容器镜像详解

1. 镜像组成

一个标准的OCI容器镜像由index, manifest, config, image layers这几个部分组成。
以docker镜像为例，下载的镜像文件保存在/var/lib/docker/目录下面
在这里插入图片描述
image/overlay2子目录下面保存着镜像相关的一些元数据
在这里插入图片描述
在下面的介绍主要以nginx:latest镜像为例子

docker inspect nginx:latest
[
    {
        "Id": "sha256:605c77e624ddb75e6110f997c58876baa13f8754486b461117934b24a9dc3a85",
        "RepoTags": [
            "nginx:latest"
        ],
        "RepoDigests": [
            "nginx@sha256:0d17b565c37bcbd895e9d92315a05c1c3c9a29f762b011a10c54a66cd53c9b31"
        ],
        "Parent": "",
        "Comment": "",
        "Created": "2021-12-29T19:28:29.892199479Z",
        "Container": "ca3e48389f7160bc9d9a892d316fcbba459344ee3679998739b1c3cd8e56f7da",
        "ContainerConfig": {
            "Hostname": "ca3e48389f71",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "80/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "NGINX_VERSION=1.21.5",
                "NJS_VERSION=0.7.1",
                "PKG_RELEASE=1~bullseye"
            ],
            "Cmd": [
                "/bin/sh",
                "-c",
                "#(nop) ",
                "CMD [\"nginx\" \"-g\" \"daemon off;\"]"
            ],
            "Image": "sha256:82941edee2f4d17c55563bb926387c3ae39fa1a99777f088bc9d3db885192209",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": [
                "/docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": {
                "maintainer": "NGINX Docker Maintainers <docker-maint@nginx.com>"
            },
            "StopSignal": "SIGQUIT"
        },
        "DockerVersion": "20.10.7",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "ExposedPorts": {
                "80/tcp": {}
            },
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "NGINX_VERSION=1.21.5",
                "NJS_VERSION=0.7.1",
                "PKG_RELEASE=1~bullseye"
            ],
            "Cmd": [
                "nginx",
                "-g",
                "daemon off;"
            ],
            "Image": "sha256:82941edee2f4d17c55563bb926387c3ae39fa1a99777f088bc9d3db885192209",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": [
                "/docker-entrypoint.sh"
            ],
            "OnBuild": null,
            "Labels": {
                "maintainer": "NGINX Docker Maintainers <docker-maint@nginx.com>"
            },
            "StopSignal": "SIGQUIT"
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 141479488,
        "VirtualSize": 141479488,
        "GraphDriver": {
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/6f894a7b66f4d1428817a444693b30b9adf85a2ae9dbcaffcde1a3e69c5e4017/diff:/var/lib/docker/overlay2/c645f15c9c993a5132afa4ce456d27af1a8119c380411374f57d5c8844b7d8b8/diff:/var/lib/docker/overlay2/f3b266fa677b6c553d3b2145135a3e1aa814f35b05467fe1500ad78a1a9cb166/diff:/var/lib/docker/overlay2/602a87b98d429966c3da272b5e08b95aa8a78d8505175bfd0c43f03dfd18f290/diff:/var/lib/docker/overlay2/e859f64556e9fe63bb08299584dc568b5f5a13b9a6a995de399a7fa020359423/diff",
                "MergedDir": "/var/lib/docker/overlay2/ce4bb4283e6fe5550b799d88dbec1f117e6bd8a5f33a17c8818df6e4bc76ce17/merged",
                "UpperDir": "/var/lib/docker/overlay2/ce4bb4283e6fe5550b799d88dbec1f117e6bd8a5f33a17c8818df6e4bc76ce17/diff",
                "WorkDir": "/var/lib/docker/overlay2/ce4bb4283e6fe5550b799d88dbec1f117e6bd8a5f33a17c8818df6e4bc76ce17/work"
            },
            "Name": "overlay2"
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:2edcec3590a4ec7f40cf0743c15d78fb39d8326bc029073b41ef9727da6c851f",
                "sha256:e379e8aedd4d72bb4c529a4ca07a4e4d230b5a1d3f7a61bc80179e8f02421ad8",
                "sha256:b8d6e692a25e11b0d32c5c3dd544b71b1085ddc1fddad08e68cbd7fda7f70221",
                "sha256:f1db227348d0a5e0b99b15a096d930d1a69db7474a1847acbc31f05e4ef8df8c",
                "sha256:32ce5f6a5106cc637d09a98289782edf47c32cb082dc475dd47cbf19a4f866da",
                "sha256:d874fd2bc83bb3322b566df739681fbd2248c58d3369cb25908d68e7ed6040a6"
            ]
        },
        "Metadata": {
            "LastTagTime": "0001-01-01T00:00:00Z"
        }
    }
]

2. distribution 目录

diffid-by-digest 保存了digest(layerID)->diffID的映射关系。
v2metadata-by-diffid 保存了diffid -> (digest,repository)的映射关系。
digest(layerID) 就是 pull 镜像时的 hash ID，拉取的镜像层文件是压缩文件，压缩态。
diffID 是 docker inspect 查看到的 镜像层 hash ID，此时镜像层文件是解压缩的，解压缩态。
因此虽说这两种 ID 都表示 镜像层 hash ID，但一个是压缩的，一个是解压缩的，所以 hash 运算后不一致。
拿上面nginx:latest镜像为例，RootFS中最上面一层镜像的diffID为"sha256:2edcec3590a4ec7f40cf0743c15d78fb39d8326bc029073b41ef9727da6c851f",
在这里插入图片描述
可以看到diffID和Digest（layerID），a2abf6c4d29d43a4bf9fbb769f524d0fb36a2edab49819c1bf3e76f409f953ea，可以相互对应到，

3. imagedb 目录

保存镜像的元数据信息，两个子文件夹 content/ 和 metadata/
tree /var/lib/docker/image/overlay2/imagedb可以查看此目录的结构
在这里插入图片描述
打开content/sha256目录下以镜像ID命名的文件，我们可以看到镜像的元数据信息(config文件内容)，包括了镜像架构、操作系统 、默认配置、创建时间、历史信息和rootfs等
cat /var/lib/docker/image/overlay2/imagedb/content/sha256/605c77e624ddb75e6110f997c58876baa13f8754486b461117934b24a9dc3a85，上面的

nginx:latest的image id。
{
  "architecture": "amd64",
  "config": {
    "Hostname": "",
    "Domainname": "",
    "User": "",
    "AttachStdin": false,
    "AttachStdout": false,
    "AttachStderr": false,
    "ExposedPorts": {
      "80/tcp": {}
    },
    "Tty": false,
    "OpenStdin": false,
    "StdinOnce": false,
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "NGINX_VERSION=1.21.5",
      "NJS_VERSION=0.7.1",
      "PKG_RELEASE=1~bullseye"
    ],
    "Cmd": [
      "nginx",
      "-g",
      "daemon off;"
    ],
    "Image": "sha256:82941edee2f4d17c55563bb926387c3ae39fa1a99777f088bc9d3db885192209",
    "Volumes": null,
    "WorkingDir": "",
    "Entrypoint": [
      "/docker-entrypoint.sh"
    ],
    "OnBuild": null,
    "Labels": {
      "maintainer": "NGINX Docker Maintainers <docker-maint@nginx.com>"
    },
    "StopSignal": "SIGQUIT"
  },
  "container": "ca3e48389f7160bc9d9a892d316fcbba459344ee3679998739b1c3cd8e56f7da",
  "container_config": {
    "Hostname": "ca3e48389f71",
    "Domainname": "",
    "User": "",
    "AttachStdin": false,
    "AttachStdout": false,
    "AttachStderr": false,
    "ExposedPorts": {
      "80/tcp": {}
    },
    "Tty": false,
    "OpenStdin": false,
    "StdinOnce": false,
    "Env": [
      "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
      "NGINX_VERSION=1.21.5",
      "NJS_VERSION=0.7.1",
      "PKG_RELEASE=1~bullseye"
    ],
    "Cmd": [
      "/bin/sh",
      "-c",
      "#(nop) ",
      "CMD [\"nginx\" \"-g\" \"daemon off;\"]"
    ],
    "Image": "sha256:82941edee2f4d17c55563bb926387c3ae39fa1a99777f088bc9d3db885192209",
    "Volumes": null,
    "WorkingDir": "",
    "Entrypoint": [
      "/docker-entrypoint.sh"
    ],
    "OnBuild": null,
    "Labels": {
      "maintainer": "NGINX Docker Maintainers <docker-maint@nginx.com>"
    },
    "StopSignal": "SIGQUIT"
  },
  "created": "2021-12-29T19:28:29.892199479Z",
  "docker_version": "20.10.7",
  "history": [
    {
      "created": "2021-12-21T01:22:43.418913408Z",
      "created_by": "/bin/sh -c #(nop) ADD file:09675d11695f65c55efdc393ff0cd32f30194cd7d0fbef4631eebfed4414ac97 in / "
    },
    {
      "created": "2021-12-21T01:22:43.799429634Z",
      "created_by": "/bin/sh -c #(nop)  CMD [\"bash\"]",
      "empty_layer": true
    },
    {
      "created": "2021-12-21T03:00:06.15011478Z",
      "created_by": "/bin/sh -c #(nop)  LABEL maintainer=NGINX Docker Maintainers <docker-maint@nginx.com>",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:08.071260391Z",
      "created_by": "/bin/sh -c #(nop)  ENV NGINX_VERSION=1.21.5",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:08.248669935Z",
      "created_by": "/bin/sh -c #(nop)  ENV NJS_VERSION=0.7.1",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:08.472939138Z",
      "created_by": "/bin/sh -c #(nop)  ENV PKG_RELEASE=1~bullseye",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:27.976946316Z",
      "created_by": "/bin/sh -c set -x     && addgroup --system --gid 101 nginx     && adduser --system --disabled-login --ingroup nginx --no-create-home --home /nonexistent --gecos \"nginx user\" --shell /bin/false --uid 101 nginx     && apt-get update     && apt-get install --no-install-recommends --no-install-suggests -y gnupg1 ca-certificates     &&     NGINX_GPGKEY=573BFD6B3D8FBC641079A6ABABF5BD827BD9BF62;     found='';     for server in         hkp://keyserver.ubuntu.com:80         pgp.mit.edu     ; do         echo \"Fetching GPG key $NGINX_GPGKEY from $server\";         apt-key adv --keyserver \"$server\" --keyserver-options timeout=10 --recv-keys \"$NGINX_GPGKEY\" && found=yes && break;     done;     test -z \"$found\" && echo >&2 \"error: failed to fetch GPG key $NGINX_GPGKEY\" && exit 1;     apt-get remove --purge --auto-remove -y gnupg1 && rm -rf /var/lib/apt/lists/*     && dpkgArch=\"$(dpkg --print-architecture)\"     && nginxPackages=\"         nginx=${NGINX_VERSION}-${PKG_RELEASE}         nginx-module-xslt=${NGINX_VERSION}-${PKG_RELEASE}         nginx-module-geoip=${NGINX_VERSION}-${PKG_RELEASE}         nginx-module-image-filter=${NGINX_VERSION}-${PKG_RELEASE}         nginx-module-njs=${NGINX_VERSION}+${NJS_VERSION}-${PKG_RELEASE}     \"     && case \"$dpkgArch\" in         amd64|arm64)             echo \"deb https://nginx.org/packages/mainline/debian/ bullseye nginx\" >> /etc/apt/sources.list.d/nginx.list             && apt-get update             ;;         *)             echo \"deb-src https://nginx.org/packages/mainline/debian/ bullseye nginx\" >> /etc/apt/sources.list.d/nginx.list                         && tempDir=\"$(mktemp -d)\"             && chmod 777 \"$tempDir\"                         && savedAptMark=\"$(apt-mark showmanual)\"                         && apt-get update             && apt-get build-dep -y $nginxPackages             && (                 cd \"$tempDir\"                 && DEB_BUILD_OPTIONS=\"nocheck parallel=$(nproc)\"                     apt-get source --compile $nginxPackages             )                         && apt-mark showmanual | xargs apt-mark auto > /dev/null             && { [ -z \"$savedAptMark\" ] || apt-mark manual $savedAptMark; }                         && ls -lAFh \"$tempDir\"             && ( cd \"$tempDir\" && dpkg-scanpackages . > Packages )             && grep '^Package: ' \"$tempDir/Packages\"             && echo \"deb [ trusted=yes ] file://$tempDir ./\" > /etc/apt/sources.list.d/temp.list             && apt-get -o Acquire::GzipIndexes=false update             ;;     esac         && apt-get install --no-install-recommends --no-install-suggests -y                         $nginxPackages                         gettext-base                         curl     && apt-get remove --purge --auto-remove -y && rm -rf /var/lib/apt/lists/* /etc/apt/sources.list.d/nginx.list         && if [ -n \"$tempDir\" ]; then         apt-get purge -y --auto-remove         && rm -rf \"$tempDir\" /etc/apt/sources.list.d/temp.list;     fi     && ln -sf /dev/stdout /var/log/nginx/access.log     && ln -sf /dev/stderr /var/log/nginx/error.log     && mkdir /docker-entrypoint.d"
    },
    {
      "created": "2021-12-29T19:28:28.390440509Z",
      "created_by": "/bin/sh -c #(nop) COPY file:65504f71f5855ca017fb64d502ce873a31b2e0decd75297a8fb0a287f97acf92 in / "
    },
    {
      "created": "2021-12-29T19:28:28.640632824Z",
      "created_by": "/bin/sh -c #(nop) COPY file:0b866ff3fc1ef5b03c4e6c8c513ae014f691fb05d530257dfffd07035c1b75da in /docker-entrypoint.d "
    },
    {
      "created": "2021-12-29T19:28:28.864872406Z",
      "created_by": "/bin/sh -c #(nop) COPY file:0fd5fca330dcd6a7de297435e32af634f29f7132ed0550d342cad9fd20158258 in /docker-entrypoint.d "
    },
    {
      "created": "2021-12-29T19:28:29.113627588Z",
      "created_by": "/bin/sh -c #(nop) COPY file:09a214a3e07c919af2fb2d7c749ccbc446b8c10eb217366e5a65640ee9edcc25 in /docker-entrypoint.d "
    },
    {
      "created": "2021-12-29T19:28:29.296888541Z",
      "created_by": "/bin/sh -c #(nop)  ENTRYPOINT [\"/docker-entrypoint.sh\"]",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:29.498026198Z",
      "created_by": "/bin/sh -c #(nop)  EXPOSE 80",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:29.705311925Z",
      "created_by": "/bin/sh -c #(nop)  STOPSIGNAL SIGQUIT",
      "empty_layer": true
    },
    {
      "created": "2021-12-29T19:28:29.892199479Z",
      "created_by": "/bin/sh -c #(nop)  CMD [\"nginx\" \"-g\" \"daemon off;\"]",
      "empty_layer": true
    }
  ],
  "os": "linux",
  "rootfs": {
    "type": "layers",
    "diff_ids": [
      "sha256:2edcec3590a4ec7f40cf0743c15d78fb39d8326bc029073b41ef9727da6c851f",
      "sha256:e379e8aedd4d72bb4c529a4ca07a4e4d230b5a1d3f7a61bc80179e8f02421ad8",
      "sha256:b8d6e692a25e11b0d32c5c3dd544b71b1085ddc1fddad08e68cbd7fda7f70221",
      "sha256:f1db227348d0a5e0b99b15a096d930d1a69db7474a1847acbc31f05e4ef8df8c",
      "sha256:32ce5f6a5106cc637d09a98289782edf47c32cb082dc475dd47cbf19a4f866da",
      "sha256:d874fd2bc83bb3322b566df739681fbd2248c58d3369cb25908d68e7ed6040a6"
    ]
  }
}

image元数据中layer的diffID是以低层到高层的顺序记录的

4. layerdb 目录

保存镜像层的关联关系，layerdb/sha256下的目录名称是以layer的chainID来命名的，它的计算方式为：

if layer是最底层，没有任何父layer
	chainID = diffID
else 
	chainID(n)=sha256sum(chainID[n-1)] + " " + diffID[n])

举例，查看某一镜像层 chainID 目录下的内容：
cat /var/lib/docker/image/overlay2/layerdb/sha256/2edcec3590a4ec7f40cf0743c15d78fb39d8326bc029073b41ef9727da6c851f，这是最底层，所以chainID[0] == diffID[0]。
在这里插入图片描述
size：layer文件的大小
cache-id：存储驱动通过cache-id索引到layer的实际文件内容
diff：diffID
计算第二层
chainID[1] = sha256sum(diffID[0] + " " + diffID[1])

echo -n "sha256:2edcec3590a4ec7f40cf0743c15d78fb39d8326bc029073b41ef9727da6c851f sha256:e379e8aedd4d72bb4c529a4ca07a4e4d230b5a1d3f7a61bc80179e8f02421ad8" | sha256sum | awk '{print $1}'
输出：780238f18c540007376dd5e904f583896a69fe620876cabc06977a3af4ba4fb5
在这里插入图片描述
parent：父layer的chainID
其余的和上面一样，其余层数也是如此
通过chainID可以从/var/lib/docker/overlayer2找到实际的镜像层数据。

5. repositories.json 文件

这个需要先搞清楚 registry 是镜像仓库，而 repository 代表镜像组（比如不同版本的 nginx 镜像）
该文件中描述了宿主机上所有镜像的repository元数据，主要包括镜像名、tag和镜像ID
镜像ID是Docker采用SHA256算法
查看：cat /var/lib/docker/image/overlay2/repositories.json | python -mjson.tool

{
    ...,
    "nginx": {
        "nginx:latest": "sha256:605c77e624ddb75e6110f997c58876baa13f8754486b461117934b24a9dc3a85",
        "nginx@sha256:0d17b565c37bcbd895e9d92315a05c1c3c9a29f762b011a10c54a66cd53c9b31": "sha256:605c77e624ddb75e6110f997c58876baa13f8754486b461117934b24a9dc3a85"
    },
	...
}

6. 拉取镜像

6.1 获取镜像清单

GET https://2h3po24q.mirror.aliyuncs.com/v2/library/alpine/manifests/latest HTTP/1.1
Host: 2h3po24q.mirror.aliyuncs.com
User-Agent: docker/19.03.12 go/go1.13.10 git-commit/48a66213fe kernel/5.8.0-1.el7.elrepo.x86_64 os/linux arch/amd64 UpstreamClient(Docker-Client/19.03.12 \(linux\))
Accept: application/vnd.docker.distribution.manifest.v2+json
Accept: application/vnd.docker.distribution.manifest.list.v2+json
Accept: application/vnd.oci.image.index.v1+json
Accept: application/vnd.oci.image.manifest.v1+json
Accept: application/vnd.docker.distribution.manifest.v1+prettyjws
Accept: application/json
Accept-Encoding: gzip
Connection: close

HTTP/1.1 200 OK
Content-Length: 1638
Content-Type: application/vnd.docker.distribution.manifest.list.v2+json
Docker-Content-Digest: sha256:185518070891758909c9f839cf4ca393ee977ac378609f700f60a771a2dfe321
Docker-Distribution-Api-Version: registry/2.0
Etag: "sha256:185518070891758909c9f839cf4ca393ee977ac378609f700f60a771a2dfe321"
Date: Thu, 20 Aug 2020 06:14:33 GMT
Connection: close

{"manifests":[{"digest":"sha256:a15790640a6690aa1730c38cf0a440e2aa44aaca9b0e8931a9f2b0d7cc90fd65","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"amd64","os":"linux"},"size":528},{"digest":"sha256:71465c7d45a086a2181ce33bb47f7eaef5c233eace65704da0c5e5454a79cee5","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"arm","os":"linux","variant":"v6"},"size":528},{"digest":"sha256:c929c5ca1d3f793bfdd2c6d6d9210e2530f1184c0f488f514f1bb8080bb1e82b","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"arm","os":"linux","variant":"v7"},"size":528},{"digest":"sha256:3b3f647d2d99cac772ed64c4791e5d9b750dd5fe0b25db653ec4976f7b72837c","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"arm64","os":"linux","variant":"v8"},"size":528},{"digest":"sha256:90baa0922fe90624b05cb5766fa5da4e337921656c2f8e2b13bd3c052a0baac1","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"386","os":"linux"},"size":528},{"digest":"sha256:5d950b30f229f0c53dd7dd7ed6e0e33e89d927b16b8149cc68f59bbe99219cc1","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"ppc64le","os":"linux"},"size":528},{"digest":"sha256:a5426f084c755f4d6c1d1562a2d456aa574a24a61706f6806415627360c06ac0","mediaType":"application\/vnd.docker.distribution.manifest.v2+json","platform":{"architecture":"s390x","os":"linux"},"size":528}],"mediaType":"application\/vnd.docker.distribution.manifest.list.v2+json","schemaVersion":2}

选择合适的镜像

GET https://2h3po24q.mirror.aliyuncs.com/v2/library/alpine/manifests/sha256:a15790640a6690aa1730c38cf0a440e2aa44aaca9b0e8931a9f2b0d7cc90fd65 HTTP/1.1
Host: 2h3po24q.mirror.aliyuncs.com
User-Agent: docker/19.03.12 go/go1.13.10 git-commit/48a66213fe kernel/5.8.0-1.el7.elrepo.x86_64 os/linux arch/amd64 UpstreamClient(Docker-Client/19.03.12 \(linux\))
Accept: application/vnd.docker.distribution.manifest.list.v2+json
Accept: application/vnd.oci.image.index.v1+json
Accept: application/vnd.oci.image.manifest.v1+json
Accept: application/vnd.docker.distribution.manifest.v1+prettyjws
Accept: application/json
Accept: application/vnd.docker.distribution.manifest.v2+json
Accept-Encoding: gzip
Connection: close

HTTP/1.1 200 OK
Content-Length: 528
Content-Type: application/vnd.docker.distribution.manifest.v2+json
Docker-Content-Digest: sha256:a15790640a6690aa1730c38cf0a440e2aa44aaca9b0e8931a9f2b0d7cc90fd65
Docker-Distribution-Api-Version: registry/2.0
Etag: "sha256:a15790640a6690aa1730c38cf0a440e2aa44aaca9b0e8931a9f2b0d7cc90fd65"
Date: Thu, 20 Aug 2020 06:14:33 GMT
Connection: close

{
   "schemaVersion": 2,
   "mediaType": "application/vnd.docker.distribution.manifest.v2+json",
   "config": {
      "mediaType": "application/vnd.docker.container.image.v1+json",
      "size": 1509,
      "digest": "sha256:a24bb4013296f61e89ba57005a7b3e52274d8edd3ae2077d04395f806b63d83e"
   },
   "layers": [
      {
         "mediaType": "application/vnd.docker.image.rootfs.diff.tar.gzip",
         "size": 2797541,
         "digest": "sha256:df20fa9351a15782c64e6dddb2d4a6f50bf6d3688060a34c4014b0d9a752eb4c"
      }
   ]
}

按照digest（layerID）下载镜像层信息

GET https://2h3po24q.mirror.aliyuncs.com/v2/library/alpine/blobs/sha256:a24bb4013296f61e89ba57005a7b3e52274d8edd3ae2077d04395f806b63d83e HTTP/1.1
Host: 2h3po24q.mirror.aliyuncs.com
User-Agent: docker/19.03.12 go/go1.13.10 git-commit/48a66213fe kernel/5.8.0-1.el7.elrepo.x86_64 os/linux arch/amd64 UpstreamClient(Docker-Client/19.03.12 \(linux\))
Accept-Encoding: identity
Connection: close

HTTP/1.1 307 Temporary Redirect
Content-Type: application/octet-stream
Docker-Distribution-Api-Version: registry/2.0
Location: https://acs-cn-hangzhou-mirror.oss-cn-hangzhou.aliyuncs.com/docker/registry/v2/blobs/sha256/a2/a24bb4013296f61e89ba57005a7b3e52274d8edd3ae2077d04395f806b63d83e/data?Expires=1597905273&OSSAccessKeyId=LTAI4G6zf8H9RhDWxXfrrhkr&Signature=%2FNOGZkZMkQOgkhYcaRm%2B1e5JX7w%3D
Date: Thu, 20 Aug 2020 06:14:33 GMT
Content-Length: 0
Connection: close

下载实际镜像数据

GET https://acs-cn-hangzhou-mirror.oss-cn-hangzhou.aliyuncs.com/docker/registry/v2/blobs/sha256/a2/a24bb4013296f61e89ba57005a7b3e52274d8edd3ae2077d04395f806b63d83e/data?Expires=1597905273&OSSAccessKeyId=LTAI4G6zf8H9RhDWxXfrrhkr&Signature=%2FNOGZkZMkQOgkhYcaRm%2B1e5JX7w%3D HTTP/1.1
Host: acs-cn-hangzhou-mirror.oss-cn-hangzhou.aliyuncs.com
User-Agent: docker/19.03.12 go/go1.13.10 git-commit/48a66213fe kernel/5.8.0-1.el7.elrepo.x86_64 os/linux arch/amd64 UpstreamClient(Docker-Client/19.03.12 \(linux\))
Accept-Encoding: identity
Referer: https://2h3po24q.mirror.aliyuncs.com/v2/library/alpine/blobs/sha256:a24bb4013296f61e89ba57005a7b3e52274d8edd3ae2077d04395f806b63d83e
Connection: close

HTTP/1.1 200 OK
Server: AliyunOSS
Date: Thu, 20 Aug 2020 06:14:34 GMT
Content-Type: application/octet-stream
Content-Length: 1509
Connection: close
x-oss-request-id: 5F3E14CAC6CA7E31381A44B0
Accept-Ranges: bytes
ETag: "B7B8406CA68CC818CB35F053F93CAF07"
Last-Modified: Fri, 29 May 2020 21:41:36 GMT
x-oss-object-type: Normal
x-oss-hash-crc64ecma: 6766004739995481995
x-oss-storage-class: Standard
Content-MD5: t7hAbKaMyBjLNfBT+TyvBw==
x-oss-server-time: 2
{"architecture":"amd64","config":{"Hostname":"","Domainname":"","User":"","AttachStdin":false,"AttachStdout":false,"AttachStderr
一大堆数据

参考文献

https://blog.csdn.net/qq_24433609/article/details/120763486
https://lework.github.io/2020/08/17/docker-pull/

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