96 lines
4.5 KiB
Markdown
96 lines
4.5 KiB
Markdown
---
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title: Docker and iptables
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description: The basics of how Docker works with iptables
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keywords: network, iptables
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---
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On Linux, Docker manipulates `iptables` rules to provide network isolation.
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While this is an implementation detail and you should not modify the rules
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Docker inserts into your `iptables` policies, it does have some implications
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on what you need to do if you want to have your own policies in addition to
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those managed by Docker.
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If you're running Docker on a host that is exposed to the Internet, you will
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probably want to have iptables policies in place that prevent unauthorized
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access to containers or other services running on your host. This page
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describes how to achieve that, and what caveats you need to be aware of.
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## Add iptables policies before Docker's rules
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Docker installs two custom iptables chains named `DOCKER-USER` and `DOCKER`,
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and it ensures that incoming packets are always checked by these two chains
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first.
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All of Docker's `iptables` rules are added to the `DOCKER` chain. Do not
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manipulate this chain manually. If you need to add rules which load before
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Docker's rules, add them to the `DOCKER-USER` chain. These rules are applied
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before any rules Docker creates automatically.
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Rules added to the `FORWARD` chain -- either manually, or by another
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iptables-based firewall -- are evaluated _after_ these chains. This means that
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if you expose a port through Docker, this port gets exposed no matter what
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rules your firewall has configured. If you want those rules to apply even
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when a port gets exposed through Docker, you _must_ add these rules to the
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`DOCKER-USER` chain.
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### Restrict connections to the Docker host
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By default, all external source IPs are allowed to connect to the Docker host.
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To allow only a specific IP or network to access the containers, insert a
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negated rule at the top of the `DOCKER-USER` filter chain. For example, the
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following rule restricts external access from all IP addresses except `192.168.1.1`:
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```bash
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$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.1 -j DROP
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```
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Please note that you will need to change `ext_if` to correspond with your
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host's actual external interface. You could instead allow connections from a
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source subnet. The following rule only allows access from the subnet `192.168.1.0/24`:
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```bash
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$ iptables -I DOCKER-USER -i ext_if ! -s 192.168.1.0/24 -j DROP
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```
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Finally, you can specify a range of IP addresses to accept using `--src-range`
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(Remember to also add `-m iprange` when using `--src-range` or `--dst-range`):
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```bash
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$ iptables -I DOCKER-USER -m iprange -i ext_if ! --src-range 192.168.1.1-192.168.1.3 -j DROP
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```
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You can combine `-s` or `--src-range` with `-d` or `--dst-range` to control both
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the source and destination. For instance, if the Docker daemon listens on both
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`192.168.1.99` and `10.1.2.3`, you can make rules specific to `10.1.2.3` and leave
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`192.168.1.99` open.
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`iptables` is complicated and more complicated rules are out of scope for this
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topic. See the [Netfilter.org HOWTO](https://www.netfilter.org/documentation/HOWTO/NAT-HOWTO.html)
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for a lot more information.
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## Docker on a router
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Docker also sets the policy for the `FORWARD` chain to `DROP`. If your Docker
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host also acts as a router, this will result in that router not forwarding
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any traffic anymore. If you want your system to continue functioning as a
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router, you can add explicit `ACCEPT` rules to the `DOCKER-USER` chain to
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allow it:
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```bash
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$ iptables -I DOCKER-USER -i src_if -o dst_if -j ACCEPT
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```
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## Prevent Docker from manipulating iptables
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It is possible to set the `iptables` key to `false` in the Docker engine's configuration file at `/etc/docker/daemon.json`, but this option is not appropriate for most users. It is not possible to completely prevent Docker from creating `iptables` rules, and creating them after-the-fact is extremely involved and beyond the scope of these instructions. Setting `iptables` to `false` will more than likely break container networking for the Docker engine.
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For system integrators who wish to build the Docker runtime into other applications, explore the [`moby` project](https://mobyproject.org/).
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## Setting the default bind address for containers
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By default, the Docker daemon will expose ports on the `0.0.0.0` address, i.e.
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any address on the host. If you want to change that behavior to only
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expose ports on an internal IP address, you can use the `--ip` option to
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specify a different IP address. However, setting `--ip` only changes the
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_default_, it does not _restrict_ services to that IP.
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