Networking & security
Separate DNS, routes, listeners, TLS, SSH, and firewall problems.
Work through the connection in layers: name resolution, address family, route, local socket, TLS, then HTTP. A DNS answer does not establish that a service is listening, and a successful IPv4 request says nothing about IPv6 reachability.
Use ip for local addresses and routes, curl for a reproducible client request, and the Nginx listen reference for server sockets. Consult the firewall's own documentation before changing rules on a remote machine.
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Separate DNS, routes, listeners, TLS, SSH, and firewall problems.
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OpenSSH manual collection
Check client and server configuration, authentication, forwarding, and file-transfer behavior. Match features to the installed release.
Upstream development manuals · Project documentation · Selection reviewed -
ip: addresses, links, and routes
Choose the address, link, route, or rule subcommand to inspect where the kernel will send a packet.
Linux iproute2 · Project documentation · Selection reviewed -
nftables project wiki
Learn tables, chains, hooks, and ruleset inspection before changing packet filtering on a remote host.
nftables · upstream wiki · Project documentation · Selection reviewed -
dig: query DNS deliberately
Select the record type and DNS server explicitly when comparing A, AAAA, authoritative, and recursive answers.
BIND 9 · stable documentation · Project documentation · Selection reviewed -
Nginx listen directive
Check address, port, default-server, and IPv6 socket settings when DNS resolves but an HTTP connection fails.
Upstream HTTP core module · Project documentation · Selection reviewed -
curl command-line reference
Build a repeatable HTTP diagnostic with explicit address family, timeouts, response headers, and TLS verification.
Current curl · check local version · Project documentation · Selection reviewed
How this library is selected
References are selected for a concrete operational task, with preference for project and distribution documentation. Annotations describe when to use a source; version labels make its scope explicit.
Selection reviewed means the destination, subject, and version scope were checked. It is not a claim that every upstream command has been tested. Original guides identify their own test environment. Platform filters also include references that apply across distributions.