Tag: forward port

Firewalld and how to preserve the original source IP when forwarding to internal IP

Using firewalld and the forwarding options (IP or port forward) might work not as expected if the default setup is left on the system. Consider the simple example:

main menu
Internet <-> router <-> local network

The purpose is to forward a port to a server in the local network, which should be easy enough. Let the forwarding port be 80 and the server should receive the original source IP. To archive this task the system administrator should do the following on the router with firewalld service. Here is one of the simplest methods:

  • When the router’s external IP/interface and the router’s internal IP/interface are in the same firewalld zone. The zone is named “public” in the CentOS world.

The solution uses the masquerade rule added with a rich rule (–add-rich-rule), not the masquerade option of the zone (–add-masquerade).
The default configuration will assign the external interface and the internal interface, which may be a virtual one, in the same firewalld zone such as “public”. When this happens, activating the masquerade option will break the source IP and it will be replaced by the Netfilter with the internal IP address of the router and the internal server will see all incoming connections on the forwarded port as if they were coming from the internal router IP. All different IPs coming to this port will be replaced with the router’s internal IP and forwarded to the server’s internal.

The router’s external IP/interface and the router’s internal IP/interface are in the same firewalld zone.

This solution is demonstrated with a virtual interface – bridge br0, but it may be a network interface. By default, when the bridge is created, it will be added to the default zone, which is “public” in CentOS world. Use –get-active-zones to check the active zones and the assigned interfaces.

[root@srv ~]# firewall-cmd --get-active-zones
public
  interfaces: eth0 br0
[root@srv ~]# firewall-cmd --list-all
public (active)
  target: default
  icmp-block-inversion: no
  interfaces: br0 eth0
  sources: 
  services: cockpit dhcpv6-client http https ssh
  ports: 10022/tcp
  protocols: 
  forward: yes
  masquerade: no
  forward-ports: 
  source-ports: 
  icmp-blocks: 
  rich rules:

If the options forward and masquerade are activated (i.e. yes on the above output) and a forward rule to an internal local IP (some server IP connected to the bridge br0) is introduced to the firewall, the local server will receive all connection attempts to the forwarded port, but the source IP will be overwritten with the gateway IP of the internal (local network). For example, the bridge br0 has IP 192.168.0.1 and the eth0 has Internet IP 1.1.1.1. Forwarding port 1.1.1.1:80 to a server behind the bridge br0 with IP 192.168.0.100:
Keep on reading!

Docker change the port mapping of an existing container

Unfortunately, it is not possible to change the port mapping (forwarded ports from the hosts to the container) of an existing RUNNING container!

Not only that, but you cannot change the mapped ports (forwarded ports) even when the container is stopped, so think twice when you run or start a container from the image you’ve chosen. Of course, you can always use docker’s commit command, which just creates a new image from you (running, in a sense of changes fro the original image) container and then you can run the new image with new mapped ports!

Still, there is a solution not involving the creation of new docker images and containers, but just to edit manually a configuration file while the Docker service is stopped.

So if you have several docker containers running you should stop all of them! When the Docker service stops, edit the “hostconfig.json” file! Here is the whole procedure:

  1. Stop the container.
  2. Stop the Docker container service.
  3. Edit the container’s file – hostconfig.json (usually in /var/lib/docker/containers/[ID]/hostconfig.json) and add or replace ports.
  4. Start the Docker container service.
  5. Start the docker container.

Real World Example

myuser@srv:~# sudo docker ps
CONTAINER ID        IMAGE                         COMMAND                  CREATED             STATUS                  PORTS                                                                                    NAMES
a9e21e92e2dd        gitlab/gitlab-runner:latest   "/usr/bin/dumb-init …"   2 days ago          Up 33 hours                                                                                                      gitlab-runner
5d025e7f93a4        gitlab/gitlab-ce:latest       "/assets/wrapper"        3 days ago          Up 34 hours (healthy)   0.0.0.0:80->80/tcp, 0.0.0.0:4567->4567/tcp, 0.0.0.0:1022->22/tcp   gitlab
myuser@srv:~# sudo docker ps
CONTAINER ID        IMAGE                         COMMAND                  CREATED             STATUS                  PORTS                                                                                    NAMES
a9e21e92e2dd        gitlab/gitlab-runner:latest   "/usr/bin/dumb-init …"   2 days ago          Up 33 hours                                                                                                      gitlab-runner
5d025e7f93a4        gitlab/gitlab-ce:latest       "/assets/wrapper"        3 days ago          Up 34 hours (healthy)   0.0.0.0:80->80/tcp, 0.0.0.0:4567->4567/tcp, 0.0.0.0:1022->22/tcp   gitlab
myuser@srv:~# sudo docker stop gitlab-runner
gitlab-runner
myuser@srv:~# sudo docker stop gitlab
gitlab
myuser@srv:~# sudo docker ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
myuser@srv:~# systemctl stop docker
myuser@srv:~# systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: inactive (dead) since Thu 2019-11-14 21:54:57 UTC; 5s ago
     Docs: https://docs.docker.com
  Process: 2340 ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock (code=exited, status=0/SUCCESS)
 Main PID: 2340 (code=exited, status=0/SUCCESS)

Nov 14 21:54:33 srv dockerd[2340]: time="2019-11-14T21:54:33.308531424Z" level=warning msg="a9e21e92e2dd297a68f68441353fc3bda39d0bb5564b60d402ae651fa80f5c72 cleanu
Nov 14 21:54:46 srv dockerd[2340]: time="2019-11-14T21:54:46.394643530Z" level=info msg="Container 5d025e7f93a45a50dbbaa87c55d7cdbbf6515bbe1d45ff599074f1cdcf320a0c
Nov 14 21:54:46 srv dockerd[2340]: time="2019-11-14T21:54:46.757171067Z" level=info msg="ignoring event" module=libcontainerd namespace=moby topic=/tasks/delete ty
Nov 14 21:54:47 srv dockerd[2340]: time="2019-11-14T21:54:47.031709355Z" level=warning msg="5d025e7f93a45a50dbbaa87c55d7cdbbf6515bbe1d45ff599074f1cdcf320a0c cleanu
Nov 14 21:54:57 srv systemd[1]: Stopping Docker Application Container Engine...
Nov 14 21:54:57 srv dockerd[2340]: time="2019-11-14T21:54:57.439296168Z" level=info msg="Processing signal 'terminated'"
Nov 14 21:54:57 srv dockerd[2340]: time="2019-11-14T21:54:57.447803201Z" level=info msg="Daemon shutdown complete"
Nov 14 21:54:57 srv dockerd[2340]: time="2019-11-14T21:54:57.449422219Z" level=info msg="stopping event stream following graceful shutdown" error="context canceled
Nov 14 21:54:57 srv dockerd[2340]: time="2019-11-14T21:54:57.449576789Z" level=info msg="stopping event stream following graceful shutdown" error="context canceled
Nov 14 21:54:57 srv systemd[1]: Stopped Docker Application Container Engine.
myuser@srv:~# cat /var/lib/docker/containers/5d025e7f93a45a50dbbaa87c55d7cdbbf6515bbe1d45ff599074f1cdcf320a0c/hostconfig.json 
{"Binds":["/srv/gitlab/config:/etc/gitlab","/srv/gitlab/logs:/var/log/gitlab","/srv/gitlab/data:/var/opt/gitlab"],"ContainerIDFile":"","LogConfig":{"Type":"json-file","Config":{}},"NetworkMode":"default","PortBindings":{"22/tcp":[{"HostIp":"","HostPort":"1022"}],"4567/tcp":[{"HostIp":"","HostPort":"4567"}],"80/tcp":[{"HostIp":"","HostPort":"80"}]},"RestartPolicy":{"Name":"always","MaximumRetryCount":0},"AutoRemove":false,"VolumeDriver":"","VolumesFrom":null,"CapAdd":null,"CapDrop":null,"Capabilities":null,"Dns":[],"DnsOptions":[],"DnsSearch":[],"ExtraHosts":null,"GroupAdd":null,"IpcMode":"private","Cgroup":"","Links":null,"OomScoreAdj":0,"PidMode":"","Privileged":false,"PublishAllPorts":false,"ReadonlyRootfs":false,"SecurityOpt":null,"UTSMode":"","UsernsMode":"","ShmSize":67108864,"Runtime":"runc","ConsoleSize":[0,0],"Isolation":"","CpuShares":0,"Memory":0,"NanoCpus":0,"CgroupParent":"","BlkioWeight":0,"BlkioWeightDevice":[],"BlkioDeviceReadBps":null,"BlkioDeviceWriteBps":null,"BlkioDeviceReadIOps":null,"BlkioDeviceWriteIOps":null,"CpuPeriod":0,"CpuQuota":0,"CpuRealtimePeriod":0,"CpuRealtimeRuntime":0,"CpusetCpus":"","CpusetMems":"","Devices":[],"DeviceCgroupRules":null,"DeviceRequests":null,"KernelMemory":0,"KernelMemoryTCP":0,"MemoryReservation":0,"MemorySwap":0,"MemorySwappiness":null,"OomKillDisable":false,"PidsLimit":null,"Ulimits":null,"CpuCount":0,"CpuPercent":0,"IOMaximumIOps":0,"IOMaximumBandwidth":0,"MaskedPaths":["/proc/asound","/proc/acpi","/proc/kcore","/proc/keys","/proc/latency_stats","/proc/timer_list","/proc/timer_stats","/proc/sched_debug","/proc/scsi","/sys/firmware"],"ReadonlyPaths":["/proc/bus","/proc/fs","/proc/irq","/proc/sys","/proc/sysrq-trigger"]
myuser@srv:~# nano /var/lib/docker/containers/5d025e7f93a45a50dbbaa87c55d7cdbbf6515bbe1d45ff599074f1cdcf320a0c/hostconfig.json 
myuser@srv:~# systemctl start docker
myuser@srv:~# systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2019-11-14 22:12:06 UTC; 2s ago
     Docs: https://docs.docker.com
 Main PID: 4693 (dockerd)
    Tasks: 54
   CGroup: /system.slice/docker.service
           ├─4693 /usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock
           ├─4867 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 4567 -container-ip 172.17.0.3 -container-port 4567
           ├─4881 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 443 -container-ip 172.17.0.3 -container-port 443
           ├─4895 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 1022 -container-ip 172.17.0.3 -container-port 22
           └─4907 /usr/bin/docker-proxy -proto tcp -host-ip 0.0.0.0 -host-port 80 -container-ip 172.17.0.3 -container-port 80

Nov 14 22:12:04 srv dockerd[4693]: time="2019-11-14T22:12:04.034007956Z" level=warning msg="Your kernel does not support swap memory limit"
Nov 14 22:12:04 srv dockerd[4693]: time="2019-11-14T22:12:04.034062799Z" level=warning msg="Your kernel does not support cgroup rt period"
Nov 14 22:12:04 srv dockerd[4693]: time="2019-11-14T22:12:04.034074070Z" level=warning msg="Your kernel does not support cgroup rt runtime"
Nov 14 22:12:04 srv dockerd[4693]: time="2019-11-14T22:12:04.034361581Z" level=info msg="Loading containers: start."
Nov 14 22:12:04 srv dockerd[4693]: time="2019-11-14T22:12:04.344354207Z" level=info msg="Default bridge (docker0) is assigned with an IP address 172.17.0.0/16. Dae
Nov 14 22:12:05 srv dockerd[4693]: time="2019-11-14T22:12:05.916782317Z" level=info msg="Loading containers: done."
Nov 14 22:12:05 srv dockerd[4693]: time="2019-11-14T22:12:05.988204406Z" level=info msg="Docker daemon" commit=9013bf583a graphdriver(s)=overlay2 version=19.03.4
Nov 14 22:12:05 srv dockerd[4693]: time="2019-11-14T22:12:05.988317448Z" level=info msg="Daemon has completed initialization"
Nov 14 22:12:06 srv dockerd[4693]: time="2019-11-14T22:12:06.010801856Z" level=info msg="API listen on /var/run/docker.sock"
Nov 14 22:12:06 srv systemd[1]: Started Docker Application Container Engine.
myuser@srv:~# sudo docker start gitlab-runner
gitlab-runner
myuser@srv:~# sudo docker start gitlab
gitlab
myuser@srv:~# sudo docker ps
CONTAINER ID        IMAGE                         COMMAND                  CREATED             STATUS                             PORTS                                                                                    NAMES
a9e21e92e2dd        gitlab/gitlab-runner:latest   "/usr/bin/dumb-init …"   2 days ago          Up 19 seconds                                                                                                               gitlab-runner
5d025e7f93a4        gitlab/gitlab-ce:latest       "/assets/wrapper"        3 days ago          Up 19 seconds (health: starting)   0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp, 0.0.0.0:4567->4567/tcp, 0.0.0.0:1022->22/tcp   gitlab
myuser@srv:~# wget --no-check-certificate https://192.168.0.238/
--2019-11-14 22:13:30--  https://192.168.0.238/
Connecting to 192.168.0.238:443... connected.
    WARNING: certificate common name ‘gitlab.ahelpme.com’ doesn't match requested host name ‘192.168.0.238’.
HTTP request sent, awaiting response... 302 Found
Location: https://192.168.0.238/users/sign_in [following]
--2019-11-14 22:13:30--  https://192.168.0.238/users/sign_in
Reusing existing connection to 192.168.0.238:443.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: ‘index.html’

index.html                                     [ <=>                                                                                     ]  12.41K  --.-KB/s    in 0s      

2019-11-14 22:13:31 (134 MB/s) - ‘index.html’ saved [12708]

Change the ports or add more ports in “PortBindings”. The syntax is pretty straightforward just mind the comas, [] and {}.

"PortBindings":{"22/tcp":[{"HostIp":"","HostPort":"1022"}],"4567/tcp":[{"HostIp":"","HostPort":"4567"}],"80/tcp":[{"HostIp":"","HostPort":"80"}]}

Here we change the mapping from “host port 1022 to 22” to “host port 2222 to 22” just replacing the “1022” to “2222”:

"PortBindings":{"22/tcp":[{"HostIp":"","HostPort":"2222"}],"4567/tcp":[{"HostIp":"","HostPort":"4567"}],"80/tcp":[{"HostIp":"","HostPort":"80"}]}

And the second example is in addition to the 2222 change we want to add another mapping “host from 443 to 443” (open the HTTPS), just add new group with the above syntax:

"PortBindings":{"22/tcp":[{"HostIp":"","HostPort":"2222"}],"4567/tcp":[{"HostIp":"","HostPort":"4567"}],"80/tcp":[{"HostIp":"","HostPort":"80"}],"443/tcp":[{"HostIp":"","HostPort":"443"}]}

A note!

Probably there may be an idea not to be easy to add mapped ports when you think one of the main Docker goals is to isolate services per a Docker instance. It sounds strange to have a docker container for one service exporting a number of ports (or a single port) and later why you would need to expose another port? For another service in the same container, but you should use a separate container, not the same one!
But more and more Docker containers are used also to deliver a fine-tuned environment of a whole platform, which provides multiple services in a single docker container. Let’s take an example – GitLab, which offers installation in a Docker container hosting more than 10 services in a single container!