Error map
ICMP Destination unreachable (Port unreachable) Networking: ICMP Destination unreachable (Port unreachable) — UDP packet sent to a port where nothing is listening
Networking · layer 2, Network
Centre: this error. First ring: errors it is linked with (9). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (9)
Networking: ping — Request timed out / 100% packet loss (host drops or filters ICMP)
No echo reply arrived in time: the host is down, unreachable, or simply ignores ping.
Networking: PING: transmit failed. General failure. / ping: sendmsg: Operation not permitted
The operating system could not even send the packet, so the problem is local (adapter, route or firewall), not the remote host.
Networking: nc -vz hangs then times out — firewall silently DROPs packets (REJECT would answer "Connection refused")
A connection attempt gets no answer at all, which points to a firewall dropping the packets rather than a closed port.
Networking: netstat -su: packet receive errors, receive buffer errors — UDP datagrams dropped by a full socket buffer
UDP packets arrive faster than the application reads them, so the kernel throws the extras away.
Networking: Handshake for peer did not complete after 5 seconds, retrying (try 2) — WireGuard tunnel never establishes
The WireGuard peers can't complete the cryptographic handshake, so no traffic flows.
Networking: ping — Destination Host Unreachable / Destination net unreachable (ICMP type 3)
A router (or your own PC) tells you it has no way to deliver the packet to that address.
Networking: TTL expired in transit — routing loop between routers
A packet ran out of hops before arriving, which usually means two routers keep sending it back and forth.
Networking: ping: local error: message too long, mtu=1500 / Packet needs to be fragmented but DF set
A packet is larger than the path MTU and is marked "don't fragment", so a router discards it.
Networking: dig: status: REFUSED / recursion requested but not available — the DNS server won't resolve queries for you
The DNS server answered, but its policy refuses to look up names for your address.
Two steps away (26)
Cloudflare Error 1020: Access Denied
A Cloudflare firewall/WAF custom rule blocked the request — the visitor’s IP, country, user agent or request pattern matched a bl…
driver failed programming external connectivity on endpoint: iptables: No chain/target/match by that name
Docker’s iptables rules were wiped (firewalld/ufw reload, iptables flush) while Docker was running, so it can’t publish ports.
ssh: connect to host x port 22: Connection timed out
SSH packets get no reply — a firewall/security group drops them, the server is off, or the IP is wrong.
Docker Desktop: WSL 2 installation is incomplete / WSL update required
Docker Desktop on Windows needs WSL 2 and its Linux kernel; they’re missing, outdated or disabled.
Networking: Network cable unplugged / `ip link` shows NO-CARRIER state DOWN / Ethernet: Network cable unplugged
The network card sees no electrical link, so nothing above it (IP, DNS, browser) can work.
Networking: No Wi-Fi networks found — the Wi-Fi adapter is missing, disabled or its driver failed
The operating system doesn't see a working wireless adapter, so no networks can be listed.
connect: No route to host (EHOSTUNREACH)
The destination host isn’t reachable — it’s down, on another subnet without routing, or a firewall rejects with “host unreachable…
Azure SQL: Cannot open server 'x' requested by the login. Client with IP address 'y' is not allowed to access the server (Error 40615)
The Azure SQL server firewall doesn’t allow your client’s IP address.
ERR_CONNECTION_TIMED_OUT: This site can’t be reached — example.com took too long to respond
The browser waited for the server but got no response — the server is down/unreachable, a firewall drops traffic, or DNS points t…
Networking: `arp -n` shows (incomplete) / Destination Host Unreachable — no ARP reply from the next hop
The machine asked "who has this IP?" and nobody on the local network answered.
Networking: Port forwarding configured but the port stays closed — carrier-grade NAT (CGNAT) or double NAT
Your router forwards the port correctly, but another NAT in front of it (your ISP or a second router) blocks inbound connections.
Networking: netstat -s: segments retransmitted / Wireshark: TCP Retransmission, TCP Dup ACK — packet loss on the path
Packets are being lost, so TCP resends them and throughput collapses.
Networking: wlan0: WPA: 4-way handshake failed - pre-shared key may be incorrect / WRONG_KEY
The Wi-Fi association started but key negotiation failed, almost always a wrong password.
Networking: SSH/HTTPS connects but hangs on large data — path MTU discovery black hole
Small packets pass but full-size ones vanish, so logins work and then transfers or page loads stall.
Networking: VPN is connected but I can't reach the internal network — missing routes, split tunnel or overlapping subnets
The tunnel is up, but traffic for the office/VPC subnets isn't sent through it or comes back the wrong way.
HTTP 404 Not Found
The server can’t find anything at the requested URL.
HTTP 405 Method Not Allowed
The URL exists, but it doesn’t accept the HTTP method you used (e.g. POST to a GET-only route).
connect: Network is unreachable (ENETUNREACH)
The host has no route to the destination network — no default gateway, interface down, or trying IPv6 without IPv6 connectivity.
Cloudflare Error 1019: Compute server error (Workers)
A Cloudflare Worker routed the request to a loop or hit an internal compute error.
SMTP: 554 5.4.14 Hop count exceeded - possible mail loop
The message passed through too many mail servers (usually over 25), which almost always means two systems forward mail to each ot…
Networking: %OSPF-5-ADJCHG: Nbr 10.0.0.2 from EXSTART to DOWN — OSPF neighbor stuck in EXSTART/EXCHANGE (MTU mismatch)
Two OSPF routers see each other but never finish exchanging databases, cycling between EXSTART and DOWN.
Cloudflare Error 1033: Argo Tunnel error (Cloudflare Tunnel not connected)
The hostname routes to a Cloudflare Tunnel, but no healthy cloudflared connector is running for it.
cloudflared: Unable to reach the origin service. The service may be down or it may not be responding to traffic from cloudflared (502)
The tunnel is connected, but cloudflared can’t reach your local service at the address in the ingress rule.
ACM certificate stuck in Pending validation
ACM is waiting to see the DNS (or email) validation record — it wasn’t created, was created in the wrong DNS zone, or has a typo.
dial tcp: lookup registry-1.docker.io: no such host / Temporary failure in name resolution during docker build
DNS isn’t working for the Docker daemon or inside build containers, so pulls and package downloads fail.
DNS not working inside a Kubernetes pod (could not resolve host / nslookup: can't resolve)
Pods can’t resolve service names or external hosts — CoreDNS is down, blocked by NetworkPolicy, or the upstream resolver fails.
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