What Really Happens When You Enter a URL? A Deep Dive into DNS Resolution and Server Setup

What Happens When a Browser Requests a URL

The client first ensures network connectivity, then the user types a URL and presses Enter. The browser performs a local DNS lookup to translate the domain name into an IP address, initiates a TCP three‑way handshake, sends an HTTP request, receives the response, renders the page, and finally closes the TCP connection with a four‑way handshake.

0. Ensure internet connectivity
1. Enter URL in browser (e.g., www.yuchaoit.cn)
2. Perform DNS lookup to obtain IP
3. Start TCP three‑way handshake with the server
4. Send HTTP request (may involve multiple requests)
5. Server processes request and sends HTTP response(s)
6. Browser parses and renders the response
7. Close connection (TCP four‑way handshake)

DNS Hierarchy and Domain Structure

Domain names are organized in a tree‑like hierarchy. Each label is separated by a dot, forming root (.), top‑level domains (e.g., .cn, .com), second‑level domains (e.g., yuchaoit), and sub‑domains (e.g., www).

Root servers (13 globally) are the authoritative source for top‑level domain information.

# Root zone example (simplified)
.    IN NS a.root-servers.net.
.    IN NS b.root-servers.net.
... (total 13 root servers)

DNS Resolution Process

The resolver follows these steps:

Check the local /etc/hosts file for a static mapping.

If not found, query the configured local DNS server (often provided by the ISP).

The local server checks its cache; if missing, it recursively queries root servers, then TLD servers, then authoritative servers for the domain.

The final IP address is returned to the client and cached for future queries.

Key DNS Terminology

Domain name

DNS resolution

Recursive query

Iterative query

TTL (Time‑to‑Live)

Cache

Using dig to Trace DNS Resolution

Install the utility with yum install bind-utils -y and run:

# Show local DNS servers
cat /etc/resolv.conf

# Trace the full resolution path
 dig +trace www.yuchaoit.cn

# Sample output (truncated)
.            2974 IN NS a.root-servers.net.
... (root servers)
cn.          172800 IN NS a.dns.cn.
... (TLD servers)
www.yuchaoit.cn. 600 IN A 123.206.16.61

The output demonstrates the query flow from root to TLD to authoritative servers, ending with the final A record.

Setting Up Your Own DNS Server with BIND

Steps (Linux example):

Prepare two machines: a server (e.g., 172.16.1.61) and a client (e.g., 172.16.1.7).

Install BIND: yum install bind bind-utils -y.

Disable firewalls or open port 53.

Edit /etc/named.conf to listen on all interfaces and allow queries.

Create a zone file for your domain (e.g., laoliulinux.cn.zone) and include it in /etc/named.rfc1912.zones.

Define records (A, NS, SOA) in the zone file.

Start the service: systemctl start named and verify with netstat -tunlp | grep named.

Configure clients to use the new DNS server by setting nameserver 172.16.1.61 in /etc/resolv.conf.

# Example zone file (laoliulinux.cn.zone)
$TTL 1D
@   IN SOA @ rname.invalid. (
        0 ; serial
        1D ; refresh
        1H ; retry
        1W ; expire
        3H ) ; minimum
    IN NS @
    IN A 172.16.1.61
www IN A 172.16.1.61
linux IN A 172.16.1.61

Deploying a Lightweight DNS Server with dnsmasq

dnsmasq provides a simple alternative to BIND.

Install: yum install dnsmasq -y.

Create /etc/dnsmasq.conf and set upstream DNS, listen address, and additional hosts file.

Prepare /etc/resolv.dnsmasq.conf (upstream) and /etc/hosts.dnsmasq.conf (local mappings).

Start the service: systemctl start dnsmasq.

Point client /etc/resolv.conf to the dnsmasq server (e.g., nameserver 172.16.1.61).

# /etc/dnsmasq.conf (relevant lines)
resolv-file=/etc/resolv.dnsmasq.conf
listen-address=172.16.1.61
addn-hosts=/etc/hosts.dnsmasq.conf

# Example hosts file
172.16.1.61 master-61
172.16.1.7 web-7
... (other internal hosts)

Both BIND and dnsmasq can be used to resolve internal domain names, provide caching, and support custom A records for services within a private network.