Neutron external network with routing (no NAT)

Reading time ~4 minutes

TO-DO, fix this blog post to explain “–disable-snat” that does the same, and probably also supports FIP (I need to check)

usage: neutron router-gateway-set [-h] [--request-format {json}]
                                  ROUTER EXTERNAL-NETWORK

Set the external network gateway for a router.

positional arguments:
  ROUTER                ID or name of the router.
  EXTERNAL-NETWORK      ID or name of the external network for the gateway.

optional arguments:
  -h, --help            show this help message and exit
  --request-format {json}
                        DEPRECATED! Only JSON request format is supported.
  --disable-snat        Disable source NAT on the router gateway.
  --fixed-ip subnet_id=SUBNET,ip_address=IP_ADDR
                        Desired IP and/or subnet on external network:
                        subnet_id=<name_or_id>,ip_address=<ip>. You can
                        specify both of subnet_id and ip_address or specify
                        one of them as well. You can repeat this option.

In this blog post I will explain how to connect private tenant networks to an external network without the use of NAT or DNAT (floating ips) via a neutron router.

With the following configuration you will have routers that don’t do NAT on ingress or egress connections. You won’t be able to use floating ips too, at least in the explained configuration, you could add a second external network and a gateway port, plus some static routes to let the router steer traffic over each network.

This can be done, with just one limitation, tenant networks connected in such topology cannot have overlapping ips. And also, the upstream router(s) must be informed about the internal networks so they can add routes themselves to those networks. That can be done manually, or automatically by periodically talking to the openstack API (checking the router interfaces, the subnets etc..) but I’ll skip that part in this blog post.

We’re going to assume that our external provider network is “public”, with the subnet “public_subnet” and that the CIDR of such network is with a gateway on Please excuse me because I’m not using the new openstack commands, I can make an updated post later if somebody is interested in that.

Step by step

Create the virtual router

source keystonerc_admin

neutron router-create router_test

# please note that you can manipulate static routes on that router if you need:
# neutron router-update --help
#  [--route destination=CIDR,nexthop=IP_ADDR | --no-routes]

Create the private network for our test, and add it to the router

neutron net-create private_test
$PRIV_NET=$(neutron net-list | awk ' /private_test/ { print $2 }')

neutron subnet-create --name private_subnet $PRIV_NET
neutron router-interface-add router_test private_subnet

Create a security group, and add an instance

neutron security-group-create test
neutron security-group-rule-create test --direction ingress

nova boot --flavor m1.tiny --image cirros --nic net-id=$PRIV_NET \
          --security-group test cirros
sleep 10
nova console-log cirros

Please note that in the console-log you may find that everything went well… and that the instance has an address through DHCP.

Create a port for the router on the external network, do it manually so we can specify the exact address we want.

SUBNET_ID=$(neutron subnet-list | awk ' /public_subnet/ { print $2 }')
neutron port-create public --fixed-ip \
    subnet_id=$SUBNET_ID,ip_address= ext-router-port

Add it to the router

PORT_ID=$(neutron port-show ext-router-port | awk '/ id / { print $4 } ')
neutron router-interface-add router_test port=$PORT_ID

We can test it locally (assuming our host has access to

# test local...
ip route add via

# and assuming that our instance was created with

[root@server ~(keystone_admin)]# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=63 time=0.621 ms
64 bytes from icmp_seq=2 ttl=63 time=0.298 ms

Extra note, if you want to avoid issues with overlapping tenant network IPs, I recommend you to have a look at the subnet pool functionality of neutron which started in Kilo.

I hope you enjoyed it, and that this is helpful, let me know in the comments :)

Cirros Image mirror

Every time I try to download a cirros image from[cirros cloud]( for use in myOpenStack devel envi...… Continue reading

We moved again! hello jekyll, bye tumblr

Published on January 03, 2018

The three islands at Amalfi coast

Published on November 12, 2017