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From: Marco Moock <mm@dorfdsl.de>
Newsgroups: comp.mail.sendmail
Subject: Re: rdns resolving not working for IPv4
Date: Tue, 7 Apr 2026 15:32:17 +0200
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On 06.04.2026 15:18 Uhr Claus A=C3=9Fmann wrote:

> Marco Moock  wrote:
>=20
> > Messages are being rejected because sendmail thinks the PTR doesn't
> > match the hostname, but DNS works properly. =20
>=20
> Maybe DNS works properly, but whatever your "systemd" crap
> is doing is most likely broken.
>=20
> Example;
> If /etc/nsswitch.conf uses myhostname for the hosts service and it
> is listed after dns, then a temporary dns failure will result in a
> permanent error - which obviously is completely bogus and something
> systemd introduced based on a poor decision by some developer(s)

hosts:          files myhostname resolve [!UNAVAIL=3Dreturn] dns

That is the default when installing Debian.

> Moreover, why (and how) do you reject message based on this?

Certain spammer-controlled systems have improper rdns - normal mail
relays have set PTR records properly.
I used FEATURE(`require_rdns')dnl in the past for that.

> > > /dnslookup smtp.eternal-september.org =20
>=20
> Why do you only provide part of the debugging?
> What is "dnslookup"? (a map  but what's the K line in the cf file)?
> What is the output of the command?

u@pi-dach:~$ echo '/dnslookup smtp.eternal-september.org AAAA'  | sudo send=
mail  -bt -d8.64 =20
_res.options =3D 41012c3, HasWildcardMX =3D 0
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> dns_lookup_int(smtp.eternal-september.org, 1, AAAA, 0)
dns_lookup_int(smtp.eternal-september.org, 1, AAAA, 0)=3D83
parse_dns_reply: ac=3D1, ad=3D0
dns2he: ad=3D0
> u@pi-dach:~$ echo '/dnslookup smtp.eternal-september.org A'  | sudo
> sendmail  -bt -d8.64 =20
_res.options =3D 41012c3, HasWildcardMX =3D 0
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> dns_lookup_int(smtp.eternal-september.org, 1, A, 0)
dns_lookup_int(smtp.eternal-september.org, 1, A, 0)=3D71
parse_dns_reply: ac=3D1, ad=3D0
parse_dns_reply: IPv4=3D95.216.221.164
dns2he: ad=3D0
> u@pi-dach:~$=20

 > u@pi-dach:~$ echo '/dnslookup 164.221.216.95.in-addr.arpa PTR '  | sudo =
sendmail  -bt -d8.64 =20
_res.options =3D 41012c3, HasWildcardMX =3D 0
ADDRESS TEST MODE (ruleset 3 NOT automatically invoked)
Enter <ruleset> <address>
> dns_lookup_int(164.221.216.95.in-addr.arpa, 1, PTR, 0)
dns_lookup_int(164.221.216.95.in-addr.arpa, 1, PTR, 0)=3D96
parse_dns_reply: ac=3D1, ad=3D0
parse_dns_reply: type=3DPTR, host=3Dsmtp.eternal-september.org
dns2he: ad=3D0
> u@pi-dach:~$=20


> What is used to show that the IP does not resolve properly?

STARTTLS=3Dserver, relay=3D[95.216.221.164]
Normally, the host name is listed there if DNS lookup is working.


--=20
kind regards
Marco

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