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dns-roundrobin-webapp

Multi-server ASP.NET web application load-balanced via DNS Round Robin, where users authenticate using JWT and remain logged in across all backend nodes thanks to centralized session persistence.

This project demonstrates horizontal scaling, DNS-based load balancing, and state sharing across instances in a fully containerized environment.

DNS Setup

The following steps describe how to configure the development machine (Debian/Ubuntu/Mint) so the BIND9 DNS container and the backend API containers can be reached from any device in the LAN.

The key strategy is IP aliasing: assigning extra LAN IPs directly to the host interface and mapping each backend container to one of these IPs.

1. Disable systemd-resolved DNS stub (to free port 53)

Debian-based distros run systemd-resolved, which binds to port 53 on localhost. This conflicts with the BIND9 container that also needs to listen on port 53.

Edit the config:

sudo nano /etc/systemd/resolved.conf

Set:

DNSStubListener=no

(Optional) If you want Docker/BIND9 as system DNS:

DNS=127.0.0.1

Restart service:

sudo systemctl restart systemd-resolved

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2. Add available LAN IPs to the host interface (IP aliasing)

This project requires each backend API instance to be reachable via its own IP address in the LAN. These IPs must belong to your local subnet.

2.1 Identify your main interface and subnet

Run:

ifconfig

Example Output:

wlp8s0: inet 10.94.217.143 netmask 255.255.255.0

This means your LAN is: 10.94.217.0/24

2.2 Scan the network to see which IPs are already in use

sudo arp-scan --localnet

2.3 Choose 3 unused IPs

Example:

• 10.94.217.151
• 10.94.217.152
• 10.94.217.153

2.4 Add these IP aliases to the host network interface

sudo ip addr add 10.94.217.151/24 dev wlp8s0
sudo ip addr add 10.94.217.152/24 dev wlp8s0
sudo ip addr add 10.94.217.153/24 dev wlp8s0

They become fully routable inside your LAN and reachable by your phone, laptop, or any other device.

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3. Bind backend containers to those IPs

In backend/compose.yaml, map each API container to its dedicated LAN IP.

Example:

API 1

ports:
  - "10.94.217.151:80:8080"

API 2

ports:
  - "10.94.217.152:80:8080"

API 3

ports:
  - "10.94.217.153:80:8080"

Now each container has a “real” LAN IP endpoint:

• http://10.94.217.151
• http://10.94.217.152
• http://10.94.217.153

And all are externally visible from your phone.

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4. Configure the BIND9 DNS for Round Robin

In the zone file for meutrabalho.com.br:

$TTL 2

$ORIGIN meutrabalho.com.br.

@    IN SOA  ns1.meutrabalho.com.br. admin.meutrabalho.com.br. (
           2025112500 ; serial YYYYMMDDnn
           1h        ; refresh
           5m        ; retry
           1w     ; expire
           1          ; minimum TTL / negative caching
)
     IN NS   ns1.meutrabalho.com.br.
ns1  IN A    10.94.217.143   ; IP of your DNS host

; A records for apex/root domain
@    IN A    10.94.217.151
@    IN A    10.94.217.152
@    IN A    10.94.217.153

; Records for subdomains
www    IN A    10.94.217.151
www    IN A    10.94.217.152
www    IN A    10.94.217.153

Note: The very low TTL forces clients to request DNS answers frequently, enabling Round Robin switching.

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5. Configure your client devices

On any device that should access the app:

Set the DNS to your host machine (running the bind9 container):

• DNS: 10.94.217.143

Now the domain:

• www.meutrabalho.com.br

will resolve to:

• 10.94.217.151
• 10.94.217.152
• 10.94.217.153

...in a round-robin cycle.

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API Setup

1. Configure User Secrets

{
  "ConnectionStrings": {
    "DefaultConnection": "Server=localhost,1433; Database=UserManagementDb; MultipleActiveResultSets=True; User ID=sa;Password='examplePassword1234'; Encrypt=False;",
    "DbTestConnection": "Server=localhost,1433; Database=UserManagementDbTest; MultipleActiveResultSets=True; User ID=sa;Password='examplePassword1234'; Encrypt=False;"
  },
  "JwtConfig": {
    "Secret": "a-string-secret-at-least-256-bits-long",
    "Issuer": "http://localhost:5201/",
    "Audience": "http://localhost:5201/",
    "AccessTokenExpirationMinutes": "15",
    "RefreshTokenExpirationDays": "7"
  }
}