Articles & Guides
Useful resources for web developers and system administrators
Networking
SMTP Errors 550 and 554: Causes and Fixes
SMTP errors 550 and 554 explained: what the codes mean, enhanced 5.x.x status codes, rejection causes (SPF, DKIM, DMARC, blacklist) and how to fix them fast.
DNS
SOA DNS Record: Fields, Serial, and How to Read It
Understand the SOA DNS record: all fields (MNAME, RNAME, serial, refresh, retry, expire, minimum), the YYYYMMDDnn serial format, zone transfers, and dig checks.
DNS
SRV DNS Record: Syntax, Priority, Examples
What an SRV DNS record is per RFC 2782: the _service._proto.name syntax, how priority and weight work, examples for SIP and XMPP, and common mistakes.
DNS
TXT DNS Record: What It's For and How to Add It
What a TXT DNS record is and why you need it (verification, SPF, DKIM, DMARC, BIMI), the 255-character limit, quoting syntax, and how to add and verify it.
DNS
NS DNS Record: What It Is and How to Check It
What an NS DNS record is and how zone delegation works: primary and secondary servers, glue records, checking NS with dig and WHOIS, changing NS servers and common mistakes.
Networking
What Is MTU: Fragmentation and How to Fix Issues
What MTU and MSS are, typical values (1500, 1492, jumbo 9000), Path MTU Discovery and black holes. How to measure MTU with ping and set it on your OS.
Networking
Email Ports Explained: 25, 465, 587, 110, 143, 993, 995
Understand email ports: SMTP (25, 587, 465), POP3 (110, 995), IMAP (143, 993), STARTTLS vs implicit TLS, why port 25 is blocked, and which port to use.
Networking
Traceroute: How to Read the Output and Find Problems
How to read traceroute and tracert output: hop numbers, RTT, asterisks. Learn to pinpoint where the problem is — your network, your ISP, or the server.
Networking
High Ping and Packet Loss: Causes and Fixes
Why ping is high and packets drop, what values are normal, how to measure with ping and traceroute, and how to isolate the problem — your network, ISP, or the server.
DNS
CAA DNS Record: Control Which CAs Issue Your SSL
What a CAA DNS record is, why it matters, the issue/issuewild/iodef syntax, and how to check it. Stop unauthorized SSL certificate issuance for your domain.