Day: November 20, 2025

Flushing DNS is an essential network maintenance task that ensures your device accesses the most up-to-date domain information. Outdated DNS records in your cache can lead to issues such as slow website loading, connection errors, and the inability to reach recently updated websites. This guide covers how to flush DNS on Windows, macOS, Linux, and mobile devices, provides troubleshooting tips, discusses advanced methods, and explains public DNS cache clearing. We’ll also show you how HasheTools can simplify your DNS management.

What Is Flushing DNS?

When you visit a website, your computer performs a DNS lookup to translate a domain name (like example.com) into an IP address. Your system stores this information locally in a DNS cache to speed up subsequent visits. Over time, cached entries can become outdated, causing connectivity issues. Flushing DNS clears these cached records and forces your system to fetch fresh DNS information.

Key Terms:

• DNS Cache: Temporary storage of domain-to-IP mappings.
• TTL (Time-to-Live): Duration a DNS record remains in cache.
• Flush DNS / Clear DNS Cache / Reset DNS Cache: Different terms for the same process.

Why You Should Flush DNS

Flushing your DNS cache resolves several common network problems:

1. Accessing Updated Websites: When a website changes servers or IP addresses, your cached DNS may point to the old location.
2. Fixing Connection Errors: Outdated DNS records can cause 404 Not Found errors or failed connections.
3. Speed & Performance: Connecting to a closer or updated server can improve load times.
4. Security: Removing cached DNS entries helps prevent DNS spoofing or cache poisoning attacks.
5. Network Troubleshooting: Clearing DNS is often the first step in diagnosing internet issues.

How to Flush DNS: Step-by-Step

Windows

Flushing DNS on Windows depends on your version. All commands require administrator privileges.

Windows 11 & 10

1. Right-click the Start menu → select Windows Terminal (Admin) or Command Prompt (Admin).
2. Type: ipconfig /flushdns
3. Press Enter. You’ll see: “Successfully flushed the DNS Resolver Cache”
4. Type exit to close the terminal.

Additional commands for network reset:

ipconfig /release
ipconfig /renew

Windows 8 / 8.1

1. Press Windows + X → select Command Prompt (Admin).
2. Run ipconfig /flushdns.

Windows 7 / Vista

1. Start → All Programs → Accessories → Right-click Command Prompt → Run as Administrator.
2. Run ipconfig /flushdns.

Windows XP / 2000 / NT

1. Start → Run → type cmd.exe → Enter.
2. Run ipconfig /flushdns.

macOS and Mac OS X

Open Terminal from Applications > Utilities and run the appropriate command for your version:

macOS Version	Command
macOS 12 (Monterey), 11 (Big Sur), 10.15 (Catalina)	sudo killall -HUP mDNSResponder
macOS 10.14 (Mojave)	sudo killall -HUP mDNSResponder
macOS 10.13 – 10.12 (High Sierra, Sierra)	sudo killall -HUP mDNSResponder
Mac OS X 10.11 (El Capitan)	sudo killall -HUP mDNSResponder
Mac OS X 10.10 (Yosemite)	sudo discoveryutil udnsflushcaches
Mac OS X 10.6 (Snow Leopard)	sudo dscacheutil -flushcache
Mac OS X 10.5 and below	sudo lookupd -flushcache

Linux

Linux DNS cache commands depend on the service used:

• systemd-resolve (Ubuntu/Debian): sudo systemd-resolve --flush-caches
• NSCD: sudo /etc/init.d/nscd restart
• BIND: sudo rndc flush

Mobile Devices

• iOS: Restart your device or toggle Airplane Mode.
• Android: Restart your device or use network reset options.

Alternative Ways to Flush DNS

1. Restart Your Device: Easiest method if you lack admin privileges.
2. Reset Network Connection: Reconnect WiFi or unplug/plug Ethernet.
3. Change DNS Server: Switching to a public DNS like Google 8.8.8.8 or Cloudflare 1.1.1.1 automatically refreshes the cache.
4. Modify Hosts File: Temporarily force specific domain resolution.

Clearing Public DNS Resolver Cache

• Cloudflare 1.1.1.1: Visit their Purge Cache tool → enter domain → select record type → clear.
• Google Public DNS 8.8.8.8: Visit Google DNS Flush Cache → enter domain → flush.
• OpenDNS: Use OpenDNS CacheCheck → enter domain → refresh cache.

Troubleshooting DNS Issues

• Page Not Found (404): Old cache points to deleted page; flush DNS.
• Unable to Connect: Cached IP may be wrong; flush DNS or reset network.
• Old Website Version: Cached data shows outdated content; flush DNS to see updates.
• Network Performance Issues: Cached DNS might point to a distant server; flushing can optimize routing.

Using HasheTools to Check and Flush DNS

HasheTools Flush DNS Tool makes clearing DNS simple:

• No command line needed
• Works on Windows, macOS, Linux
• Clears local cache instantly
• Links to DNS propagation checker and other network diagnostic tools

Other useful HasheTools DNS tools:

• DNS Lookup
• Reverse DNS Lookup
• Domain Expiry Check

FAQs About Flushing DNS

Q1: Is it safe to flush DNS?
Yes. Flushing your DNS cache is safe and does not harm your system.

Q2: Does flushing DNS improve internet speed?
It doesn’t increase raw bandwidth, but it can resolve slow access due to outdated cached records.

Q3: How often should I flush DNS?
Only when you encounter connectivity issues, outdated content, or network changes.

Q4: Can I flush DNS without admin access?
Yes, by restarting your device or using the HasheTools online tool.

Q5: Why am I still seeing the old website after flushing DNS?
Your browser cache may still hold old content; clear the browser cache as well.

Pro Tips

• Combine flush DNS with ipconfig /release and ipconfig /renew for a complete network reset.
• Use public DNS servers for faster and more reliable resolution.
• Keep a regular maintenance schedule for servers and DNS settings to avoid cache issues.

Conclusion

Flushing DNS is a critical step in network maintenance, troubleshooting, and ensuring up-to-date website access. With step-by-step instructions for Windows, macOS, Linux, and mobile devices, along with HasheTools online tools, you can maintain optimal DNS performance and avoid connectivity problems.

Email deliverability depends heavily on your DNS configuration. If your mail server is even slightly misconfigured, you may run into warnings that directly affect inbox placement. One of the most common alerts seen during SMTP tests is:

“Reverse DNS does not match SMTP banner.”

This warning appears on platforms like MXToolbox, HasheTools SMTP tests, and other email diagnostics tools. Although it may look technical or confusing, the issue is simple: Your mail server is identifying itself using a hostname that does not match its reverse DNS record.

This mismatch does not always break your email service, but it can affect your sender reputation and may cause your messages to land in spam.

Let’s break it down in an easy, clear, and actionable way.

What Is Reverse DNS (rDNS)?

Reverse DNS, also called PTR record lookup, is a DNS mechanism that maps an IP address back to a hostname.

While a normal DNS (A record) converts hostname → IP,

reverse DNS converts IP → hostname.

Simple Example

• A record: mail.example.com → 192.0.2.10
• PTR record (rDNS): 192.0.2.10 → mail.example.com

Mail servers rely on this reverse lookup to verify the identity of the server trying to send email. If a mail server cannot verify the reverse DNS or finds mismatching information, it becomes suspicious.

This forms the basis of the warning.

What Is an SMTP Banner?

When your mail server connects to another server to send an email, it introduces itself using a greeting message called the SMTP banner.

Example SMTP banner:

220 mail.example.com ESMTP Postfix

The most important part here is the hostname:

mail.example.com

Receiving servers expect this hostname to match your reverse DNS and forward DNS.

If it does not match, the server flags it as a potential spam indicator.

Why Does the “Reverse DNS Does Not Match SMTP Banner” Warning Appear?

This warning appears when:

SMTP banner hostname ≠ PTR record hostname

Common reasons include:

1. Incorrect SMTP hostname

Your mail server might be configured with a default hostname like:

• localhost
• server.yourhostprovider.com
• ip-123-45-67-89.hostingprovider.net

Instead of your actual mailing domain.

2. Wrong PTR record

The PTR record may point to:

• an old hostname
• your hosting provider’s hostname
• a generic IP-based hostname

Instead of your mail server hostname.

3. Missing forward DNS

Even if the PTR is correct, if the matching A record is missing, it still triggers a warning.

4. Using an ISP mail server or shared hosting

Cloud providers like AWS, DigitalOcean, Vultr, and shared hosts often use generic reverse DNS names.

5. Misconfigured mail server during setup

During Postfix, Exim, or cPanel setup, if the SMTP banner was not updated properly.

Why Is This Mismatch a Problem?

While this warning is not always fatal, it strongly impacts:

1. Email Deliverability

Gmail, Outlook, Yahoo, and most corporate mail servers check for DNS consistency. A mismatch raises suspicion and lowers trust.

2. Spam Filtering

Spam engines score your server based on:

• reverse DNS consistency
• proper SMTP banner
• matching hostname

A mismatch increases your spam score.

3. Email Reputation

Long-term sending with mismatches can reduce IP reputation, causing:

• delays
• soft bounces
• junk folder placement

4. MXToolbox / HasheTools Warnings

When you test using HasheTools SMTP Test or MXToolbox, you’ll see:

“Reverse DNS does not match SMTP Banner”

This is a direct indication that your DNS identity is not aligned.

How to Check This Warning Using HasheTools or MXToolbox

You can verify the issue quickly:

Step 1: Run HasheTools SMTP Test

Go to: https://www.hashetools.com/tools/smtp-test

Enter your mail server address and run the scan.

Look for this line:

Reverse DNS does not match SMTP Banner

Step 2: Run Reverse DNS Lookup

Use HasheTools Reverse DNS Lookup: https://www.hashetools.com/tools/reverse-dns-lookup

Check if the hostname matches your SMTP banner.

Step 3: Check your forward DNS

Look for the A record pointing to your IP.

When these three don’t align, the mismatch appears.

How to Fix “Reverse DNS Does Not Match SMTP Banner”

The fix is simple:

All three must match:

A record = PTR record = SMTP banner

Follow these exact steps:

1. Fix Your SMTP Banner (HELO/EHLO)

Set the correct hostname in your mail server settings:

Example: mail.yourdomain.com

How to update:

• Postfix → /etc/postfix/main.cf → myhostname = mail.yourdomain.com
• Exim → cPanel → Exim Configuration → SMTP Banner
• Plesk → Mail Settings → Outbound Mail Mode
• Windows SMTP → Server properties → Identification

2. Fix Your PTR Record (Reverse DNS)

PTR records can only be updated by your:

• hosting provider
• ISP
• VPS/cloud provider

Request them to set:

Your IP → mail.yourdomain.com

Example request: Please update the PTR record for IP 192.0.2.10 to: mail.example.com

3. Make Sure Forward DNS Exists

For the hostname: mail.yourdomain.com

Create an A record: mail → your IP

4. Match All Three Correctly

Final correct structure must be:

Component	Should Be
SMTP Banner	mail.yourdomain.com
A Record	mail.yourdomain.com → IP
PTR Record	IP → mail.yourdomain.com

If even one differs → a warning appears.

5. Test Again

Use HasheTools SMTP Test again: https://www.hashetools.com/tools/smtp-test

If all matches: Warning disappears.

Correct Example of DNS + SMTP Setup

Forward DNS (A Record): mail.example.com → 192.0.2.10

Reverse DNS (PTR Record): 192.0.2.10 → mail.example.com

SMTP Banner: 220 mail.example.com ESMTP Postfix

This is the ideal setup.

What Happens If You Ignore This Warning?

If you ignore this mismatch, your email server may:

• land in spam (especially in Gmail & Outlook)
• face slow delivery
• get domain reputation issues
• fail verification checks
• get flagged by corporate mail filters
• lose email trust score

For businesses and transactional emails, ignoring this warning is never recommended.

FAQs

1. Why does the “Reverse DNS does not match SMTP banner” warning appear?

This warning appears when your mail server’s hostname (SMTP banner) does not match the hostname returned by its reverse DNS (PTR record). Both must point to the same FQDN for proper authentication.

2. Does this mismatch affect email delivery?

Yes. Most spam filters, including Gmail, Outlook, and corporate mail gateways, treat DNS inconsistencies as a sign of an untrustworthy mail server. This can push emails into spam or cause delivery delays.

3. How do I fix a reverse DNS mismatch?

You must update:

1. SMTP banner hostname
2. A record of your mail server
3. PTR record for your IP

All three should match the same domain name, such as mail.yourdomain.com.

4. Can I update the PTR record myself?

No. PTR records are controlled by the IP address owner, usually your hosting provider, VPS provider, or ISP. You must request that they update the reverse DNS.

5. How long do DNS and PTR updates take to propagate?

A record updates within minutes. PTR record changes may take 15 minutes to 24 hours, depending on the provider.

6. If everything else is correct, is this warning still serious?

Even if SPF, DKIM, and DMARC are perfect, a hostname mismatch still affects server trust. It won’t always block your emails, but it does reduce inbox performance, especially for bulk or transactional emails.

Conclusion

A “Reverse DNS does not match SMTP banner” warning simply means your mail server is presenting an identity that does not match the hostname returned by its reverse DNS record. While this issue doesn’t break email sending, it directly influences deliverability, trust, and your overall sender reputation. Modern mail providers rely heavily on hostname alignment to verify whether a server is genuine or potentially harmful.

By ensuring your SMTP banner, A record, and PTR record all match the same hostname, you create a clean and trustworthy DNS configuration. This small fix helps your emails reach inboxes faster, reduces spam flags, and strengthens your server’s credibility.

Tools like HasheTools SMTP Test and DNS utilities make it easy to detect and resolve these issues quickly, ensuring your mail server stays optimized and reliable at all times.