Check if your forward and reverse dns match

When sending email it’s a best practice that your sending ips reverse and forward resolutions match.

When looking up IPv4 addresses you use a special domain called so if you want to know the ptr of just run this

See the name = in the answer, that’s the PTR not to see if we have a confirmed forward and are good to go we need to see if the domains A-record match the IP we looked up. Run this

See the Address part in the answer it matches the ip we checked PTR on. That means that they are matching.



How to check Cisco Senderbase reputation from commandline

I have created an API for all sorts of email and ip related info, read more here Api for talos reputation (previous, senderscore, reputation authority and blacklists.

Since this no longer work i have created a solution in c# for now, not as “fancy” but it does the job.  Go here to read the post Public senderbase dns lookup is closed, here is how to look it up in c# using HttpClient and HtmlAgilityPack 

Senderbase is similar to Senderscore it is a reputation score for you IP address and/or domain you can use the website to lookup your IP. But if you just wan’t to check it from the command line this is how you do it.

First little about how the reputation is calculated.

Reputation scores in SenderBase may range from -10 to +10, reflecting the likelihood that a sending IP address is trying to send spam. Highly negative scores indicate senders who are very likely to be sending spam; highly positive scores indicate senders who are unlikely to be sending spam.

SenderBase combines multiple sources of information to determine a “reputation score” for any IP address. This information includes:

  • Email volume information provided by tens of thousands of organizations that regularly receive Internet email
  • Spam complaints received by the SpamCop service
  • Information on other DNS-based blacklists

This is the command to get the score if your IP is

Here is the tricky part, if you do a lookup on the website you will get a grouped reputation Poor, Neutral and Good. So how do you know where the score you get in the dns lookup fits?

This is my findings:

Poor is between -10 and -1

Neutral is between -1 and 2

Good is between 2 and 10

If you have any other knowledge about the classification from the scoring please let me know in the comments.



Check your IPs senderscore from the commandline

So you  have heard about senderscore but you think it’s tedious work to go to the website and log in and/or verifying the captcha..

Don’t worry you can the same way as checking dns blacklists (DNSBL) you can check your score from the command line.

Just reverse your IP and do a dns lookup, like this:

Unlike the DNSBL lookup where you where listed if you got a response the score is part of the Address response. So in the exemple above my score is 99.

You take the last part of the response address and that is your score. When you get Non-existent domain as an answer your IP hasn’t sent out enough email for senderscore to do a measure.

This can be done with dig exactly the same way as done in this post about DNSBL

This is updated twice a day at ~12:00 GMT and ~22:00 GMT so no need to look more than once after these times.


How to check DNS-blacklists (dnsbl) from commandline (cmd / terminal)

There is a lot of DNS-blacklists out there. Sometimes you just want to check if your IP is listed in one.

A really easy way to do that is to use your shell, terminal, command prompt. Below i will show two examples using nslookup in windows and dig in OSX (Unix).


The basics is that if you get a response your IP is listed. There are different type of listings please refer to each services documentation.

You will typically get a response of 127.0.0.x where x has different meaning in different services.

Sometimes they will publish a TXT-record with the A-record to provide some more information about the listing or information on how to delist. So that i something that can be useful to lookup if listed.