What is the 10.42.0.1 address in my k3s cluster? [closed]

Question

I'm trying to figure out what the address 10.42.0.1 is resolving to ( or what it belongs to ). I have a very basic k3s cluster with all the defaults enabled ( so the default pod cidr is 10.42.0.0/24 I think ). None of my pods in the cluster are indicating they own that address. When I curl the address from a pod in side of my cluster, it seems to resolve to the traefik deployed load balancer, but I'm trying to figure out the mechanics behind this, mostly for curiosity.

For reference, this is what's currently in the cluster

    NAMESPACE      NAME                                        READY   STATUS      RESTARTS      AGE   IP           NODE         NOMINATED NODE   READINESS GATES
kube-system    helm-install-traefik-crd-rrt22              0/1     Completed   0             19h   <none>       k8s-master   <none>           <none>
kube-system    helm-install-traefik-lxtmr                  0/1     Completed   1             19h   <none>       k8s-master   <none>           <none>
kube-system    svclb-traefik-9fa171fb-26n2r                2/2     Running     2 (59m ago)   19h   10.42.0.11   k8s-master   <none>           <none>
kube-system    traefik-64f55bb67d-5b4nv                    1/1     Running     1 (59m ago)   19h   10.42.0.13   k8s-master   <none>           <none>
kube-system    coredns-77ccd57875-dg92g                    1/1     Running     1 (59m ago)   19h   10.42.0.10   k8s-master   <none>           <none>
kube-system    local-path-provisioner-957fdf8bc-vjrxt      1/1     Running     2 (58m ago)   19h   10.42.0.12   k8s-master   <none>           <none>
kube-system    metrics-server-648b5df564-btzmp             1/1     Running     2 (58m ago)   19h   10.42.0.9    k8s-master   <none>           <none>
kube-system    svclb-traefik-9fa171fb-gkktp                2/2     Running     2 (53m ago)   19h   10.42.1.3    k8s-agent    <none>           <none>
istio-system   istiod-5f859db56c-2nsbz                     1/1     Running     0             51m   10.42.1.4    k8s-agent    <none>           <none>
kube-system    svclb-istio-ingressgateway-8b3c00eb-zd5cw   0/3     Pending     0             51m   <none>       <none>       <none>           <none>
kube-system    svclb-istio-ingressgateway-8b3c00eb-cbznb   0/3     Pending     0             51m   <none>       <none>       <none>           <none>
istio-system   istio-ingressgateway-794b8468f5-pbd8z       1/1     Running     0             51m   10.42.0.14   k8s-master   <none>           <none>
default        client-59f4b74cd-sxct5                      2/2     Running     0             19m   10.42.1.6    k8s-agent    <none>           <none>

NAMESPACE      NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP         PORT(S)                                      AGE   SELECTOR
default        kubernetes             ClusterIP      10.43.0.1       <none>              443/TCP                                      19h   <none>
kube-system    kube-dns               ClusterIP      10.43.0.10      <none>              53/UDP,53/TCP,9153/TCP                       19h   k8s-app=kube-dns
kube-system    metrics-server         ClusterIP      10.43.65.157    <none>              443/TCP                                      19h   k8s-app=metrics-server
kube-system    traefik                LoadBalancer   10.43.95.159    10.0.0.4,10.0.0.5   80:31462/TCP,443:31401/TCP                   19h   app.kubernetes.io/instance=traefik-kube-system,app.kubernetes.io/name=traefik
istio-system   istiod                 ClusterIP      10.43.169.1     <none>              15010/TCP,15012/TCP,443/TCP,15014/TCP        51m   app=istiod,istio=pilot
istio-system   istio-ingressgateway   LoadBalancer   10.43.221.192   <pending>           15021:31385/TCP,80:31036/TCP,443:30469/TCP   51m   app=istio-ingressgateway,istio=ingressgateway
default        httpd                  ClusterIP      10.43.254.191   <none>              80/TCP                                       38m   app=httpd

Answer 1

I recently began trying out k3s and wondered this as well.
Your laptop, the host, has a bridge network interface cni0 into the private network that the containers are running in. Your host's IP is 10.42.0.1 on that private network.

$ ip addr show dev cni0
188: cni0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc noqueue state UP group default qlen 1000
    link/ether 06:d3:9b:0c:ee:be brd ff:ff:ff:ff:ff:ff
    inet 10.42.0.1/24 brd 10.42.0.255 scope global cni0
       valid_lft forever preferred_lft forever
    inet6 fe80::4d3:9bff:fe0c:eebe/64 scope link
       valid_lft forever preferred_lft forever

Each of those containers have a namespace & routing table and their default gateway to any other IP is your host: 10.42.0.1

$ sudo ip -all netns exec route
...
netns: cni-bd52b5cc-b724-fd82-01c7-bcf0f0d97f49
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         10.42.0.1       0.0.0.0         UG    0      0        0 eth0
10.42.0.0       0.0.0.0         255.255.255.0   U     0      0        0 eth0
10.42.0.0       10.42.0.1       255.255.0.0     UG    0      0        0 eth0

So, you'll see that traffic will first go through your host, and then say, your home router, on the way to the internet.

$ kubectl exec -it dnsutils -- traceroute google.com.
traceroute to google.com (142.250.114.101), 30 hops max, 46 byte packets
 1  10.42.0.1 (10.42.0.1)  0.017 ms  0.014 ms  0.013 ms
 2  192.168.1.1 (192.168.1.1)  4.172 ms  1.658 ms  0.721 ms
 3  *  *  *
 4  *  *  *
 5  *  *  *
 6  *  *  *
 7  *  *  *
 8  syn-035-146-031-064.res.spectrum.com (35.146.31.64)  82.547 ms  94.083 ms  417.137 ms
 9  lag-63.rcr01hstqtx02.netops.charter.com (24.164.209.122)  21.506 ms  523.018 ms  30.659 ms
10  syn-131-150-063-015.res.spectrum.com (131.150.63.15)  42.605 ms  syn-024-093-037-083.inf.spectrum.com (24.93.37.83)  40.012 ms  syn-131-150-063-015.res.spectrum.com (131.150.63.15)  39.808 ms
11  *  *  *
12  142.251.237.112 (142.251.237.112)  38.418 ms  72.14.237.46 (72.14.237.46)  45.226 ms  142.251.76.36 (142.251.76.36)  43.672 ms
13  108.170.233.119 (108.170.233.119)  28.496 ms  142.250.60.238 (142.250.60.238)  38.363 ms  108.170.228.91 (108.170.228.91)  28.948 ms
14  108.170.228.82 (108.170.228.82)  39.146 ms  108.170.233.119 (108.170.233.119)  80.752 ms  108.170.233.117 (108.170.233.117)  30.590 ms
15  142.251.76.47 (142.251.76.47)  39.273 ms  108.170.229.87 (108.170.229.87)  32.167 ms  142.250.233.171 (142.250.233.171)  34.306 ms
16  142.250.224.11 (142.250.224.11)  36.546 ms  216.239.43.144 (216.239.43.144)  32.527 ms  209.85.252.210 (209.85.252.210)  28.142 ms
17  142.250.224.27 (142.250.224.27)  40.756 ms  142.250.224.25 (142.250.224.25)  33.482 ms  142.250.224.13 (142.250.224.13)  32.428 ms
18  *  *  *
19  *  *  *
20  *  *  *
21  *  *  *
22  *  *  *
23  *  *  *
24  *  *  *
25  *  *  *
26  *  *  *
27  rr-in-f101.1e100.net (142.250.114.101)  30.500 ms  *  36.067 ms

Services and pods have IP addresses.

$ kubectl get services -A
NAMESPACE     NAME                  TYPE        CLUSTER-IP      EXTERNAL-IP   PORT(S)                      AGE
default       kubernetes            ClusterIP   10.43.0.1       <none>        443/TCP                      2d23h
kube-system   kube-dns              ClusterIP   10.43.0.10      <none>        53/UDP,53/TCP,9153/TCP       2d23h
kube-system   metrics-server        ClusterIP   10.43.3.140     <none>        443/TCP                      2d23h

$ kubectl get pods -A -o=custom-columns=NAME:metadata.name,STATUS:status.phase,IP:status.podIP
NAME                                      STATUS    IP
dnsutils                                  Running   10.42.0.16
coredns-ccb96694c-7vgbs                   Running   10.42.0.3
local-path-provisioner-5cf85fd84d-g5q46   Running   10.42.0.5
metrics-server-5985cbc9d7-9mkb6           Running   10.42.0.6

I found this YouTube video on this topic informative.