599 Network Connect Timeout Error
The HTTP 599 Network Connect Timeout Error status code is an unofficial HTTP status code used by some proxies to signal a connection timeout when attempting to reach an upstream server.
Usage
The 599 Network Connect Timeout Error status code is an informal convention indicating the proxy was unable to establish a TCP connection to the upstream server within the allowed timeout. Unlike 598 Network Read Timeout Error, where the connection succeeds but the read times out, a 599 means the connection itself was never established.
This status code is not defined in any RFC or formal specification. Some proxy implementations and custom middleware adopt the code to provide more granular timeout reporting to downstream clients.
Note
This status code is not standardized. The behavior and presence of this code depends entirely on the proxy software in use.
SEO impact
Search engines treat 599 responses as server errors. Persistent connection timeouts reduce crawl rate and prevent indexing of affected URLs. Resolving the upstream connectivity issue restores normal crawling.
Example
A proxy attempts to connect to an upstream server. The TCP connection does not complete within the proxy's connect timeout window.
Request
GET /api/status HTTP/1.1
Host: www.example.re
Accept: application/json
Response
HTTP/1.1 599 Network Connect Timeout Error
Date: Sun, 02 Mar 2026 11:20:00 GMT
Content-Type: text/plain
Network connect timeout while attempting to
reach upstream server
How to fix
The fix depends on the proxy software generating the 599 response. Unlike 598 Network Read Timeout Error, the TCP connection itself never completed.
Verify the upstream is listening. Confirm the
upstream server process is running and bound to the
expected port. Run ss -tlnp or netstat -tlnp on the
upstream host to check active listeners. A stopped
service or a wrong port binding causes immediate
connection failures.
Increase the proxy connect timeout. In Nginx,
adjust proxy_connect_timeout (default 60 seconds):
location /api/ {
proxy_pass http://upstream;
proxy_connect_timeout 10s;
}
In HAProxy, the equivalent is timeout connect. Keep
the connect timeout short (5-15 seconds) since TCP
handshakes complete in milliseconds on a healthy
network. A long connect timeout masks underlying
connectivity problems.
Check firewall and routing. Ensure no firewall
rules between the proxy and upstream silently drop SYN
packets. Run traceroute or mtr between the two
hosts to identify routing failures or packet loss. If
the proxy and upstream are in different networks or
cloud regions, verify security group rules and VPC
peering configurations allow traffic on the upstream
port.
Confirm DNS resolution. If the proxy resolves the upstream hostname via DNS, verify the returned IP address is correct. Stale DNS cache entries pointing to a decommissioned server cause persistent connect timeouts. Flush the proxy's DNS cache or reduce the TTL on the upstream's DNS record.
Add upstream redundancy. Configure multiple upstream servers in the proxy's upstream block. If one instance is unreachable, the proxy fails over to a healthy instance instead of returning a 599 to the client.
Takeaway
The 599 Network Connect Timeout Error status code is an unofficial proxy error indicating the proxy was unable to establish a TCP connection to the upstream server before the connect timeout expired.
