indrc.cz - DNS, WHOIS, DMARC and SPF of the domain name

Summary

Registrar: REG-WEB4U
Status
Created on: September 7, 2021
Expires on: September 7, 2027
Name servers: ns.web4u.cz, ns2.web4u.cz
Mail servers: indrc-cz.mail.protection.outlook.com
SPF: Soft
DKIM: No key found among the common selectors
Website: See the website

Authorized senders (SPF)

Soft

E-mails sent by any other server are accepted but flagged as suspicious.

SPF lists the servers allowed to send e-mails for indrc.cz.

Rule	Value	Result
include	spf.protection.outlook.com	Allowed
include	spf.orwin.cz	Allowed
all	—	Suspicious

Top-level DNS lookups: 2 of 10 allowed

Record

v=spf1 include:spf.protection.outlook.com include:spf.orwin.cz ~all

MX, SPF, DKIM and DMARC: the e-mail records

Four DNS records make a domain's e-mail work. MX says where e-mails sent to indrc.cz arrive; SPF, DKIM and DMARC let mail servers check that an e-mail claiming to come from indrc.cz was really sent by this domain, which protects it against spoofing.

MX

An MX (Mail Exchanger) record designates a server that receives e-mails sent to the domain. When someone writes to an @indrc.cz address, their mail server queries the DNS for the domain's MX records, then delivers the message to the server they point to.

Example record

indrc.cz  MX  10 mx1.indrc.cz.
indrc.cz  MX  20 mx2.indrc.cz.

What the record contains

10, 20: Priority: the server with the lowest number is tried first. Servers with equal priority share the load.
mx1.indrc.cz.: Host name of the mail server. It must point to an A or AAAA record: an IP address or an alias (CNAME) is not allowed here.
0 .: An MX "0 .", called a null MX, declares that the domain accepts no e-mail: sending servers give up at once instead of retrying.

Declaring at least two servers with different priorities avoids losing e-mails during an outage: sending servers retry for several days and fall back on the backup server. Without any MX, they try to deliver directly to the IP address of the domain.

SPF

SPF (Sender Policy Framework) is a TXT record listing the servers allowed to send e-mails on behalf of the domain. The receiving server checks that the sender is on this list. The final "all" rule says what to do with other servers: reject them (-all), flag their e-mails as suspicious (~all) or make no decision (?all).

Example record

indrc.cz  TXT  "v=spf1 mx include:_spf.google.com ip4:192.0.2.10 -all"

What the record contains

v=spf1: Identifies the record as an SPF policy. It must come first.
ip4, ip6: Allows a specific IP address or an address range, for example ip4:192.0.2.0/24.
a, mx: Allows the addresses of the domain itself (A record) or those of its mail servers (MX).
include: Allows the servers of another domain, usually a sending service: include:_spf.google.com for Google Workspace.
-all, ~all: Final rule, applied to every other server. Its qualifier decides what happens to them: - rejects them, ~ flags their e-mails as suspicious, ? makes no decision, + accepts them (avoid it).

A domain must publish only one SPF record, and its evaluation cannot exceed 10 DNS lookups (include, a, mx, exists, redirect). Beyond that, mail servers treat it as invalid.

DKIM

DKIM (DomainKeys Identified Mail) signs every outgoing e-mail with a private key. The matching public key is published in a TXT record at selector._domainkey.indrc.cz, so the recipient can check that the message really comes from the domain and was not altered on the way. The selector is chosen by each sending service and cannot be guessed: Dataweaz checks the most common ones (Google Workspace, Microsoft 365, Mailchimp, SendGrid…), so a key published under another selector may exist without appearing here.

Example record

google._domainkey.indrc.cz  TXT  "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA…"

What the record contains

v=DKIM1: Protocol version. Optional but recommended, in which case it must come first.
k: Key algorithm: rsa, the most common, or ed25519.
p: Public key, base64-encoded. An empty value means the key has been revoked.
t: Flags: y marks a key in testing, whose failures must not be penalized; s forbids its use for subdomains.

Each sending service publishes its key under its own selector (google, selector1, k1…), which allows several keys and seamless rotation. A 2048-bit RSA key is recommended today.

DMARC

DMARC (Domain-based Message Authentication, Reporting and Conformance) builds on SPF and DKIM. Published on _dmarc.indrc.cz, it tells mail servers what to do with an e-mail that fails these checks (let it through, send it to spam or reject it) and where to send reports.

Example record

_dmarc.indrc.cz  TXT  "v=DMARC1; p=quarantine; rua=mailto:[email protected]; adkim=r; aspf=r"

What the record contains

v=DMARC1: Identifies the record as a DMARC policy. It must come first.
p: Policy applied to e-mails that fail the checks: none (monitoring only), quarantine (spam folder) or reject.
sp: Subdomain policy. Without it, subdomains follow the domain's policy.
rua: Address that receives daily aggregate reports about e-mails sent on behalf of the domain, whether they pass the checks or not.
ruf: Address that receives detailed failure reports, message by message. Few providers still send them.
pct: Percentage of e-mails subject to the policy, for a gradual rollout. 100 by default.
adkim, aspf: DKIM and SPF alignment: r (relaxed, a subdomain of the same domain is enough) or s (strict, the domain must be identical).

To roll out DMARC without blocking legitimate e-mails, start with p=none and an rua address, analyze the reports to find every service sending on behalf of the domain, then move to quarantine and finally to reject.

DNS records of indrc.cz

IP addresses (A and AAAA)

An A record maps the domain name to an IPv4 address: the address of the server that browsers contact to display the website. The AAAA record does the same for an IPv6 address. A domain can have several of them to spread visits across several servers.

Type	IP address
A	46.36.41.174

Name servers (NS)

NS records designate the authoritative DNS servers of the domain: they answer every question about its other records (A, MX, TXT…). They are provided by the registrar, the hosting provider or a dedicated DNS service, and several servers are declared so that the domain keeps working if one of them fails.

Server	IP address
ns.web4u.cz	185.14.224.241
ns2.web4u.cz	85.207.53.12

Mail servers (MX)

MX records point to the servers that receive e-mails sent to the domain, that is to @indrc.cz addresses. Each server has a priority: the one with the lowest number is tried first, the others take over if it fails. Without an MX record, the domain cannot receive e-mails.

Priority	Server	IP address
0	indrc-cz.mail.protection.outlook.com	52.101.91.159

TXT records

TXT records hold free text. They are used to prove ownership of the domain to online services (Google, Microsoft, Facebook…) and to publish e-mail security rules: SPF on the domain itself, DMARC on _dmarc.indrc.cz.

Name	Content
indrc.cz	v=spf1 include:spf.protection.outlook.com include:spf.orwin.cz ~all
indrc.cz	MS=ms35297516

Related IP addresses

IPv4 addresses used by indrc.cz: website, name servers, mail servers and senders authorized by SPF.

IP address	Used for	/24 block
46.36.41.174	Website	46.36.41.0/24
185.14.224.241	Name server ns.web4u.cz	185.14.224.0/24
85.207.53.12	Name server ns2.web4u.cz	85.207.53.0/24
52.101.91.159	Mail server indrc-cz.mail.protection.outlook.com	52.101.91.0/24

WHOIS of the domain indrc.cz

%  (c) 2006-2021 CZ.NIC, z.s.p.o.
% 
% Intended use of supplied data and information
% 
% Data contained in the domain name register, as well as information
% supplied through public information services of CZ.NIC association,
% are appointed only for purposes connected with Internet network
% administration and operation, or for the purpose of legal or other
% similar proceedings, in process as regards a matter connected
% particularly with holding and using a concrete domain name.
% 
% Full text available at:
% http://www.nic.cz/page/306/intended-use-of-supplied-data-and-information/
% 
% See also a search service at http://www.nic.cz/whois/
% 
% 
% Whoisd Server Version: 3.15.0
% Timestamp: Sat Oct 03 07:02:41 2026

domain:       indrc.cz
registrant:   TROPPOVA
admin-c:      TROPPOVA
nsset:        WEB4U
keyset:       WEB4U
registrar:    REG-WEB4U
registered:   07.09.2021 17:10:24
changed:      07.09.2021 21:20:49
expire:       07.09.2027

contact:      TROPPOVA
name:         Eva Troppová
address:      Bulhary 13
address:      Bulhary
address:      69189
address:      CZ
registrar:    REG-MOJEID
created:      07.09.2021 17:07:05

nsset:        WEB4U
nserver:      ns2.web4u.cz (85.207.53.12, 2a02:a40:100:28ff::12)
nserver:      ns.web4u.cz (185.14.224.241, 2001:1568:fd::241)
tech-c:       WEB4U
registrar:    REG-WEB4U
created:      03.10.2007 14:53:59
changed:      11.12.2024 19:13:20

contact:      WEB4U
org:          Web4U s.r.o.
name:         František Princ
address:      Korunní 2569/108
address:      Vinohrady
address:      Praha 10
address:      10100
address:      CZ
registrar:    REG-WEB4U
created:      03.10.2007 14:48:14
changed:      24.04.2023 10:14:55

keyset:       WEB4U
dnskey:       257 3 14 7Zllkn95exAgF2rGMLeDUfHSJHKmZ0U4Cr2I492Gdg/Dn4LX5F0HWrRsM+ssztoWHiMz+mQcfuAfRVPkADkzIDFxKxEQ8krciYIU6ft9iOpv8m5kzzjxd+nTYMxoYil3
tech-c:       WEB4U
registrar:    REG-WEB4U
created:      23.09.2009 14:06:52
changed:      25.06.2025 12:25:01

% Query time: 725 msec
% WHEN: Sat Oct 03 05:02:40 UTC 2026