Error map
nc/telnet hangs: firewall DROP vs REJECT Networking: nc -vz hangs then times out — firewall silently DROPs packets (REJECT would answer "Connection refused")
Networking · layer 2, Network
Centre: this error. First ring: errors it is linked with (12). 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 (12)
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…
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…
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.
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.
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: 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: NAT gateway ErrorPortAllocation / SNAT port exhaustion — intermittent outbound connection timeouts
Many instances share few public IPs, and the NAT gateway runs out of source ports for outbound connections.
Networking: Connection works, then fails after being idle — firewall/NAT idle timeout dropped the TCP session (use keepalive)
A connection that sat idle was forgotten by a firewall or NAT device, so the next packet gets reset or vanishes.
Networking: TCP: request_sock_TCP: Possible SYN flooding on port 80. Sending cookies. Check SNMP counters.
The server's pending-connection queue filled, so the kernel switched to SYN cookies to survive.
Networking: ICMP Destination unreachable (Port unreachable) — UDP packet sent to a port where nothing is listening
A host answered a UDP packet with an ICMP message saying no program is listening on that port.
SMTP: telnet smtp.example.com 25 — Connection timed out (cloud provider blocks port 25)
Connections from your server to other mail servers on port 25 time out because your cloud host or ISP blocks outbound SMTP to fig…
Two steps away (36)
Error: connect ECONNREFUSED 127.0.0.1:5432
Nothing is listening at that host:port — the target service is down or you’re connecting to the wrong address.
HTTP 407 Proxy Authentication Required
A proxy between you and the server requires credentials before it will forward your request.
HTTP 408 Request Timeout
The server closed the connection because the client took too long to send the complete request.
HTTP 403 Forbidden
The server understood the request but refuses to authorize it — you (or the server process) are not allowed to access this resour…
HTTP 429 Too Many Requests
You’ve hit a rate limit — too many requests in a given time window. Slow down and retry later.
Cloudflare Error 520: Web Server Returned an Unknown Error
Cloudflare connected to your origin, but the origin returned an empty, unknown or invalid response.
Error: socket hang up / read ECONNRESET
The other side closed the TCP connection abruptly while your client was still using it.
ssh: connect to host x port 22: Connection refused
The server answered, but nothing is listening on the SSH port — sshd isn’t running, it uses another port, or a firewall actively…
ping: google.com: Temporary failure in name resolution
The Linux host can’t resolve domain names — no working DNS server is configured or reachable.
SQL Server: Login failed for user 'x'. (Error 18456)
SQL Server rejected the login — wrong password, wrong authentication mode, or a disabled/missing login.
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.
Azure SQL: Login failed for user '<token-identified principal>' (Microsoft Entra / managed identity)
Entra ID authentication succeeded, but there’s no database user for that identity (or it lacks permission to the database).
ssh: connect to host github.com port 22: Connection timed out
Outbound SSH on port 22 is blocked (firewall, corporate or public Wi-Fi).
kex_exchange_identification: Connection closed by remote host
The SSH connection was closed before the handshake — server overload, rate limiting, or a proxy/firewall interfering.
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 1005: Access denied — The owner of this website has banned the autonomous system number (ASN) your IP address is in
The site owner blocked your network provider (ASN) — common for VPNs, cloud/data-center IPs and some mobile carriers.
Cloudflare Error 1006 / 1007 / 1008: Access denied — Your IP address has been banned
The site owner (or a Cloudflare security setting) blocked your IP address.
Cloudflare Error 1012: Access denied
The site owner blocked access with a rule that matched your request (often due to malicious activity detected from your IP).
Networking: Wi-Fi keeps disconnecting / signal -80 dBm / wlan0: deauthenticating by local choice (reason=3)
The wireless link is too weak or too noisy to hold, so the client drops and rejoins repeatedly.
Networking: Interface down — SFP transceiver Loss of Signal / no receive light
A fibre or SFP link is down because the transceiver receives no light.
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.
Docker: Bind for 0.0.0.0:80 failed: port is already allocated
Another container or host process already publishes that host port.
nginx: [emerg] bind() to 0.0.0.0:80 failed (98: Address already in use)
Nginx (or another server) can’t listen on the port because another process already holds it — often Apache, another Nginx, or a s…
Networking: Server reachable from the internet but not from inside the LAN via its public IP — NAT loopback / hairpin NAT not working
Devices on your own network can't use the router's public address to reach a server that is also on that network.
client_loop: send disconnect: Broken pipe (SSH session drops when idle)
The SSH connection was cut — usually an idle timeout on a NAT router, firewall or the server, sometimes a network change.
Redis: Error: read ECONNRESET / redis.exceptions.ConnectionError: Connection reset by peer after idle time
A middlebox or Redis closed an idle connection, and the pooled client reused it.
Networking: Your Remote Desktop Services session has ended. Possibly the network connection was lost / Remote Desktop disconnected, error code 0x904
An established Remote Desktop session dropped because the network under it stalled or the session was reset.
Cloudflare-branded 502 Bad Gateway / 504 Gateway Timeout ("cloudflare" footer)
A plain-looking 502/504 page with “cloudflare” at the bottom comes from Cloudflare’s network itself (or an origin response it cou…
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: connect: Cannot assign requested address — ephemeral port range exhausted by TIME_WAIT connections
The machine ran out of local ports for new outgoing connections to the same destination.
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: 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.
SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited
The receiving server (often Gmail or Outlook) is temporarily throttling your mail because the volume or reputation of your sendin…
SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html
The recipient server deliberately rejected the first delivery attempt from an unknown sender (greylisting) and expects a legitima…
SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing
The server hit an internal problem (disk, database, content filter, DNS) while handling your message and asked you to retry later.
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