Networking Runbooks
DNS resolution, TCP connections, TLS handshakes, and packet loss.
25 scenarios ยท search and filter below, or jump via the quick nav.
ping app-server works but ping app-server.internal does not (or vice versa)
curl: Connection refused immediately (no timeout delay)
curl: (60) SSL certificate problem: certificate has expired
curl: (51) SSL: no alternative certificate subject name matches target host name
dig/nslookup returns NXDOMAIN for a domain that was recently updated
dig returns status: SERVFAIL for one or more domains
curl: Connection timed out after 30-120s instead of refused
curl: (56) Connection reset by peer
curl: (35) error:14094410:ssl routines:ssl3_read_bytes:sslv3 alert handshake failure
curl: (60) SSL certificate problem: unable to get local issuer certificate
Old Java/Python client gets 'no cipher suites in common' error
curl returns HTTP 502 Bad Gateway
curl returns HTTP 504 Gateway Timeout after exactly 60s (default nginx proxy timeout)
Connections to a specific port fail despite an ACCEPT rule existing for the source
'connect: Network is unreachable' when the app tries to connect via IPv6
dig and nslookup resolve the hostname correctly but curl/browser gets ERR_NAME_NOT_RESOLVED
WebSocket or long-poll connection drops after ~300s or ~3600s of inactivity
netstat shows high RetransSegs counter increasing rapidly
curl: (35) error:14094410:ssl routines:ssl3_read_bytes:sslv3 alert handshake failure when connecting to mTLS endpoint
Browser shows certificate warning for a vhost -- certificate CN doesn't match the hostname
High rate of TCP TIME_WAIT connections on the server
dmesg shows 'nf_conntrack: table full, dropping packet'
Small HTTP requests succeed but large responses or file transfers hang or fail
'No route to host' errors between hosts on the same subnet