underscores in A queries

[Kevin K]

Hi,

I've been parsing my query logs to watch for unusual/unexpected lookups, and I notice quite a few A queries with underscores, often in patterns like

_.domainname.com

often followed by

_.xyz.domainname.com

or

_.domainname.com.mydomain.com

Can someone tell me what these are and what the underscores mean?


thanks

Kevin

[John W. Blue via bind-users]

It would seem that underscores is one of those characters in DNS that leads a double life.

RFC’s say that underscores are disallowed for use in hostnames but SRV records use it to indicate service type et al. And then you have the acm-validations.aws geniuses who use it their hostnames to validate domain ownership to issue SSL certs never mind it that the format completely screws up the design and architecture of your subzones.

:/

(not a fan of Route53 BTW .. and now they say they can “do” DNSSEC. lol)

So while there is more to talk about with underscores the real answer to your question is what do those records resolve to? SIP or TCP or whatever? Using the DNS query answer will provide the clue as to why those questions are being asked.

John

From: bind-users [mailto:bind-users-***@lists.isc.org] On Behalf Of Kevin K
Sent: Friday, April 09, 2021 1:28 PM
To: bind-***@lists.isc.org
Subject: underscores in A queries

Hi,

I've been parsing my query logs to watch for unusual/unexpected lookups, and I notice quite a few A queries with underscores, often in patterns like

_.domainname.com

often followed by

_.xyz.domainname.com

or

_.domainname.com.mydomain.com

Can someone tell me what these are and what the underscores mean?


thanks

Kevin

[Lee]

Post by John W. Blue via bind-users
It would seem that underscores is one of those characters in DNS that leads a double life.
RFC’s say that underscores are disallowed for use in hostnames

Right. But it's **hostnames** and not everyone enforces that rule :(

Post by John W. Blue via bind-users
but SRV
records use it to indicate service type et al.

SRV records aren't hostnames, nor are CNAME records, TXT, etc.

I've got this bit in my notes re "check-names response fail;"
# also see dns-***@lists.dns-oarc.net
# [dns-operations] about the underline in hostname
# where the consensus is to not do this check on resolvers

Regards,
Lee
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[Ondřej Surý]

Those are qname minimization queries.

Because DNS implementations (especially in load-balancers) are so broken, the qname minimizing resolver can’t ask for:

<domain> IN NS

because that often doesn’t work, but when it asks:

_.<domain> IN A

the resolver will get the correct answer.

Unfortunately, this is the world we are living in...

Ondrej
--
Ondřej SurÃœ (He/Him)

Post by Kevin K
Hi,
I've been parsing my query logs to watch for unusual/unexpected lookups, and I notice quite a few A queries with underscores, often in patterns like
_.domainname.com
often followed by
_.xyz.domainname.com
or
_.domainname.com.mydomain.com
Can someone tell me what these are and what the underscores mean?
thanks
Kevin
_______________________________________________
Please visit https://lists.isc.org/mailman/listinfo/bind-users to unsubscribe from this list
ISC funds the development of this software with paid support subscriptions. Contact us at https://www.isc.org/contact/ for more information.
bind-users mailing list
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