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🗓️ 04102025 0030
📎 networking_dns_tools

/etc/hosts File

Local DNS override file that is checked before DNS servers.

What is it?​

The /etc/hosts file is a local DNS override file that maps hostnames to IP addresses.

Key point: Your system checks this file before querying DNS servers!

DNS Resolution Order​

  1. Browser cache
  2. Operating system cache
  3. /etc/hosts file ⬅️ THIS FILE - checked before DNS!
  4. DNS resolver (networking_interfaces's /etc/resolv.conf - configured DNS servers like 8.8.8.8)

This is why editing /etc/hosts bypasses all networking_dns_tools queries!

File Location​

  • Linux/Mac: /etc/hosts
  • Windows: C:\Windows\System32\drivers\etc\hosts

Format​

# IP_ADDRESS    HOSTNAME    [ALIASES]
127.0.0.1       localhost
192.168.1.100   myserver.local myserver
93.184.216.34   example.com www.example.com

Structure:

  • Lines starting with # are comments
  • Format: IP_ADDRESS + whitespace + HOSTNAME + optional ALIASES
  • Multiple aliases allowed per line
  • Case-insensitive (hostnames)

Common Use Cases​

Local Development​

127.0.0.1       myapp.local
127.0.0.1       api.myapp.local
127.0.0.1       db.myapp.local

→ Access local services with friendly names instead of localhost:8080

Block Domains (Ad Blocking)​

0.0.0.0         ads.example.com
0.0.0.0         tracking.example.com
0.0.0.0         malware-site.com

→ Browser tries to connect to 0.0.0.0 (nowhere) = domain blocked!

DNS Workaround (Your NXDOMAIN Fix!)​

93.184.216.34   example.com

→ When DNS fails with NXDOMAIN, this forces the resolution locally

Testing Before DNS Propagation​

54.123.45.67    newserver.example.com

→ Test new server before DNS records are updated globally

Multiple Aliases​

192.168.1.50    jenkins.local jenkins ci

→ Access via jenkins.local, jenkins, or ci

When to Use​

✅ Good for:

  • Temporary DNS issue workarounds
  • Local development environments (avoid port numbers!)
  • Testing before DNS propagation
  • Blocking unwanted domains (privacy/security)
  • Overriding DNS for specific domains
  • Quick testing without modifying DNS servers

❌ Not recommended for:

  • Permanent solutions (fix DNS properly!)
  • Domains with load balancers (they use multiple IPs)
  • Domains where IP changes frequently (CDNs)
  • Production environments (doesn't scale)
  • Sharing across team (each person needs to edit manually)

Editing the File​

# Linux/Mac
sudo nano /etc/hosts
# or
sudo vim /etc/hosts

# Windows (as Administrator)
notepad C:\Windows\System32\drivers\etc\hosts

Note: Requires root/administrator privileges (it's a system file).

Example Session​

$ sudo nano /etc/hosts

# Add this line:
192.168.1.100   myserver.local

# Save (Ctrl+O, Enter) and exit (Ctrl+X)

Testing Changes​

Flush DNS Cache​

After editing, flush cached DNS entries:

# Mac
sudo dscacheutil -flushcache
sudo killall -HUP mDNSResponder  # Also restart DNS responder

# Linux (systemd)
sudo systemd-resolve --flush-caches

# Linux (nscd)
sudo /etc/init.d/nscd restart

# Windows (as Administrator)
ipconfig /flushdns

Verify Changes​

Test with ping (networking_connectivity):

$ ping myserver.local
PING myserver.local (192.168.1.100): 56 data bytes
64 bytes from 192.168.1.100: icmp_seq=0 ttl=64 time=1.234 ms

→ Resolves to your /etc/hosts entry!

Test with dig (networking_dns_tools):

$ dig myserver.local
# Will still query DNS (doesn't use /etc/hosts)
# But curl, ping, browsers DO use /etc/hosts

→ Note: dig bypasses /etc/hosts! Use getent instead:

$ getent hosts myserver.local
192.168.1.100   myserver.local

→ This uses /etc/hosts

Test with curl (networking_http_tools):

$ curl http://myserver.local
# Uses /etc/hosts, will connect to 192.168.1.100

NXDOMAIN Workaround Example​

The Problem: DNS can't resolve domain (NXDOMAIN error)

Your Fix:

# 1. Get IP from working DNS server
$ dig example.com @8.8.8.8
;; ANSWER SECTION:
example.com.  86400  IN  A  93.184.216.34

→ DNS server 8.8.8.8 knows the IP!

# 2. Add to /etc/hosts
$ sudo nano /etc/hosts
# Add this line:
93.184.216.34    example.com
# 3. Flush DNS cache
$ sudo dscacheutil -flushcache  # Mac
# 4. Test with ping
$ ping example.com
PING example.com (93.184.216.34): 56 data bytes
64 bytes from 93.184.216.34: icmp_seq=0 ttl=117 time=12.4 ms

→ Works! System used /etc/hosts instead of broken DNS

Why This Worked​

  1. Your DNS resolver (from networking_interfaces's /etc/resolv.conf) was returning NXDOMAIN
  2. /etc/hosts is checked BEFORE DNS queries
  3. System found example.com in /etc/hosts → used that IP
  4. Never queried broken DNS server!

When to Remove the Fix​

Once DNS is working again:

# Test if DNS works now
$ dig example.com
# If it returns the correct IP, remove from /etc/hosts

$ sudo nano /etc/hosts
# Remove or comment out the line:
# 93.184.216.34    example.com

Interpreting Behavior​

How to Tell if /etc/hosts is Being Used​

Check the file:

$ cat /etc/hosts | grep example.com
93.184.216.34   example.com

→ Entry exists

Use getent (respects /etc/hosts):

$ getent hosts example.com
93.184.216.34   example.com

→ System uses this IP

Compare with dig (bypasses /etc/hosts):

$ dig +short example.com
93.184.216.35   # Different IP!

→ DNS says different IP, but system will use /etc/hosts (93.184.216.34)

Common Issues​

Changes not taking effect?

  1. Flush DNS cache (see above)
  2. Check syntax (no typos, correct whitespace)
  3. Verify file permissions: ls -l /etc/hosts (should be readable by all)

Application ignores /etc/hosts?

  • Some apps (like dig, nslookup) bypass /etc/hosts intentionally
  • Most apps (browsers, ping, curl) respect it

Real-World Examples​

Local Kubernetes Development​

127.0.0.1       app.local
127.0.0.1       api.app.local
127.0.0.1       db.app.local

→ Access services by name instead of localhost:8080, localhost:3000, etc.

Blocking Telemetry​

0.0.0.0  telemetry.microsoft.com
0.0.0.0  vortex.data.microsoft.com

Staging Environment Testing​

54.123.45.67   staging.myapp.com
54.123.45.68   api.staging.myapp.com

→ Test staging servers before DNS points to them

References