README.md

   1 # LXC
   2
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   5
   6 LXC is the well-known and heavily tested low-level Linux container runtime. It
   7 is in active development since 2008 and has proven itself in critical
   8 production environments world-wide. Some of its core contributors are the same
   9 people that helped to implement various well-known containerization features
  10 inside the Linux kernel.
  11
  12 ## System Containers
  13
  14 LXC's main focus is system containers. That is, containers which offer an
  15 environment as close as possible as the one you'd get from a VM but without the
  16 overhead that comes with running a separate kernel and simulating all the
  17 hardware.
  18
  19 This is achieved through a combination of kernel security features such as
  20 namespaces, mandatory access control and control groups.
  21
  22 ## Unprivileged Containers
  23
  24 Unprivileged containers are containers that are run without any privilege. This
  25 requires support for user namespaces in the kernel that the container is run
  26 on. LXC was the first runtime to support unprivileged containers after user
  27 namespaces were merged into the mainline kernel.
  28
  29 In essence, user namespaces isolate given sets of UIDs and GIDs. This is
  30 achieved by establishing a mapping between a range of UIDs and GIDs on the host
  31 to a different (unprivileged) range of UIDs and GIDs in the container. The
  32 kernel will translate this mapping in such a way that inside the container all
  33 UIDs and GIDs appear as you would expect from the host whereas on the host
  34 these UIDs and GIDs are in fact unprivileged. For example, a process running as
  35 UID and GID 0 inside the container might appear as UID and GID 100000 on the
  36 host. The implementation and working details can be gathered from the
  37 corresponding user namespace man page.
  38
  39 Since unprivileged containers are a security enhancement they naturally come
  40 with a few restrictions enforced by the kernel. In order to provide a fully
  41 functional unprivileged container LXC interacts with 3 pieces of setuid code:
  42
  43 - lxc-user-nic (setuid helper to create a veth pair and bridge it on the host)
  44 - newuidmap (from the shadow package, sets up a uid map)
  45 - newgidmap (from the shadow package, sets up a gid map)
  46
  47 Everything else is run as your own user or as a uid which your user owns.
  48
  49 In general, LXC's goal is to make use of every security feature available in
  50 the kernel. This means LXC's configuration management will allow experienced
  51 users to intricately tune LXC to their needs.
  52
  53 A more detailed introduction into LXC security can be found under the following link
  54
  55 - https://linuxcontainers.org/lxc/security/
  56
  57 ### Removing all Privilege
  58
  59 In principle LXC can be run without any of these tools provided the correct
  60 configuration is applied. However, the usefulness of such containers is usually
  61 quite restricted. Just to highlight the two most common problems:
  62
  63 1. Network: Without relying on a setuid helper to setup appropriate network
  64    devices for an unprivileged user (see LXC's `lxc-user-nic` binary) the only
  65    option is to share the network namespace with the host. Although this should
  66    be secure in principle, sharing the host's network namespace is still one
  67    step of isolation less and increases the attack vector.
  68    Furthermore, when host and container share the same network namespace the
  69    kernel will refuse any sysfs mounts. This usually means that the init binary
  70    inside of the container will not be able to boot up correctly.
  71
  72 2. User Namespaces: As outlined above, user namespaces are a big security
  73    enhancement. However, without relying on privileged helpers users who are
  74    unprivileged on the host are only permitted to map their own UID into
  75    a container. A standard POSIX system however, requires 65536 UIDs and GIDs
  76    to be available to guarantee full functionality.
  77
  78 ## Configuration
  79
  80 LXC is configured via a simple set of keys. For example,
  81
  82 - `lxc.rootfs`
  83 - `lxc.mount.entry`
  84
  85 LXC namespaces configuration keys by using single dots. This means complex
  86 configuration keys such as `lxc.network` expose various subkeys such as
  87 `lxc.network.type`, `lxc.network.link`, `lxc.network.ipv6`, and others for even
  88 more fine-grained configuration.
  89
  90 LXC is used as the default runtime for [LXD](https://github.com/lxc/lxd),
  91 a container hypervisor exposing a well-designed and stable REST-api on top of
  92 it.
  93
  94 ## Kernel Requirements
  95
  96 LXC runs on any kernel from 2.6.32 onwards. All it requires is a functional
  97 C compiler. LXC works on all architectures that provide the necessary kernel
  98 features. This includes (but isn't limited to):
  99
 100 - i686
 101 - x86_64
 102 - ppc, ppc64, ppc64le
 103 - s390x
 104 - armvl7, arm64
 105
 106 LXC also supports at least the following C standard libraries:
 107
 108 - glibc
 109 - musl
 110 - bionic (Android's libc)
 111
 112 ## Backwards Compatibility
 113
 114 LXC has always focused on strong backwards compatibility. In fact, the API
 115 hasn't been broken from release `1.0.0` onwards. Main LXC is currently at
 116 version `2.*.*`.
 117
 118 ## Reporting Security Issues
 119
 120 The LXC project has a good reputation in handling security issues quickly and
 121 efficiently. If you think you've found a potential security issue, please
 122 report it by e-mail to all of the following persons:
 123
 124 - serge.hallyn (at) ubuntu (dot) com
 125 - stgraber (at) ubuntu (dot) com
 126 - christian.brauner (at) ubuntu (dot) com
 127
 128 For further details please have a look at
 129
 130 - https://linuxcontainers.org/lxc/security/
 131
 132 ## Becoming Active in LXC development
 133
 134 We always welcome new contributors and are happy to provide guidance when
 135 necessary. LXC follows the kernel coding conventions. This means we only
 136 require that each commit includes a `Signed-off-by` line. The coding style we
 137 use is identical to the one used by the Linux kernel. You can find a detailed
 138 introduction at:
 139
 140 - https://www.kernel.org/doc/html/v4.10/process/coding-style.html
 141
 142 and should also take a look at the [CONTRIBUTING](CONTRIBUTING) file in this
 143 repo.
 144
 145 If you want to become more active it is usually also a good idea to show up in
 146 the LXC IRC channel `#lxc-dev` on `Freenode`. We try to do all development out
 147 in the open and discussion of new features or bugs is done either in
 148 appropriate Github issues or on IRC.
 149
 150 When thinking about making security critical contributions or substantial
 151 changes it is usually a good idea to ping the developers first and ask whether
 152 a PR would be accepted.
 153
 154 ## Semantic Versioning
 155
 156 LXC and its related projects strictly adhere to a [semantic
 157 versioning](http://semver.org/) scheme.
 158
 159 ## Downloading the current source code
 160
 161 Source for the latest released version can always be downloaded from
 162
 163 - https://linuxcontainers.org/downloads/
 164
 165 You can browse the up to the minute source code and change history online
 166
 167 - https://github.com/lxc/lxc
 168
 169 ## Building LXC
 170
 171 Without considering distribution specific details a simple
 172
 173     ./autogen.sh && ./configure && make && sudo make install
 174
 175 is usually sufficient.
 176
 177 In order to test current git master of LXC it is usually a good idea to compile with
 178
 179     ./autogen.sh && ./configure && make
 180
 181 in a convenient directory and set `LD_LIBRARY_PATH="${BUILD_DIR}"/lxc/src/lxc/.libs`.
 182
 183 ## Getting help
 184
 185 When you find you need help, the LXC projects provides you with several options.
 186
 187 ### Discuss Forum
 188
 189 We maintain an discuss forum at
 190
 191 - https://discuss.linuxcontainers.org/
 192
 193 where you can get support.
 194
 195 ### IRC
 196
 197 You can find support by joining `#lxcontainers` on `Freenode`.
 198
 199 ### Mailing Lists
 200
 201 You can check out one of the two LXC mailing list archives and register if
 202 interested:
 203
 204 - http://lists.linuxcontainers.org/listinfo/lxc-devel
 205 - http://lists.linuxcontainers.org/listinfo/lxc-users