CodeParkAn ephemeral port is a temporary local port that the kernel gives a client when it connects without choosing a port. The kernel picks it from a range. On Linux, the default net.ipv4.ip_local_port_range is 32768 to 60999. That is about 28,000 ports.
How they run out: the four-tuple (source IP, source port, destination IP, destination port) must be unique. So every connection to the same destination IP and port needs its own local port. Imagine a service that opens a new connection for every outbound request to one upstream, like a single database proxy or API:
- Each request uses one local port.
- After a connection closes, its port stays in TIME_WAIT state for 60 seconds and can't be reused.
- At about 470 new connections per second, the whole range is used up.
New connections then fail with EADDRNOTAVAIL ("cannot assign requested address").
The fix: stop opening and closing connections all the time. Use HTTP keep-alive agents and connection pools, so connections get reused.
These options give more room, but they don't remove the cause:
- widening the port range
- enabling
tcp_tw_reuse - spreading traffic over more destination IPs
- Ephemeral ports come from ip_local_port_range, which has about 28,000 ports by default.
- Each connection to the same destination needs a different local port.
- TIME_WAIT keeps a port busy for 60 seconds after the connection closes.
- Running out of ports gives the EADDRNOTAVAIL error.
- Fix it with keep-alive and connection pooling.