Network Connect Timeout Error
Production Risk
Low — proxy-specific; investigate upstream server availability and firewall rules.
599 Network Connect Timeout Error is an informal status code used by some HTTP proxies to signal that a network connection timeout occurred while attempting to connect to the upstream server. It is the connection-phase counterpart to 598 (read timeout) and 504 (gateway timeout).
- 1The proxy could not establish a TCP connection to the upstream server within the configured connect timeout.
- 2The upstream server is overloaded and not accepting new connections.
- 3A firewall is silently dropping connection attempts (causing timeout rather than immediate refusal).
- 4DNS resolved successfully but the upstream IP is unreachable.
A proxy attempts to connect to an upstream server that is overloaded and not completing TCP handshakes.
GET /api/data HTTP/1.1 Host: example.com # Proxy → upstream TCP connect timeout
expected output
HTTP/1.1 599 Network Connect Timeout Error
Fix 1
Check upstream server availability
WHEN The upstream server may be down.
# Test connectivity to upstream nc -zv upstream-server 443 curl -v --connect-timeout 5 https://upstream-server/
Why this works
Confirms whether the upstream server is accepting TCP connections.
Fix 2
Increase connect timeout in proxy configuration
WHEN The upstream is slow to accept connections under load.
# nginx proxy_connect_timeout 30s;
Why this works
Gives the upstream more time to complete the TCP handshake before the proxy gives up.
✕ Do not use 599 in new API or application code
Use 504 Gateway Timeout — it is the IETF-standardised equivalent.
Not defined in any IETF RFC. Use 504 in any new proxy or application designs.
Informal convention (no IETF RFC)
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