What does the RST flag mean in TCP?

RST = Reset - it immediately aborts the connection (no graceful handshake), typically after an error, timeout, or a packet sent to a closed port.

RST = Reset

The RST flag immediately terminates the connection without the normal handshake.

Purpose:

• Aborts a connection due to error
• Rejects invalid connection attempts
• Responds to packets on closed ports

Common causes:

• Connection timeout
• Application crash
• Packet sent to closed port
• Firewall blocking connection

Tip: RST is the "emergency stop" button - something went wrong, abort immediately!

Go deeper:

• RFC 9293 §3.5.3 — Reset Generation — the authoritative rules for when a host sends RST to abort or reject a connection.
• TCP segment structure — Flags — Wikipedia's control-bit table placing RST in context.