Developer Reference Explorer
Searchable reference for HTTP headers, common port numbers, process exit codes and Linux signals.
About this tool
Developer Reference Explorer
Searchable reference for HTTP headers, common port numbers, process exit codes and Linux signals.
Pick the reference and search by name or number to see what it means. This page brings together 4 separate developer reference explorer pages (Exit codes, HTTP headers, Linux signals, Port numbers) so you can compare them without hunting for the right one.
How to use Developer Reference Explorer
- 1
Open Developer Reference Explorer on AltFTool — it loads instantly in your browser.
- 2
Choose your reference at the top: Exit codes, HTTP headers, Linux signals, Port numbers. On a phone it is a drop-down list.
- 3
Fill in the developer reference explorer for that reference; each of the 4 options keeps its own rules, data and defaults.
- 4
Read the result, then switch reference to compare. The page link keeps your reference (?v=exit-code), so you can bookmark or share it.
Why use Developer Reference Explorer
All 4 in one place
Exit codes, HTTP headers, Linux signals and the rest sit behind one picker, so switching reference takes a tap instead of a new search.
Built for each option
Every reference keeps its own rules, numbers and defaults rather than one generic template.
Shareable view
The link remembers the reference you picked, so a bookmark or a shared link opens straight on it.
Common use cases
- An SRE whose Kubernetes pod shows exit code 137 and needs to confirm it was SIGKILL — usually the OOM killer — before raising memory limits
- Debugging a CORS failure by checking exactly which Access-Control-* headers the server must return for a preflight OPTIONS request
- An engineer whose container ignores docker stop and needs to understand why PID 1 discards an unhandled SIGTERM
- Identifying what service a firewall log entry on port 3389 or 5432 most likely belongs to
Frequently asked questions
Which reference options does this tool cover?
4: Exit codes, HTTP headers, Linux signals, Port numbers. Pick one at the top of the tool; the page link updates (for example ?v=exit-code) so you can bookmark or share that exact view.
What does exit code 137 mean?
Exit code 137 means the process was killed by SIGKILL (signal 9), because shells report death-by-signal-N as 128 + N and 128 + 9 = 137. In Docker and Kubernetes it almost always indicates the kernel OOM killer terminated the container for exceeding its memory limit — check kubectl describe pod for an OOMKilled reason.
What is the difference between a request header and a response header?
A request header is sent by the client to the server (like Accept, Authorization or Cookie), while a response header travels the other way (like Set-Cookie, Location or Cache-Control on a response). Some headers, such as Content-Type and Content-Length, describe the message body and legitimately appear in both directions.
What is the difference between SIGTERM and SIGKILL?
SIGTERM (signal 15) is a polite, catchable termination request — a process can trap it, finish work and exit cleanly. SIGKILL (signal 9) cannot be caught, blocked or ignored: the kernel terminates the process immediately with no cleanup. That is why docker stop sends SIGTERM first and only escalates to SIGKILL after the grace period (10 seconds by default, 30 in Kubernetes).
What are well-known, registered and dynamic ports?
RFC 6335 splits the 0-65535 port space into three ranges: 0-1023 are system or well-known ports managed by IANA, 1024-49151 are user or registered ports that anyone can register, and 49152-65535 are dynamic or private ports that are never assigned and are used for ephemeral client connections.