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+# Docker Containers, Linux Features and Security
+
+OK. Let's take it from the top.<br/>
+
+We will be exploring in which ways we can make an application container, more specifically a docker container, more secure.<br/>
+We will not talk about firewalls and apparmor because they are tools that enhance security on the host in general and not specific to a docker application container. Be that as it may, it still means a secure host is always better than a non-secure host.<br/>
+We will focus on Linux containers since Freebsd containers are still experimental(see [here](https://wiki.freebsd.org/Docker) and [here](https://github.com/samuelkarp/runj)). Yes, windows containers exist.<br/>
+
+Before we begin, Linux docker containers are Linux. They are using most of the functionality that existed before application containers in the form of docker were a thing. Knowing Linux better means you know Linux Docker containers better. We will reference this fact a couple of time later on.<br/>
+
+## Base Image
+
+We start with the first building block of a new docker image, The base image. By far the most used base images are the Alpine docker base image, followed by Debian and Ubuntu docker base images.
+These distros have two major differences that we want to focus on:
+
+- C standard library implementation
+- the userspace utility implementation
+
+Debian and Ubuntu(we are not forgetting that Ubuntu itself is a Debian derivative) both use glibc, as in gnu's [libc](https://www.gnu.org/software/libc/) implementation. Alpine uses [musl-libc](https://www.musl-libc.org/) as its C standard library implementation.<br/>
+The major difference here which will come into play later on again is glibc has been around for much longer, so it has to keep backwards compatibility for a much longer period of time and for far more many things. Also the general attitude with the glibc team is that they have to support everything since if they don't then who will?<br/>
+Libmusl on the other hand, does not try to support everything under the sun, a relatively newer project comparatively, and, keep their codebase lean.<br/>
+As a result not all applications are supported by libmusl but a good number of them are.<br/>
+In simpler terms, libmusl has a far smaller attack surface compared to glic.<br/>
+
+On to our second point, which is the cli utilities' implementation. Debian and Ubuntu use gnu's [Coreutils](https://www.gnu.org/software/coreutils/) while Alpine uses [Busybox](https://busybox.net/).<br/>
+Here we have the same situation as before, The GNU coreutils are bigger, do more and have a larger attack surface. Busybox is smaller, does not support as many features as GNU Coreutils but do support enough of them to make them useful. Needless to say, busybox is small and lean hence it has a smaller attack surface.<br/>
+
+For some intuitive observation, you can look at the some popular images that come in both Debian and Alpine flavours on dockerhub. Take a look at the number of reported vulnerabilities for both bases. The theme we observe is simple. The bigger the attack surface the bigger the number of vulnerabilities.<br/>
+
+Alpine images are small, lean and functional, just like libmusl and busybox but there are still quite a few things on an alpine image that are extraneous. We can take them out and have a perfectly functioning application container.<br/>
+
+That's how we get [distroless](https://github.com/GoogleContainerTools/distroless).<br/>
+Distroless base images follow the same pattern as alpine base docker images, as in, less functionality while still keeping enough functionality to be able to do the job and minimizing the attack surface.
+Minimizing a base image like this means that the base images are very specialized so we have base images for golang, python, java and the like.<br/>
+
+## Dokcer Runtimes
+
+What is a docker runtime?
+
+- runc
+- nvidia
+- gvisor
+
+### gviros's runsc
+
+## Capabilites and Syscalls
+
+[man 7 capabilites](https://manpages.debian.org/bookworm/manpages/capabilities.7.en.html)
+[man 2 sycalls](https://manpages.debian.org/bookworm/manpages-dev/syscalls.2.en.html)
+
+### capabilities in the wild
+
+[man 8 setcap](https://manpages.debian.org/bookworm/libcap2-bin/setcap.8.en.html)
+
+### syscall Filtering in the wild
+
+[Bubblewrap](https://github.com/containers/bubblewrap)
+
+Let's see how we can
+
+```c
+#include <errno.h>
+#include <fcntl.h>
+#include <inttypes.h>
+#include <seccomp.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+// https://blog.mnus.de/2020/05/sandboxing-soldatserver-with-bubblewrap-and-seccomp/
+
+void log_all_syscalls(void) {
+  scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_LOG);
+  seccomp_arch_add(ctx, SCMP_ARCH_X86_64);
+  seccomp_export_bpf(ctx, 1);
+  seccomp_export_pfc(ctx, 2);
+  seccomp_release(ctx);
+}
+
+int log_current_seccomp(void) {
+  int rc = -1;
+  scmp_filter_ctx ctx;
+  int filter_fd;
+
+  ctx = seccomp_init(SCMP_ACT_KILL);
+  if (ctx == NULL)
+    goto out;
+
+  filter_fd = open("/tmp/seccomp_filter.bpf",
+                   O_CREAT | O_WRONLY | O_NOFOLLOW | O_TRUNC, S_IRWXU);
+  if (filter_fd == -1) {
+    rc = -errno;
+    goto out;
+  }
+
+  rc = seccomp_export_bpf(ctx, filter_fd);
+  if (rc < 0) {
+    close(filter_fd);
+    goto out;
+  }
+  close(filter_fd);
+
+  filter_fd = open("/tmp/seccomp_filter.pfc",
+                   O_CREAT | O_WRONLY | O_NOFOLLOW | O_TRUNC, S_IRWXU);
+  if (filter_fd == -1) {
+    rc = -errno;
+    goto out;
+  }
+
+  rc = seccomp_export_pfc(ctx, filter_fd);
+  if (rc < 0) {
+    close(filter_fd);
+    goto out;
+  }
+  close(filter_fd);
+
+out:
+  seccomp_release(ctx);
+  return -rc;
+}
+
+int main(int argc, char **argv) {
+  if (argc == 3) {
+    if (!strcmp("--filter", argv[1])) {
+      if (!strcmp("current", argv[2])) {
+        log_current_seccomp();
+      } else if (!strcmp("logging", argv[2])) {
+        log_all_syscalls();
+      } else {
+      }
+    }
+  } else {
+    printf("going with the default filter kind which is logging.\n");
+    log_all_syscalls();
+  }
+}
+```
+
+```bash
+gcc -lseccomp
+```
+
+### Namespaces in the Wild
+
+```sh
+#!/usr/bin/dash
+
+NS=home_ns
+IF=wlp0s20f3
+PHY=phy0
+
+sudo ip netns add ${NS} || true
+sudo iw phy ${PHY} set netns "$(sudo ip netns exec home_ns sh -c 'sleep 1 >&- & echo "$!"')"
+# sudo ip link set ${IF} netns ${NS}
+sudo ip netns exec ${NS} ip link set ${IF} up
+sudo ip netns exec ${NS} ip link set dev lo up
+sudo ip netns exec ${NS} dhclient ${IF}
+
+ip netns exec ${NS} ping -4 9.9.9.9
+ip netns exec ${NS} ping -4 google.com
+ip netns exec ${NS} curl -4 icanhazip.com
+```
+
+```sh
+sudo ip netns exec home_ns curl -4 icanhaveip.com
+```
+
+### Docker syscall filtering
+
+### BPF
+
+## SBOM and Provenance Attestation
+
+### Conclusion
+
+```Dockerfile
+FROM alpine:3.19 as builder
+RUN apk update && \
+      apk upgrade && \
+      apk add go git
+WORKDIR /milla
+COPY go.sum go.mod /milla/
+RUN go mod download
+COPY *.go /milla/
+RUN go build
+
+FROM alpine:3.19
+ENV HOME /home/user
+RUN set -eux; \
+  adduser -u 1001 -D -h "$HOME" user; \
+  mkdir "$HOME/.irssi"; \
+  chown -R user:user "$HOME"
+COPY --from=builder /milla/milla "$HOME/milla"
+RUN chown user:user "$HOME/milla"
+ENTRYPOINT ["home/user/milla"]
+```
+
+```Dockerfile
+FROM golang:1.21 as builder
+WORKDIR /milla
+COPY go.sum go.mod /milla/
+RUN go mod download
+COPY *.go /milla/
+RUN CGO_ENABLED=0 go build
+
+FROM gcr.io/distroless/static-debian12
+COPY --from=builder /milla/milla "/usr/bin/milla"
+ENTRYPOINT ["milla"]
+```
+
+```yaml
+services:
+  terra:
+    image: milla_distroless_vendored
+    build:
+      context: .
+      dockerfile: ./Dockerfile_distroless_vendored
+    deploy:
+      resources:
+        limits:
+          memory: 128M
+    logging:
+      driver: "json-file"
+      options:
+        max-size: "100m"
+    networks:
+      - terranet
+    user: 1000:1000
+    restart: unless-stopped
+    entrypoint: ["/usr/bin/milla"]
+    command: ["--config", "/config.toml"]
+    volumes:
+      - ./config.toml:/config.toml
+      - /etc/localtime:/etc/localtime:ro
+    cap_drop:
+      - ALL
+    environment:
+      - HTTPS_PROXY=http://172.17.0.1:8120
+      - https_proxy=http://172.17.0.1:8120
+      - HTTP_PROXY=http://172.17.0.1:8120
+      - http_proxy=http://172.17.0.1:8120
+  postgres:
+    image: postgres:16-alpine3.19
+    deploy:
+      resources:
+        limits:
+          memory: 4096M
+    logging:
+      driver: "json-file"
+      options:
+        max-size: "200m"
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:5455:5432/tcp"
+    volumes:
+      - terra_postgres_vault:/var/lib/postgresql/data
+      - ./scripts/:/docker-entrypoint-initdb.d/:ro
+    environment:
+      - POSTGRES_PASSWORD_FILE=/run/secrets/pg_pass_secret
+      - POSTGRES_USER_FILE=/run/secrets/pg_user_secret
+      - POSTGRES_INITDB_ARGS_FILE=/run/secrets/pg_initdb_args_secret
+      - POSTGRES_DB_FILE=/run/secrets/pg_db_secret
+    networks:
+      - terranet
+      - dbnet
+    secrets:
+      - pg_pass_secret
+      - pg_user_secret
+      - pg_initdb_args_secret
+      - pg_db_secret
+    runtime: runsc
+  pgadmin:
+    image: dpage/pgadmin4:8.6
+    deploy:
+      resources:
+        limits:
+          memory: 1024M
+    logging:
+      driver: "json-file"
+      options:
+        max-size: "100m"
+    environment:
+      - PGADMIN_LISTEN_PORT=${PGADMIN_LISTEN_PORT:-5050}
+      - PGADMIN_DEFAULT_EMAIL=${PGADMIN_DEFAULT_EMAIL:-devi@terminaldweller.com}
+      - PGADMIN_DEFAULT_PASSWORD_FILE=/run/secrets/pgadmin_pass
+      - PGADMIN_DISABLE_POSTFIX=${PGADMIN_DISABLE_POSTFIX:-YES}
+    ports:
+      - "127.0.0.1:5050:5050/tcp"
+    restart: unless-stopped
+    volumes:
+      - terra_pgadmin_vault:/var/lib/pgadmin
+    networks:
+      - dbnet
+    secrets:
+      - pgadmin_pass
+networks:
+  terranet:
+    driver: bridge
+  dbnet:
+volumes:
+  terra_postgres_vault:
+  terra_pgadmin_vault:
+secrets:
+  pg_pass_secret:
+    file: ./pg/pg_pass_secret
+  pg_user_secret:
+    file: ./pg/pg_user_secret
+  pg_initdb_args_secret:
+    file: ./pg/pg_initdb_args_secret
+  pg_db_secret:
+    file: ./pg/pg_db_secret
+  pgadmin_pass:
+    file: ./pgadmin/pgadmin_pass
+```
+
+## Further Reading
+
+- [man 7 cgroups](https://manpages.debian.org/bookworm/manpages/cgroups.7.en.html)
+- [man 7 namespaces](https://manpages.debian.org/bookworm/manpages/namespaces.7.en.html)
+- system containers using [lxc/incus](https://github.com/lxc/incus)
+- [katacontainers](https://katacontainers.io/)