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engine: describe DNAT limitation of DOCKER-USER iptables chain 

Signed-off-by: David Karlsson <david.karlsson@docker.com>
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David Karlsson 2023-05-03 14:41:00 +02:00
parent 625a56ec02
commit 44424f2ed1
1 changed files with 29 additions and 7 deletions
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network/iptables.md
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@ -19,19 +19,41 @@ describes how to achieve that, and what caveats you need to be aware of.
Docker installs two custom iptables chains named `DOCKER-USER` and `DOCKER`, Docker installs two custom iptables chains named `DOCKER-USER` and `DOCKER`,
and it ensures that incoming packets are always checked by these two chains and it ensures that incoming packets are always checked by these two chains
first. first. These chains are part of the `FORWARD` chain.
All of Docker's `iptables` rules are added to the `DOCKER` chain. Do not All of Docker's `iptables` rules are added to the `DOCKER` chain. Do not
manipulate this chain manually. If you need to add rules which load before manipulate this chain manually. If you need to add rules which load before
Docker's rules, add them to the `DOCKER-USER` chain. These rules are applied Docker's rules, add them to the `DOCKER-USER` chain. These rules are applied
before any rules Docker creates automatically. before any rules Docker creates automatically.
Rules added to the `FORWARD` chain -- either manually, or by another Other rules added to the `FORWARD` chain, either manually, or by another
iptables-based firewall -- are evaluated _after_ these chains. This means that iptables-based firewall, are evaluated after the `DOCKER-USER` and `DOCKER` chains.
if you expose a port through Docker, this port gets exposed no matter what This means that if you expose a port through Docker,
rules your firewall has configured. If you want those rules to apply even this port gets exposed no matter what rules your firewall has configured.
when a port gets exposed through Docker, you _must_ add these rules to the If you want rules to apply even when a port gets exposed through Docker,
`DOCKER-USER` chain. you must add these rules to the `DOCKER-USER` chain.
### Match the original IP and ports for requests
When packets arrive to the `DOCKER-USER` chain, they have already passed through
a Destination Network Address Translation (DNAT) filter. That means that the
`iptables` flags you use can only match internal IP addresses and ports of
containers.
If you want to match traffic based on the original IP and port in the network
request, you must use the
[`conntrack` iptables extension](https://ipset.netfilter.org/iptables-extensions.man.html#lbAO).
For example:
```console
$ sudo iptables -I DOCKER-USER -p tcp -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
$ sudo iptables -I DOCKER-USER -p tcp -m conntrack -ctorigsrc 1.2.3.4 --ctorigdstport 80 -j ACCEPT
```
> **Important**
>
> Using the `conntrack` extension may result in degraded performance.
{: .important }
### Restrict connections to the Docker host ### Restrict connections to the Docker host