Solutions
The starting solution can be found here.
The final solution can be found here.
Overview
Now before we get to our agent-side implementation, there’s actually one more thing we need to address here on the server.
Let’s just think about this for a second:
- Our server runs say on HTTPS, and so does our agent
- We signal our intent to switch both to DNS by hitting the
/switchon port 8080 - Our server then sends
trueto the agent - (TODO) Our agent will interpret this value and reach out to connect to the server over DNS
But there is of course something missing here… We don’t have a DNS server to respond. So a transition should also involve creating the new server. Now there are two ways to do this - the right way, and the expedited way.
The right way assumes we only have our HTTP server running (in the example above), then when the trigger is received it creates a DNS Server. Then, once it’s confirmed the new connection with the agent has been established over DNS it will kill the HTTPS server.
But, there is a much simpler way - we can just start both servers when our application begins, and keep both open. This of course represents the expedited way, and it’s what I’ll opt to do in this situation since it’ll save us quite a bit of work, and have the exact same outcome.
That being said, it’s not great practice and it’s not something that scales really well - it’s a definite uptick in technical debt, the equivalent of sweeping dust under the rug.
That being the case, I do 100% encourage you to think about how to do it in “the right way”. This would be an excellent exercise to perform following the completion of this workshop, and by the end you will have all the fundamental building blocks to figure out how to do this.
OK, with that out of the way, let’s just make a simple adjustment to our server’s main so that we start not only the server that’s specified in the protocol field of config.yaml, but both irrespective of that field’s value.
Server’s main
In our server’s main, this is the code that currently:
- Creates a server based on the value in
cfg - Starts that specific server
- Following the reception of the termination signal stops only that server
// Create server using interface's factory function
server, err := models.NewServer(cfg)
if err != nil {
log.Fatalf("Failed to create server: %v", err)
}
// Start the server in own goroutine
go func() {
log.Printf("Starting %s server on %s", cfg.Protocol, cfg.ServerAddr)
if err := server.Start(); err != nil {
log.Fatalf("Server error: %v", err)
}
}()
// Wait for interrupt signal
sigChan := make(chan os.Signal, 1)
signal.Notify(sigChan, os.Interrupt)
<-sigChan
// Graceful shutdown
log.Println("Shutting down server...")
if err := server.Stop(); err != nil {
log.Printf("Error stopping server: %v", err)
}
We just want to change that code to:
// Create BOTH servers regardless of config
log.Printf("Starting both protocol servers on %s", cfg.ServerAddr)
// Create HTTPS server
httpsCfg := *cfg
httpsCfg.Protocol = "https"
httpsServer, err := models.NewServer(&httpsCfg)
if err != nil {
log.Fatalf("Failed to create HTTPS server: %v", err)
}
// Create DNS server
dnsCfg := *cfg
dnsCfg.Protocol = "dns"
dnsServer, err := models.NewServer(&dnsCfg)
if err != nil {
log.Fatalf("Failed to create DNS server: %v", err)
}
// Start HTTPS server in goroutine
go func() {
log.Printf("Starting HTTPS server on %s (TCP)", cfg.ServerAddr)
if err := httpsServer.Start(); err != nil {
log.Fatalf("HTTPS server error: %v", err)
}
}()
// Start DNS server in goroutine
go func() {
log.Printf("Starting DNS server on %s (UDP)", cfg.ServerAddr)
if err := dnsServer.Start(); err != nil {
log.Fatalf("DNS server error: %v", err)
}
}()
// Wait for interrupt signal
sigChan := make(chan os.Signal, 1)
signal.Notify(sigChan, os.Interrupt)
<-sigChan
// Graceful shutdown
log.Println("Shutting down both servers...")
if err := httpsServer.Stop(); err != nil {
log.Printf("Error HTTPS stopping server: %v", err)
}
if err := dnsServer.Stop(); err != nil {
log.Printf("Error DNS stopping server: %v", err)
}
Again, for posterity, here is the entire function:
func main() {
// Command line flag for config file path
configPath := flag.String("config", pathToYAML, "path to configuration file")
flag.Parse()
// Load configuration
cfg, err := config.LoadConfig(*configPath)
if err != nil {
log.Fatalf("Failed to load config: %v", err)
}
// Load our control API
control.StartControlAPI()
// Create BOTH servers regardless of config
log.Printf("Starting both protocol servers on %s", cfg.ServerAddr)
// Create HTTPS server
httpsCfg := *cfg
httpsCfg.Protocol = "https"
httpsServer, err := models.NewServer(&httpsCfg)
if err != nil {
log.Fatalf("Failed to create HTTPS server: %v", err)
}
// Create DNS server
dnsCfg := *cfg
dnsCfg.Protocol = "dns"
dnsServer, err := models.NewServer(&dnsCfg)
if err != nil {
log.Fatalf("Failed to create DNS server: %v", err)
}
// Start HTTPS server in goroutine
go func() {
log.Printf("Starting HTTPS server on %s (TCP)", cfg.ServerAddr)
if err := httpsServer.Start(); err != nil {
log.Fatalf("HTTPS server error: %v", err)
}
}()
// Start DNS server in goroutine
go func() {
log.Printf("Starting DNS server on %s (UDP)", cfg.ServerAddr)
if err := dnsServer.Start(); err != nil {
log.Fatalf("DNS server error: %v", err)
}
}()
// Wait for interrupt signal
sigChan := make(chan os.Signal, 1)
signal.Notify(sigChan, os.Interrupt)
<-sigChan
// Graceful shutdown
log.Println("Shutting down both servers...")
if err := httpsServer.Stop(); err != nil {
log.Printf("Error HTTPS stopping server: %v", err)
}
if err := dnsServer.Stop(); err != nil {
log.Printf("Error DNS stopping server: %v", err)
}
}
Test
We can test this real quick by running the server:
❯ go run ./cmd/server
2025/08/24 09:35:43 Starting Control API on :8080
2025/08/24 09:35:43 Starting both protocol servers on 127.0.0.1:8443
2025/08/24 09:35:43 Starting HTTPS server on 127.0.0.1:8443 (TCP)
2025/08/24 09:35:43 Starting DNS server on 127.0.0.1:8443 (UDP)
We can see from the output that both servers were started on the same port 8443 - HTTPS on TCP, and DNS on UDP.
We can also confirm that this is actually case with lsof:
❯ lsof -i :8443
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
server 65243 faanross 5u IPv4 0xcdbcc35684b1111c 0t0 UDP localhost:pcsync
server 65243 faanross 6u IPv4 0x3edc32002434b2af 0t0 TCP localhost:pcsync(LISTEN)