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Lorenzo Garuti
Lorenzo Garuti

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A bash solution for docker and iptables conflict

#docker #firewall #iptables #linux

If you’ve ever tried to setup firewall rules on the same machine where docker daemon is running you may have noticed that docker (by default) manipulate your iptables chains.
If you want the full control of your iptables rules this might be a problem.

Docker and iptables

Docker is utilizing the iptables "nat" to resolve packets from and to its containers and "filter" for isolation purposes, by default docker creates some chains in your iptables setup:

sudo iptables -L Chain INPUT (policy ACCEPT) target prot opt source destination Chain FORWARD (policy DROP) target prot opt source destination DOCKER-USER all -- anywhere anywhere DOCKER-ISOLATION-STAGE-1 all -- anywhere anywhere ACCEPT all -- anywhere anywhere ctstate RELATED,ESTABLISHED DOCKER all -- anywhere anywhere ACCEPT all -- anywhere anywhere ACCEPT all -- anywhere anywhere Chain OUTPUT (policy ACCEPT) target prot opt source destination Chain DOCKER (1 references) target prot opt source destination Chain DOCKER-INGRESS (0 references) target prot opt source destination Chain DOCKER-ISOLATION-STAGE-1 (1 references) target prot opt source destination DOCKER-ISOLATION-STAGE-2 all -- anywhere anywhere RETURN all -- anywhere anywhere Chain DOCKER-ISOLATION-STAGE-2 (1 references) target prot opt source destination DROP all -- anywhere anywhere RETURN all -- anywhere anywhere Chain DOCKER-USER (1 references) target prot opt source destination RETURN all -- anywhere anywhere
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now for example we have the need to expose our nginx container to the world:

docker run --name some-nginx -d -p 8080:80 nginx:latest 47a12adff13aa7609020a1aa0863b0dff192fbcf29507788a594e8b098ffe47a docker ps CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 47a12adff13a nginx:latest "/docker-entrypoint.…" 27 seconds ago Up 24 seconds 0.0.0.0:8080->80/tcp, :::8080->80/tcp some-nginx
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and now we can reach our nginx default page:

curl -v http://192.168.25.200:8080 * Trying 192.168.25.200:8080... * TCP_NODELAY set * Connected to 192.168.25.200 (192.168.25.200) port 8080 (#0) > GET / HTTP/1.1 > Host: 192.168.25.200:8080 > User-Agent: curl/7.68.0 > Accept: */* > * Mark bundle as not supporting multiuse < HTTP/1.1 200 OK < Server: nginx/1.21.1 < Date: Thu, 14 Oct 2021 10:31:38 GMT < Content-Type: text/html < Content-Length: 612 < Last-Modified: Tue, 06 Jul 2021 14:59:17 GMT < Connection: keep-alive < ETag: "60e46fc5-264" < Accept-Ranges: bytes < <!DOCTYPE html> <html> <head> <title>Welcome to nginx!</title> <style> body { width: 35em; margin: 0 auto; font-family: Tahoma, Verdana, Arial, sans-serif; } ... * Connection #0 to host 192.168.25.200 left intact
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NOTE the connection test is made using an external machine, not the same machine where the docker container is running.

The "magic" iptables rules added also allow our containers to reach the outside world:

docker run --rm nginx curl ipinfo.io/ip % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 15 100 15 0 0 94 0 --:--:-- --:--:-- --:--:-- 94 1.2.3.4
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Now check what happened to our iptables rules:

iptables -L ... Chain DOCKER (1 references) target prot opt source destination ACCEPT tcp -- anywhere 172.17.0.2 tcp dpt:http ...
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a new rule is appeared, but is not the only rule added to our chains.

To get a more detailed view of our iptables chain we can dump the full iptables rules with iptables-save:

# Generated by iptables-save v1.8.4 on Thu Oct 14 12:32:46 2021 *mangle :PREROUTING ACCEPT [33102:3022248] :INPUT ACCEPT [33102:3022248] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [32349:12119113] :POSTROUTING ACCEPT [32357:12120329] COMMIT # Completed on Thu Oct 14 12:32:46 2021 # Generated by iptables-save v1.8.4 on Thu Oct 14 12:32:46 2021 *nat :PREROUTING ACCEPT [1:78] :INPUT ACCEPT [1:78] :OUTPUT ACCEPT [13:1118] :POSTROUTING ACCEPT [13:1118] :DOCKER - [0:0] :DOCKER-INGRESS - [0:0] -A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER -A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER -A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE -A POSTROUTING -s 172.17.0.2/32 -d 172.17.0.2/32 -p tcp -m tcp --dport 80 -j MASQUERADE -A DOCKER -i docker0 -j RETURN -A DOCKER ! -i docker0 -p tcp -m tcp --dport 8080 -j DNAT --to-destination 172.17.0.2:80 COMMIT # Completed on Thu Oct 14 12:32:46 2021 # Generated by iptables-save v1.8.4 on Thu Oct 14 12:32:46 2021 *filter :INPUT ACCEPT [4758:361293] :FORWARD DROP [0:0] :OUTPUT ACCEPT [4622:357552] :DOCKER - [0:0] :DOCKER-INGRESS - [0:0] :DOCKER-ISOLATION-STAGE-1 - [0:0] :DOCKER-ISOLATION-STAGE-2 - [0:0] :DOCKER-USER - [0:0] -A FORWARD -j DOCKER-USER -A FORWARD -j DOCKER-ISOLATION-STAGE-1 -A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A FORWARD -o docker0 -j DOCKER -A FORWARD -i docker0 ! -o docker0 -j ACCEPT -A FORWARD -i docker0 -o docker0 -j ACCEPT -A DOCKER -d 172.17.0.2/32 ! -i docker0 -o docker0 -p tcp -m tcp --dport 80 -j ACCEPT -A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2 -A DOCKER-ISOLATION-STAGE-1 -j RETURN -A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP -A DOCKER-ISOLATION-STAGE-2 -j RETURN -A DOCKER-USER -j RETURN COMMIT # Completed on Thu Oct 14 12:32:46 2021
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in our dump we can see some other rules added by docker:

DOCKER-INGRESS (nat table)

-A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE -A POSTROUTING -s 172.17.0.2/32 -d 172.17.0.2/32 -p tcp -m tcp --dport 80 -j MASQUERADE -A DOCKER -i docker0 -j RETURN -A DOCKER ! -i docker0 -p tcp -m tcp --dport 8080 -j DNAT --to-destination 172.17.0.2:80
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DOCKER-USER (filter table)

-A FORWARD -j DOCKER-USER -A FORWARD -j DOCKER-ISOLATION-STAGE-1 -A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A FORWARD -o docker0 -j DOCKER -A FORWARD -i docker0 ! -o docker0 -j ACCEPT -A FORWARD -i docker0 -o docker0 -j ACCEPT -A DOCKER -d 172.17.0.2/32 ! -i docker0 -o docker0 -p tcp -m tcp --dport 80 -j ACCEPT -A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2 -A DOCKER-ISOLATION-STAGE-1 -j RETURN -A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP -A DOCKER-ISOLATION-STAGE-2 -j RETURN -A DOCKER-USER -j RETURN
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to explore in detail how iptables and docker work:

The problem

But what happen if we stop and restart our firewall?

systemctl stop ufw|firewalld # <- the service (ufw or firewalld) may change from distro to distro systemctl stop ufw|firewalld curl -v http://192.168.25.200:8080 * Trying 192.168.25.200:8080... * TCP_NODELAY set docker run --rm nginx curl ipinfo.io/ip % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 0 0 0 0 0 0 0 0 --:--:-- 0:00:06 --:--:-- 0
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we can see that:

  • our container is not reachable from the outside world
  • our container is not able to reach internet

The solution

The solution for this problem is a simple bash script (combined to an awk script) to manage our iptables rules.
In short the script parse the output of the iptables-save command and preserve a set of chains. The chains preserved are:

for table nat:

  • POSTROUTING
  • PREROUTING
  • DOCKER
  • DOCKER-INGRESS
  • OUTPUT

for table filter:

  • FORWARD
  • DOCKER-ISOLATION-STAGE-1
  • DOCKER-ISOLATION-STAGE-2
  • DOCKER
  • DOCKER-INGRESS
  • DOCKER-USER

Install iptables-docker

The first step is to clone this repository

Local install (sh)

NOTE this kind of install use a static file (src/iptables-docker.sh). By default only ssh access to local machine is allowd. To allow specific traffic you have to edit manually this file with your own rules:

 # Other firewall rules # insert here your firewall rules $IPT -A INPUT -p tcp --dport 1234 -m state --state NEW -s 0.0.0.0/0 -j ACCEPT
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NOTE2 if you use a swarm cluster uncomment the lines under Swarm mode - uncomment to enable swarm access (adjust source lan) and adjust your LAN subnet

To install iptables-docker on a local machine, clone this repository and run sudo sh install.sh

sudo sh install.sh Set iptables to iptables-legacy Disable ufw,firewalld Synchronizing state of ufw.service with SysV service script with /lib/systemd/systemd-sysv-install. Executing: /lib/systemd/systemd-sysv-install disable ufw Failed to stop firewalld.service: Unit firewalld.service not loaded. Failed to disable unit: Unit file firewalld.service does not exist. Install iptables-docker.sh Create systemd unit Enable iptables-docker.service Created symlink /etc/systemd/system/multi-user.target.wants/iptables-docker.service → /etc/systemd/system/iptables-docker.service. start iptables-docker.service
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Automated install (ansible)

You can also use ansible to deploy iptables-docker everywhere. To do this adjust the settings under group_vars/main.yml.

Label Default Description
docker_preserve yes Preserve docker iptables rules
swarm_enabled no Tells to ansible to open the required ports for the swarm cluster
ebable_icmp_messages yes Enable response to ping requests
swarm_cidr 192.168.1.0/24 Local docker swarm subnet
ssh_allow_cidr 0.0.0.0/0 SSH alloed subnet (default everywhere)
iptables_allow_rules [] List of dict to dynamically open ports. Each dict has the following key: desc, proto, from, port. See group_vars/all.yml for examples
iptables_docker_uninstall no Uninstall iptables-docker

Now create the inventory (hosts.ini file) or use an inline inventory and run the playbook:

ansible-playbook -i hosts.ini site.yml
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Usage

To start the service use:

sudo systemctl start iptables-docker or sudo iptables-docker.sh start
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To stop the srevice use:

sudo systemctl stop iptables-docker or sudo iptables-docker.sh stop
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Test iptables-docker

Now if you turn off the firewall with sudo systemctl stop iptables-docker and if you check the iptable-save output, you will see that the docker rules are still there:

sudo iptables-save # Generated by iptables-save v1.8.4 on Thu Oct 14 15:52:30 2021 *mangle :PREROUTING ACCEPT [346:23349] :INPUT ACCEPT [346:23349] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [340:24333] :POSTROUTING ACCEPT [340:24333] COMMIT # Completed on Thu Oct 14 15:52:30 2021 # Generated by iptables-save v1.8.4 on Thu Oct 14 15:52:30 2021 *nat :PREROUTING ACCEPT [0:0] :INPUT ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :POSTROUTING ACCEPT [0:0] :DOCKER - [0:0] :DOCKER-INGRESS - [0:0] -A PREROUTING -m addrtype --dst-type LOCAL -j DOCKER -A OUTPUT ! -d 127.0.0.0/8 -m addrtype --dst-type LOCAL -j DOCKER -A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE -A POSTROUTING -s 172.17.0.2/32 -d 172.17.0.2/32 -p tcp -m tcp --dport 80 -j MASQUERADE -A DOCKER -i docker0 -j RETURN -A DOCKER ! -i docker0 -p tcp -m tcp --dport 8080 -j DNAT --to-destination 172.17.0.2:80 COMMIT # Completed on Thu Oct 14 15:52:30 2021 # Generated by iptables-save v1.8.4 on Thu Oct 14 15:52:30 2021 *filter :INPUT ACCEPT [357:24327] :FORWARD DROP [0:0] :OUTPUT ACCEPT [355:26075] :DOCKER - [0:0] :DOCKER-INGRESS - [0:0] :DOCKER-ISOLATION-STAGE-1 - [0:0] :DOCKER-ISOLATION-STAGE-2 - [0:0] :DOCKER-USER - [0:0] -A FORWARD -j DOCKER-USER -A FORWARD -j DOCKER-ISOLATION-STAGE-1 -A FORWARD -o docker0 -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A FORWARD -o docker0 -j DOCKER -A FORWARD -i docker0 ! -o docker0 -j ACCEPT -A FORWARD -i docker0 -o docker0 -j ACCEPT -A DOCKER -d 172.17.0.2/32 ! -i docker0 -o docker0 -p tcp -m tcp --dport 80 -j ACCEPT -A DOCKER-ISOLATION-STAGE-1 -i docker0 ! -o docker0 -j DOCKER-ISOLATION-STAGE-2 -A DOCKER-ISOLATION-STAGE-1 -j RETURN -A DOCKER-ISOLATION-STAGE-2 -o docker0 -j DROP -A DOCKER-ISOLATION-STAGE-2 -j RETURN -A DOCKER-USER -j RETURN COMMIT # Completed on Thu Oct 14 15:52:30 2021
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our container is still accesible form the outside:

 curl -v http://192.168.25.200:8080 * Trying 192.168.25.200:8080... * TCP_NODELAY set * Connected to 192.168.25.200 (192.168.25.200) port 8080 (#0) > GET / HTTP/1.1 > Host: 192.168.25.200:8080 > User-Agent: curl/7.68.0 > Accept: */* > * Mark bundle as not supporting multiuse < HTTP/1.1 200 OK < Server: nginx/1.21.1 < Date: Thu, 14 Oct 2021 13:53:33 GMT < Content-Type: text/html < Content-Length: 612 < Last-Modified: Tue, 06 Jul 2021 14:59:17 GMT < Connection: keep-alive < ETag: "60e46fc5-264" < Accept-Ranges: bytes
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and our container can reach internet:

docker run --rm nginx curl ipinfo.io/ip % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 15 100 15 0 0 94 0 --:--:-- --:--:-- --:--:-- 94 my-public-ip-address
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Important notes

Before install iptables-docker please read this notes:

  • both local instal and ansible install configure your system to use iptables-legacy
  • by default only port 22 is allowed
  • ufw and firewalld will be permanently disabled
  • filtering on all docker interfaces is disabled

Docker interfaces are:

  • vethXXXXXX interfaces
  • br-XXXXXXXXXXX interfaces
  • docker0 interface
  • docker_gwbridge interface

Extending iptables-docker

You can extend or modify iptables-docker by editing:

  • src/iptables-docker.sh for the local install (sh)
  • roles/iptables-docker/templates/iptables-docker.sh.j2 template file for the automated install (ansible)

Uninstall

Local install (sh)

Run uninstall.sh

Automated install (ansible)

set the variable "iptables_docker_uninstall" to "yes" into group_vars/all.yml and run the playbook.

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