highAdvancedUSE#tcp#timewait#sockets#ephemeral#sysctl

TCP TIME_WAIT socket exhaustion — ephemeral port exhaustion

Cannot assign requested address errors when connecting out

Likely source:performanceconnectivity

Focused incident mode — strips away everything but this runbook.

Symptoms

  • Cannot assign requested address errors when connecting out
  • ss -tan shows thousands of TIME_WAIT sockets
  • Short-lived HTTP connections exhaust the ephemeral port range

TIME_WAIT is a protocol safety mechanism — it prevents old packets from corrupting new connections. Tweaking it (tw_reuse) is safe; disabling it entirely is not.

Evidence to grab

Collect these artifacts before you change anything — they prove (or rule out) each cause.

  • TIME_WAIT socket count

    ss -tan state time-wait | wc -l

  • Ephemeral port range

    cat /proc/sys/net/ipv4/ip_local_port_range

Diagnostic flow

Run the steps in order, marking each claim confirmed or refuted. The evidence you gather feeds the ranked causes below.

Guided decision tree is ready.

Steps with commands, expected output, and pass/fail branches.

Ranked root causes

Evidence checklist

  • Application opens a new TCP connection per HTTP request
  • TIME_WAIT count grows under load

Confirm before fixing

ss -tan state time-wait | wc -l
READ

Confirm high TIME_WAIT count

Fix

sysctl -w net.ipv4.tcp_tw_reuse=1 && sysctl -w net.ipv4.tcp_fin_timeout=15
CHANGE

Enable TIME_WAIT reuse and reduce FIN timeout

Resolution runbook

Apply from top to bottom after the cause is confirmed. Review every command before running it — risk tags are shown on each.

1
ss -tan state time-wait | wc -l
READ

Count TIME_WAIT sockets

2
cat /proc/sys/net/ipv4/ip_local_port_range
READ

Check the ephemeral port range

3
sysctl -w net.ipv4.tcp_tw_reuse=1
CHANGE

Enable TIME_WAIT socket reuse

4
sysctl -w net.ipv4.ip_local_port_range='1024 65535'
CHANGE

Expand the available port range

Prevention

  • Use HTTP connection keep-alive or connection pooling to reduce connection churn.
  • Set net.ipv4.tcp_tw_reuse=1 on servers with high outbound connection rates.
  • Expand ip_local_port_range for high-churn proxy servers.
  • Monitor TIME_WAIT count via node_exporter and alert on sustained high values.

Related runbooks

Same stack, same failure family — worth a skim.

Address already in use — port binding conflicthigh

bind: Address already in use error on application startup

LinuxFoundation3 causes
Open runbook
Too many open files — EMFILE error on processhigh

EMFILE: Too many open files error in application logs

LinuxIntermediateUSE3 causes
Open runbook

Commands marked READ CHANGE DESTRUCTIVE — never run DESTRUCTIVE commands against production data you cannot restore. Copy → verify → execute.