Text & documents
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Text convertercamelCase, snake_case...
Text DiffCompare two texts
LaTeXPreview mathematical formulas
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DiagramsVisio-style visual editor, Mermaid import
String escaperJS, JSON, SQL, Regex, HTML, URL
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Numbers & maths
CalculatorExpressions, calculation sheet and plots
StatisticsCalculator, distributions, simulators
Physics SimulatorPendulum, energy, integrators
Base converterBase 2 to 64
Unit systembits, bytes, SI, IEC
Subnet / CIDRNetwork calculator
Timestamp and datesUnix, time zones, formats
Date calculatorDays between dates, exact age, day of the week
Data & formats
Format converterJSON, YAML, Properties, ENV, CSV
JSON FormatterFormat and validate JSON
Regex TesterRegular expressions
JSON DiffStructurally compare two JSONs
Code formatterSQL, CSS, HTML, JavaScript
DB StudioVisual Postgres database in the browser
SQL LabValidate, run and optimize SQL with real Postgres
Giant file viewerOpen huge CSV/logs without loading them
Visual ETL flowTransform data by chaining nodes
Log analyzerMake sense of a log: traces, patterns and AI reduction
Knowledge graphRDF, SPARQL and ontologies
Atoms and chemistryPeriodic table, orbitals and valence shell
Security
Text anonymizerRedact PII and secrets (reversible)
JWT InspectorDecode JWT tokens
Auth flowsBasic, JWT and OAuth2 step by step
Hash generatorSHA-256, SHA-384, SHA-512
Password & UUID generatorPasswords and UUIDs
Hidden text (X-ray)Invisibles, hidden instructions and fake letters
Agent file auditorCLAUDE.md, skills, MCP, hooks and permissions
Can I paste this?Secrets and cookies in a text, log or HAR before sharing it
GitHub Actions auditorCommand injection, unsafe pull_request_target, unpinned actions
Headers and CSP auditorCSP, HSTS, cookies (Secure/HttpOnly/SameSite) from a pasted curl -I
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MCP catalogue auditorContext cost, annotations, official registry compliance
Access LabIdentity, RBAC and governance — vendor-neutral, multi-cloud
SAML decoder and auditorSignature, expiry, audience, and the XML Signature Wrapping attack
Email headers and DMARC auditorSPF, DKIM, DMARC — from an email’s headers or its DNS records
Side-channel labAttack a secret by timing and a key by power analysis
Authentication flow labAttack a real JWT (alg=none, RS256→HS256 confusion) on an interactive canvas
Defense in depthThe 7 security layers, and what Jarroba already covers with a real attack
Development & DevOps
CronGenerator and validator
Docker → ComposeConvert docker run to Compose
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Random dataNames, emails, IPs, UUIDs...
ElectronicsCircuit simulator: DC, transient, AC, MOSFETs and breadboard
From the transistor to the chipFrom a cell's noise margin to the clock: SRAM, read path, IR drop, standard cell and setup/hold
Logic GatesAND, OR, NOT and more gate simulator
Architecture SimulatorDiagram CI/CD, cloud, AI/MCP and simulate load
Quantum simulatorQubits, gates, superposition and entanglement
HammingError detection
Nginx/Apache SPA configGenerate config to serve a SPA
SemverSemantic version ranges
CORS ExplainerAnalyse your server's CORS headers
API workbenchSend requests, convert collections and compare versions
Cert inspectorAnalyse X.509 certificates in PEM format
Rate LimiterSimulate Token Bucket and Fixed Window
Assembly simulatorSimplified MIPS ISA · step-by-step · registers & memory
Operating systemsScheduling and page replacement, step by step and with sources
AutomataFrom regular expression to NFA, DFA and minimal DFA, plus CYK, step by step
Capture labNetwork captures (.pcap) explained: TCP, DNS and HTTP, packet by packet
Context ForgePack and shield your project for AI
Visual terminalClick and get the command: Linux, macOS, PowerShell
Project workshopSimulate your Makefile: what it rebuilds and why
Project X-rayWhat Spring will do with your code, and which dependencies are stale
Spring DoctorField injection, and the rest of the Spring Boot conventions, file by file
Test auditorOf these passing tests, which ones cannot fail?
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Artificial Intelligence
LLM costsSimulate monthly spend
Practice validityWhat still holds and what was superseded
AgentsSpec, permissions, verification and evals
Local AI cameraDetects objects and alerts, without uploading anything
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Does this text say anything?How much you can cut without losing a single idea
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Regression & MLELeast squares, overfitting, ridge
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Hypothesis testingp-values, t-test, χ², ANOVA and power
OCR — Image to textExtract text from images, 100% local
Finance
InflationOfficial CPI, purchasing power and savings in today's euros
Economy simulatorFirms, prices, monopolies and limited resources
Mortgage / loanPayment, real APRC of each offer and early repayment
Split expensesWho owes whom and how much
Self-employed (Spain)Social Security, IRPF, VAT and how much to set aside
Health & Wellness
BMI & healthBMI, ideal weight, BMR and sleep cycles
Productivity
PomodoroTime-block work technique
StopwatchWith laps, stages and history
Games & Entertainment
Dice3D dice from 2 to 9,999 faces, with projection
ScorekeeperPoints per player with game timer
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TournamentLeagues, groups, knockouts and Swiss, with standings
Multimedia & design
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Spreadsheet auditorRecalculates and compares with what's saved
Business
Meeting costHow much does each meeting really cost?
SLA / UptimeAllowed downtime, error budget and alerts
Burn rate / RunwayRunway, default alive and break-even
A/B TestAnalyse, plan and don't fool yourself by peeking
DORA MetricsYour team according to every DORA report, 2019–2025
UTM BuilderUTM links with their predicted GA4 channel
Spaces and resourcesTables, racks, beds or desks: share them out and see if they hold up
How to use
What it is

A network capture (.pcap or .pcapng, from Wireshark or tcpdump) explained packet by packet: what happens to each TCP connection, each DNS query and each HTTP request, and why, with the numbers that decide it. The capture is read in your browser and never leaves your device nor is saved.

The same rules as Wireshark

The TCP analysis is translated from Wireshark 4.4’s (its code, not just its manual) and checked against tshark packet by packet on 95 captures: retransmission, fast and spurious retransmission, out of order, duplicate ACKs, zero window, probes, keep-alive and RST. What it does not do: “Window Full”, SACK and MPTCP.

What Wireshark does not tell you

Four things that confuse and are not in the textbooks: a retransmission on timeout cannot be seen from the receiver (the lost segment never reaches the capture); “fast” requires the last duplicate ACK to have been seen less than 20 ms earlier, so it depends on where you capture; Wireshark’s iRTT counts the RST+ACK of a refused connection as the end of the handshake, and is not computed if the SYN has timestamp 0. When it happens, you will see it in the summary.

Practice

The Practice tab asks questions about the capture you have open, yours too: which packets have each problem, one it does NOT have (so as not to invent) and the iRTT. Tick “Hide the analysis” so you do not see the answers while you reply.

Capture labNetwork captures (.pcap) explained: TCP, DNS and HTTP, packet by packet

Capture

Fast retransmission after duplicate ACKs · captured at the client · built for teaching and checked with tshark

Drag your capture here or open it with the button: it is read in your browser, never leaves your device and is not saved.

What is going on

  • Fast retransmission:
  • Duplicate ACK: , , ,
Initial RTT (iRTT): 5.1 msDNS blog.prueba.test: NOERROR → 203.0.119.211 GET /login: 200

What Wireshark does not tell you

At packet 9 a piece is missing that was lost BEFORE reaching the capture point. If its resend arrives on a timeout and in order, from here it will look like a normal segment arriving late: such a retransmission can only be seen capturing at the sender.

N.ºTimeSourceDestinationProtoInfoAnalysis
10.000000192.168.1.4810.0.0.247DNS? blog.prueba.test (id 0x49f4)
20.01700010.0.0.247192.168.1.48DNSNOERROR · blog.prueba.test → CNAME edge.blog.prueba.test, A 203.0.119.211 (id 0x49f4)
30.018000192.168.1.48203.0.119.211TCP52028 → 80 [SYN] Seq=0 Win=64240 Len=0
40.023000203.0.119.211192.168.1.48TCP80 → 52028 [SYN, ACK] Seq=0 Ack=1 Win=64240 Len=0
50.023100192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=1 Ack=1 Win=64240 Len=0
60.023300192.168.1.48203.0.119.211HTTPGET /login HTTP/1.1 (Host: blog.prueba.test) · 52028 → 80 [PSH, ACK] Seq=1 Ack=1 Win=64240 Len=86
70.028300203.0.119.211192.168.1.48TCP80 → 52028 [PSH, ACK] Seq=1 Ack=87 Win=64240 Len=1000
80.028400192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=1001 Win=64240 Len=0
90.028700203.0.119.211192.168.1.48TCP80 → 52028 [PSH, ACK] Seq=2001 Ack=87 Win=64240 Len=1000Previous segment missing
100.028800192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=1001 Win=64240 Len=0Duplicate ACK #1
110.029100203.0.119.211192.168.1.48TCP80 → 52028 [PSH, ACK] Seq=3001 Ack=87 Win=64240 Len=1000
120.029200192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=1001 Win=64240 Len=0Duplicate ACK #2
130.029500203.0.119.211192.168.1.48TCP80 → 52028 [PSH, ACK] Seq=4001 Ack=87 Win=64240 Len=1000
140.029600192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=1001 Win=64240 Len=0Duplicate ACK #3
150.029900203.0.119.211192.168.1.48TCP80 → 52028 [PSH, ACK] Seq=5001 Ack=87 Win=64240 Len=831
160.030000192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=1001 Win=64240 Len=0Duplicate ACK #4
170.035000203.0.119.211192.168.1.48HTTPHTTP/1.1 200 OK · 80 → 52028 [PSH, ACK] Seq=1001 Ack=87 Win=64240 Len=1000Fast retransmission
180.035100192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=87 Ack=5832 Win=64240 Len=0
190.035300192.168.1.48203.0.119.211TCP52028 → 80 [FIN, ACK] Seq=87 Ack=5832 Win=64240 Len=0
200.040300203.0.119.211192.168.1.48TCP80 → 52028 [FIN, ACK] Seq=5832 Ack=88 Win=64240 Len=0
210.040400192.168.1.48203.0.119.211TCP52028 → 80 [ACK] Seq=88 Ack=5833 Win=64240 Len=0

Seq and Ack are relative (as in Wireshark): they count from the initial number of each direction.