Mastering Linux TCP Settings: Key /proc/sys/net Parameters Explained

Linux Kernel TCP Parameters

/proc/sys/net/ipv4/

The directory /proc/sys/net/ipv4/ contains files that control TCP behavior. Adjusting these settings can improve security, routing, and performance.

The tcp_syn_retries parameter defines how many SYN retransmissions the client will perform before giving up.

Example modification: sysctl net.ipv4.tcp_syn_retries=2 Verification can be done by attempting an SSH connection to a non‑existent host and observing the retransmissions with Wireshark.

Other Important IPv4 TCP Settings

Name

Default

Recommended

Description tcp_syn_retries 5

1‑2

Number of SYN packets sent before abandoning a new connection (max 255). tcp_synack_retries 5

1‑2

Number of SYN+ACK packets sent before giving up on an incoming connection. tcp_keepalive_time 7200

600

Interval (seconds) between keepalive probes. tcp_keepalive_probes 9

3

Number of keepalive probes before declaring the connection dead. tcp_keepalive_intvl 74

15

Interval (seconds) between successive keepalive probes when no response is received. tcp_retries1 3

3

Retries before giving up on an unacknowledged SYN‑ACK (minimum RFC value). tcp_retries2 15

5

Retries before aborting an established connection (depends on RTO, typically 13‑30 min). tcp_orphan_retries 7

3

Retries before discarding an orphaned socket (50 s‑16 min). tcp_fin_timeout 60

2

Time (seconds) a socket stays in FIN‑WAIT‑2 state. tcp_max_tw_buckets 180000

36000

Maximum number of TIME‑WAIT sockets; raising helps under heavy load. tcp_tw_recycle 0

1

Enable fast recycling of TIME‑WAIT sockets (use with caution). tcp_tw_reuse 0

1

Allow reuse of TIME‑WAIT sockets for new connections. tcp_max_orphans 8192

32768

Maximum number of orphaned TCP sockets; increase on memory‑rich systems. tcp_abort_on_overflow 0

0

Send RST when the daemon is too busy to accept new connections. tcp_syncookies 0

1

Enable SYN‑cookies to mitigate SYN‑flood attacks (requires kernel config). tcp_window_scaling 1

1

Enable variable TCP window scaling (important for high‑speed links). tcp_timestamps 1

1

Enable RFC 1323 timestamps for better RTT calculation and protection against old sequence numbers. tcp_sack 1

1

Enable Selective Acknowledgement to improve loss recovery on WAN links. tcp_fack 1

1

Enable Forward Acknowledgement for faster congestion avoidance. tcp_ecn 0

0

Enable Explicit Congestion Notification. tcp_reordering 3

6

Maximum number of out‑of‑order packets before triggering fast‑retransmit. tcp_low_latency 0

0

Allow the stack to favor low latency over throughput (usually disabled). ip_forward 0

1

Enable IP forwarding for NAT. ip_local_port_range 32768 61000

1024 65000

Range of ports used for outgoing connections. ip_conntrack_max 65535

65535

Maximum number of tracked IPv4 connections.

/proc/sys/net/core/

This directory holds parameters that control how the kernel interacts with the network layer.

Name

Default

Recommended

Description netdev_max_backlog 1024

16384

Maximum packets queued per NIC when arrival rate exceeds processing rate. somaxconn 128

16384

Maximum size of the listen backlog queue for server sockets. wmem_default 129024

129024

Default send buffer size (bytes). rmem_default 129024

129024

Default receive buffer size (bytes). rmem_max 129024

873200

Maximum TCP receive buffer (bytes). wmem_max 129024

873200

Maximum TCP send buffer (bytes).

How to Modify Kernel Parameters

Directly echo a value into the file (temporary, lost after reboot): echo "1" > /proc/sys/net/ipv4/tcp_syn_retries Add the setting to /etc/sysctl.conf and apply with sysctl -p (persistent).

Use the sysctl command directly, e.g. sysctl net.ipv4.tcp_syn_retries=2.

Viewing Linux Network Statistics

Network interface statistics can be read from /sys/class/net/eth0/statistics/ (replace eth0 with the desired interface):

sys/class/net/eth0/statistics/rx_packets   # packets received
sys/class/net/eth0/statistics/tx_packets   # packets transmitted
sys/class/net/eth0/statistics/rx_bytes     # bytes received
sys/class/net/eth0/statistics/tx_bytes     # bytes transmitted
sys/class/net/eth0/statistics/rx_dropped   # packets dropped on receive
sys/class/net/eth0/statistics/tx_dropped   # packets dropped on transmit

Additional NIC details (MAC, speed, MTU, etc.) are available under /sys/class/net/ for each interface.