ArthurChiao's Blog

Host Disconnect Caused by Istio Sidecar Injection

Published at 2019-10-27 | Last Update 2019-10-27

1 Problem

1.1 Phenomemon

We met a network interrupt problem in our istio environment: as long as istio sidecar injection is enabled, the host will suffer a disconnection (e.g. SSH connections to this host was dropped) whenever a new pod is launching on this host. The disconnection lasts about 30 seconds, then automatically restores.

This post summarizes the trouble shooting steps we went through, the direct causes we’ve found, and the fixups we’ve made.

1.2 Environment Info

K8S cluster information:

  • Host: Linux 4.14+ (custom patched)
  • Host network: 2 NICs, bonding via OVS
  • Service mesh: istio 1.2.4
  • Network solution: Cilium 1.6.2 (custom patched) + bird 2.x (BGP agent)

Fig. 1.1 Host network topology

2 Minimum reproducing scheme

After some preliminary investigations, we narrowed down the problem to following scope:

  1. istio sidecar injection on/off directly resulted to this problem’s appear/disappear
  2. all hosts behaved the same

Based on this information, we made a minimum reproducing scheme: creating (or scaling up) a simple service, and utilizing node affinity properties to schedule the pods to specific node, so we could do capturing works at the node beforehand:

$ kubectl create -f nginx-sts.yaml

# or scale up if it already exists
$ kubectl scale sts web --replicas=2

See Appendix A for nginx-sts.yaml.

3 Trouble Shooting

As has been said, we managed to schedule the test pod to a specific node. During testing, we chose an empty node, this greatly reduced the traffic we needed to analyze.

3.1 Check ingress/egress traffic interrupt

Fig. 3.1 Ping check

First, we’d like to check whether both the egress/ingress traffic were interrupted during this period, or only one direction, so we did this:

  1. selected another node node2, where node1 and node2 could reach each other via ICMP (ping)
  2. kept pinging node2 from node1
  3. kept pinging node1 from node2

Then

  1. capture all node1<-->node2 ICMP traffic on physical NICs of node1
  2. kubectl create -f nginx-sts.yaml, schedule pod to node1

We found that the egress packets (node1->node2) was not interrupted during sidecar injection, while the ingress traffic (node2->node1) disappeared from physical NICs during that period.

This indicated that the ingress traffic to this host was either intercepted by some stuff - or dropped somewhere - before tcpdump capturing point.

3.2 Check iptables rules

Our first guess was that there might be some buggy iptables rules during sidecar injection. So we dumped all the iptables rules on the host (one dump each second) and saved to file, comparing the rules before injection, during injection and after injection, but nothing seemed abnormal.

3.3 Check drops

Quickly checked the physical NICs’s statistics, also not noticed any obvious dropping.

3.4 Capture all host traffic

With no other means, we had to capture all traffic went through the host. Thanks to that there were only our test pod on this host, the traffic was not large.

First capturing all traffic on all physical NICs:

$ tcpdump -i eth0 -w eth0.pcap
$ tcpdump -i eth1 -w eth1.pcap

Then creating service (set node affinity to this node in yaml):

$ kubectl create -f nginx-sts.yaml

Waiting for the problem’s appear and disappear (60s or so), then stopping the capturing. Analyzing the traffic:

$ tcpdump -n -e -r eth0.pcap
 15:38:52.862377 2e:ce:c1:bb:f4:d9 > 04:40:a9:dc:61:a4, ethertype IPv4 (0x0800), 10.6.1.196 > 10.6.1.194
 15:39:52.880618 04:40:a9:dc:61:a4 > 2e:ce:c1:bb:f4:d9, ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 ...
 15:38:54.874005 2e:ce:c1:bb:f4:d9 > 04:40:a9:dc:61:a4, ethertype IPv4 (0x0800), 10.6.1.196 > 10.6.1.194
 15:38:54.875537 04:40:a9:dc:61:a4 > 10:51:72:27:4b:4f, ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 ...
 15:39:22.088589 2e:ce:c1:bb:f4:d9 > 04:40:a9:dc:61:a4, ethertype ARP (0x0806), Request who-has 10.6.1.194 tell 10.6.1.196
 15:39:22.091433 04:40:a9:dc:61:a4 > 2e:ce:c1:bb:f4:d9, ethertype ARP (0x0806), Reply 10.6.1.194 is-at 04:40:a9:dc:61:a4
 15:39:22.600618 04:40:a9:dc:61:a4 > 2e:ce:c1:bb:f4:d9, ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 ...

where -n specified numerical output, and -e printed MAC address of each packets.

Some MAC and IP info:

  • 2e:ce:c1:bb:f4:d9 and 10.6.1.196: MAC and IP (host IP) of mgnt device
  • 04:40:a9:dc:61:a4 and 10.6.1.194: BGP Peer’s MAC and IP

Replace the above, we get:

$ tcpdump -n -e -r eth0.pcap
 15:38:52.862377 MGNT_MAC > BGP_PEER_MAC,           ethertype IPv4 (0x0800), 10.6.1.196 > 10.6.1.194
 15:39:52.880618 BGP_PEER_MAC > MGNT_MAC,           ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 15:38:52.897481 10:51:72:27:4b:4f > Broadcast,     ethertype ARP (0x0806), Request who-has 169.254.169.254 tell 10.6.1.196
 ...                                                
 15:38:54.874005 MGNT_MAC > BGP_PEER_MAC,           ethertype IPv4 (0x0800), 10.6.1.196 > 10.6.1.194
 15:38:54.875537 BGP_PEER_MAC > 10:51:72:27:4b:4f,  ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 15:38:54.xxxxxx MGNT_MAC > OTHER_HOST_MAC,         ethertype IPv4 (0x0800), 10.6.1.196 > 10.6.1.194
 15:38:54.xxxxxx OTHER_HOST_MAC > 10:51:72:27:4b:4f,ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
   ...
 15:39:22.088589 MGNT_MAC > BGP_PEER_MAC,           ethertype ARP (0x0806), Request who-has 10.6.1.194 tell 10.6.1.196
 15:39:22.091433 BGP_PEER_MAC > MGNT_MAC,           ethertype ARP (0x0806), Reply 10.6.1.194 is-at <BGP PEER MAC>
 15:39:22.600618 BGP_PEER_MAC > MGNT_MAC,           ethertype IPv4 (0x0800), 10.6.1.194 > 10.6.1.196
 ...

We noticed this:

  1. before sidecar injection happened (15:38:52), all host’s egress traffic was sent with src_mac=MGNT_MAC,src_ip=HOST_IP, and all ingress traffic to host IP that received had dst_mac=MGNT_MAC,dst_ip=HOST_IP, this was correct!
  2. at the time the injection happened, the host sent out an ARP request, with src_ip=HOST_IP but src_mac=10:51:72:27:4b:4f, then in the next 30s, all egress traffic of this host still used src_mac=MGNT_MAC,src_ip=HOST_IP (correct), but all responded traffic (yes, they were responded) had dst_mac=10:51:72:27:4b:4f,dst_ip=HOST_IP, due to the MAC mismatch, all those ingress traffic not arrived mgnt device (which was why our SSH connections get disconnected)
  3. 30s later, the host sent out another ARP request with correct MAC and IP: src_mac=MGNT_MAC,src_ip=HOST_IP, subsequently, the ingress traffic after this took the correct dst_mac=MGNT_MAC, and host network restored

3.5 Further Findings

So the direct reason is that the host sent a ARP request with a wrong MAC, which flushed the forwarding table of the switches in the physical network, so all subsequent ingress to HOST_IP was chose the wrong MAC by TOR, and those packets were not forwarded to mgnt.

But where did this wrong MAC 10:51:72:27:4b:4f come from? Digging further on host:

root@node:~  # ip l | grep -B 1 10:51:72:27:4b:4f
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq master ovs-system state UP mode DEFAULT qlen 1000
    link/ether 10:51:72:29:1b:50 brd ff:ff:ff:ff:ff:ff
--
7: br-bond: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether 10:51:72:29:1b:50 brd ff:ff:ff:ff:ff:ff

And,

root@node1:~  # ovs-vsctl show
    Bridge br-bond
        Port br-bond
            Interface br-bond
                type: internal
        Port "mgnt"
            Interface "mgnt"
                type: internal
        Port "bond1"
            Interface "eth1"
            Interface "eth0"
    ovs_version: "2.5.6"

root@node:~  # ovs-appctl bond/show
bond_mode: balance-slb
...
active slave mac: 10:51:72:29:1b:50(eth1)

slave eth0: enabled
        may_enable: true

slave eth1: enabled
        active slave
        may_enable: true

It can be seen that this MAC belonged to physical NIC eth1, and it was the active slave for the OVS bond during this period.

But why would the HOST used this MAC for boradcasting? Further digging:

root@node:~  # route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
...
169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth1
169.254.0.0     0.0.0.0         255.255.0.0     U     1003   0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1007   0        0 mgnt

Notice it? In the pcap file we saw the broadcasting packet with a dst_ip=169.254.169.254, when the host networking stack chose MAC for this packet, there would be three entries matching:

169.254.0.0     0.0.0.0         255.255.0.0     U     1002   0        0 eth1
169.254.0.0     0.0.0.0         255.255.0.0     U     1003   0        0 eth0
169.254.0.0     0.0.0.0         255.255.0.0     U     1007   0        0 mgnt

In this situation, the Metric (priority flag) will be checked. Since the entry via eth1 with the highest priority, that route entry would be chosen, and the src_mac thus would be eth1’s MAC, which was just 10:51:72:29:1b:50.

But why there were those rules? It turns out that 169.254.0.0/16 is what called link local IP for IPv4. When a NIC is up (e.g. ifconfig eth0 up) but no IP is assgined to it, the device will try to automatically get an IP via DHCP. The DHCP just uses this IP range. See Appendix B for more details on this.

But in thoery, if a NIC serves as a slave device of a BOND, that NIC would never need an IP address, unless it stops to serve as bond slave device. So in my understanding, if a L2 software (e.g. OVS, Linux bond) makes a bond out of eth0 and eth1, it should remove those two rules, leave only the last rule (via mgnt) there. With this understanding/guessing, we checked our Linux Bond hosts, indeeded there were no such rules, just the last one there.

4 Direct Cause

So now it should be clear:

  1. when sidecar injection happened, some still unknown behavior triggers the host sent ARP requests to 169.254
  2. OVS bond left two stale route entries targeted to 169.254, which has higher priority than the correct rule (the one via mgnt)
  3. 1 & 2 resulted the ARP packet took an incorrect MAC, which polluted the switches in the physical network
  4. this further resulted to all subsequent ingress packets to host itself (with dst_ip=HOST_IP) took the incorrect MAC (dst_mac=eth0 or eth1), thus those traffic not reached mgnt, thus our SSH disconnected
  5. 30s later, host sent another ARP with the correct MAC (src_mac=BOND0), this flushed forwarding tables of switches in physical network
  6. all ingress traffic back to normal

5 Fixup

Manually reomve those two route entries:

$ route del -net 169.254.0.0 gw 0.0.0.0 netmask 255.255.0.0 dev eth0
$ route del -net 169.254.0.0 gw 0.0.0.0 netmask 255.255.0.0 dev eth1

Re-run our test, problem disappeared.

6 Root Cause (still missing)

At least two questions need to be further investigated:

  1. why sidecar injection would trigger the ARP request to 169.254 from host?
  2. why host takes the correct MAC 30s later, while the incorrect route entries are still there?

7 Closing Words

Following experiences are valued:

  1. shrink down the reproducing scheme to minimum as possible as you can
  2. choose a node with least traffic as possible as you can

We just need bonding solution, and Linux bond meets this needs. We may remove OVS in the future (currently in using only for historical reasons).

Appendix

A: Simple Service used for problem reproduction

This yaml will schedule the pod to node1. Make sure you have correct tolerations for the taints on node1.

nginx-sts.yaml:

apiVersion: v1
kind: Service
metadata:
  name: nginx
  labels:
    app: nginx
spec:
  ports:
  - port: 80
    name: web
  clusterIP: None
  selector:
    app: nginx
---
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  serviceName: "nginx"
  replicas: 1
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx
    spec:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: kubernetes.io/hostname
                operator: In
                values:
                - node1
      tolerations:
      - effect: NoSchedule
        key: smoke
        operator: Equal
        value: test
      containers:
      - name: nginx
        image: nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web

See Wikipedia: Link-local Address.

May update this part later.