ArthurChiao's Blog

The Mysterious Container net.core.somaxconn (2022)

Published at 2022-08-06 | Last Update 2022-08-28


Try to answer several quick questions with a long post:

  1. On creating a pod in Kubernetes, if somaxconn is not specified, what the default value will be, and who will set it?
  2. If changing node’s sysctl settings, will they be propagated to pods?
  3. Are all sysctl parameters are equal in terms of initialization and propagation?

1 Background

1.1 Journey of components when creating a pod

In our environment, when creating a pod in Kubernetes (e.g. with k apply -f pod.yaml), the request will go through the following components in sequence, kube-apiserver -> kubelet -> dockerd -> containerd (-> containerd-shim) -> runc, as shown below:

Fig. Journey of components when creating a pod in Kubernetes

A few words about each component:

  1. kube-apiserver: accept and validate the pod creation request;

  2. kubelet: watch kube-apiserver, on receiving a new Pod spec, call dockerd to create containers for the Pod (and later call CNI plugins to init pod network);

  3. dockerd: a user-facing wrapper over containerd, which just delegate most jobs to containerd;

    Actually, docker can be removed from the stack, which results in a simpler chain: kube-apiserver -> kubelet -> containerd -> runc. But for historical reasons, we still have it in place now (may remove it later though).

  4. containerd: a less user-facing container manager;

    • containerd: a daemon that manages the complete container lifecycle on the host: creates, starts, stops containers, pulls and stores images, configures mounts, networking, etc. [1]
    • ctr: the command line tool ships with containerd, which has similar commands and options as docker does, we’ll see it later.
    • containerd-shim: a child process of containerd that serves a single container and takes care of the container lifecycle and exposes its functions to containerd through containerd-shim API. [2]
  5. runc: the eventual container runtime in using, which creates containers by Linux namespaces;

  6. cni-plugin: this is a binary, by default located at /opt/cni/bin/, and will be called by kubelet after pause container is created, which is responsible for allocating IP addresses, configuring network interface, … for the pod. Refer to [5] if you would like to see what Cilium CNI does.

1.2 sysctls in container and unsafe sysctls in k8s

When creating a container, each sysctl parameter of the container has its default value; to customize those settings on container creation, you can pass them via command line arguments or API parameters. Such as, with docker CLI,

$ docker run --sysctl net.ipv4.ip_forward=1 <image>

In Kubernetes, some of these parameters are named "unsafe sysctls" parameters, and must be explicitly enabled in kubelet if you’d like to customize them on Pod creation: adding --allowed-unsafe-sysctls=<list> option to kubelet, such as --allowed-unsafe-sysctls=net.core.somaxconn; then, you can specify the desired value in pod spec’s securityContext section, an example:

apiVersion: v1
kind: Pod
  name: smoke-pod-with-k8s
    app: smoke-pod-with-k8s
    sysctls:                    # <--- Put your custom sysctls in this section
    - name: net.core.somaxconn
      value: "1024"
  - name: nginx
    image: nginx-slim:0.8
    - containerPort: 80
      name: nginx

The securityContext claim will go through the pod security validation procedure in kube-apiserver; if validates, the request will be accepted.

1.3 The net.core.somaxconn parameter of pods/containers

As shown in the above spec, somaxconn is just one of those “unsafe sysctls” parameters in Kubernetes.

What is this parameter used for? According to kernel Documentation/networking/ip-sysctl.txt,

somaxconn - INTEGER
    Limit of socket listen() backlog, known in userspace as SOMAXCONN.
    Defaults to 4096. (Was 128 before linux-5.4)
    See also tcp_max_syn_backlog for additional tuning for TCP sockets.

For server side applications, if this parameter is too small, such as 128 traditionally, new TCP connecting requests from clients may be rejected on encountering ingress requests bursts.

Refer to Cloudflare’s wonderful post [3] for more information.

1.4 Problem statement

With all the above introductions, we can now describe the problem that we’re going to talk about: if you’re running a Kubernetes cluster, you may have noticed that when somaxconn is not specified in Pod specs, some pods will have a value of 128, while some others may have a value of 4096, and it seems these values are not inherited from the corresponding node’s settings, as a node may have a somaxconn setting of 128, 1024, 2048, 4096, etc (but pods on the same host have the same default value).

So our question is: if net.core.somaxconn is not specified in Pod spec,

  1. What the parameter will be initialized for the pod? And,
  2. Who (kubelet, dockerd, containerd, runc, …) will be in responsible for the initialization?

By looking at the source code from kubelet all the way down to runc, we didn’t find any place that initiated the parameter explicitly: all they do is just passing this parameter to the down stream component if it were provided, and do nothing if the parameter is missing - meaning that the parameter is opaque to them.

But to confirm this behavior, let’s get our hands dirty and do some testings along the journey.

2 Dig inside

What we’ve already seen is that some pods will get their somaxconn set as 128, and some as 4096; to see clearly if this initial setting has any relationship with the host’s one, we set somaxconn=2048 for the hosts, which is different from the both; hence, if there is somewhere that inherits this value from the host, we can immediately recognize it.

Changing the system setting of the node (we’ll schedule pods to this node later):

(node1) $ sysctl -w net.core.somaxconn=2048
net.core.somaxconn = 2048

Check the effective value:

(node1) $ sysctl net.core.somaxconn=2048
net.core.somaxconn = 2048

(node1) $ cat /proc/sys/net/core/somaxconn

Now we’re ready to start our journey.

2.1 kube-apiserver -> kubelet

As the very beginning of a pod creation action, Kubernetes validates the pod spec and passes it to kubelet to perform the real creation.

So the first thing we want to be sure is: will Kubernetes (kube-apiserver or kubelet) choose a default value for a Pod if the parameter is missing from the Pod’s spec?

Well, we can find the answer in two places:

  1. Look at the final Pod spec in Kubernetes: to determine if Kubernetes would insert a default setting when somaxconn is not specified;
  2. Look at the kubelet log: check if there is somaxconn related fields in the received Pod spec; kubelet will create pod according to this spec.

Now create a pod with spec (with no securityContext):

apiVersion: v1
kind: Pod
  name: smoke-pod-with-k8s
    app: smoke-pod-with-k8s
        - matchExpressions:
          - key:
            operator: In
            - node1
  - effect: NoSchedule
    operator: Exists
  - name: nginx
    image: nginx-slim:0.8
    - containerPort: 80
      name: nginx
(master) $ k apply -f smoke-with-k8s.yaml

(master) $ k get pod smoke-pod-with-k8s -o wide
smoke-pod-with-k8s   1/1     Running   0          14s   node1   <none>           <none>

Check if somaxconn will be automatically inserted if not provided in pod spec:

(master) $ k get pod smoke-pod-with-k8s -o yaml | grep -i somaxconn
# nothing here

No. Now check kubelet log on the node:

(node1) $ grep smoke-pod-with-k8s /var/log/kubernetes/kubelet.INFO
config.go          ] Receiving a new pod "smoke-pod-with-k8s_default(ca66a1aa)"
kubelet.go         ] SyncLoop (ADD, "api"): "smoke-pod-with-k8s_default(ca66a1aa)"
kubelet_pods.go    ] Generating status for "smoke-pod-with-k8s_default(ca66a1aa)"
volume_manager.go  ] Waiting for volumes to attach and mount for pod "smoke-pod-with-k8s_default(ca66a1aa)"
kubelet.go         ] SyncLoop (RECONCILE, "api"): "smoke-pod-with-k8s_default(ca66a1aa)"
status_manager.go  ] Patch status for pod "smoke-pod-with-k8s_default(ca66a1aa)" with 
    "status":{"$setElementOrder/conditions":[{"type":"Initialized"},{"type":"Ready"},{"type":"ContainersReady"},{"type":"PodScheduled"}],"conditions":[{"message":"containers with unready status: [nginx]","reas
on":"ContainersNotReady","status":"False","type":"Ready"},{"message":"containers with unready status: [nginx]","reason":"ContainersNotReady","status":"False","type":"ContainersReady"}],"containerStatuses":[{"image":"nginx-slim:0.8","imageID":"","name":"nginx","ready":false,"restartCount":0,"started":false,"state":{"waiting":{"reason":"ContainerCreating"}}}]}}'

Well, we get lots of detailed configurations about the pod/container, but still no somaxconn related stuffs.

Fig. Kubernetes control plane (e.g. kube-apiserver) is eliminated from our suspects list

Kubernetes control plane components eliminated. Will kubelet add this parameter during its internal handling? We can decide that by looking at the request it sends to dockerd.

2.2 kubelet -> dockerd

Will kubelet set a default value for somaxconn? Well, if it does, it must pass this parameter to dockerd, as the real container creation is done by the latter. So, we can check dockerd logs for this information.

(node1) $ grep smoke-pod-with-k8s /var/log/docker/dockerd.log
level=debug msg='Calling POST /containers/create?name=k8s_POD_smoke-pod-with-k8s_default_ca66a1aa'
level=debug msg='form data: {
level=debug msg='Calling POST /containers/create?name=k8s_nginx_smoke-pod-with-k8s_default_ca66a1aa'

There are two POST /containers/create calls,

  • the first one asks to create the pause container, which will hold the several namespaces for this Pod;
  • the second one asks to create the nginx container in the Pod spec.

No explicit somaxconn option in the POST body, otherwise we could see it in the HostConfig.Sysctls field:


// HostConfig the non-portable Config structure of a container.
// Here, "non-portable" means "dependent of the host we are running on".
// Portable information *should* appear in Config.
type HostConfig struct {
    Sysctls         map[string]string `json:",omitempty"` // List of Namespaced sysctls used for the container

As a comparison, this is how the HostConfig.Sysctls field looks like if the somaxconn is provided in Pod spec’s securityContext:

level=debug msg='form data: {

Fig. kubelet is eliminated from our suspects list

kubelet eliminated. Will dockerd add this parameter in its internal handling?

2.3 Skip k8s/kubelet/cni: create container right from docker

Now we’ve narrows down the scope to dockerd and all its downstream components. So, let’s forget all the stuffs around Kubernetes, and just start a clean container directly with docker and see what the somaxconn will be. But before this, we need to configure containerd to output more logs for us:

$ cat /etc/containerd/config.toml
  level = "debug"           # <--- set this

    shim = "containerd-shim"
    runtime = "runc"
    runtime_root = ""
    no_shim = false
    shim_debug = true       # <--- set this

# WARNING: restart containerd will cause all containers on this node to restart,
# DO NOT do this in your production nodes
$ systemctl restart containerd

Now create a container with docker:

$ sudo docker run -d --name smoke-ctn-with-docker nginx-slim:0.8 sleep 10d
$ sudo docker exec smoke-ctn-with-docker cat /proc/sys/net/core/somaxconn
4096  # <-- container value

$ cat /proc/sys/net/core/somaxconn
2048  # <-- host value

Still 4096, while the node has a value of 2048. Since this is a docker container but not a Kubernetes pod, CNI plugin is not involved, so CNI plugin (and the specific Kubernetes networking solution) is also eliminated from our suspects list.

Fig. kubelet and CNI plugin is eliminated from our suspects list

Now check the logs of containerd:

$ jourctl -u containerd -f
level=debug msg="event published" ns=moby topic=/containers/create
level=info  msg="shim containerd-shim started" address=/containerd-shim/977e0b1.sock debug=true pid=957711
level=debug msg="registering ttrpc server"
level=debug msg="serving api on unix socket" socket="[inherited from parent]"
level=debug msg="garbage collected" d="826.031µs"
level=debug msg="event published" ns=moby topic=/tasks/create
level=debug msg="event published" ns=moby topic=/tasks/start

Well, request body not captured, maybe there are other log settings that we need to adjust. But by looking into docker (moby) source code, we confirmed that it would only pass this sysctl parameter if it’s provided in HostConfig.Sysctls[] option, or provided via CLI --sysctl option. Neither did we do. This indicates that, the initial value is set by somewhere further behind.

Fig. docker is eliminated from our suspects list

Move on.

2.4 Skip docker: create container right from containerd

Again, let’s forget all the stuffs about Docker, and just start a container with containerd’s CLI and see what the somaxconn setting is.

But note that containerd is one layer lower than docker, so docker images/containers can not be manipulated by containerd directly. However, the good news is that they are much the same:

Find the right containerd.sock file

root@node1:~ $ ctr images ls # stuck then timed out
ctr: failed to dial "/run/containerd/containerd.sock": context deadline exceeded

root@node1:~ $ ls /run/containerd/containerd.sock
ls: cannot access /run/containerd/containerd.sock: No such file or directory

Well, seems we need to find the right sock, there are several ways:

# method 1: get from process parameters
root@node1:~ $ ps -ef | grep "\-shim"
root     121568   7930  0 Jul29 ?        00:00:05 containerd-shim -namespace moby \
        -workdir /var/lib/containerd/io.containerd.runtime.v1.linux/moby/4d0e1ac \
        -address /var/run/docker/libcontainerd/docker-containerd.sock \  # <-- this parameter
        -containerd-binary /usr/bin/containerd \
        -runtime-root /var/run/docker/runtime-runc

# method 2: get from config file
$ cat /etc/containerd/config.toml
  address = "/var/run/docker/libcontainerd/docker-containerd.sock"       # <-- this parameter

# method 3: get from config dump
$ containerd config dump | grep address | fgrep ".sock"
  address = "/var/run/docker/libcontainerd/docker-containerd.sock"       # <-- this parameter

Now try to use /var/run/docker/libcontainerd/docker-containerd.sock,

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock images ls
REF            TYPE                                                 DIGEST             SIZE     PLATFORMS   LABELS


Pull image

Pull the same image with containerd:

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock images pull nginx-slim:0.8

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock images ls
REF            TYPE                                                 DIGEST             SIZE     PLATFORMS   LABELS
nginx-slim:0.8 application/vnd.docker.distribution.manifest.v2+json sha256:dca6396b... 50.0 MiB linux/amd64 -

Create a container

Create a container (just run in the foreground here):

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock run --rm -t nginx-slim:0.8 tmp

In another window:

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock task ls
tmp     162866    RUNNING

Check the somaxconn setting

$ ctr -a /run/docker/libcontainerd/docker-containerd.sock task exec -t --exec-id bash_1 tmp bash
(inside container) $ cat /proc/sys/net/core/somaxconn

Still 4096. Does containerd set this parameter internally? We can check the parameters it sends to runc, and the answer will be: NO - we leave the contents to the next section.

2.5 Skip containerd: create container right from runc

To get rid of containerd, we can create a container with runc directly. Well, this is feasible of course, but not as straight forward as docker/containerd does, as it needs file bundle rather than existing images as input:

$ runc create -h

   runc create [command options] <container-id>

   The create command creates an instance of a container for a bundle.
   The bundle is a directory with a specification file named "config.json" and a root filesystem.

   --bundle value, -b value  path to the root of the bundle directory, defaults to the current directory
   --console-socket value    path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal
   --pid-file value          specify the file to write the process id to
   --no-pivot                do not use pivot root to jail process inside rootfs.  This should be used whenever the rootfs is on top of a ramdisk
   --no-new-keyring          do not create a new session keyring for the container.  This will cause the container to inherit the calling processes session key
   --preserve-fds value      Pass N additional file descriptors to the container (stdio + $LISTEN_FDS + N in total) (default: 0)

As the description said, runc reads a specification file named "config.json". The config.json for each container on the node:

(node1) $ find /run/containerd -name config.json

Peek into one:

(node1) $ ls /run/containerd/io.containerd.runtime.v1.linux/moby/eedd6341c/
address  config.json  log.json  rootfs

(node1) $ cat /run/containerd/io.containerd.runtime.v1.linux/moby/eedd6341c/config.json | jq
  "ociVersion": "1.0.1-dev",
  "process": {
    "user": {
      "uid": 0,
      "gid": 0
    "args": [
    "env": [
    "memory": {
      "limit": 2147483648,
      "swap": 2147483648,
      "disableOOMKiller": false
    "cpu": {
      "shares": 1024,
      "quota": 100000,
      "period": 100000,
      "cpus": "0-31"
    "blockIO": {
      "weight": 0
    "cgroupsPath": "/kubepods/besteffort/pod187acdb9/eedd6341c",
    "namespaces": [
        "type": "mount"
        "type": "network"
    "maskedPaths": [
    "readonlyPaths": [

Still no somaxconn settings in the config.json.

Notice that runc mounts /proc/sys into the container as readonly. However, proc files are not regular files, but read/write interfaces to the corresponding kernel parameters. So this only indicates that we can read /proc/sys/net/core/somaxconn inside container, not that the container inherits the current settings of the node.

But at this point, we still have gained something new: containerd didn’t set this for us, as config.json was created by containerd.

Fig. containerd is eliminated from our suspects list

Will runc initialize the parameter internally? The source code tells us that it won’t:

// libcontainer/standard_init_linux.go in runc

    for key, value := range l.config.Config.Sysctl {
        if err := writeSystemProperty(key, value); err != nil {
            return err

It just writes those sysctl parameters specified by config.json to the container, and doesn’t not add any new values, or initialize default values of some fields are not provided.

Fig. runc is eliminated from our suspects list

So, it seems we’ve come to an end: no component picks and sets the default value for a newly created container. What’s next to do? Well, runc will create namespaces based on kernel capabilities, among those is the network namespace (netns). Why not create a netns by hands and see what’s the value of somaxconn in that netns?

2.6 Skip runc: create container netns right from iproute2

root@node1:~ $ ip netns add smoke-netns
root@node1:~ $ ip netns ls

root@node1:~ $ ip netns exec smoke-netns sysctl net.core.somaxconn
net.core.somaxconn = 4096

Oh, still 4096.

But, this narrows things down quite a lot: maybe the parameter is specially handled/initialized by the kernel. With a quick search I found this question & answer at year 2014 [4]. Now let me explain it with kernel 5.10 code.

2.7 Skip iproute2: locate the kernel code of init new netns

In short, somaxconn is a special parameter that are registered per-netns:


static int __net_init net_defaults_init_net(struct net *net) {
    net->core.sysctl_somaxconn = SOMAXCONN; // <-- initialized with the kernel constant
    return 0;

static struct pernet_operations net_defaults_ops = {
    .init = net_defaults_init_net,

static __init int net_defaults_init(void) {
    if (register_pernet_subsys(&net_defaults_ops))
        panic("Cannot initialize net default settings");

    return 0;


When a new netns is created, .init = net_defaults_init_net will be called to initialize the netns, and the init function actually does exactly one thing as shown above, set somaxconn=SOMAXCONN for the netns, where SOMAXCONN is a kernel built time constant, for kernel 5.4+, it’s 4096:


/* Maximum queue length specifiable by listen.  */
#define SOMAXCONN   4096

And for kernel < 5.4, it’s set to 128. Suspect found!

3 Conclusion & discussion

3.1 Answers

  1. When creating a pod in Kubernetes, if somaxconn is not specified, what the default value will be and who sets it?

    The default value will be a kernel constant SOMAXCONN, initialized during container’s network namespace (netns) creation, regardless of the node setting (effective only in default netns). For kernel < 5.4, the constant is 128; for kernel 5.4+, it’s 4096.

  2. If changing node’s sysctl settings, will they be propagated to pods?

    It depends. Most values will, but as we’ve seen in this post, somaxconn won’t.

  3. Are all sysctl parameters are equal in terms of initialization and propagation?

    No. somaxconn is just such an exception.

The change was from Google:

net: increase SOMAXCONN to 4096

SOMAXCONN is /proc/sys/net/core/somaxconn default value.

It has been defined as 128 more than 20 years ago.

Since it caps the listen() backlog values, the very small value has
caused numerous problems over the years, and many people had
to raise it on their hosts after beeing hit by problems.

Google has been using 1024 for at least 15 years, and we increased
this to 4096 after TCP listener rework has been completed, more than
4 years ago. We got no complain of this change breaking any
legacy application.

Many applications indeed setup a TCP listener with listen(fd, -1);
meaning they let the system select the backlog.

Raising SOMAXCONN lowers chance of the port being unavailable under
even small SYNFLOOD attack, and reduces possibilities of side channel

Signed-off-by: Eric Dumazet <>
Cc: Willy Tarreau <>
Cc: Yue Cao <>
Signed-off-by: David S. Miller <>

Cilium bandwidth manager: setting somaxconn=4096 on agent start


baseSettings := []setting{
    {"net.core.netdev_max_backlog", "1000"},
    {"net.core.somaxconn", "4096"},
    {"net.core.default_qdisc", "fq"},
    {"net.ipv4.tcp_max_syn_backlog", "4096"},
    // Temporary disable setting bbr for now until we have a kernel fix for pacing out of Pods as described in #15324.
    // Then, kernels with the fix can use bbr, and others cubic.
    {"net.ipv4.tcp_congestion_control", "cubic"},

But note that bandwidth manager could only be enabled on kernel 5.10+.


  1. Why and How to Use containerd From Command Line,, 2022
  2. CVE-2020-15275: New Vulnerability Exploits containerd-shim API,, 2020
  3. SYN packet handling in the wild, Cloudflare, 2018
  4. Refresh net.core.somaxcomm (or any sysctl property) for docker containers, Stackoverflow, 2014
  5. Cilium Code Walk Through: CNI Create Network