Error map

Possible SYN flooding on port 80. Sending cookies. Networking: TCP: request_sock_TCP: Possible SYN flooding on port 80. Sending cookies. Check SNMP counters.

Networking · layer 2, Network

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Centre: this error. First ring: errors it is linked with (8). 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.

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Possible SYN flooding on port 80. Sending cookies. — Networking: TCP: request_sock_TCP: Possible SYN flooding on port 80. Sending cookies. Check SNMP counters. (Networking)Possible SY…Cloudflare 502 — Cloudflare-branded 502 Bad Gateway / 504 Gateway Timeout ("cloudflare" footer) (Cloudflare)Cloudflare 502nc/telnet hangs: firewall DROP vs REJECT — Networking: nc -vz hangs then times out — firewall silently DROPs packets (REJECT would answer "Connection refused") (Networking)nc/telnet hangs: fi…nf_conntrack: table full, dropping packet — Networking: kernel: nf_conntrack: table full, dropping packet — new connections fail on a Linux firewall/NAT host (Networking)nf_conntrack: table…TCP ZeroWindow / TCP Window Full — Networking: Wireshark: TCP ZeroWindow, TCP Window Full — the receiver can't keep up and tells the sender to stop (Networking)TCP ZeroWindow / TC…Many sockets in CLOSE_WAIT — Networking: Thousands of sockets in CLOSE_WAIT — the application is not closing its side of the connection (Networking)Many sockets in CLO…Connections hang on large transfers (PMTU black hole) — Networking: SSH/HTTPS connects but hangs on large data — path MTU discovery black hole (Networking)Connections hang on…Cannot assign requested address (EADDRNOTAVAIL) — Networking: connect: Cannot assign requested address — ephemeral port range exhausted by TIME_WAIT connections (Networking)Cannot assign reque…times the listen queue of a socket overflowed — Networking: netstat -s: 1523 times the listen queue of a socket overflowed — accept queue full, connections dropped (Networking)times the listen qu…502 — HTTP 502 Bad Gateway (HTTP)502504 — HTTP 504 Gateway Timeout (HTTP)504520 — Cloudflare Error 520: Web Server Returned an Unknown Error (Cloudflare)5201020 — Cloudflare Error 1020: Access Denied (Cloudflare)1020ssh port 22 Connection timed out — ssh: connect to host x port 22: Connection timed out (Linux)ssh port 22 Connect…No route to host — connect: No route to host (EHOSTUNREACH) (Linux)No route to hostDNS in pod — DNS not working inside a Kubernetes pod (could not resolve host / nslookup: can't resolve) (Kubernetes)DNS in podarp -a shows (incomplete) — Networking: `arp -n` shows (incomplete) / Destination Host Unreachable — no ARP reply from the next hop (Networking)arp -a shows (incom…NAT gateway SNAT port exhaustion (ErrorPortAllocation) — Networking: NAT gateway ErrorPortAllocation / SNAT port exhaustion — intermittent outbound connection timeouts (Networking)NAT gateway SNAT po…High TCP retransmissions / packet loss — Networking: netstat -s: segments retransmitted / Wireshark: TCP Retransmission, TCP Dup ACK — packet loss on the path (Networking)High TCP retransmis…UDP RcvbufErrors / packet receive errors — Networking: netstat -su: packet receive errors, receive buffer errors — UDP datagrams dropped by a full socket buffer (Networking)UDP RcvbufErrors / …too many open files — Go: accept tcp [::]:8080: accept4: too many open files / open x: too many open files (Go)too many open filesSFP link down / Loss of Signal (LOS) — Networking: Interface down — SFP transceiver Loss of Signal / no receive light (Networking)SFP link down / Los…MAC flapping / broadcast storm (spanning tree loop) — Networking: %SW_MATM-4-MACFLAP_NOTIF: Host is flapping between ports / broadcast storm (Networking)MAC flapping / broa…Frag needed and DF set (ICMP type 3 code 4) — Networking: ping: local error: message too long, mtu=1500 / Packet needs to be fragmented but DF set (Networking)Frag needed and DF …VPN connected but internal subnets unreachable — Networking: VPN is connected but I can't reach the internal network — missing routes, split tunnel or overlapping subnets (Networking)VPN connected but i…OSPF neighbor stuck in EXSTART/EXCHANGE — Networking: %OSPF-5-ADJCHG: Nbr 10.0.0.2 from EXSTART to DOWN — OSPF neighbor stuck in EXSTART/EXCHANGE (MTU mismatch) (Networking)OSPF neighbor stuck…Network cable unplugged / NO-CARRIER — Networking: Network cable unplugged / `ip link` shows NO-CARRIER state DOWN / Ethernet: Network cable unplugged (Networking)Network cable unplu…Wi-Fi keeps disconnecting (weak signal, -80 dBm) — Networking: Wi-Fi keeps disconnecting / signal -80 dBm / wlan0: deauthenticating by local choice (reason=3) (Networking)Wi-Fi keeps disconn…4-way handshake timeout / Incorrect password — Networking: wlan0: WPA: 4-way handshake failed - pre-shared key may be incorrect / WRONG_KEY (Networking)4-way handshake tim…upstream timed out — Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream (Nginx / Apache)upstream timed outWARNING: The TCP backlog setting of 511 cannot be enforced — Redis: # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128 (Redis)WARNING: The TCP ba…

Directly related (8)

Cloudflare 502Cloudflare

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…

nc/telnet hangs: firewall DROP vs REJECTNetworking

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.

nf_conntrack: table full, dropping packetNetworking

Networking: kernel: nf_conntrack: table full, dropping packet — new connections fail on a Linux firewall/NAT host

The connection-tracking table is full, so the kernel drops packets for any new connection.

TCP ZeroWindow / TCP Window FullNetworking

Networking: Wireshark: TCP ZeroWindow, TCP Window Full — the receiver can't keep up and tells the sender to stop

The receiving application isn't reading data fast enough, so its TCP buffer filled and the sender must pause.

Many sockets in CLOSE_WAITNetworking

Networking: Thousands of sockets in CLOSE_WAIT — the application is not closing its side of the connection

The other end closed the connection, but your program never called close(), so the socket stays open.

Connections hang on large transfers (PMTU black hole)Networking

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.

Cannot assign requested address (EADDRNOTAVAIL)Networking

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.

times the listen queue of a socket overflowedNetworking

Networking: netstat -s: 1523 times the listen queue of a socket overflowed — accept queue full, connections dropped

The application accepts connections slower than they arrive, so the kernel's accept queue overflows.

Two steps away (22)

502HTTP

HTTP 502 Bad Gateway

A proxy or gateway (Nginx, load balancer, CDN) got an invalid response — or no response — from the upstream server behind it.

504HTTP

HTTP 504 Gateway Timeout

A proxy or gateway waited too long for the upstream server to respond and gave up.

520Cloudflare

Cloudflare Error 520: Web Server Returned an Unknown Error

Cloudflare connected to your origin, but the origin returned an empty, unknown or invalid response.

1020Cloudflare

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…

ssh port 22 Connection timed outLinux

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.

No route to hostLinux

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…

DNS in podKubernetes

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.

arp -a shows (incomplete)Networking

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.

NAT gateway SNAT port exhaustion (ErrorPortAllocation)Networking

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.

High TCP retransmissions / packet lossNetworking

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.

UDP RcvbufErrors / packet receive errorsNetworking

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.

too many open filesGo

Go: accept tcp [::]:8080: accept4: too many open files / open x: too many open files

The process reached its file-descriptor limit — usually leaked connections, files or response bodies.

SFP link down / Loss of Signal (LOS)Networking

Networking: Interface down — SFP transceiver Loss of Signal / no receive light

A fibre or SFP link is down because the transceiver receives no light.

MAC flapping / broadcast storm (spanning tree loop)Networking

Networking: %SW_MATM-4-MACFLAP_NOTIF: Host is flapping between ports / broadcast storm

A layer-2 loop is making frames circle the network, saturating links and making the whole VLAN crawl.

Frag needed and DF set (ICMP type 3 code 4)Networking

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.

VPN connected but internal subnets unreachableNetworking

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.

OSPF neighbor stuck in EXSTART/EXCHANGENetworking

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.

Network cable unplugged / NO-CARRIERNetworking

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.

Wi-Fi keeps disconnecting (weak signal, -80 dBm)Networking

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.

4-way handshake timeout / Incorrect passwordNetworking

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.

upstream timed outNginx / Apache

Nginx: upstream timed out (110: Connection timed out) while reading response header from upstream

Nginx waited longer than proxy_read_timeout (60 s default) for the backend (app server, PHP-FPM) to respond, then returned 504.

WARNING: The TCP backlog setting of 511 cannot be enforcedRedis

Redis: # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128

The kernel limits the listen queue below Redis' `tcp-backlog`, so bursts of new connections may be dropped.

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