Captive portals operate through multiple layers of network interception and redirection. When a device connects to such a network, the following sequence occurs:

// Example of DNS interception (Python pseudocode)
def captive_portal_intercept(request):
    if not request.client.has_authenticated:
        redirect_to_portal(request.ip)
    else:
        allow_regular_traffic(request)

The system typically involves these technical elements:

• DNS hijacking (resolving all domains to portal IP)
• HTTP/HTTPS interception
• MAC address tracking
• Session cookies or tokens

Here's how modern OSes detect captive portals by attempting to access a known endpoint:

// JavaScript example for portal detection
fetch('http://connectivitycheck.gstatic.com/generate_204')
  .then(response => {
    if (response.status !== 204) {
      // Portal detected
      window.location = response.url;
    }
  });

For developers creating apps that need to handle captive networks:

// Python example using requests with portal handling
import requests

def internet_connectivity_check():
    try:
        response = requests.get('http://example.com', timeout=5)
        if len(response.content) < 1000:  # Likely portal page
            return False
        return True
    except:
        return False

For automated testing scenarios where you need to authenticate through the portal:

// Automating form submission (Python + Selenium example)
from selenium import webdriver

def portal_authenticate(username, password):
    driver = webdriver.Chrome()
    driver.get('http://google.com')  # Will redirect to portal
    driver.find_element_by_name('username').send_keys(username)
    driver.find_element_by_name('password').send_keys(password)
    driver.find_element_by_tag_name('form').submit()

Important security aspects developers should know:

• HTTPS breaks the traditional captive portal flow (hence HTTP interception)
• Certificate pinning can prevent portal redirection
• Always validate portal pages before submitting credentials

Basic captive portal implementation using Linux and iptables:

# Basic iptables rules for captive portal
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 80 -j DNAT \
--to-destination 192.168.1.1:80
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 443 -j DNAT \
--to-destination 192.168.1.1:443

When you connect to a public WiFi network, the gateway (usually a router or firewall) intercepts all HTTP traffic through these key steps:

1. Initial DNS Query: Your device resolves domains through the captive portal's DNS server
2. HTTP Redirect: All HTTP requests get 302-redirected to the portal's authentication page
3. Traffic Whitelist: Only specific domains (like payment processors) are allowed through

// Example of how a router might implement this in iptables:
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 80 \
         -j DNAT --to-destination 10.0.0.1:80
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 443 \
         -j DNAT --to-destination 10.0.0.1:443

Modern operating systems actively check for captive portals by attempting to access specific test URLs:

• Apple uses http://captive.apple.com/hotspot-detect.html
• Microsoft uses http://www.msftconnecttest.com/connecttest.txt
• Android pings http://connectivitycheck.gstatic.com/generate_204

# Python example to detect captive portals
import requests

def check_captive_portal():
    try:
        r = requests.get("http://connectivitycheck.gstatic.com/generate_204", timeout=3)
        return r.status_code != 204
    except:
        return True

Different networks implement captive portals in distinct ways:

For automation scenarios, you can script the authentication process:

// Node.js example for hotel WiFi authentication
const puppeteer = require('puppeteer');

async function autoAuth(credentials) {
  const browser = await puppeteer.launch();
  const page = await browser.newPage();
  
  try {
    await page.goto('http://neverssl.com', {waitUntil: 'networkidle0'});
    const redirectUrl = page.url();
    
    if(redirectUrl.includes('hotelwifi.com/auth')) {
      await page.type('#username', credentials.user);
      await page.type('#password', credentials.pass);
      await page.click('#accept_terms');
      await page.click('#submit_button');
    }
  } finally {
    await browser.close();
  }
}

Captive portals create unique security challenges:

• HTTPS interception requires installing custom CA certificates
• MAC address tracking persists even after network disconnection
• Cookie-based sessions can be hijacked if not properly secured

# Bash command to monitor portal interactions
tcpdump -i wlan0 -s 0 -A 'tcp[((tcp[12:1] & 0xf0) >> 2):4] = 0x47455420' | 
grep -E 'Host:|GET /|POST /|HTTP/'