Kubernetes对Container Capabilities的支持

相信玩过docker的同学，都对Docker Container Capability有过了解。我曾经在项目中因为修改网卡配置权限问题，需要配置容器的Capabilities(NET_ADMIN)，当时做过一些研究。今天突然有个同事问我，在Kubernetes中怎么配置容器的Capabilities呢？本文就全面介绍一下这方面的内容。

Docker Container Capabilities

在docker run命令中，我们可以通过--cap-add和--cap-drop来给容器添加linux Capabilities。下面表格中的列出的Capabilities是docker默认给容器添加的，用户可以通过--cap-drop去除其中一个或者多个。

Docker’s capabilities	Linux capabilities	Capability Description
SETPCAP	CAP_SETPCAP	Modify process capabilities.
MKNOD	CAP_MKNOD	Create special files using mknod(2).
AUDIT_WRITE	CAP_AUDIT_WRITE	Write records to kernel auditing log.
CHOWN	CAP_CHOWN	Make arbitrary changes to file UIDs and GIDs (see chown(2)).
NET_RAW	CAP_NET_RAW	Use RAW and PACKET sockets.
DAC_OVERRIDE	CAP_DAC_OVERRIDE	Bypass file read, write, and execute permission checks.
FOWNER	CAP_FOWNER	Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file.
FSETID	CAP_FSETID	Don’t clear set-user-ID and set-group-ID permission bits when a file is modified.
KILL	CAP_KILL	Bypass permission checks for sending signals.
SETGID	CAP_SETGID	Make arbitrary manipulations of process GIDs and supplementary GID list.
SETUID	CAP_SETUID	Make arbitrary manipulations of process UIDs.
NET_BIND_SERVICE	CAP_NET_BIND_SERVICE	Bind a socket to internet domain privileged ports (port numbers less than 1024).
SYS_CHROOT	CAP_SYS_CHROOT	Use chroot(2), change root directory.
SETFCAP	CAP_SETFCAP	Set file capabilities.

下面表格中列出的Capabilities是docker默认删除的Capabilities，用户可以通过--cap-add添加其中一个或者多个。

Docker’s capabilities	Linux capabilities	Capability Description
SYS_MODULE	CAP_SYS_MODULE	Load and unload kernel modules.
SYS_RAWIO	CAP_SYS_RAWIO	Perform I/O port operations (iopl(2) and ioperm(2)).
SYS_PACCT	CAP_SYS_PACCT	Use acct(2), switch process accounting on or off.
SYS_ADMIN	CAP_SYS_ADMIN	Perform a range of system administration operations.
SYS_NICE	CAP_SYS_NICE	Raise process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes.
SYS_RESOURCE	CAP_SYS_RESOURCE	Override resource Limits.
SYS_TIME	CAP_SYS_TIME	Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock.
SYS_TTY_CONFIG	CAP_SYS_TTY_CONFIG	Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.
AUDIT_CONTROL	CAP_AUDIT_CONTROL	Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.
MAC_OVERRIDE	CAP_MAC_OVERRIDE	Allow MAC configuration or state changes. Implemented for the Smack LSM.
MAC_ADMIN	CAP_MAC_ADMIN	Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM).
NET_ADMIN	CAP_NET_ADMIN	Perform various network-related operations.
SYSLOG	CAP_SYSLOG	Perform privileged syslog(2) operations.
DAC_READ_SEARCH	CAP_DAC_READ_SEARCH	Bypass file read permission checks and directory read and execute permission checks.
LINUX_IMMUTABLE	CAP_LINUX_IMMUTABLE	Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags.
NET_BROADCAST	CAP_NET_BROADCAST	Make socket broadcasts, and listen to multicasts.
IPC_LOCK	CAP_IPC_LOCK	Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
IPC_OWNER	CAP_IPC_OWNER	Bypass permission checks for operations on System V IPC objects.
SYS_PTRACE	CAP_SYS_PTRACE	Trace arbitrary processes using ptrace(2).
SYS_BOOT	CAP_SYS_BOOT	Use reboot(2) and kexec_load(2), reboot and load a new kernel for later execution.
LEASE	CAP_LEASE	Establish leases on arbitrary files (see fcntl(2)).
WAKE_ALARM	CAP_WAKE_ALARM	Trigger something that will wake up the system.
BLOCK_SUSPEND	CAP_BLOCK_SUSPEND	Employ features that can block system suspend.

比如，我们可以通过给给容器add NET_ADMIN Capability，使得我们可以对network interface进行modify，对应的docker run命令如下：

$ docker run -it --rm --cap-add=NET_ADMIN ubuntu:14.04 ip link add dummy0 type dummy

Kubernetes SecurityContext

在Kubernetes对Pod的定义中，用户可以add/drop Capabilities在Pod.spec.containers.sercurityContext.capabilities中添加要add的Capabilities list和drop的Capabilities list。

比如，我要添加NET_ADMIN Capability，删除KILL Capability，则对应的Pod定义如下：

apiVersion: v1
kind: Pod
metadata:
  name: hello-world
spec:
  containers:
  - name: friendly-container
    image: "alpine:3.4"
    command: ["/bin/echo", "hello", "world"]
    securityContext:
      capabilities:
        add:
        - NET_ADMIN
        drop:
        - KILL

总结

Kubernetes通过在Pod.spec.containers.sercurityContext.capabilities中配置容器待add和drop的Capabilities，最终借助docker container Capabilities的能力，完成容器的Capabilities权限控制。