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ProtocolCodes
421
FTP

421 Service Not Available

Service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.

On this page

When You See This Code

When the FTP server is overloaded, shutting down for maintenance, or has reached its maximum number of concurrent connections.

How to Fix

Wait a few minutes and try reconnecting. If persistent, check if the server has a connection limit and whether your IP is being rate-limited.

Specification

View RFC / Specification →

Comparisons

FTP 421 Service Not Available vs 530 Not Logged In

Compare With

110 Restart Marker Reply 120 Service Ready Soon 125 Data Connection Open 150 File Status OK 200 Command OK 202 Command Superfluous 211 System Status 212 Directory Status 213 File Status 214 Help Message 215 System Type 220 Service Ready

Guides

FTP Reply Codes: Understanding File Transfer Responses DNS & Networking
TCP Three-Way Handshake and Connection Lifecycle DNS & Networking
Essential Network Debugging Tools: ping, traceroute, mtr, and tcpdump DNS & Networking
RFC 959: FTP Protocol Deep Dive Protocol Deep Dives
TLS 1.3: Faster, Simpler, More Secure Protocol Deep Dives
Migrating from FTP to SFTP: A Complete Transition Guide Migration & Upgrades
HTTP Access Log Management: Rotation, Parsing, and Analysis Production Infrastructure

Specifications

RFC 959 File Transfer Protocol (FTP)

Glossary

Status Code TLS Handshake TCP/IP Three-Way Handshake Port Number Firewall Proxy Server (Forward Proxy) Socket Request-Response Model RFC (Request for Comments) Connection-Oriented Half-Duplex Text vs Binary Protocol FTP Active Mode FTP Passive Mode FTP Control Channel FTP Data Channel SFTP (SSH File Transfer Protocol) FTPS (FTP Secure) FTP Reply Code FTP Anonymous Access FTP Transfer Mode (ASCII vs Binary) FTP Resume (REST Command) SCP (Secure Copy Protocol) FTP Chroot Jail WebDAV (Web Distributed Authoring and Versioning) rsync FTP MLSD (Machine-Readable Listing) FTP NAT Traversal Transfer Integrity FTP over TLS (RFC 4217) File Locking Bandwidth Throttling TLS Versions

Frequently Asked Questions

What is FTP 421 Service Not Available?

Service not available, closing control connection. The server is shutting down or has reached its connection limit and cannot serve this request.

When do you see FTP 421 Service Not Available?

When the FTP server is overloaded, shutting down for maintenance, or has reached its maximum number of concurrent connections.

How to fix FTP 421 Service Not Available?

Wait a few minutes and try reconnecting. If persistent, check if the server has a connection limit and whether your IP is being rate-limited.