| Native VLAN mismatch / wrong VLAN, no connectivity |
Networking: %CDP-4-NATIVE_VLAN_MISMATCH / device on the wrong VLAN can't reach the gateway |
A port or trunk carries a different VLAN than expected, so hosts silently land on the wrong network. |
| Port err-disabled (psecure-violation) |
Networking: %PM-4-ERR_DISABLE: psecure-violation error detected on Gi0/5, putting Gi0/5 in err-disable state |
The switch shut the port down because a security or protection feature detected a violation. |
| Rogue DHCP server hands out wrong gateway/DNS |
Networking: Clients receive the wrong default gateway or DNS server — a rogue DHCP server (often a home router) answers first |
A second DHCP server on the network is giving clients bad settings, so some machines have no internet or odd… |
| Idle connections silently dropped by firewall/NAT |
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 vanish… |
| DHCPDISCOVER: no free leases |
Networking: dhcpd: DHCPDISCOVER from aa:bb:cc:dd:ee:ff via eth0: network 192.168.1.0/24: no free leases |
The DHCP server has no unused addresses left in the scope. |
| dig: status: SERVFAIL (DNSSEC validation failed) |
Networking: dig: ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL — resolver failed to answer (DNSSEC validation or upstream failure) |
The DNS resolver couldn't get a valid answer, often because DNSSEC signatures are broken or the authoritative… |
| Duplex mismatch: late collisions / CRC errors |
Networking: Duplex mismatch — late collisions, runts and CRC errors on the switch port |
One side runs full duplex and the other half duplex, so the link works but is slow and drops packets. |
| WireGuard: Handshake did not complete after 5 seconds |
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. |
| 169.254.x.x self-assigned IP (DHCP failed) |
Networking: IPv4 address 169.254.x.x (Autoconfiguration / APIPA) — the PC couldn't get an address from DHCP |
With no DHCP answer, the computer gave itself a link-local 169.254.x.x address, which only works on the local… |
| nf_conntrack: table full, dropping packet |
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. |
| nfs: server not responding, still trying |
Networking: kernel: nfs: server nas01 not responding, still trying / nfs: server nas01 OK |
An NFS client can't get replies from the server, so processes using the mount hang in uninterruptible sleep. |
| NAT gateway SNAT port exhaustion (ErrorPortAllocation) |
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. |
| nc/telnet hangs: firewall DROP vs REJECT |
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 clo… |
| High TCP retransmissions / packet loss |
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 errors |
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. |
| Network cable unplugged / NO-CARRIER |
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. |
| Ping: Request timed out |
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. |
| PING: transmit failed. General failure. |
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), no… |
| Port forwarding not working (CGNAT / double NAT) |
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… |
| IPv6 enabled but broken: slow connections / Happy Eyeballs |
Networking: Sites load slowly or hang for seconds — broken IPv6 (AAAA resolves, IPv6 route missing) falling back to IPv4 |
The client prefers IPv6, but the IPv6 path is broken, so every connection waits for a timeout before using IP… |
| Possible SYN flooding on port 80. Sending cookies. |
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. |
| Wrong subnet mask: hosts can't reach each other |
Networking: Two hosts on one LAN can't talk — subnet mask or network address mismatch |
The hosts disagree about which addresses are local, so one tries to route traffic that should go straight acr… |
| Internal hostnames fail on VPN (split DNS / systemd-resolved) |
Networking: VPN connected but internal names like db.corp.local don't resolve — split DNS / 127.0.0.53 stub resolver not using the VPN DNS |
The VPN works by IP, but internal hostnames fail because queries still go to your normal DNS server. |
| 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 |
The tunnel is up, but traffic for the office/VPC subnets isn't sent through it or comes back the wrong way. |
| The specified network name is no longer available (64) |
Networking: Windows error 64: The specified network name is no longer available — SMB session dropped |
A connection to a Windows file share was cut while in use. |
| Windows has detected an IP address conflict |
Networking: Windows has detected an IP address conflict / Duplicate address detected by 00:aa:bb:cc:dd:ee |
Two devices on the same network are using the same IP address, so traffic reaches the wrong one or neither. |
| 4-way handshake timeout / Incorrect password |
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. |
| arp -a shows (incomplete) |
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. |
| 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) |
Two OSPF routers see each other but never finish exchanging databases, cycling between EXSTART and DOWN. |
| MAC flapping / broadcast storm (spanning tree loop) |
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. |
| DHCPDISCOVER ... No DHCPOFFERS received |
Networking: dhclient: DHCPDISCOVER on eth0 ... No DHCPOFFERS received. No working leases in persistent database - sleeping. |
The client broadcast a DHCP request and no server answered. |
| ICMP Destination unreachable (Port unreachable) |
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. |
| NO_PROPOSAL_CHOSEN (IPsec/IKE) |
Networking: IKE: received NO_PROPOSAL_CHOSEN — IPsec peers can't agree on encryption, hash or DH group |
The two VPN gateways offered encryption settings that have nothing in common, so the tunnel never comes up. |
| SFP link down / Loss of Signal (LOS) |
Networking: Interface down — SFP transceiver Loss of Signal / no receive light |
A fibre or SFP link is down because the transceiver receives no light. |
| No Wi-Fi adapter found / Wi-Fi option missing |
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. |
| Reply from 192.168.1.1: Destination host unreachable |
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. |
| 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 |
A packet is larger than the path MTU and is marked "don't fragment", so a router discards it. |
| Public IP/domain unreachable from inside the LAN (NAT hairpin) |
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 netw… |
| Many sockets in CLOSE_WAIT |
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. |
| Reply from x: TTL expired in transit |
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. |
| TCP ZeroWindow / TCP Window Full |
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. |
| Cannot assign requested address (EADDRNOTAVAIL) |
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. |
| dig: status: REFUSED (recursion not available) |
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. |
| Domain still resolves to the old IP (DNS propagation/cache) |
Networking: DNS changed but the domain still resolves to the old IP — TTL and resolver caching ("propagation") |
You updated a DNS record, but resolvers and devices keep serving the old answer until its TTL expires. |
| DNS lookups take 5 seconds (timeout then retry) |
Networking: Every connection pauses about 5 seconds — resolv.conf nameserver timeout, then fallback to the next DNS server |
The first DNS server in the list isn't answering, so each lookup waits out a timeout before trying the next o… |
| 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 |
The application accepts connections slower than they arrive, so the kernel's accept queue overflows. |
| Connections hang on large transfers (PMTU black hole) |
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. |
| Wi-Fi keeps disconnecting (weak signal, -80 dBm) |
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. |
| Windows: Default gateway is not available |
Networking: Windows: Default gateway is not available / Diagnose Connection Problems: DHCP not enabled |
The PC can't reach its router, so it has no path out of the local network. |
| Your Remote Desktop Services session has ended (network problem) |
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. |