ptrcheck

Check that every address record (A/AAAA) in a DNS zone has a valid and acceptable corresponding PTR record.

Introduction

Remembering to set correct reverse DNS is sometimes hard, yet sometimes it is very important, especially if you are sending email from those addresses. Other times it is just a matter of pride! ptrcheck allows you to run constant, bulk checks on a whole DNS zone to give assurance that all PTR records are as they should be.

Typical run

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Verbose run

Without the verbose option this would actually have been silent as all PTRs were "good".

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And another verbose run, this time with only partial success.

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Installation

Some binaries are available at the releases page of GitHub.

This is a Rust application, so after cloning this repo you can build it from source with cargo like:

$ cargo build --release

The binary should then be found in the target/release/ directory. Put it on your path or run it from anywhere.

Usage

ptrcheck 0.1.1
Andy Smith <[email protected]>

Check that all address records in a DNS zone have valid and
acceptable PTR records associated

Usage: ptrcheck [OPTIONS] --server <SERVER> --zone <ZONE>

Options:
  -b, --badre <BADRE>
          Regular expression for unacceptable PTRs
  -c, --color <COLOR>
          Use colored output [default: auto] [possible
          values: auto, always, never]
  -s, --server <SERVER>
          Server to do AXFR against (in form "IP:port";
          ":port" optional)
  -v, --verbose
          Be more verbose
  -z, --zone <ZONE>
          Zone to check PTR records for
  -h, --help
          Print help
  -V, --version
          Print version

The required arguments are --server and --zone. ptrcheck gets its zone data by zone transfer (AXFR), so you'll need to be able to do a transfer from a name server that is authoritative for your zone.

Server to do zone transfer from

This is specified with --server <SERVER>. The <SERVER> part should be an IP address and port number separated by a colon. IPv6 addresses should be wrapped in square brackets. The :<PORT> part can be omitted in which case port 53 will be used. Host names are not supported.

Examples:

$ ptrcheck --server 127.0.0.1:5353 --zone example.com
$ ptrcheck --server [::1] --zone example.com

Zone to check

This is specified with --zone <ZONE>. It should be the zone as it exists in the DNS, which means for example that IDN domains would need to be converted to Punycode.

Detecting bad PTR content

The default behavior is to consider only missing (or unqueryable) PTR records to be "bad". Often though, hosting providers supply default reverse DNS when none is set by the customer, and these usually will resolve both ways. Just checking that a PTR record exists may not be enough for your purposes. In the case of a mail server, for example, it is a very bad idea to operate with a "generic" reverse DNS of any kind.

To help with this the --badre option allows you to supply a regular expression that identifies "bad" PTR content.

Example:

$ ptrcheck \
  --server [::1] \
  --zone example.com \
  --badre 'linodeusercontent|vps\.ovh\.net'

Use of DNS

ptrcheck does a zone transfer directly from the host and port that you specify with --server, but it then queries for PTR records using your normal system resolver as you have configured in /etc/resolv.conf (or equivalent on Windows).

In particular this means:

• You'll get cached answers from your resolver if they are present
• Your resolver may have a different view of the Internet than the zone administrator expects. For example, if a public zone contains internal hostnames these may point at IP addresses you cannot ever resolve PTR records for, and that is not necessarily an error.

Exit codes

• 0
  • When no problems were detected
• 1
  • When any "bad" PTR records were detected
• 2
  • DNS protocol error with initial AXFR such as "Refused" or "NotAuth"

Limitations

Things I'll probably try to improve

These things affect me so I will probably get around to improving them at some point.

• This is synchronous, single-threaded code. A zone is transferred and then individual PTR queries are made one after another with default timeouts. This is quite slow. Async would likely improve this. Filed as GitHub issue #3.
• Should be able to specify a host name for the DNS server to query. Filed as GitHub issue #4.
• There should be an option for a silent mode. You would use the exit code to tell if there were a problem or not. Filed as GitHub issue #5.
• An option to ignore some address blocks would be useful, in order to exclude the RFC1918 private blocks from checking, for example. Filed as GitHub issue #6.
• Zone content should be optionally obtainable from a file instead of a zone transfer. This would be a bit faster, would facilitate more testing, and would make the tool useful for people who don't have AXFR access to their name servers. Filed as GitHub issue #7.
• Did I say testing? There's no tests here. There really should be, but I wasn't up to mocking a DNS server and I wanted to get something achieved.
• Port should default to 53 if not supplied. Closed as GitHub issue #1.

Known issues that aren't a personal priority

These limitations don't bother me for my use cases so I'm unlikely to fix them, but I still welcome assistance. Please file an issue if any of this actually affects you (no promises though).

• There's no TSIG support so zone transfers are authenticated by IP address alone.
• There isn't any support for DNS-over-TLS or DNS-over-HTTPS either.
• I can imagine someone wanting to specify a different resolver to use other than what they have in their /etc/resolv.conf, but it's not a need I have personally.
• ptrcheck currently only checks that there is some PTR content. Some might like an optional stricter check that requires that a successful chain of hostname1 → IP → hostname2 → IP exist where hostname1 and hostname2 may or may not be the same thing, i.e. that at least one forward and reverse mapping is in agreement.

Disclaimer

I'm not a proficient Rust developer. I did this just for practice and I'm well aware that it contains some poor code, plus no doubt some more poor code that I'm not yet aware of. I welcome constructive feedback and assistance, though I may not be capable of acting upon it. Please contact me or file a GitHub issue for any bugs, feature requests or other feedback.