在我的 Docker 容器中，为什么我在没有 NET_BIND_SERVICE 功能的情况下仍然可以绑定端口 1？

Question

我在用着Ubuntu 18.04 Desktop。以下是有关我的问题的更多详细信息。

最近，我正在编写一些想要执行此操作的测试代码：当它作为非特权用户运行时，测试代码尝试绑定特权端口（在我的例子中为端口 1）并期望绑定失败。

在我的主机上，我当前的非特权用户有以下capsh --print输出：

Current: =
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=1000(ywen)
gid=1000(ywen)
groups=4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),999(docker),1000(ywen)

因此，当我尝试使用当前非特权用户绑定端口 1 时，我可以得到预期的权限拒绝错误：

Current: =
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=1000(ywen)
gid=1000(ywen)
groups=4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare),999(docker),1000(ywen)

因为我的测试代码最终将在 Docker 容器内运行，所以我使用以下命令构建了一个映像Dockerfile：

ARG UBUNTU_VERSION=18.04
FROM ubuntu:${UBUNTU_VERSION}
ARG USER_NAME=ywen
ARG USER_ID=1000
ARG GROUP_ID=1000

RUN apt-get update

# Install the needed packages.
RUN DEBIAN_FRONTEND=noninteractive apt-get -y install \
    bash-completion \
    libcap2-bin \
    openssh-server \
    openssh-client \
    sudo \
    tree \
    vim

# Add a non-privileged user.
RUN groupadd -g ${GROUP_ID} ${USER_NAME} && \
    useradd -r --create-home -u ${USER_ID} -g ${USER_NAME} ${USER_NAME}

# Give the non-privileged user the privilege to run `sudo` without a password.
RUN echo "${USER_NAME} ALL=(ALL:ALL) NOPASSWD: ALL" > /etc/sudoers.d/${USER_NAME}

# Switch to the non-root user.
USER ${USER_NAME}

# The default command when the container is run.
CMD ["/bin/sleep", "infinity"]

通过运行以下docker build命令：

docker build -f ./Dockerfile.ubuntu --tag port-binding .

生成的图像称为port-binding:latest.

然后我运行它，首先使用此处列出的默认功能：

docker run --rm -it --name binding port-binding /bin/bash

然后我登录到容器并运行capsh --print。我有：

Current: = cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+i
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=1000(ywen)
gid=1000(ywen)
groups=

目前，我有这个cap_net_bind_service能力。因此，当我运行本文开头的测试代码时，端口绑定可以成功，并且没有收到任何错误：

Python 3.6.9 (default, Oct  8 2020, 12:12:24) 
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket as s
>>> o = s.socket(s.AF_INET)
>>> o.bind(("127.0.0.1", 1))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [Errno 13] Permission denied
>>> exit()

我认为成功是意料之中的，因为容器有这个cap_net_bind_service能力。所以我停止了容器并启动了一个新的容器，该容器删除了cap_net_bind_service：

docker run --rm -it --cap-drop=NET_BIND_SERVICE --name binding port-binding /bin/bash

在新容器内，capsh --print没有显示cap_net_bind_service：

Current: = cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap+i
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
Securebits: 00/0x0/1'b0
 secure-noroot: no (unlocked)
 secure-no-suid-fixup: no (unlocked)
 secure-keep-caps: no (unlocked)
uid=1000(ywen)
gid=1000(ywen)
groups=

但是当我运行测试代码时，我发现我仍然可以成功绑定端口1：

ARG UBUNTU_VERSION=18.04
FROM ubuntu:${UBUNTU_VERSION}
ARG USER_NAME=ywen
ARG USER_ID=1000
ARG GROUP_ID=1000

RUN apt-get update

# Install the needed packages.
RUN DEBIAN_FRONTEND=noninteractive apt-get -y install \
    bash-completion \
    libcap2-bin \
    openssh-server \
    openssh-client \
    sudo \
    tree \
    vim

# Add a non-privileged user.
RUN groupadd -g ${GROUP_ID} ${USER_NAME} && \
    useradd -r --create-home -u ${USER_ID} -g ${USER_NAME} ${USER_NAME}

# Give the non-privileged user the privilege to run `sudo` without a password.
RUN echo "${USER_NAME} ALL=(ALL:ALL) NOPASSWD: ALL" > /etc/sudoers.d/${USER_NAME}

# Switch to the non-root user.
USER ${USER_NAME}

# The default command when the container is run.
CMD ["/bin/sleep", "infinity"]

然而，通过阅读以下帖子，我认为删除NET_BIND_SERVICE应该是正确的做法。显然，我在某个地方犯了错误。有人可以告诉我我做错了什么吗？

• 能力(7)
• https://superuser.com/a/892391/224429
• https://serverfault.com/a/112798/125167

Answer 1

我遇到了相反的问题 - 想绑定到端口 80 但不能。两天的调试导致了这个： https: //github.com/moby/moby/pull/41030 - 自 docker 20.03.0 起，容器的默认 sysctl net.ipv4.ip_unprivileged_port_start 设置为 0，与 cap_net_bind_service 具有相同的效果- 容器内的所有进程现在可以绑定到（容器的）任何端口，即使作为非特权用户也是如此。可以通过docker run --sysctl net.ipv4.ip_unprivileged_port_start=0 ...docker-compose.yml 设置外部设置

  sysctls:
    - net.ipv4.ip_unprivileged_port_start=0

将其设置为 1024 以获得与 docker 20.03.0 之前版本相同的行为