Articles & Guides
Useful resources for web developers and system administrators
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: SMTP 25, 465, 587 and IMAP 993, POP3 995
Which email port to use: 587 or 465 to send, 993 (IMAP) or 995 (POP3) to receive, 25 only between servers. Gmail settings, errors, test commands.
Networking
How to Read Traceroute: Hops, Asterisks, RTT and Who's at Fault
How to read traceroute and tracert output: hop columns, RTT, * asterisks and !H flags, plus how to tell if the fault is on your side, your ISP's or the site's.
Networking
High Ping and Packet Loss: Find the Cause and Fix It
Fix ping spikes and packet loss: test with ping and traceroute, find whether it starts at your Wi-Fi, router, ISP or the server, then fix that step.
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.
Networking
Fix ERR_CONNECTION_CLOSED: Causes and Solutions
ERR_CONNECTION_CLOSED in Chrome means the server dropped the connection. Causes: TCP RST, antivirus, proxy, MTU, TLS. Fix it as a user and on the server.
Networking
Fix ERR_QUIC_PROTOCOL_ERROR: QUIC / HTTP3 Error Guide
What ERR_QUIC_PROTOCOL_ERROR means in Chrome, why QUIC/HTTP3 fails, and how to fix it: disable the QUIC flag, check UDP/443, the Alt-Svc header and server bugs.
Networking
Site Blocked in Russia? Owner's Guide to Check and Appeal
Is your site blocked in Russia? Check the Roskomnadzor register, tell a block from a hosting or DNS outage, find the grounds and appeal step by step.