Network Connectivity Errors

Understanding Network errors

Network errors are precise if you read them literally. Connection refused means a machine answered and actively rejected: nothing is listening on that port. Timeout means nothing answered at all, which points at a firewall silently dropping packets or a wrong address. Connection reset means the peer accepted then abruptly closed, often a proxy, a protocol mismatch, or an application crash. Distinguishing those three saves most of the debugging time.

How to debug Network errors

1. Test layer by layer: ping for reachability, nc -zv host port for the port, then the application protocol. Do not skip to the application layer.
2. Confirm what is listening locally with ss -tulpn. "Connection refused" on localhost almost always means the service bound to 127.0.0.1 when it needed 0.0.0.0, or vice versa.
3. Capture packets when the error is ambiguous: sudo tcpdump -ni any port 443 and host x.x.x.x. A RST is visible; a silent drop is visible as unanswered SYNs.
4. For intermittent failures under load, check ephemeral port exhaustion and conntrack table limits (ss -s, conntrack -S) before blaming the network.
5. Suspect MTU when small requests succeed and large ones hang. Test with ping -M do -s 1472 and reduce until packets pass.

Tools worth reaching for

• ss -tulpn
• nc -zv
• tcpdump
• mtr
• curl --connect-timeout
• iptables -L -n -v

All 35 Network errors

• Bandwidth limit exceededNetwork bandwidth quota has been exceeded, causing connection throttling or blocking.
• bind: address already in use (EADDRINUSE)Another process is already listening on the port, or a previous server instance did not shut down cleanly.
• chrony/NTP: clock not synchronised (Leap status: Not synchronised) NewThe host's clock is free running because it cannot reach a time source or has rejected the ones it has. Skew breaks…
• Circuit breaker openCircuit breaker pattern activated due to repeated failures, temporarily blocking requests to protect service.
• Connection pool exhaustedAll available connections in the connection pool are in use, unable to create new connections.
• Connection refusedThe target server is not accepting connections on the specified port. The service may be down, blocked by a firewall…
• Connection reset by peerThe remote server abruptly closed the connection, often due to server overload or configuration issues.
• Connection timeoutThe connection attempt took too long and was aborted. Network connectivity issues or server overload.
• Database connection timeoutDatabase connection attempt exceeded the timeout limit, often due to network latency or server load.
• Database deadlockDatabase transaction deadlock occurred, causing request to fail or timeout.
• Docker network errorDocker containers cannot communicate due to network configuration issues or DNS resolution problems.
• Elasticsearch cluster unavailableElasticsearch cluster is not responding, nodes are down, or cluster health is red/yellow with no available shards.
• Firewall blocking connectionNetwork firewall is blocking the connection attempt to the destination port or IP address.
• gRPC unavailablegRPC service is temporarily unavailable, often due to server overload or network issues.
• IPv6: Network is unreachable / no route to hostThe client prefers IPv6 or receives AAAA records but has no working IPv6 path. This can add long fallback delays or…
• kubectl: connection refusedkubectl cannot connect to the Kubernetes API server. Cluster may be down, kubeconfig incorrect, or network…
• Linux: duplicate IPv4 address detected on the network NewTwo hosts answer ARP for the same address, so switches flap the MAC between ports and traffic reaches whichever…
• Linux: possible SYN flooding on port X, sending cookies NewThe kernel's accept queue for a listening socket overflowed. Under a real flood that is the defence working, but far…
• Memory allocation failedServer ran out of memory while processing the request, causing allocation failures.
• MongoDB connection timeoutMongoDB client cannot establish connection within the specified timeout period. Server overload, network issues, or…
• MTU mismatch / fragmentation needed but DF setA link in the path has a smaller MTU than the sender used and the packet has Don't Fragment set. Common with VPNs…
• MySQL: Access denied for userMySQL authentication failed due to incorrect username, password, or insufficient privileges for the specified host.
• Network changedThe network configuration changed during the request, causing connection failure.
• Network is unreachableNo route to the destination network. Routing table may be missing entries or network interface is down.
• Port unreachableThe specified port on the target host is not accessible, likely blocked by firewall or service not running.
• psql: connection to server refusedPostgreSQL server is not accepting connections. Server may be down, wrong host/port, or connection limits exceeded.
• QUIC handshake failed / UDP 443 blockedHTTP/3 over QUIC requires UDP/443. Corporate networks and firewalls often block it, forcing fallback to TCP or causing…
• Redis connection errorUnable to connect to Redis server, often due to authentication, network, or configuration issues.
• Service mesh routing errorService mesh (Istio, Linkerd) failed to route request properly due to configuration or policy issues.
• ssh: connect to host X port 22: Connection refusedSSH daemon is not running on the target host, port 22 is blocked, or host is unreachable.
• WebRTC: ICE connection failed NewThe two peers could not find a working path between them. Direct and STUN reflexive candidates fail behind symmetric…
• WebSocket connection failedUnable to establish WebSocket connection, often due to proxy issues or unsupported protocols.
• WebSocket frame too largeWebSocket frame size exceeds the maximum allowed limit, connection terminated.
• WebSocket protocol errorViolation of WebSocket protocol specification, often due to malformed frames or incorrect handshake.
• WebSocket upgrade failedHTTP connection could not be upgraded to WebSocket protocol, often due to proxy or server configuration.

Other categories

• AI 35Rate limits, context windows, GPU memory and model-serving failures.
• Ansible 10Unreachable hosts, become passwords, undefined variables and Jinja2 failures.
• API 14Auth headers, payload limits, versioning, idempotency and webhook signatures.
• Apple 10Command line tools, dyld, Homebrew permissions, notarisation and Keychain.
• Auth 11OIDC, SAML, Auth0, Okta, Keycloak, passkeys and MFA failures.
• BigData 11Spark, Kafka, Airflow, Snowflake, Flink and Databricks failures.
• C# 12NuGet restore, null references, EF Core migrations, async deadlocks and Blazor…
• C++ 11Segfaults, linker errors, memory corruption and template deduction failures.
• Caching 10Cache stampedes, stale content, Varnish and CloudFront failures.
• CI/CD 18GitHub Actions, GitLab CI, Jenkins, CircleCI: permissions, runners and…
• Client 21CORS, mixed content, module resolution, memory limits and framework runtime…
• Cloud 25IAM permissions, quotas, service limits and credential failures.
• Dart 10Null safety, pub version solving and build toolchain problems.
• Database 41Connections, deadlocks, constraints, replication and memory limits.
• DNS 10NXDOMAIN, SERVFAIL, timeouts, propagation and delegation problems.
• Docker 27Daemon connectivity, disk space, image pulls, ports and architecture mismatches.
• Elixir 9GenServer timeouts, supervision failures and Mix compilation problems.
• Email 8Delivery failures, relay denial, authentication, SPF, DKIM and DMARC.
• Frontend 23Hydration mismatches, bundler resolution, layout shift and font loading.
• Git 20Merge conflicts, rejected pushes, detached HEAD, LFS and repository corruption.
• Go 19Nil map assignment, concurrent map access, context cancellation and deadlocks.
• GraphQL 13Validation, depth limits, N+1 queries and fragment problems.
• gRPC 10Status codes, deadlines, message limits, TLS and HTTP/2 transport failures.
• HTTP 494xx client errors, 5xx server errors, redirects, headers and protocol problems.
• ICMP 23Destination unreachable, time exceeded, fragmentation needed and redirects.
• Ingress 8404 default backend, missing TLS secrets, IngressClass and path matching.
• Java 19Class loading, dependency resolution, connection pools and JVM version…
• JavaScript 42npm resolution, async pitfalls, hydration, memory limits and runtime type…
• Kubernetes 34CrashLoopBackOff, ImagePullBackOff, OOMKilled, RBAC, scheduling and storage.
• Logging 9Log4j, Logback, Fluentd, Logstash and CloudWatch ingestion problems.
• MessageQueue 14Kafka, RabbitMQ, SQS, NATS and Celery: lag, rebalancing and poison messages.
• Mobile 17Gradle, CocoaPods, Xcode signing, Metro bundler and toolchain mismatches.
• Monitoring 12Prometheus scrapes, Grafana data sources, OpenTelemetry exporters and agent…
• Performance 6GC pauses, thread pool starvation and event loop blocking.
• PHP 11Memory limits, execution timeouts, autoloading, Composer and PDO connections.
• Proxy 17nginx, Envoy, HAProxy, Traefik, Caddy and Cloudflare upstream failures.
• Python 35Imports, virtual environments, encoding, concurrency and dependency conflicts.
• Regex 8Catastrophic backtracking, back references, escaping and engine differences.
• Ruby 11Bundler, migrations, native extensions, encoding and asset compilation.
• Rust 19Borrow checker, ownership moves, trait bounds and lifetime mismatches.
• Scala 6Dependency resolution, binary compatibility and type inference failures.
• Security 25JWT validation, CSRF, OAuth grants, SELinux, SSH host keys and CSP.
• Serverless 11Lambda timeouts, package size limits, VPC networking and cold starts.
• Shell 13Command not found, permissions, quoting, expansion and Makefile syntax.
• Storage 13S3 permissions, NFS mounts, quotas, signed URLs and volume attachment.
• Svelte 10Store subscriptions, load functions and server/client boundaries.
• System 26Disk space, systemd units, file descriptors, OOM killer and scheduled jobs.
• Terraform 18State locks, provider auth, drift, dependency cycles and plan-time unknowns.
• Testing 18Jest, pytest, JUnit, Cypress and Playwright: fixtures, snapshots and timeouts.
• TLS 24Untrusted authorities, expiry, hostname mismatch, chains and cipher negotiation.
• TypeScript 19Assignability, missing declarations, strict null checks and generic constraints.
• Virtualization 8VirtualBox, VMware, Hyper-V, WSL, KVM and hypervisor conflicts.
• Web3 7Gas estimation, nonce management and reverted transactions.
• WebAssembly 7Compile errors, memory bounds and host binding mismatches.
• WebServer 10nginx, Apache, IIS and Caddy: binding, permissions, rewrites and TLS.
• Windows 10Installer failures, missing runtimes, update errors and permission problems.