When the Domain Name System was designed thirty years ago the concept behind it was pretty simple. It’s mostly just a distributed database that lets you map hostname / query-type pairs to values. If you want to know the IP address of cnn.com, you look up {cnn.com, A} and get back a couple of IP addresses. If you want to know where to send mail for aol.com users, you look up {aol.com, MX}...
PTR records are easy to over look and they have a significant impact on your ability to deliver mail without them. Some ISP and mailbox providers will reject mail from IP addresses that do not have a PTR record created. PTR records are a type of DNS record that resolves an IP address to a fully qualified domain name or FQDN. The PTR records are also called Reverse DNS records. If you are...
I’ve never met a spammer that claims to be a spammer. Most that I’ve met claim to be legitimate marketers (or high volume email deployers). But there are things spammers do that I never expect to see a legitimate marketer doing. I’ve written about these things throughout the blog (tag: TWSD), but it’s probably time to actually pull them together into a single post. Use...
Like many bits of technology, email is often set-and-forget. Everything is checked and rechecked during setup, and then no one goes back and looks at it again. But mail programs are not static, and people make changes. These changes don’t really break things, but over time they can create their own set of problems. Setting aside some time every quarter or even every year to check and make...
Recipients can’t click through if you don’t exist A tale of misconfigured DNS wrecking someone’s campaign. I got mail this morning from A Large Computer Supplier, asking me to fill in a survey about them. I had some feedback for them, mostly along the lines of “It’s been two decades since I bought anything other than rackmount servers from you, maybe I’m not a...
This time it appears that DNS for major websites, including the NY Times, has been compromised. Attackers put in DNS entries that redirected visitors to a malware site. The compromise has been fixed and the fake DNS entries corrected. However, people may still have the old data in their DNS caches and security experts are suggesting everyone flush their DNS cache to make sure the fake data is...
I’ve had a couple folks come to me recently for help troubleshooting SPF failures. The error messages said the SPF record was invalid, but by all checks it was valid. Eventually, we tracked the issue down to how many include files were in the SPF record. The SPF specification specifically limits the number of lookups that can happen during a SPF check. SPF implementations MUST limit the...
Back in the dark ages of email delivery the only thing that really mattered to get your email into the inbox was having a good IP reputation. If your IP sent good mail most of the time, then that mail got into the inbox and all was well with the world. All that mattered was that good IP reputation. Even better for the people who wanted to game the system and get their spam into the inbox, there...
A number of sources are reporting this morning that there was a problem with some domains in the .com zone yesterday. These problems caused the DNS records of these domains to become corrupted. The records are now fixed. Some of the domains, however, had long TTLs. If a recursive resolver pulled the corrupted records, it could take up to 2 days for the new records to naturally age out. Folks can...
When you look up a host name, a mailserver or anything else there are three types of reply you can get. The way they’re described varies from tool to tool, but they’re most commonly referred to using the messages dig returns – NXDOMAIN, NOERROR and SERVFAIL. NXDOMAIN is the simplest – it means that there’s no DNS record that matches your query (or any other query for...
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