NX_REG4):
    • bits [0..16] are used to store selected Service Endpoint port number in OVS group entry. They are consumed in EndpointDNATTable.
    • bits [17..18] are used to store the state of a Service request packet. Marks in this field include,
      • 0b001: packet needs to do Endpoint selection.
      • 0b010: packet has done Endpoint selection.
      • 0b011: packet has done Endpoint selection and the selection result needs to be cached.

Network Policy Implementation

Several tables of the pipeline are dedicated to K8s Network Policy implementation (EgressRuleTable, EgressDefaultTable, IngressRuleTable and IngressDefaultTable).

The Antrea implementation of K8s Network Policy, including the communication channel between the Controller and Agents, and how a Network Policy is mapped to OVS flows at each Node, will be described in details in a separate document. For the present document, we will use the Network Policy example below, and explain how these simple ingress and egress rules map to individual flows as we describe the relevant tables of our pipeline.

apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: test-network-policy
  namespace: default
spec:
  podSelector:
    matchLabels:
      app: nginx
  policyTypes:
  - Ingress
  - Egress
  ingress:
  - from:
    - podSelector:
        matchLabels:
          app: nginx
    ports:
    - protocol: TCP
      port: 80
  egress:
  - to:
    - podSelector:
        matchLabels:
          app: nginx
    ports:
    - protocol: TCP
      port: 80

This Network Policy is applied to all Pods with the nginx app label in the default Namespace. For these Pods, it only allows TCP traffic on port 80 from and to Pods which also have the nginx app label. Because Antrea will only install OVS flows for this Network Policy on Nodes for which some of the Pods are the target of the policy, we have scheduled 2 nginx Pods on the same Node. They received IP addresses 10.10.1.2 and 10.10.1.3 from the Antrea CNI, so you will see these addresses show up in the OVS flows.

Antrea-native Policies Implementation

In addition to the above tables created for K8s NetworkPolicy, Antrea creates additional dedicated tables to support the Antrea-native policies (AntreaPolicyEgressRuleTable and AntreaPolicyIngressRuleTable).

Consider the following Antrea ClusterNetworkPolicy (ACNP) in the Application tier as an example for the remainder of this document.

apiVersion: crd.antrea.io/v1alpha1
kind: ClusterNetworkPolicy
metadata:
  name: cnp0
spec:
  priority: 10
  tier: application # defaults to application tier if not specified
  appliedTo:
    - podSelector:
        matchLabels:
          app: server
  ingress:
    - action: Drop
      from:
        - podSelector:
            matchLabels:
              app: notClient
      ports:
        - protocol: TCP
          port: 80
  egress:
    - action: Allow
      to:
        - podSelector:
            matchLabels:
              app: dns
      ports:
        - protocol: UDP
          port: 53

This ACNP is applied to all Pods with the app: server label in all Namespaces. For these Pods, it drops TCP traffic on port 80 from all Pods which have the app: notClient label. In addition to the ingress rules, this policy also allows egress UDP traffic on port 53 to all Pods with the label app: dns. Similar to K8s NetworkPolicy, Antrea will only install OVS flows for this ACNP on Nodes for which some of the Pods are the target of the policy. Thus, we have scheduled three Pods (appServer, appDns, appNotClient) on the same Node and they have the following IP addresses:

  • appServer: 10.10.1.6
  • appNotClient: 10.10.1.7
  • appDns: 10.10.1.8

Tables

OVS pipeline

ClassifierTable (0)

This table is used to determine which "category" of traffic (tunnel, local gateway or local Pod) the packet belongs to. This is done by matching on the ingress port for the packet. The appropriate value is then written to bits [0..3] in NXM_NX_REG0: 0 for tunnel, 1 for local gateway and 2 for local Pod. This information is used by matches in subsequent tables. For a packet received from the tunnel port, bit 19 in NXM_NX_REG0 is set to 1, to indicate MAC rewrite should be performed for the packet in L3ForwardingTable.

If you dump the flows for this table, you may see the following:

1. table=0, priority=200,in_port=2 actions=load:0x1->NXM_NX_REG0[0..3],resubmit(,10)
2. table=0, priority=200,in_port=1 actions=load:0->NXM_NX_REG0[0..3],load:0x1->NXM_NX_REG0[19],resubmit(,30)
3. table=0, priority=190,in_port=4 actions=load:0x2->NXM_NX_REG0[0..3],resubmit(,10)
4. table=0, priority=190,in_port=3 actions=load:0x2->NXM_NX_REG0[0..3],resubmit(,10)
5. table=0, priority=0 actions=drop

Flow 1 is for traffic coming in on the local gateway. Flow 2 is for traffic coming in through an overlay tunnel (i.e. from another Node). The next two flows (3 and 4) are for local Pods.

Local traffic then goes to SpoofGuardTable, while tunnel traffic from other Nodes goes to ConntrackTable. The table-miss flow entry will drop all unmatched packets (in practice this flow entry should almost never be used).

SpoofGuardTable (10)

This table prevents IP and ARP spoofing from local Pods. For each Pod (as identified by the ingress port), we ensure that:

  • for IP traffic, the source IP and MAC addresses are correct, i.e. match the values configured on the interface when Antrea set-up networking for the Pod.
  • for ARP traffic, the advertised IP and MAC addresses are correct, i.e. match the values configured on the interface when Antrea set-up networking for the Pod.

Because Antrea currently relies on kube-proxy to load-balance traffic destined to Services, implementing that kind of IP spoofing check for traffic coming-in on the local gateway port is not as trivial. Traffic from local Pods destined to Services will first go through the gateway, get load-balanced by the kube-proxy datapath (DNAT) then sent back through the gateway. This means that legitimate traffic can be received on the gateway port with a source IP belonging to a local Pod. We may add some fine-grained rules in the future to accommodate for this, but for now we just allow all IP traffic received from the gateway. We do have an ARP spoofing check for the gateway however, since there is no reason for the host to advertise a different MAC address on antrea-gw0.

If you dump the flows for this table, you may see the following:

1. table=10, priority=200,ip,in_port=2 actions=resubmit(,23)
2. table=10, priority=200,arp,in_port=2,arp_spa=10.10.0.1,arp_sha=3a:dd:79:0f:55:4c actions=resubmit(,20)
3. table=10, priority=200,arp,in_port=4,arp_spa=10.10.0.2,arp_sha=ce:99:ca:bd:62:c5 actions=resubmit(,20)
4. table=10, priority=200,arp,in_port=3,arp_spa=10.10.0.3,arp_sha=3a:41:49:42:98:69 actions=resubmit(,20)
5. table=10, priority=200,ip,in_port=4,dl_src=ce:99:ca:bd:62:c5,nw_src=10.10.0.2 actions=resubmit(,23)
6. table=10, priority=200,ip,in_port=3,dl_src=3a:41:49:42:98:69,nw_src=10.10.0.3 actions=resubmit(,23)
7. table=10, priority=0 actions=drop

After this table, ARP traffic goes to ARPResponderTable, while IP traffic goes to ServiceHairpinTable. Traffic which does not match any of the rules described above will be dropped by the table-miss flow entry.

ARPResponderTable (20)

The main purpose of this table is to reply to ARP requests from the local gateway asking for the MAC address of a remote peer gateway (another Node's gateway). This ensures that the local Node can reach any remote Pod, which in particular is required for Service traffic which has been load-balanced to a remote Pod backend by kube-proxy. Note that the table is programmed to reply to such ARP requests with a "Global Virtual MAC" ("Global" because it is used by all Antrea OVS bridges), and not with the actual MAC address of the remote gateway. This ensures that once the traffic is received by the remote OVS bridge, it can be directly forwarded to the appropriate Pod without actually going through the gateway. The Virtual MAC is used as the destination MAC address for all the traffic being tunnelled.

If you dump the flows for this table, you may see the following:

1. table=20, priority=200,arp,arp_tpa=10.10.1.1,arp_op=1 actions=move:NXM_OF_ETH_SRC[]->NXM_OF_ETH_DST[],mod_dl_src:aa:bb:cc:dd:ee:ff,load:0x2->NXM_OF_ARP_OP[],move:NXM_NX_ARP_SHA[]->NXM_NX_ARP_THA[],load:0xaabbccddeeff->NXM_NX_ARP_SHA[],move:NXM_OF_ARP_SPA[]->NXM_OF_ARP_TPA[],load:0xa0a0101->NXM_OF_ARP_SPA[],IN_PORT
2. table=20, priority=190,arp actions=NORMAL
3. table=20, priority=0 actions=drop

Flow 1 is the "ARP responder" for the peer Node whose local Pod subnet is 10.10.1.0/24. If we were to look at the routing table for the local Node, we would see the following "onlink" route:

10.10.1.0/24 via 10.10.1.1 dev antrea-gw0 onlink

A similar route is installed on the gateway (antrea-gw0) interface every time the Antrea Node Route Controller is notified that a new Node has joined the cluster. The route must be marked as "onlink" since the kernel does not have a route to the peer gateway 10.10.1.1: we trick the kernel into believing that 10.10.1.1 is directly connected to the local Node, even though it is on the other side of the tunnel.

Flow 2 ensures that OVS handle the remainder of ARP traffic as a regular L2 learning switch (using the normal action). In particular, this takes care of forwarding ARP requests and replies between local Pods.

The table-miss flow entry (flow 3) will drop all other packets. This flow should never be used because only ARP traffic should go to this table, and ARP traffic will either match flow 1 or flow 2.

ServiceHairpinTable (23)

When a backend Pod of a Service accesses the Service, and the Pod itself is selected as the destination, then we have the hairpin case, in which the source IP should be SNAT'd with a virtual hairpin IP in hairpinSNATTable. The source and destination IP addresses cannot be the same, otherwise the connection will be broken. It will be explained in detail in hairpinSNATTable. For response packets, the destination IP is the virtual hairpin IP, so the destination IP should be changed back to the IP of the backend Pod. Then the response packets can be forwarded back correctly.

If you dump the flows for this table, you should see the flows:

1. table=23, priority=200,ip,nw_dst=169.254.169.252 actions=move:NXM_OF_IP_SRC[]->NXM_OF_IP_DST[],load:0x1->NXM_NX_REG0[18],resubmit(,30)
2. table=23, priority=0 actions=resubmit(,24)

Flow 1 is used to match packet whose destination IP is virtual hairpin IP and change the destination IP of the matched packet by loading register NXM_OF_IP_SRC to NXM_OF_IP_DST. Bit 18 in NXM_NX_REG0 is set to 0x1, which indicates that the packet should be output to the port on which it was received, which is done in L2ForwardingOutTable.

ConntrackTable (30)

The sole purpose of this table is to invoke the ct action on all packets and set the ct_zone (connection tracking context) to a hard-coded value, then forward traffic to ConntrackStateTable. If you dump the flows for this table, you should only see 1 flow:

1. table=30, priority=200,ip actions=ct(table=31,zone=65520)

A ct_zone is simply used to isolate connection tracking rules. It is similar in spirit to the more generic Linux network namespaces, but ct_zone is specific to conntrack and has less overhead.

After invoking the ct action, packets will be in the "tracked" (trk) state and all connection tracking fields will be set to the correct value. Packets will then move on to ConntrackStateTable.

Refer to this document for more information on connection tracking in OVS.

ConntrackStateTable (31)

This table handles "tracked" packets (packets which are moved to the tracked state by the previous table ConntrackTable) and "untracked" packets (packets is not in tracked state).

This table serves the following purposes:

If you dump the flows for this table, you should see the following:

1. table=31, priority=200,ct_state=-new+trk,ct_mark=0x21,ip actions=load:0x1->NXM_NX_REG0[19],resubmit(,50)
2. table=31, priority=190,ct_state=+inv+trk,ip actions=drop
3. table=31, priority=190,ct_state=-new+trk,ip actions=resubmit(,50)
4. table=31, priority=0 actions=resubmit(,40),resubmit(,41)

Flow 1 is used to forward tracked Service packets to EgressRuleTable directly, without passing SessionAffinityTable, ServiceLBTable and EndpointDNATTable. The flow also sets bit 19 in NXM_NX_REG0 to 0x1, which indicates that the destination and source MACs of the matched packets should be rewritten in l3ForwardingTable.

Flow 2 is used to drop packets which is reported as invalid by conntrack.

Flow 3 is used to forward tracked non-Service packets to EgressRuleTable directly, without passing SessionAffinityTable, ServiceLBTable and EndpointDNATTable.

Flow 4 is used to match the first packet of untracked connection and forward it to SessionAffinityTable and ServiceLBTable.

SessionAffinityTable (40)

If service.spec.sessionAffinity of a Service is None, this table will set the value of bits [16..18] in NXM_NX_REG4 to 0b001, which indicates that the Service needs to do Endpoint selection. If you dump the flow, you should see the flow:

table=40, priority=0 actions=load:0x1->NXM_NX_REG4[16..18]

If service.spec.sessionAffinity of a Service is ClientIP, when a client accesses the Service for the first time, a learned flow with hard timeout which equals service.spec.sessionAffinityConfig.clientIP.timeoutSeconds of the Service will be generated in this table. This will be explained in detail in chapter ServiceLBTable.

ServiceLBTable (41)

This table is used to implement Service Endpoint selection. Note that, currently, only ClusterIP Service request from Pods is supported. NodePort, LoadBalancer and ClusterIP whose client is from K8s Node will be supported in the future.

When a ClusterIP Service is created with service.spec.sessionAffinity set to None, if you dump the flows, you should see the following flow:

1. table=41, priority=200,tcp,reg4=0x10000/0x70000,nw_dst=10.107.100.231,tp_dst=443 actions=load:0x2->NXM_NX_REG4[16..18],load:0x1->NXM_NX_REG0[19],group:5

Among the match conditions of the above flow:

  • reg4=0x10000/0x70000, value of bits [16..18] in NXM_NX_REG4 is 0b001, which is used to match Service packet whose state is to do Endpoint selection. The value of bits [16..18] in NXM_NX_REG4 is set in SessionAffinityTable by flow table=40, priority=0 actions=load:0x1->NXM_NX_REG4[16..18].

The actions of the above flow:

  • load:0x2->NXM_NX_REG4[16..18] is used to set the value of bits [16..18] in NXM_NX_REG4 to 0b002, which indicates that Endpoint selection "is performed". Note that, Endpoint selection has not really been done yet - it will be done by group action. The current action should have been done in target OVS group entry after Endpoint selection. However, we set the bits here, for the purpose of supporting more Endpoints in an OVS group. Please check PR #2101 to learn more information.
  • load:0x1->NXM_NX_REG0[19] is used to set the value of bit 19 in NXM_NX_REG0 to 0x1, which means that the source and destination MACs need to be rewritten.
  • group:5 is used to set the target OVS group. Note that, the target group needs to be created first before the flow is created.

Dump the group entry with command ovs-ofctl dump-groups br-int 5, you should see the following:

group_id=5,type=select,\
bucket=bucket_id:0,weight:100,actions=load:0xa0a0002->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,42),\
bucket=bucket_id:1,weight:100,actions=load:0xa0a0003->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,42),\
bucket=bucket_id:2,weight:100,actions=load:0xa0a0004->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,42)

For the above OVS group, there are three buckets which have the same weight. Every bucket has the same chance to be selected since they have the same weight. The selected bucket will load Endpoint IPv4 address to NXM_NX_REG3, Endpoint port number to bits [0..15] in NXM_NX_REG4. Then the matched packet will be resubmitted to EndpointDNATTable.

When a ClusterIP Service is created with service.spec.sessionAffinity set to ClientIP, you may see the following flows:

1. table=41, priority=200,tcp,reg4=0x10000/0x70000,nw_dst=10.107.100.231,tp_dst=443 actions=load:0x3->NXM_NX_REG4[16..18],load:0x1->NXM_NX_REG0[19],group:5
2. table=41, priority=190,tcp,reg4=0x30000/0x70000,nw_dst=10.107.100.231,tp_dst=443 actions=\
   learn(table=40,hard_timeout=300,priority=200,delete_learned,cookie=0x2040000000008, \
     eth_type=0x800,nw_proto=6,NXM_OF_TCP_DST[],NXM_OF_IP_DST[],NXM_OF_IP_SRC[],\
     load:NXM_NX_REG3[]->NXM_NX_REG3[],load:NXM_NX_REG4[0..15]->NXM_NX_REG4[0..15],load:0x2->NXM_NX_REG4[16..18],load:0x1->NXM_NX_REG0[19]),\
   load:0x2->NXM_NX_REG4[16..18],\
   resubmit(,42)

When a client (assumed that the source IP is 10.10.0.2) accesses the ClusterIP for the first time, the first packet of the connection will be matched by flow 1. Note that the action load:0x3->NXM_NX_REG4[16..18] indicates that the Service Endpoint selection result needs to be cached.

Dump the group entry with command ovs-ofctl dump-groups br-int 5, you should see the following:

group_id=5,type=select,\
bucket=bucket_id:0,weight:100,actions=load:0xa0a0002->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,41),\
bucket=bucket_id:1,weight:100,actions=load:0xa0a0003->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,41),\
bucket=bucket_id:2,weight:100,actions=load:0xa0a0004->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],resubmit(,41)

Note the action resubmit(,41) resubmits the first packet of a ClusterIP Service connection back to ServiceLBTable, not resubmits the packet to EndpointDNATTable. Then the packet will be matched by flow 2 since value of bits [16..18] in NXM_NX_REG4 is 0b011. One action of the flow is to generate a learned flow in SessionAffinityTable, the other action is to resubmit the packet to EndpointDNATTable.

Now if you dump flows of table SessionAffinityTable, you may see the following flows:

1. table=40, hard_timeout=300, priority=200,tcp,nw_src=10.10.0.2,nw_dst=10.107.100.231,tp_dst=443 \
   actions=load:0xa0a0002->NXM_NX_REG3[],load:0x23c1->NXM_NX_REG4[0..15],load:0x2->NXM_NX_REG4[16..18],load:0x1->NXM_NX_REG0[19]
2. table=40, priority=0 actions=load:0x1->NXM_NX_REG4[16..18]

Note that, flow 1 (the generated learned flow) has higher priority than flow 2 in table SessionAffinityTable. When a particular client accesses the ClusterIP once again, the first packet of the connection will be matched by flow 1 due to the match condition nw_src=10.10.0.2.

The actions of flow 1:

  • load:0xa0a0004->NXM_NX_REG3[] is used to load Endpoint IPv4 address to NXM_NX_REG3.
  • load:0x50->NXM_NX_REG4[0..15] is used to load Endpoint port number to bits [0..15] in NXM_NX_REG4.
  • load:0x2->NXM_NX_REG4[16..18] is used to set the value of bits [16..18] in NXM_NX_REG4 to 0b010, which indicates that the Service has done Endpoint selection.
  • load:0x1->NXM_NX_REG0[18] is used to set the value of bit 18 in NXM_NX_REG4 to 0x1, which indicates that the source and destination MACs need to be rewritten.

Note that, if the value of bits [16..18] in NXM_NX_REG4 is 0b010 (set by action load:0x2->NXM_NX_REG4[16..18] in table SessionAffinityTable), then packet will not be matched by any flows in table ServiceLBTable except the last one. The last one just forwards the packet to table EndpointDNATTable without selecting target OVS group. Then connections from a particular client will always access the same backend Pod within the session timeout setting by service.spec.sessionAffinityConfig.clientIP.timeoutSeconds.

EndpointDNATTable (42)

The table implements DNAT for Service traffic after Endpoint selection for the first packet of a Service connection.

If you dump the flows for this table, you should see flows like the following:

1. table=42, priority=200,tcp,reg3=0xc0a84d64,reg4=0x2192b/0x7ffff actions=ct(commit,table=45,zone=65520,nat(dst=192.168.77.100:6443),exec(load:0x21->NXM_NX_CT_MARK[]))
2. table=42, priority=200,tcp,reg3=0xc0a84d65,reg4=0x2286d/0x7ffff actions=ct(commit,table=45,zone=65520,nat(dst=192.168.77.101:10349),exec(load:0x21->NXM_NX_CT_MARK[]))
3. table=42, priority=200,tcp,reg3=0xa0a0004,reg4=0x20050/0x7ffff actions=ct(commit,table=45,zone=65520,nat(dst=10.10.0.4:80),exec(load:0x21->NXM_NX_CT_MARK[]))
4. table=42, priority=200,tcp,reg3=0xa0a0102,reg4=0x20050/0x7ffff actions=ct(commit,table=45,zone=65520,nat(dst=10.10.1.2:80),exec(load:0x21->NXM_NX_CT_MARK[]))
5. table=42, priority=200,udp,reg3=0xa0a0002,reg4=0x20035/0x7ffff actions=ct(commit,table=45,zone=65520,nat(dst=10.10.0.2:53),exec(load:0x21->NXM_NX_CT_MARK[]))
6. table=42, priority=190,reg4=0x20000/0x70000 actions=load:0x1->NXM_NX_REG4[16..18],resubmit(,41)
7. table=42, priority=0 actions=resubmit(,45)

For flow 1-5, DNAT is performed with the IPv4 address stored in NXM_NX_REG3 and port number stored in bits[0..15] in NXM_NX_REG4 by ct commit action. Note that, the match condition reg4=0x2192b/0x7ffff is a union value. The value of bits [0..15] is port number. The value of bits [16..18] is 0b010, which indicates that Service has done Endpoint selection. Service ct_mark 0x21 is also marked.

If none of the flows described above are hit, flow 6 is used to forward packet back to table ServiceLBTable to select Endpoint again.

Flow 7 is used to match non-Service packet.

AntreaPolicyEgressRuleTable (45)

For this table, you will need to keep in mind the ACNP specification that we are using.

This table is used to implement the egress rules across all Antrea-native policies, except for policies that are created in the Baseline Tier. Antrea-native policies created in the Baseline Tier will be enforced after K8s NetworkPolicies, and their egress rules are installed in the EgressDefaultTable and EgressRuleTable respectively, i.e.

Baseline Tier     ->  EgressDefaultTable(60)
K8s NetworkPolicy ->  EgressRuleTable(50)
All other Tiers   ->  AntreaPolicyEgressRuleTable(45)

Since the example ACNP resides in the Application tier, if you dump the flows for table 45, you should see something like this:

1. table=45, priority=64990,ct_state=-new+est,ip actions=resubmit(,61)
2. table=45, priority=14000,conj_id=1,ip actions=load:0x1->NXM_NX_REG5[],ct(commit,table=61,zone=65520,exec(load:0x1->NXM_NX_CT_LABEL[32..63]))
3. table=45, priority=14000,ip,nw_src=10.10.1.6 actions=conjunction(1,1/3)
4. table=45, priority=14000,ip,nw_dst=10.10.1.8 actions=conjunction(1,2/3)
5. table=45, priority=14000,udp,tp_dst=53 actions=conjunction(1,3/3)
6. table=45, priority=0 actions=resubmit(,50)

Similar to K8s NetworkPolicy implementation, AntreaPolicyEgressRuleTable also relies on the OVS built-in conjunction action to implement policies efficiently.

The above example flows read as follow: if the source IP address is in set {10.10.1.6}, and the destination IP address is in the set {10.10.1.8}, and the destination TCP port is in the set {53}, then use the conjunction action with id 1, which stores the conj_id 1 in ct_label[32..63] for egress metrics collection purposes, and forwards the packet to EgressMetricsTable, then L3ForwardingTable. Otherwise, go to EgressRuleTable if no conjunctive flow above priority 0 is matched. This corresponds to the case where the packet is not matched by any of the Antrea-native policy egress rules in any tier (except for the "baseline" tier).

If the conjunction action is matched, packets are "allowed" or "dropped" based on the action field of the policy rule. If allowed, they follow a similar path as described in the following EgressRuleTable section.

Unlike the default of K8s NetworkPolicies, Antrea-native policies have no such default rules. Hence, they are evaluated as-is, and there is no need for a AntreaPolicyEgressDefaultTable.

EgressRuleTable (50)

For this table, you will need to keep mind the Network Policy specification that we are using. We have 2 Pods running on the same Node, with IP addresses 10.10.1.2 to 10.10.1.3. They are allowed to talk to each other using TCP on port 80, but nothing else.

This table is used to implement the egress rules across all Network Policies. If you dump the flows for this table, you should see something like this:

1. table=50, priority=210,ct_state=-new+est,ip actions=goto_table:70
2. table=50, priority=200,ip,nw_src=10.10.1.2 actions=conjunction(2,1/3)
3. table=50, priority=200,ip,nw_src=10.10.1.3 actions=conjunction(2,1/3)
4. table=50, priority=200,ip,nw_dst=10.10.1.2 actions=conjunction(2,2/3)
5. table=50, priority=200,ip,nw_dst=10.10.1.3 actions=conjunction(2,2/3)
6. table=50, priority=200,tcp,tp_dst=80 actions=conjunction(2,3/3)
7. table=50, priority=190,conj_id=2,ip actions=load:0x2->NXM_NX_REG5[],ct(commit,table=61,zone=65520,exec(load:0x2->NXM_NX_CT_LABEL[32..63]))
8. table=50, priority=0 actions=goto_table:60

Notice how we use the OVS built-in conjunction action to implement policies efficiently. This enables us to do a conjunctive match across multiple dimensions (source IP, destination IP, port) efficiently without "exploding" the number of flows. By definition of a conjunctive match, we have at least 2 dimensions. For our use-case we have at most 3 dimensions.

The only requirements on conj_id is for it to be a unique 32-bit integer within the table. At the moment we use a single custom allocator, which is common to all tables that can have NetworkPolicy flows installed (45, 50, 60, 85, 90 and 100). This is why conj_id is set to 2 in the above example (1 was allocated for the egress rule of our Antrea-native NetworkPolicy example in the previous section).

The above example flows read as follow: if the source IP address is in set {10.10.1.2, 10.10.1.3}, and the destination IP address is in the set {10.10.1.2, 10.10.1.3}, and the destination TCP port is in the set {80}, then use the conjunction action with id 2, which goes to [EgressMetricsTable], and then L3ForwardingTable. Otherwise, packet goes to EgressDefaultTable.

If the Network Policy specification includes exceptions (except field), then the table will include multiple flows with conjunctive match, corresponding to each CIDR that is present in from or to fields, but not in except field. Network Policy implementation details are not covered in this document.

If the conjunction action is matched, packets are "allowed" and forwarded directly to L3ForwardingTable. Other packets go to EgressDefaultTable. If a connection is established - as a reminder all connections are committed in ConntrackCommitTable - its packets go straight to L3ForwardingTable, with no other match required (see flow 1 above, which has the highest priority). In particular, this ensures that reply traffic is never dropped because of a Network Policy rule. However, this also means that ongoing connections are not affected if the K8s Network Policies are updated.

One thing to keep in mind is that for Service traffic, these rules are applied after the packets have gone through the local gateway and through kube-proxy. At this point the ingress port is no longer the Pod port, but the local gateway port. Therefore we cannot use the port as the match condition to identify if the Pod has been applied a Network Policy - which is what we do for the IngressRuleTable -, but instead have to use the source IP address.

EgressDefaultTable (60)

This table complements EgressRuleTable for Network Policy egress rule implementation. In K8s, when a Network Policy is applied to a set of Pods, the default behavior for these Pods become "deny" (it becomes an isolated Pod). This table is in charge of dropping traffic originating from Pods to which a Network Policy (with an egress rule) is applied, and which did not match any of the allowlist rules.

Accordingly, based on our Network Policy example, we would expect to see flows to drop traffic originating from our 2 Pods (10.10.1.2 and 10.10.1.3), which is confirmed by dumping the flows:

1. table=60, priority=200,ip,nw_src=10.10.1.2 actions=drop
2. table=60, priority=200,ip,nw_src=10.10.1.3 actions=drop
3. table=60, priority=0 actions=goto_table:61

This table is also used to implement Antrea-native policy egress rules that are created in the Baseline Tier. Since the Baseline Tier is meant to be enforced after K8s NetworkPolicies, the corresponding flows will be created at a lower priority than K8s default drop flows. For example, a baseline rule to drop egress traffic to 10.0.10.0/24 for a Namespace will look like the following:

1. table=60, priority=80,ip,nw_src=10.10.1.11 actions=conjunction(5,1/2)
2. table=60, priority=80,ip,nw_src=10.10.1.10 actions=conjunction(5,1/2)
3. table=60, priority=80,ip,nw_dst=10.0.10.0/24 actions=conjunction(5,2)
4. table=60, priority=80,conj_id=5,ip actions=load:0x3->NXM_NX_REG5[],load:0x1->NXM_NX_REG0[20],resubmit(,61)

The table-miss flow entry, which is used for non-isolated Pods, forwards traffic to the next table EgressMetricsTable, then (L3ForwardingTable).

L3ForwardingTable (70)

This is the L3 routing table. It implements the following functionality:

  • Tunnelled traffic coming-in from a peer Node and destined to a local Pod is directly forwarded to the Pod. This requires setting the source MAC to the MAC of the local gateway interface and setting the destination MAC to the Pod's MAC address. Then the packets will go to L3DecTTLTable for decrementing the IP TTL value. Such packets can be identified by bit 19 of the NXM_NX_REG0 register (which was set to 1 in the ClassifierTable) and the destination IP address (which should match the IP address of a local Pod). We therefore install one flow for each Pod created locally on the Node. For example:
table=70, priority=200,ip,reg0=0x80000/0x80000,nw_dst=10.10.0.2 actions=mod_dl_src:e2:e5:a4:9b:1c:b1,mod_dl_dst:12:9e:a6:47:d0:70,goto_table:72
  • All tunnelled traffic destined to the local gateway (i.e. for which the destination IP matches the local gateway's IP) is forwarded to the gateway port by rewriting the destination MAC (from the Global Virtual MAC to the local gateway's MAC).
table=70, priority=200,ip,reg0=0x80000/0x80000,nw_dst=10.10.0.1 actions=mod_dl_dst:e2:e5:a4:9b:1c:b1,goto_table:80
  • All reply traffic of connections initiated through the gateway port, i.e. for which the first packet of the connection (SYN packet for TCP) was received through the gateway. Such packets can be identified by the packet's direction in ct_state and the ct_mark value 0x20 which is committed in ConntrackCommitTable when the first packet of the connection was handled. A flow will overwrite the destination MAC to the local gateway MAC to ensure that they get forwarded through the gateway port. This is required to handle the following cases:
    • reply traffic for connections from a local Pod to a ClusterIP Service, which are handled by kube-proxy and go through DNAT. In this case the destination IP address of the reply traffic is the Pod which initiated the connection to the Service (no SNAT by kube-proxy). We need to make sure that these packets are sent back through the gateway so that the source IP can be rewritten to the ClusterIP ("undo" DNAT). If we do not use connection tracking and do not rewrite the destination MAC, reply traffic from the backend will go directly to the originating Pod without going first through the gateway and kube-proxy. This means that the reply traffic will arrive at the originating Pod with the incorrect source IP (it will be set to the backend's IP instead of the Service IP).
    • when hair-pinning is involved, i.e. connections between 2 local Pods, for which NAT is performed. One example is a Pod accessing a NodePort Service for which externalTrafficPolicy is set to Local using the local Node's IP address, as there will be no SNAT for such traffic. Another example could be hostPort support, depending on how the feature is implemented.
table=70, priority=210,ct_state=+rpl+trk,ct_mark=0x20,ip actions=mod_dl_dst:e2:e5:a4:9b:1c:b1,goto_table:80
  • All traffic destined to a remote Pod is forwarded through the appropriate tunnel. This means that we install one flow for each peer Node, each one matching the destination IP address of the packet against the Pod subnet for the Node. In case of a match the source MAC is set to the local gateway MAC, the destination MAC is set to the Global Virtual MAC and we set the OF tun_dst field to the appropriate value (i.e. the IP address of the remote gateway). Traffic then goes to L3DecTTLTable. For a given peer Node, the flow may look like this:
table=70, priority=200,ip,nw_dst=10.10.1.0/24 actions=mod_dl_src:e2:e5:a4:9b:1c:b1,mod_dl_dst:aa:bb:cc:dd:ee:ff,load:0x1->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],load:0xc0a80102->NXM_NX_TUN_IPV4_DST[],goto_table:72

If none of the flows described above are hit, traffic goes directly to L2ForwardingCalcTable. This is the case for external traffic, whose destination is outside the cluster (such traffic has already been forwarded to the local gateway by the local source Pod, and only L2 switching is required), as well as for local Pod-to-Pod traffic.

table=70, priority=0 actions=goto_table:80

When the Egress feature is enabled, extra flows will be added to L3ForwardingTable, which send the egress traffic from Pods to external network to SNATTable. The following two flows match traffic to local Pods and traffic to the local Node IP respectively, and keep them in the normal forwarding path (to L2ForwardingCalcTable), so they will not be sent to SNATTable:

table=70, priority=200,ip,reg0=0/0x80000,nw_dst=10.10.1.0/24 actions=goto_table:80
table=70, priority=200,ip,reg0=0x2/0xffff,nw_dst=192.168.1.1 actions=goto_table:80

The following two flows send the traffic not matched by other flows to SNATTable. One of the flows is for egress traffic from local Pods; another one is for egress traffic from remote Pods, which is tunnelled to this Node to be SNAT'd with a SNAT IP configured on the Node. In the latter case, the flow also rewrites the destination MAC to the local gateway interface MAC.

table=70, priority=190,ip,reg0=0x2/0xf actions=goto_table:71
table=70, priority=190,ip,reg0=0/0xf actions=mod_dl_dst:e2:e5:a4:9b:1c:b1,goto_table:71

SNATTable (71)

This table is created only when the Egress feature is enabled. It includes flows to implement Egresses and select the right SNAT IPs for egress traffic from Pods to external network.

When no Egress applies to Pods on the Node, and no SNAT IP is configured on the Node, SNATTable just has two flows. One drops egress traffic tunnelled from remote Nodes that does not match any SNAT IP configured on this Node, and the default flow that sends egress traffic from local Pods, which do not have any Egress applied, to L2ForwardingCalcTable. Such traffic will be SNAT'd with the default SNAT IP (by an iptables masquerade rule).

table=71, priority=190,ct_state=+new+trk,ip,reg0=0/0xf actions=drop
table=71, priority=0 actions=goto_table:80

When there is an Egress applied to a Pod on the Node, a flow will be added for the Pod's egress traffic. If the SNAT IP of the Egress is configured on the local Node, the flow sets an 8 bits ID allocated for the SNAT IP to pkt_mark. The ID is for iptables SNAT rules to match the packets and perfrom SNAT with the right SNAT IP (Antrea Agent adds an iptables SNAT rule for each local SNAT IP that matches the ID).

table=71, priority=200,ct_state=+new+trk,ip,in_port="pod1-7e503a" actions=load:0x1->NXM_NX_PKT_MARK[0..7],goto_table:80

When the SNAT IP of the Egress is on a remote Node, the flow will tunnel the packets to the remote Node with the tunnel's destination IP to be the SNAT IP. The packets will be SNAT'd on the remote Node. The same as a normal tunnel flow in L3ForwardingTable, the flow will rewrite the packets' source and destination MAC addresses, load the SNAT IP to NXM_NX_TUN_IPV4_DST, and send the packets to L3DecTTLTable.

table=71, priority=200,ct_state=+new+trk,ip,in_port="pod2-357c21" actions=mod_dl_src:e2:e5:a4:9b:1c:b1,mod_dl_dst:aa:bb:cc:dd:ee:ff,load:0x1->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],load:0xc0a80a66->NXM_NX_TUN_IPV4_DST[],goto_table:72

Last, when a SNAT IP configured for Egresses is on the local Node, an additional flow is added in SNATTable for egress traffic from remote Node that should use the SNAT IP. The flow matches the tunnel destination IP (which should be equal to the SNAT IP), and sets the 8 bits ID of the SNAT IP to pkt_mark.

table=71, priority=200,ct_state=+new+trk,ip,tun_dst="192.168.10.101" actions=load:0x1->NXM_NX_PKT_MARK[0..7],goto_table:80

L3DecTTLTable (72)

This is the table to decrement TTL for the IP packets destined to remote Nodes through a tunnel, or the IP packets received from a tunnel. But for the packets that enter the OVS pipeline from the local gateway and are destined to a remote Node, TTL should not be decremented in OVS on the source Node, because the host IP stack should have already decremented TTL if that is needed.

If you dump the flows for this table, you should see flows like the following:

1. table=72, priority=210,ip,reg0=0x1/0xf, actions=goto_table:80
2. table=72, priority=200,ip, actions=dec_ttl,goto_table:80
3. table=72, priority=0, actions=goto_table:80

The first flow is to bypass the TTL decrement for the packets from the gateway port.

L2ForwardingCalcTable (80)

This is essentially the "dmac" table of the switch. We program one flow for each port (tunnel port, gateway port, and local Pod ports), as you can see if you dump the flows:

1. table=80, priority=200,dl_dst=aa:bb:cc:dd:ee:ff actions=load:0x1->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],goto_table:105
2. table=80, priority=200,dl_dst=e2:e5:a4:9b:1c:b1 actions=load:0x2->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],goto_table:105
3. table=80, priority=200,dl_dst=12:9e:a6:47:d0:70 actions=load:0x3->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],goto_table:90
4. table=80, priority=200,dl_dst=ba:a8:13:ca:ed:cf actions=load:0x4->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],goto_table:90
5. table=80, priority=0 actions=goto_table:105

For each port flow (1 through 5 in the example above), we set bit 16 of the NXM_NX_REG0 register to indicate that there was a matching entry for the destination MAC address and that the packet must be forwarded. In the last table of the pipeline (L2ForwardingOutTable), we will drop all packets for which this bit is not set. We also use the NXM_NX_REG1 register to store the egress port for the packet, which will be used as a parameter to the output OpenFlow action in L2ForwardingOutTable.

The packets that match local Pods' MAC entries will go to the first table (AntreaPolicyIngressRuleTable when AntreaPolicy is enabled, or IngressRuleTable when AntreaPolicy is not enabled) for NetworkPolicy ingress rules. Other packets will go to ConntrackCommitTable. Specifically, packets to the gateway port or the tunnel port will also go to ConntrackCommitTable and bypass the NetworkPolicy ingress rule tables, as NetworkPolicy ingress rules are not enforced for these packets on the source Node.

What about L2 multicast / broadcast traffic? ARP requests will never reach this table, as they will be handled by the OpenFlow normal action in the ArpResponderTable. As for the rest, if it is IP traffic, it will hit the "last" flow in this table and go to ConntrackCommitTable; and finally the last table of the pipeline (L2ForwardingOutTable), and get dropped there since bit 16 of the NXM_NX_REG0 will not be set. Traffic which is non-ARP and non-IP (assuming any can be received by the switch) is actually dropped much earlier in the pipeline (SpoofGuardTable). In the future, we may need to support more cases for L2 multicast / broadcast traffic.

AntreaPolicyIngressRuleTable (85)

This table is very similar to AntreaPolicyEgressRuleTable, but implements the ingress rules of Antrea-native Policies. Depending on the tier to which the policy belongs to, the rules will be installed in a table corresponding to that tier. The ingress table to tier mappings is as follows:

Baseline Tier     ->  IngressDefaultTable(100)
K8s NetworkPolicy ->  IngressRuleTable(90)
All other Tiers   ->  AntreaPolicyIngressRuleTable(85)

Again for this table, you will need to keep in mind the ACNP specification that we are using. Since the example ACNP resides in the Application tier, if you dump the flows for table 85, you should see something like this:

1. table=85, priority=64990,ct_state=-new+est,ip actions=resubmit(,105)
2. table=85, priority=14000,conj_id=4,ip actions=load:0x4->NXM_NX_REG3[],load:0x1->NXM_NX_REG0[20],resubmit(,101)
3. table=85, priority=14000,ip,nw_src=10.10.1.7 actions=conjunction(4,1/3)
4. table=85, priority=14000,ip,reg1=0x19c actions=conjunction(4,2/3)
5. table=85, priority=14000,tcp,tp_dst=80 actions=conjunction(4,3/3)
6. table=85, priority=0 actions=resubmit(,90)

As for AntreaPolicyEgressRuleTable, flow 1 (highest priority) ensures that for established connections packets go straight to IngressMetricsTable, then L2ForwardingOutTable, with no other match required.

The rest of the flows read as follows: if the source IP address is in set {10.10.1.7}, and the destination OF port is in the set {412} (which correspond to IP addresses {10.10.1.6}), and the destination TCP port is in the set {80}, then use conjunction action with id 4, which loads the conj_id 4 into NXM_NX_REG3, a register used by Antrea internally to indicate the disposition of the packet is Drop, and forward the packet to IngressMetricsTable for it to be dropped.

Otherwise, go to IngressRuleTable if no conjunctive flow above priority 0 is matched. This corresponds to the case where the packet is not matched by any of the Antrea-native policy ingress rules in any tier (except for the "baseline" tier). One notable difference is how we use OF ports to identify the destination of the traffic, while we use IP addresses in AntreaPolicyEgressRuleTable to identify the source of the traffic. More details regarding this can be found in the following IngressRuleTable section.

As seen in AntreaPolicyEgressRuleTable, the default action is to evaluate K8s Network Policy IngressRuleTable and a AntreaPolicyIngressDefaultTable does not exist.

IngressRuleTable (90)

This table is very similar to EgressRuleTable, but implements ingress rules for Network Policies. Once again, you will need to keep mind the Network Policy specification that we are using. We have 2 Pods running on the same Node, with IP addresses 10.10.1.2 to 10.10.1.3. They are allowed to talk to each other using TCP on port 80, but nothing else.

If you dump the flows for this table, you should see something like this:

1. table=90, priority=210,ct_state=-new+est,ip actions=goto_table:101
2. table=90, priority=210,pkt_mark=0x1/0x1 actions=goto_table:105
3. table=90, priority=200,ip,nw_src=10.10.1.2 actions=conjunction(3,1/3)
4. table=90, priority=200,ip,nw_src=10.10.1.3 actions=conjunction(3,1/3)
5. table=90, priority=200,ip,reg1=0x3 actions=conjunction(3,2/3)
6. table=90, priority=200,ip,reg1=0x4 actions=conjunction(3,2/3)
7. table=90, priority=200,tcp,tp_dst=80 actions=conjunction(3,3/3)
8. table=90, priority=190,conj_id=3,ip actions=load:0x3->NXM_NX_REG6[],ct(commit,table=101,zone=65520,exec(load:0x3->NXM_NX_CT_LABEL[0..31]))
9. table=90, priority=0 actions=goto_table:100

As for EgressRuleTable, flow 1 (highest priority) ensures that for established connections - as a reminder all connections are committed in ConntrackCommitTable - packets go straight to IngressMetricsTable, then L2ForwardingOutTable, with no other match required.

Flow 2 ensures that the traffic initiated from the host network namespace cannot be dropped because of Network Policies. This ensures that K8s liveness probes can go through. An iptables rule in the mangle table of the host network namespace is responsible for marking the locally-generated packets with the 0x1/0x1 mark. Note that the flow will be different for Windows worker Node or when OVS userspace (netdev) datapath is used. This is because either there is no way to add mark for particular traffic (i.e. Windows) or matching the mark in OVS is not properly supported (i.e. netdev datapath). As a result, the flow will match source IP instead, however, NodePort Service access by external clients will be masqueraded as a local gateway IP to bypass Network Policies. This may be fixed after AntreaProxy can serve NodePort traffic.

The rest of the flows read as follows: if the source IP address is in set {10.10.1.2, 10.10.1.3}, and the destination OF port is in the set {3, 4} (which correspond to IP addresses {10.10.1.2, 10.10.1.3}, and the destination TCP port is in the set {80}, then use conjunction action with id 3, which stores the conj_id 3 in ct_label[0..31] for egress metrics collection purposes, and forwards the packet to IngressMetricsTable, then L2ForwardingOutTable. Otherwise, go to IngressDefaultTable. One notable difference is how we use OF ports to identify the destination of the traffic, while we use IP addresses in EgressRuleTable to identify the source of the traffic. We do this as an increased security measure in case a local Pod is misbehaving and trying to access another local Pod using the correct destination MAC address but a different destination IP address to bypass an egress Network Policy rule. This is also why the Network Policy ingress rules are enforced after the egress port has been determined.

IngressDefaultTable (100)

This table is similar in its purpose to EgressDefaultTable, and it complements IngressRuleTable for Network Policy ingress rule implementation. In K8s, when a Network Policy is applied to a set of Pods, the default behavior for these Pods become "deny" (it becomes an isolated Pod). This table is in charge of dropping traffic destined to Pods to which a Network Policy (with an ingress rule) is applied, and which did not match any of the allowlist rules.

Accordingly, based on our Network Policy example, we would expect to see flows to drop traffic destined to our 2 Pods (3 and 4), which is confirmed by dumping the flows:

1. table=100, priority=200,ip,reg1=0x3 actions=drop
2. table=100, priority=200,ip,reg1=0x4 actions=drop
3. table=100, priority=0 actions=goto_table:105

Similar to the EgressDefaultTable, this table is also used to implement Antrea-native policy ingress rules that are created in the Baseline Tier. Since the Baseline Tier is meant to be enforced after K8s NetworkPolicies, the corresponding flows will be created at a lower priority than K8s default drop flows. For example, a baseline rule to isolate ingress traffic for a Namespace will look like the following:

table=100, priority=80,ip,reg1=0xb actions=conjunction(6,2/3)
table=100, priority=80,ip,reg1=0xc actions=conjunction(6,2/3)
table=100, priority=80,ip,nw_src=10.10.1.9 actions=conjunction(6,1/3)
table=100, priority=80,ip,nw_src=10.10.1.7 actions=conjunction(6,1/3)
table=100, priority=80,tcp,tp_dst=8080 actions=conjunction(6,3/3)
table=100, priority=80,conj_id=6,ip actions=load:0x6->NXM_NX_REG3[],load:0x1->NXM_NX_REG0[20],resubmit(,101)

The table-miss flow entry, which is used for non-isolated Pods, forwards traffic to the next table (ConntrackCommitTable).

ConntrackCommitTable (105)

As mentioned before, this table is in charge of committing all new connections which are not dropped because of Network Policies. If you dump the flows for this table, you should see something like this:

1. table=105, priority=200,ct_state=+new+trk,ip,reg0=0x1/0xf actions=ct(commit,table=108,zone=65520,exec(load:0x20->NXM_NX_CT_MARK[]))
2. table=105, priority=190,ct_state=+new+trk,ip actions=ct(commit,table=108,zone=65520)
3. table=105, priority=0 actions=goto_table:108

Flow 1 ensures that we commit connections initiated through the gateway interface and mark them with a ct_mark of 0x20. This ensures that ConntrackStateTable can perform its functions correctly and rewrite the destination MAC address to the gateway's MAC address for connections which require it. Such connections include Pod-to-ClusterIP traffic. Note that the 0x20 mark is applied to all connections initiated through the gateway (i.e. for which the first packet of the connection was received through the gateway) and that ConntrackStateTable will perform the destination MAC address for the reply traffic of all such connections. In some cases (the ones described for ConntrackStateTable), this rewrite is necessary. For others (e.g. a connection from the host to a local Pod), this rewrite is not necessary but is also harmless, as the destination MAC is already correct.

Flow 2 commits all other new connections.

All traffic then goes to HairpinSNATTable.

HairpinSNATTable (108)

The table is used to handle Service hairpin case, which indicates that the packet should be output to the port on which it was received.

If you dump the flows for this table, you should see the flows:

1. table=108, priority=200,ip,nw_src=10.10.0.4,nw_dst=10.10.0.4 actions=mod_nw_src:169.254.169.252,load:0x1->NXM_NX_REG0[18],resubmit(,110)
2. table=108, priority=200,ip,nw_src=10.10.0.2,nw_dst=10.10.0.2 actions=mod_nw_src:169.254.169.252,load:0x1->NXM_NX_REG0[18],resubmit(,110)
3. table=108, priority=200,ip,nw_src=10.10.0.3,nw_dst=10.10.0.3 actions=mod_nw_src:169.254.169.252,load:0x1->NXM_NX_REG0[18],resubmit(,110)
4. table=108, priority=0 actions=resubmit(,110)

Flow 1-3 are used to match Service packets from Pods. The source IP of the matched packets by flow 1-3 should be SNAT'd with a virtual hairpin IP since the source and destination IP addresses should not be the same. Without SNAT, response packets from a Pod will not be forwarded back to OVS pipeline as the destination IP is the Pod's own IP, then the connection is interrupted because the conntrack state is only stored in OVS ct zone, not in the Pod. With SNAT, the destination IP will be the virtual hairpin IP and forwarded back to OVS pipeline. Note that, bit 18 in NXM_NX_REG0 is set to 0x1, and it is consumed in L2ForwardingOutTable to output the packet to the port on which it was received with action IN_PORT.

L2ForwardingOutTable (110)

It is a simple table and if you dump the flows for this table, you should only see 2 flows:

1. table=110, priority=200,ip,reg0=0x10000/0x10000 actions=output:NXM_NX_REG1[]
2. table=110, priority=0, actions=drop

The first flow outputs all unicast packets to the correct port (the port was resolved by the "dmac" table, L2ForwardingCalcTable). IP packets for which L2ForwardingCalcTable did not set bit 16 of NXM_NX_REG0 will be dropped.

Tables (AntreaProxy is disabled)

OVS pipeline

DNATTable (40)

This table is created only when AntreaProxy is disabled. Its only job is to send traffic destined to Services through the local gateway interface, without any modifications. kube-proxy will then take care of load-balancing the connections across the different backends for each Service.

If you dump the flows for this table, you should see something like this:

1. table=40, priority=200,ip,nw_dst=10.96.0.0/12 actions=load:0x2->NXM_NX_REG1[],load:0x1->NXM_NX_REG0[16],goto_table:105
2. table=40, priority=0 actions=goto_table:45

In the example above, 10.96.0.0/12 is the Service CIDR (this is the default value used by kubeadm init). This flow is not actually required for forwarding, but to bypass EgressRuleTable and EgressDefaultTable for Service traffic on its way to kube-proxy through the gateway. If we omitted this flow, such traffic would be unconditionally dropped if a Network Policy is applied on the originating Pod. For such traffic, we instead enforce Network Policy egress rules when packets come back through the gateway and the destination IP has been rewritten by kube-proxy (DNAT to a backend for the Service). We cannot output the Service traffic to the gateway port directly as we haven't committed the connection yet; instead we store the port in NXM_NX_REG1 - similarly to how we process non-Service traffic in L2ForwardingCalcTable - and forward it to ConntrackCommitTable. By committing the connection we ensure that reply traffic (traffic from the Service backend which has already gone through kube-proxy for source IP rewrite) will not be dropped because of Network Policies.

The table-miss flow entry (flow 2) for this table forwards all non-Service traffic to the next table, AntreaPolicyEgressRuleTable.