--- title: Guides description: 60 Guides articles — Step-by-step guides to the free WatchFor tools: a real problem, a real run, how to read the result and what to do next. canonical: https://watchfor.io/blog/category/guides --- [All posts](/blog) # Guides Step-by-step guides to the free WatchFor tools: a real problem, a real run, how to read the result and what to do next. [All](/blog)[Monitoring](/blog/category/monitoring)[Networking](/blog/category/networking)[Performance](/blog/category/performance)[Security](/blog/category/security)[Reliability](/blog/category/reliability)[Email](/blog/category/email)[DevOps](/blog/category/devops)[Engineering](/blog/category/engineering)[Guides](/blog/category/guides) [All articles](/blog/all) 60 article s · page 1 of 3 [GuidesOct 04, 2026 ## Why can't my server reach the gateway? How to calculate a subnet with the Subnet Calculator A freshly configured server reaches some machines on its LAN but not the gateway or the printer down the hall. Very often the subnet mask is wrong. To calculate a subnet, apply the mask to the address: you get the network address, the broadcast address and the range of hosts that are on-link. Here's how to do it in one step with the free Subnet Calculator, how to read every field, and a Python one-liner that gives the same answer.WatchFor Team6 min read](/blog/how-to-calculate-a-subnet)[GuidesOct 04, 2026 ## Can we still ship this week? Check with the Error Budget Calculator Two incidents have used 31 minutes of downtime twelve days into the month, and a risky migration is planned for Friday. With a 99.9% SLO over 30 days the error budget is 43 minutes 12 seconds; 72% is gone and the burn rate is 1.79×, so at this pace it runs out on day 17. Here's how to calculate an error budget and burn rate with the free Error Budget Calculator, and what to decide from the numbers.WatchFor Team6 min read](/blog/how-to-calculate-an-error-budget)[GuidesOct 04, 2026 ## Can we restore 500 GB in an hour? Do the maths with the Bandwidth Calculator Your recovery plan says the database is back within an hour, and the backup is 500 GB in offsite storage. Over a 1 Gbps link that takes 1 hour 14 minutes, so the plan fails before anything else goes wrong. Here's how to calculate bandwidth and transfer time with the free Bandwidth Calculator, why Mbps and MB/s differ by eight, and how to measure your real throughput from the command line.WatchFor Team6 min read](/blog/how-to-calculate-bandwidth)[GuidesOct 04, 2026 ## "Permissions 0644 are too open"? Fix it with the Chmod Calculator SSH refuses your key with "UNPROTECTED PRIVATE KEY FILE! Permissions 0644 are too open". The fix is chmod 600: read and write for you, nothing for anyone else. Here's how to work out any chmod value with the free Chmod Calculator, what each digit means, which modes SSH accepts, and the commands to check and fix permissions.WatchFor Team6 min read](/blog/how-to-calculate-chmod-permissions)[GuidesOct 04, 2026 ## Which IPs does this allowlist cover? How to calculate CIDR ranges with the CIDR Calculator A vendor tells you to allowlist 140.82.112.0/20, and your firewall wants a start and end address, a netmask or a list of /24s. To calculate a CIDR range, keep the first N bits of the address and vary the rest: a /20 is 4,096 addresses from 140.82.112.0 to 140.82.127.255. Here's how to get the range, masks and splits in one step with the free CIDR Calculator, and a Python one-liner that checks it.WatchFor Team5 min read](/blog/how-to-calculate-cidr-ranges)[GuidesOct 04, 2026 ## Did the vendor breach its SLA? Check the downtime with the SLA Calculator A provider was down for 52 minutes last month and its SLA promises 99.95% monthly availability. Did it breach? Calculate the SLA downtime allowance (21 minutes 36 seconds in a 30-day month), compare it with the outage, and work out the availability you actually got. Here's how with the free SLA Calculator, how service credits usually work, and the formula to check it yourself.WatchFor Team5 min read](/blog/how-to-calculate-sla-downtime)[GuidesOct 04, 2026 ## How much downtime is 99.9% uptime? Calculate it with the Uptime Calculator Before you promise customers 99.9% uptime, turn it into minutes: 99.9% allows 43 minutes 12 seconds of downtime in a 30-day month and 8 hours 45 minutes 36 seconds a year. Here's how to calculate uptime and downtime with the free Uptime Calculator, what a target costs in lost revenue, how much each extra nine buys, and the one-line formula to check it yourself.WatchFor Team6 min read](/blog/how-to-calculate-uptime-and-downtime)[GuidesOct 04, 2026 ## Custom domain not connecting? How to check a CNAME record with the CNAME Lookup Your CDN, Shopify, Zendesk or status page says the custom domain isn't configured, even though you added the CNAME. To check a CNAME record, look up the hostname and confirm it aliases to the exact target the vendor gave you. Here's how to do it with the free CNAME Lookup, a real CDN example, what the result can and can't show, and the dig commands that reveal every hop of the chain.WatchFor Team5 min read](/blog/how-to-check-a-cname-record)[GuidesOct 04, 2026 ## DKIM failing? How to check a DKIM record with the DKIM Checker DMARC reports or message headers say dkim=fail or "no key", and you need to know whether the public key is published. To check a DKIM record, find the selector in the message's DKIM-Signature header, then look up the TXT record at selector._domainkey.yourdomain. Here's how to do it with the free DKIM Checker, how to read the key, and how to check its size with openssl.WatchFor Team6 min read](/blog/how-to-check-a-dkim-record)[GuidesOct 04, 2026 ## Is our domain being spoofed? How to check a DMARC record with the DMARC Checker Customers receive phishing that looks like it came from your domain, or Gmail asks bulk senders for DMARC. To check a DMARC record, look up the TXT record at _dmarc.yourdomain and read its policy: p=none only monitors, quarantine sends failing mail to spam, reject blocks it. Here's how to do it with the free DMARC Checker and what every tag means.WatchFor Team5 min read](/blog/how-to-check-a-dmarc-record)[GuidesOct 04, 2026 ## Can search engines find my sitemap? How to check with the Sitemap Checker New pages take weeks to show up in search, and you're not sure crawlers even know your sitemap exists. To check a sitemap, look where crawlers look: /sitemap.xml, the common index paths, and the Sitemap: line in robots.txt. Here's how to do it in seconds with the free Sitemap Checker, a real case where a large site's sitemap was invisible at every standard location, and the one-line fix.WatchFor Team5 min read](/blog/how-to-check-a-sitemap)[GuidesOct 04, 2026 ## New subdomain not resolving? Check a SOA record with the SOA Record Lookup You created a subdomain, it works for you, and a colleague or a certificate authority still gets 'no such domain'. The answer is in the zone's SOA record: its minimum field and TTL set how long resolvers cache a 'doesn't exist' answer. Here's how to check a SOA record with the free SOA Record Lookup, how to read all seven fields, and the dig commands that compare serials across nameservers.WatchFor Team6 min read](/blog/how-to-check-a-soa-record)[GuidesOct 04, 2026 ## Is my site on IPv6? How to check AAAA records with the AAAA Record Lookup Some users on mobile or IPv6-only networks time out while everyone else is fine, or you're simply not sure your site has IPv6 at all. To check AAAA records, look up the domain's IPv6 addresses: none means IPv4 only, and one that points at a server nobody configured means slow or failing connections. Here's how to do it with the free AAAA Record Lookup, a real domain with no IPv6, and the dig commands that tell 'no AAAA' from 'no such domain'.WatchFor Team5 min read](/blog/how-to-check-aaaa-records)[GuidesOct 04, 2026 ## Is my domain pointing to the right IP? Check an A record with the A Record Lookup You moved your site to a new host or set up a custom domain on GitHub Pages, Netlify or a VPS, and the site doesn't load, or loads only sometimes. To check an A record, look up the IPv4 addresses the domain resolves to and compare them with the exact list your host gives you, no more and no fewer. Here's how to do it with the free A Record Lookup, a real GitHub Pages check, and the dig and curl commands that confirm it.WatchFor Team5 min read](/blog/how-to-check-an-a-record)[GuidesOct 04, 2026 ## Will another sender break SPF? How to check an SPF record with the SPF Checker Marketing wants to add one more email tool, and someone has to say whether the SPF record can take another include. To check an SPF record, confirm there is exactly one v=spf1 record, read its ending (~all or -all) and count the DNS lookups against the limit of 10, including the ones hidden inside each include. Here's how, with the free SPF Checker and dig.WatchFor Team6 min read](/blog/how-to-check-an-spf-record)[GuidesOct 04, 2026 ## Is my SSL certificate valid? How to check it with the SSL Certificate Check The browser says "Your connection is not private" on one hostname and works on the next. To check an SSL certificate, connect to the server the way a browser does and read what it actually serves: who issued it, the dates, and every name it covers. Here's how to do it in seconds with the free SSL Certificate Check, how to read a wildcard SAN list, and the openssl commands that confirm it.WatchFor Team7 min read](/blog/how-to-check-an-ssl-certificate)[GuidesOct 04, 2026 ## Is my site serving Brotli? How to check with the Brotli Checker You switched Brotli on in nginx or at your CDN, and now you want proof that visitors actually get it. To check Brotli compression, request the page with Accept-Encoding: br and read the Content-Encoding header that comes back. Here's how to do it in seconds with the free Brotli Checker, why a "No" doesn't mean your site is uncompressed, and the curl commands that measure the bytes.WatchFor Team6 min read](/blog/how-to-check-brotli-compression)[GuidesOct 04, 2026 ## CAA record prevents issuance? How to check CAA records with the CAA Record Lookup Let's Encrypt, your hosting panel or your CA refuses to issue a certificate with 'CAA record prevents issuance', or the normal certificate works and the wildcard fails. To check CAA records, look up the domain's issue and issuewild entries and confirm your CA's identifier is listed for the kind of certificate you want. Here's how to do it with the free CAA Record Lookup, a real domain with a split policy, and the exact records to add.WatchFor Team6 min read](/blog/how-to-check-caa-records)[GuidesOct 04, 2026 ## Lighthouse says 100, users say slow? Check with the Core Web Vitals Checker Your laptop's Lighthouse run scores 100, yet phone users say the page is slow or jumps around. To check Core Web Vitals properly, test the same page with the mobile profile, not just desktop, and read LCP and CLS against Google's thresholds. Here's how with the free Core Web Vitals Checker, a real page that scored 100 on desktop and 65 on mobile, and what to fix first.WatchFor Team6 min read](/blog/how-to-check-core-web-vitals)[GuidesOct 04, 2026 ## Has my DNS change propagated? How to check with the DNS Propagation Checker You changed a record, it works on your laptop, and someone else still lands on the old server. To check DNS propagation, ask the same question from many places at once and compare the answers with what your DNS host serves. Here's how to do it in seconds with the free DNS Propagation Checker, how to read a result that shows different answers in different places, and the dig commands that confirm it.WatchFor Team7 min read](/blog/how-to-check-dns-propagation) --- Canonical page: https://watchfor.io/blog/category/guides · Site guide: https://watchfor.io/llms.txt