Web Development

18-Jan-09 9:46

Is it possible to have an email server without at least one MX record?

I used to check if the username was associated with an email account but decided that it was leaving an opening for a malicious person to write a robot to use this check to get the clients IP address listed as a spammer so I dropped that check and am writing a new one which uses the DNS records only.

The reason I ask about MX records is because I am writing an email verification routine which first verifies the format of an email address with a regular expression and then parses out the domain name.

I then do a nslookup for the mail exchange (type = mx) records associated with that domain name. If there are mx records associated with the domain I check to see if there is an address record (type = a) for at least one of those mail exchange servers.

It is not perfect but it at least filters out the obvious fake email addresses.

I just want to make sure that there needs to be at least one mail exchange (type = mx) record that resolves to a valid address record (type = a) in order to be an email server.

Does anyone know of any exceptions to that rule?

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Aman Bhullar18-Jan-09 18:28

Aman Bhullar

18-Jan-09 18:28

I don't think so.
Every mail server has MX records

JimmyRopes19-Jan-09 1:34

19-Jan-09 1:34

Thanks for the reply.

Amandeep Singh Bhullar wrote:
Every mail server has MX records

That is what I thought but I just wanted to make sure that someone didn't know of a way to have a mail server without them.

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Jörgen Andersson19-Jan-09 4:24

Jörgen Andersson

19-Jan-09 4:24

No it isn't.

But keep in mind that it does not need to have a public A record to be able to recieve mail.
I would try to initiate an SMTP session instead to see if there is a corresponding mailserver to the mx-record.

JimmyRopes19-Jan-09 4:34

19-Jan-09 4:34

Thanks for the reply.

Jörgen Andersson wrote:
But keep in mind that it does not need to have a public A record to be able to recieve mail.

Interesting. I thought I was finished but must now investigate further.

Jörgen Andersson wrote:
I would try to initiate an SMTP session instead to see if there is a corresponding mailserver to the mx-record.

I get failures when I try to open a socket on these mail servers. I have tried on ports 25 and 80 so far but have failed on a range of mail servers associated with different domains so it is not peculiar to just one server.

Can you suggest an alternate way of initiating an SMTP session that I can try?

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Jörgen Andersson19-Jan-09 5:00

Jörgen Andersson

19-Jan-09 5:00

I should be port 25.
But quite often the ISPs are blocking port 25 on their network to all mailservers except their own, for all users with dynamic IP, in order to reduce spam.
As this is bad for Web hotels they often have opened port 2525 instead.

JimmyRopes19-Jan-09 6:55

19-Jan-09 6:55

I tried port 25 first and when it failed I tried 80 just to see if it would work, but it didn't.

I'll take a look at 2525, but if it is not uniformly open I really can't use it.

I suppose as long as I find an MX record for the domain it should be OK to assume that the domain in the email address is at least valid. I could additionally open a socket to the domain itself on port 80 to insure that it is active, that I know I can do. I don't know if the additional check is necessary once I find a mail exchange server for the domain, but it can't hurt.

Thanks for the information.

Simply Elegant Designs JimmyRopes Designs
Think inside the box! ProActive Secure Systems

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Jörgen Andersson19-Jan-09 10:08

Jörgen Andersson

19-Jan-09 10:08

I don't know anything about your testing procedures, but Microsoft has some troubleshooting info here[^] which might be helpful. (You did mention exchange in your original post)

JimmyRopes19-Jan-09 10:55

19-Jan-09 10:55

Thanks for the link.

Although I am using php on an apache server I may be able to adapt what is said in the article to get the php program to retrieve the required data.

I see what you mean about an email server not having to have an “A” record, actually the domain does not have to have a “mail exchange” entry.

In fact it was msdn.com where I found this to be the case. Instead of mail exchange records the nslookup type=mx command returns:

Non-authoritative answer:
*** Can't find msdn.com: No answer

Authoritative answers can be found from:
msdn.com
origin = ns1.msft.net
mail addr = msnhst.microsoft.com
serial = 2008110101
refresh = 1800
retry = 900
expire = 1209600
minimum = 3600

The Non-authoritative answer section is where you find the mail exchange records. I need to look into the “origin” and "mail addr" entries to see if either has an "A" record or “mail exchange server” records associated with them later tonight or tomorrow, depending on how I feel after dinner.

I feel like I am getting close. Thanks for the help.

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Think inside the box! ProActive Secure Systems

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Roger Wright20-Jan-09 16:59

Roger Wright

20-Jan-09 16:59

You might try sending a message using the target domain (test@target.tld) to see if it responds. Some servers may not respond to a message that isn't directed to the hosted domain.

"A Journey of a Thousand Rest Stops Begins with a Single Movement"

JimmyRopes21-Jan-09 14:47

Setting Access Rights in Virtual Folder

21-Jan-09 14:47

Thanks for the reply.

I do not want to query the actual email server because it can be used maliciously. Picture a botnet sending 15,000 emails a day to the mail server causing my clients IP to be blacklisted as a spammer. Then picture it happening to many servers until the IP address appears on almost every blacklist.

I used to have a verification routine that contacted the email server, although not sending email, but decided it was open for abuse so I am writing a verification routine that only uses DNS servers to verify the domain name. It is not as robust a solution but much safer.

I am honest with the client about what I can do safely, and the limitations of doing it this way.

I eliminate the roger@las.vegas but not the mickey.mouse@disney.com

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Brendan Vogt16-Jan-09 8:34

16-Jan-09 8:34

Hi,

I have a virtual directory for one of my websites locally. I need to be able to add images and delete images to the Image folder. I can remember how to give permissions to this folder. Please can some body advise me?

Regards,
Brendan

Jon Rista16-Jan-09 9:26

Jon Rista

16-Jan-09 9:26

I assume your using ASP.NET/IIS...if that is the case, add NETWORK SERVICE to the folder on your hard drive that the virtual directory maps to. If you have changed the account that your IIS AppPool uses, use that account instead of the default NETWORK SERVICE account.

Brendan Vogt16-Jan-09 20:13

16-Jan-09 20:13

Thanks for the reply. I have no idea where to do this, could you maybe direct me please?

Thanks
Brendan

Jon Rista16-Jan-09 20:22

Jon Rista

16-Jan-09 20:22

All IIS virtual folders map to some physical directory on your hard drive. Just go to the mapped directories parent, right-click the mapped directory, and choose properties. In the security tab, add the appropriate user, and give them the right permissions (read/write/modify probably).

Brendan Vogt16-Jan-09 22:14

16-Jan-09 22:14

Hi Jon,

I know I always right clicked in Windows Explorer, and then I select the user and assign the permissions to whatever user. When I right click now then I don't see this part where I see the different users.

When I right click and then click on properties, the only tabs that display is General, Sharing, Web Sharing and Customize.

I am using Windows XP Pro, I think it is IIS 5.

Brendan

Abhijit Jana17-Jan-09 2:47

Abhijit Jana

17-Jan-09 2:47

.NET Enthusiast wrote:
When I right click and then click on properties, the only tabs that display is General, Sharing, Web Sharing and Customize.

1 .Select Sharing Tab
2. Select Shared this folder option Button
3. Click on Permission Button
4. Add the User/Groups
------

.NET Enthusiast wrote:
I am using Windows XP Pro, I think it is IIS 5.

Then Application pool does not work for you. Because AFAIK : Application pool has been introduce from IIS 6.0.

cheers,
Abhijit
CodeProject MVP
My Recent Article : Exploring Session in ASP.Net

Brendan Vogt18-Jan-09 0:24

18-Jan-09 0:24

Hi,

Still no success. Under the sharing folder there is no "Share this folder" option. This option is available on the Web Sharing tab, but it doesn't allow you to choose the user/groups. Do you want me to send you screen shots?

Brendan

Brendan Vogt18-Jan-09 0:40

18-Jan-09 0:40

I managed!!

I had to go and set the folder options to not allow simple file sharing. No when I right click properties and select sharing it gives me extra options for roles selection to different user.