Newer versions of docker have --cap-add, what CAP's can be added?

Question

Newer versions of docker (I think 1.2 and later) have a --cap-add feature.

This gives fine grained control of feature capabilities without opening up everything with --privileged=true.

I've googled it, but can't find a list of capabilities and what they mean. Can anyone help?

Answer 1

Funny enough... I googled this for an hour and couldn't find an answer. I ask the question here and found it in 2 secs.

http://man7.org/linux/man-pages/man7/capabilities.7.html

When passing these to docker, you need to drop the CAP_ in the name.

i.e. to allow mounting inside a docker container

docker run --cap-add SYS_ADMIN ...

Answer 2

The following might be useful: https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities

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The following is extracted from the above page (3 Feb 2022):

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By default, Docker has a default list of capabilities that are kept. The following table lists the Linux capability options which are allowed by default and can be dropped.

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

The next table shows the capabilities which are not granted by default and may be added.

Capability Key	Capability Description
AUDIT_CONTROL	Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.
AUDIT_READ	Allow reading the audit log via multicast netlink socket.
BLOCK_SUSPEND	Allow preventing system suspends.
BPF	Allow creating BPF maps, loading BPF Type Format (BTF) data, retrieve JITed code of BPF programs, and more.
CHECKPOINT_RESTORE	Allow checkpoint/restore related operations. Introduced in kernel 5.9.
DAC_READ_SEARCH	Bypass file read permission checks and directory read and execute permission checks.
IPC_LOCK	Lock memory (mlock(2), mlockall(2), mmap(2), shmctl(2)).
IPC_OWNER	Bypass permission checks for operations on System V IPC objects.
LEASE	Establish leases on arbitrary files (see fcntl(2)).
LINUX_IMMUTABLE	Set the FS_APPEND_FL and FS_IMMUTABLE_FL i-node flags.
MAC_ADMIN	Allow MAC configuration or state changes. Implemented for the Smack LSM.
MAC_OVERRIDE	Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM).
NET_ADMIN	Perform various network-related operations.
NET_BROADCAST	Make socket broadcasts, and listen to multicasts.
PERFMON	Allow system performance and observability privileged operations using perf_events, i915_perf and other kernel subsystems
SYS_ADMIN	Perform a range of system administration operations.
SYS_BOOT	Use reboot(2) and kexec_load(2), reboot and load a new kernel for later execution.
SYS_MODULE	Load and unload kernel modules.
SYS_NICE	Raise process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes.
SYS_PACCT	Use acct(2), switch process accounting on or off.
SYS_PTRACE	Trace arbitrary processes using ptrace(2).
SYS_RAWIO	Perform I/O port operations (iopl(2) and ioperm(2)).
SYS_RESOURCE	Override resource Limits.
SYS_TIME	Set system clock (settimeofday(2), stime(2), adjtimex(2)); set real-time (hardware) clock.
SYS_TTY_CONFIG	Use vhangup(2); employ various privileged ioctl(2) operations on virtual terminals.
SYSLOG	Perform privileged syslog(2) operations.
WAKE_ALARM	Trigger something that will wake up the system.