Deploying AWS Site-to-Site on OpenWRT

• Ansible Role Used (gekmihesg/ansible-openwrt)

I want to connect to resources on AWS from my home with the least operational overhead, leading me to deploy AWS Site-to-Site for connecting resources from my home to a VPC.

The Environment

Some resources I want to access are;

• G4dn.xlarge EC2 instances used for streaming games
• t2.micro EC2 instances hosting Home Assistant
• RDS (PostgreSQL) instances for hosting OpenStreetMap data

Home Environment

When setting up a connection from AWS to my home, I have to consider the following specifications;

• I live in West London, relatively close to the eu-west-2 data center
  • I have a VPC in eu-west-2 running on the 10.1.0.0/16 network
• I use a publicly-offered ISP for accessing the internet
• There are two hops (routers) between the public internet and my home network
  • The first hop is the router provided by the ISP to connect to the internet
    • This network lives on the 192.168.0.0/24 subnet
  • The second hop is my own off-the-shelf router from ASUS running OpenWRT
    • My home network lives on the 10.0.0.0/24 subnet
    • The router has 8MB of usable storage for packages and configuration

Setting up AWS Site-to-Site

AWS Site-to-Site is one of Amazon’s offerings for bridging an external network to a VPC over the public internet. Some alternatives are;

• AWS Client VPN (based on OpenVPN)
  • More expensive
  • More complex, often tends to be slower without optimization
• Self-managed VPN
  • Allows use of any VPN technology, such as Wireguard
  • Allows custom metric monitoring
  • Requires management of VPC topologies and firewalls
  • Can be more expensive

I chose to use the Site-to-Site in this occasion so I could learn about how IPSec works in more detail, and saw it as a challenge in deploying to OpenWRT. It’s also a lot cheaper than a firewall license, EC2 rental and public IP charges.

Deploying a Virtual Private Gateway

A Virtual Private Gateway is the AWS-side endpoint of an IPSec tunnel. It also hosts the configuration of the local BGP instance, and is what drives the propagation of routes between the IPSec tunnels and the VPC routing tables.

resource "aws_vpn_gateway" "main" {
  vpc_id = data.aws_vpc.main.id
}

There’s not much to configure with the VPG, so I left it with its’ defaults.

Deploying a Customer Gateway

A customer gateway represents the local end of the IPSec tunnel and the BGP daemon running on it. In my case, this is the OpenWRT router.

resource "aws_customer_gateway" "openwrt" {
  bgp_asn    = 65000
  ip_address = "<WAN Address of OpenWRT>"
  type       = "ipsec.1"
}

Deploying a Site-to-Site VPN Connection

The VPN Connection itself is what connects a VPG (AWS Endpoint) to a customer gateway (Local endpoint) in the form of an IPSec VPN connection.

resource "aws_vpn_connection" "main" {
  customer_gateway_id = aws_customer_gateway.openwrt.id
  vpn_gateway_id      = aws_vpn_gateway.main.id
  type                = aws_customer_gateway.openwrt.type

  tunnel1_ike_versions = ["ikev1", "ikev2"]
  tunnel1_phase1_dh_group_numbers = [2, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]
  tunnel1_phase1_encryption_algorithms = ["AES128", "AES128-GCM-16", "AES256", "AES256-GCM-16"]
  tunnel1_phase1_integrity_algorithms = ["SHA1", "SHA2-256", "SHA2-384", "SHA2-512"]
  tunnel1_phase2_dh_group_numbers = [2, 5, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]
  tunnel1_phase2_encryption_algorithms = ["AES128", "AES128-GCM-16", "AES256", "AES256-GCM-16"]
  tunnel1_phase2_integrity_algorithms = ["SHA1", "SHA2-256", "SHA2-384", "SHA2-512"]

}

These (tunnel1_*) are the default values set by AWS and should be locked down. For the purpose of testing, I left them all to their defaults. This settings are directly tied to the IPSec encryption settings described below.

Connecting OpenWRT via IPSec

Ansible Role Variables

I’ve designed my Ansible role to be able to configure AWS IPSec tunnels with the bare minimum configuration. All information that the role requires is provided by Terraform upon provisioning of the AWS Site-to-Site configuration.

bgp_remote_as: "64512"
ipsec_tunnels:
  - ifid: "301"
    name: xfrm0
    inside_addr: <Inside IPv4>
    gateway: <Endpoint Tunnel 1>
    psk: <PSK Tunnel 1>
  - ifid: "302"
    name: xfrm1
    inside_addr: <Inside IPv4>
    gateway: <Endpoint Tunnel 2>
    psk: <PSK Tunnel 2>

• bgp_remote_as refers to the ASN of the Virtual Private Gateway, and is strictly used by the BGP Daemon offered by Quagga.
  • BGP is used to propagate routes to-and-from AWS.
  • When a Site-to-Site VPN is configured to use Dynamic routing, it will state that the tunnel is Down if AWS cannot reach the BGP instance.
• ipsec_tunnels is used by XFRM and strongSwan to;
  • Build one XFRM interface per-tunnel
  • Build one alias interface bound to each XFRM interface for static routing
  • Configure the static routing of the XFRM interfaces
  • Configure the BGP daemon neighbours
  • Configure one IPSec endpoint per-tunnel
  • Configure one IPSec tunnel for each XFRM interface

Required packages

I used three components for this workflow to function, and a last one for debugging security association errors.

• strongswan-full
  • strongSwan provides an IPSec implementation for OpenWRT with full support for UCI. The -full variation of the package is overkill, but you never know!
• quagga-bgpd
  • A BGP implementation light enough to run on OpenWRT. quagga comes in as a dependency
• luci-proto-xfrm
  • A virtual interface for use by IPSec, where a tunnel requires a vif to bind to.
• ip-full
  • Provides an xfrm argument for debugging IPSec connections with.

name: install required packages
opkg:
  name: "{{ item }}"
loop:
  - strongswan-full
  - quagga-bgpd
  - luci-proto-xfrm
  - ip-full

Adding the XFRM Interface

name: configure xfrm adapter
uci:
  command: section
  key: network
  type: interface
  name: "{{ item.name }}"
  value:
    tunlink: 'wan'
    mtu: '1300'
    proto: xfrm
    ifid: "{{ item.ifid }}"
loop: "{{ ipsec_tunnels }}"

config interface 'xfrm0'
	option ifid '301'
	option tunlink 'wan'
	option mtu '1300'
	option proto 'xfrm'

config interface 'xfrm1'
	option tunlink 'wan'
	option mtu '1300'
	option proto 'xfrm'
	option ifid '302'

I use the uci task to deploy adapter configurations. I create one interface per-tunnel provided by AWS.

• tunlink sets the IPSec tunnel to connect to & from the wan interface
• mtu is 1300 by default, I didn’t need to configure this value
• ifid is defined as strongSwan will use this to bind an IPSec tunnel to a network interface. This is separate from the name of the interface.

AWS needs to communicate with the BGP instance running on OpenWRT. The value of Inside IPv4 CIDR instructs AWS which IPs to listen on for their BGP instance, and which IP to connect to for fetching routes. The CIDRs will be restricted to the /30 prefix, which provides the range of 4 IP addresses, 2 of which are usable.

As an example, here is the Inside IPv4 CIDR of 169.254.181.60/30 and what that means.

With this known, we know that;

Configuring the IP Address on the IPSec tunnel

I create an alias on top of the originating XFRM interface so that I can utilize the static protocol within UCI to configure static routing in a declarative way.

name: create xfrm alias for static addressing
uci:
  command: section
  key: network
  type: interface
  name: "{{ item.name }}_s"
  value:
    proto: static
    ipaddr:
      - "{{ item.inside_addr | ipaddr('net') | ipaddr(2) }}"
    device: "@{{ item.name }}"
loop: "{{ ipsec_tunnels }}"

config interface 'xfrm0_s'
	option proto 'static'
	option device '@xfrm0'
	list ipaddr '169.254.211.46/30'

config interface 'xfrm1_s'
	option proto 'static'
	list ipaddr '169.254.181.62/30'
	option device '@xfrm1'

I use ipaddr('net') | ipaddr(2) to simplify my Ansible configuration. inside_addr is 169.254.181.60/30 and these functions simply increase the IP address by two, giving the result of 169.254.181.62/30.

This will ensure two things;

• The xfrm interface persistently holds the 169.254.181.62/30 IP Address
• The Linux routing table holds a route of 169.254.181.60/30 via the xfrm interface

This resolves the issue of OpenWRT knowing what IP Address to use and how to route the traffic.

Setting up IPSec

Because I’m using strongSwan, I can also use UCI to configure the IPSec tunnel. With this workflow, IPSec configuration is broken down into three elements;

• Endpoint
  • Primarily what’s known as “IKE Phase 1”. This is the “How I will connect to the other end”.
• Tunnel
  • Primarily known as “IKE Phase 2”. This is the “How do I pass traffic through to the other end”.
• Encryption
  • A set of rules to describe how to handle the cryptography.

IPSec Encryption

What’s defined here drives whether Phase 1 will succeed, and must match the AWS VPN Encryption settings.

name: define ipsec encryption
uci:
  command: section
  key: ipsec
  type: crypto_proposal
  name: "aws"
  value:
    is_esp: '1'
    dh_group: modp1024
    hash_algorithm: sha1

config crypto_proposal 'aws'
	option is_esp '1'
	option dh_group 'modp1024'
	option encryption_algorithm 'aes128'
	option hash_algorithm 'sha1'

In my case, I’m;

• Using AES128 for encryption of the traffic
• Using SHA1 as the integrity algorithm for ensuring packets are correct upon arrival
• Naming the crypto_proposal aws for use by the Endpoint and the Tunnel

AES128 and SHA1 are supported by the configuration defined on the VPN configuration above.

Declaring the IPSec Endpoint

name: configure ipsec remote
uci:
  command: section
  key: ipsec
  type: remote
  name: "{{ item.name }}_ep"
  value:
    enabled: "1"
    gateway: "{{ item.gateway }}"
    local_gateway: "<Public IP>"
    local_ip: "10.0.0.1"
    crypto_proposal:
      - aws
    tunnel:
      - "{{ item.name }}"
    authentication_method: psk
    pre_shared_key: "{{ item.psk }}"
    fragmentation: yes
    keyingretries: '3'
    dpddelay: '30s'
    keyexchange: ikev2
loop: "{{ ipsec_tunnels }}"

config remote 'xfrm0_ep'
	option enabled '1'
	option gateway '<Tunnel 1 IP>'
	option local_gateway '<Public IP>'
	option local_ip '10.0.0.1'
	list crypto_proposal 'ike2'
	list tunnel 'xfrm0'
	option authentication_method 'psk'
	option pre_shared_key '<PSK>'
	option fragmentation '1'
	option keyingretries '3'
	option dpddelay '30s'
	option keyexchange 'ikev2'

config remote 'xfrm1_ep'
	option enabled '1'
	option gateway '<Tunnel 2 IP>'
	option local_gateway '<Public IP>'
	option local_ip '10.0.0.1'
	list crypto_proposal 'ike2'
	list tunnel 'xfrm1'
	option authentication_method 'psk'
	option pre_shared_key '<PSK>'
	option fragmentation '1'
	option keyingretries '3'
	option dpddelay '30s'
	option keyexchange 'ikev2'

• The gateway is known as the Outside IP Address on AWS
• local_gateway points to the WAN Address of OpenWRT
• local_ip points to the LAN address of OpenWRT
• crypto_proposal points to aws (Defined above)
• tunnel points to the name of the interface that this IPSec endpoint represents.
  • Since there are two IPSec endpoints, two of these remotes are created. I use the interface name (from xfrm) across all duplicates to make sure that it’s visibly clear what’s being used where.
• pre_shared_key is the PSK that gets generated (or set) within the VPN Tunnel.
  • This is unique per-tunnel, meaning that there should be two different PSKs per Site-to-site VPN connection. They can be found under the Modify VPN Tunnel Options selection.

Configuring the IPSec Tunnel

The tunnel instructs strongSwan how to bind the IPSec tunnel to an interface. The key here is the ifid of the XFRM interfaces defined earlier.

name: configure ipsec tunnel
uci:
  command: section
  key: ipsec
  type: tunnel
  name: "{{ item.name }}"
  value:
    startaction: start
    closeaction: start
    crypto_proposal: aws
    dpdaction: start
    if_id: "{{ item.ifid }}"
    local_ip: "10.0.0.1"
    local_subnet:
      - 0.0.0.0/0
    remote_subnet:
      - 0.0.0.0/0
loop: "{{ ipsec_tunnels }}"

config tunnel 'xfrm0'
	option startaction 'start'
	option closeaction 'start'
	option crypto_proposal 'ike2'
	option dpdaction 'start'
	option if_id '301'
	option local_ip '10.0.0.1'
	list local_subnet '0.0.0.0/0'
	list remote_subnet '0.0.0.0/0'

config tunnel 'xfrm1'
	option startaction 'start'
	option closeaction 'start'
	option crypto_proposal 'ike2'
	option dpdaction 'start'
	option if_id '302'
	option local_ip '10.0.0.1'
	list local_subnet '0.0.0.0/0'
	list remote_subnet '0.0.0.0/0'

• Like the AWS configuration, I define the local_subnet and remote_subnet to 0.0.0.0/0. This is so I can focus on testing connectivity.
• if_id points to the XFRM interface that’s representing the tunnel in iteration.
  • The if_id must match the tunnel in iteration, as the Inside IPv4 CIDRs have been bound to an interface.

Configuring BGP on OpenWRT

I opted for Quagga when using BGP on OpenWRT.

router bgp 65000
bgp router-id {{ ipsec_inside_cidrs[0] | ipaddr('net') | ipaddr(2) | split('/') | first }}
{% for ipsec_inside_cidr in ipsec_inside_cidrs %}
neighbor {{ ipsec_inside_cidr | ipaddr('net') | ipaddr(1) | split('/') | first }} remote-as {{ bgp_remote_as }}
neighbor {{ ipsec_inside_cidr | ipaddr('net') | ipaddr(1) | split('/') | first }} soft-reconfiguration inbound
neighbor {{ ipsec_inside_cidr | ipaddr('net') | ipaddr(1) | split('/') | first }} distribute-list localnet in
neighbor {{ ipsec_inside_cidr | ipaddr('net') | ipaddr(1) | split('/') | first }} distribute-list all out
neighbor {{ ipsec_inside_cidr | ipaddr('net') | ipaddr(1) | split('/') | first }} ebgp-multihop 2
{% endfor %}

/etc/quagga/bgpd.conf – Rendered Template

router bgp 65000
bgp router-id 169.254.211.46
neighbor 169.254.211.45 remote-as 64512
neighbor 169.254.211.45 soft-reconfiguration inbound
neighbor 169.254.211.45 distribute-list localnet in
neighbor 169.254.211.45 distribute-list all out
neighbor 169.254.211.45 ebgp-multihop 2
neighbor 169.254.181.61 remote-as 64512
neighbor 169.254.181.61 soft-reconfiguration inbound
neighbor 169.254.181.61 distribute-list localnet in
neighbor 169.254.181.61 distribute-list all out
neighbor 169.254.181.61 ebgp-multihop 2

• Like earlier, I use ipaddr('net') | ipaddr(1) to increment the IP address from the CIDR
• remote-as defines the AWS-side ASN.
  • BGP at its’ core defines routes based on path to AS, a layer on-top of IP Addresses.
  • It’s designed to work with direct connections, not over-the-internet.
    • ISPs & exchanges will, however, use BGP at their level to forward the traffic on.
• router bgp states what the ASN of the OpenWRT router is. Because I used the default of 65000 from AWS, I place that here.
• bgp router-id is set to the first XFRM interface’s IP address, since the same BGP instance will be shared by both tunnels in the event that one tunnel goes down. AWS does not do a validation check on the router-id.

Verifying the connection to IPSec

Using the swanctl command, I can identify whether my applied configuration is successful when logged into my OpenWRT router using SSH.

Start swanctl

I don’t use the legacy ipsec init script, instead, directly using the swanctl one. Under the hood, this will convert the UCI configuration into a strongSwan configuration located at /var/swanctl/swanctl.conf

/etc/init.d/swanctl start
ipsec statusall

Routing traffic to & from the XFRM Interface

I finally need to instruct OpenWRT to forward packets that are destined to xfrm0 or xfrm1 to be allowed. The fact that the Linux routing table will state that 10.1.0.0/24 is accessed via xfrm0, which is applied via BGP is enough to know that either xfrm0 or xfrm1 is the interface required.

By default, a flag of REJECT is defined. By applying the following firewall rule, packet successfully go through to the AWS VPC.

name: install firewall zone
uci:
  command: section
  key: firewall
  type: zone
  find_by:
    name: 'tunnel'
  value:
    input: REJECT
    output: ACCEPT
    forward: REJECT
    network:
      - xfrm0
      - xfrm1

name: install firewall forwarding
uci:
  command: section
  key: firewall
  type: forwarding
  find_by:
    dest: 'tunnel'
  value:
    src: lan

config zone
	option name 'tunnel'
	option input 'REJECT'
	option output 'ACCEPT'
	option forward 'REJECT'
	list network 'xfrm0' 'xfrm1'

config forwarding
	option src 'lan'
	option dest 'tunnel'

Final tasks

The final steps of the Ansible playbook is to instruct the UCI framework to save the changes to the disk, and to reload the configuration of all services required.

name: commit changes
uci:
  command: commit

name: enable required services
service:
  name: "{{ item }}"
  enabled: yes
  state: reloaded
loop:
  - swanctl
  - quagga
  - network

I then invoke the Ansible playbook by using a local-exec provisioner on a null_resource within terraform, where the AWS Site-to-Site resource is a dependency. Along the lines of:

resource "null_resource" "cluster" {
  provisioner "local-exec" {
    command = <<EOT
  ansible-playbook \
    -I ${var.openwrt_address}, \
    -e 'aws_tunnel_ips=${aws_vpn_connection.main.tunnel1_address},${aws_vpn_connection.tunnel2_address}' 
    playbook.yaml \
    -e 'aws_psk=${aws_vpn_connection.main.tunnel1_preshared_key},${aws_vpn_connection.main.tunnel2_preshared_key}' 
    playbook.yaml
    EOT
  }
}

This is a shortened version of what I have, but by simply piping the Ansible playbook with the outputs of the AWS Site-to-Site Resource, my router is automatically configured correctly when I create a Site-to-Site resource.

With IPSec now deployed, I can communicate directly with my resources hosted on AWS as if it were local.