When I connect to my WiFi router (TIM) my machine is logged on the internal DNS to the router and the PTR with the value: MBP-di-Antonio.homenet.telecomitalia.it.

I get the PTR value via the command: dig -x $MY_IP_ADDRESS


;; ANSWER SECTION: 0    IN  PTR MBP-di-Antonio.homenet\.telecomitalia\.it.

The presence of the character \ on the PTR record creates some problems with the e-mail client.

MBP-di-Antonio:~ amusarra$ telnet smtp.dontesta.it 25
Connected to smtp.dontesta.it.
Escape character is '^]'.
220 smtpcmd01.ad.aruba.it bizsmtp ESMTP server ready
EHLO MBP-di-Antonio.homenet\.telecomitalia\.it.
501 EHLO requires valid address

I still believe that the character \ is not allowed by DNS standards, right?

  • 2
    That's a bug alright, probably in the script which creates this PTR record. I guess it's outside of your control, though, and that this information is hosted by your ISP or DNS provider.
    – tripleee
    Jun 17, 2017 at 16:58
  • Report this problem to the system administrator in charge of that record — which would be your ISP, I guess. If they don't fix the issue, you should be able to work around it by defining your own DNS entry on your machine. What distribution are you using? Which mail client? Jun 17, 2017 at 19:38
  • The problem is the DHCP server on the router that incorrectly configures the PTR. I do not have access to the router. However, I solved the configuration of the DNS manually. My workstation is macOS and Apple Mail as client email. Jun 18, 2017 at 21:13
  • I still believe that the character \ is not allowed by DNS standards, right? Jun 18, 2017 at 21:16

2 Answers 2


\ is allowed in DNS records (but in your specific case, its presence seems more of a bug during zone provisioning than anything else), but not in hostnames. DNS is 8-bit clean, it can manage records with any character. There are however stronger constraints for hostnames. See this question and its answers: https://stackoverflow.com/questions/25199202/which-rfcs-specify-the-syntax-of-hostnames-and-constraints-on-dns-hostnames


Hostnames can contain letters [a-z/A-Z], numerals [0-9], and the hyphen [-]. The underscore [_] is also legal, and required for certain records. (e.g. SRV)

That's it. No other character is legal in a DNS hostname.

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