Port 674 carries ACAP (Application Configuration Access Protocol), a protocol designed to solve a problem that was urgent in 1997 but feels quaint today: how do you keep your email settings, address books, and preferences synchronized across multiple computers?
ACAP was the answer. Store everything on a remote server. Access it from anywhere. Change a setting once, and it updates everywhere. This was cloud sync before we called it that.
The protocol worked. It was technically sound. But it never achieved the widespread adoption its creators hoped for. By 2004, the IETF working group had ceased activity. Today, ACAP exists primarily as a historical artifact—a port number in the IANA registry and a reminder that not every well-designed protocol finds its audience.
The Problem ACAP Tried to Solve
Imagine the late 1990s. You have an email account. You check it from your desktop at work, your laptop at home, maybe a computer at the library. Every time you sit down at a new machine, you have to reconfigure everything:
- IMAP server settings
- Your signature
- Filters and folders
- Your address book
- Personal preferences
Change your password? Update it on every device. Add a contact? Enter it again on each machine. The information was scattered, duplicated, and constantly out of sync.
ACAP was designed to centralize this. One canonical source. One place to update. Access it from anywhere with proper authentication. The server holds your configuration, and clients fetch it as needed.
How ACAP Works
ACAP operates as a client-server protocol over TCP port 674. A client connects to an ACAP server to store and retrieve configuration data organized into a hierarchical namespace.
Key features include:
Inheritance — Default values can be set at higher levels and overridden at lower levels, reducing duplication and making management easier.
Access control lists — Fine-grained permissions allow some information to be shared (like group calendars) while keeping personal data private.
Synchronization — Clients can detect changes and update their local copies without downloading everything.
The protocol was designed primarily for IMAP email clients, but its scope was broader—any application could use ACAP to store and sync configuration data.
The History
ACAP was defined in RFC 2244, published in November 1997 by Chris Newman (Innosoft) and John Myers (Netscape).1 It emerged from the IETF ACAP working group as a companion to IMAP—email was becoming portable, but settings weren't.
The protocol saw implementation in at least four clients and three servers. But it never achieved critical mass. By April 2004, the IETF working group ceased activity.2 ACAP never reached the popularity of LDAP (Lightweight Directory Access Protocol) or SyncML, both of which addressed similar synchronization problems.
Why ACAP Faded
ACAP wasn't broken. It worked as designed. But adoption requires more than technical correctness:
Complexity — Setting up an ACAP server added infrastructure overhead. Many users and organizations opted for simpler solutions, even if they were less elegant.
Competition — LDAP already existed and was widely deployed for directory services. SyncML emerged for device synchronization. Proprietary solutions from email providers offered easier onboarding.
The web happened — Webmail services like Hotmail and Gmail shifted email access to the browser. Your settings lived on their servers by default. No protocol needed.
Mobile changed everything — Smartphones and cloud services made centralized configuration storage the default, not the exception. Apple, Google, and Microsoft built proprietary sync into their ecosystems.
ACAP solved the right problem but arrived at the wrong moment. The world moved toward centralized web services faster than decentralized protocols could gain traction.
Port 674 Today
Port 674 remains officially assigned to ACAP in the IANA registry. You won't find much traffic on it. The protocol is rarely deployed in modern systems.
To check if anything is listening on port 674:
Most systems will return nothing. The port is assigned but dormant—a permanent record of a protocol that tried to solve tomorrow's problem with yesterday's architecture.
What ACAP Teaches Us
ACAP is a case study in the gap between technical merit and market adoption. The protocol was well-designed. It addressed a genuine need. It had backing from major organizations like Netscape. And it still faded into obscurity.
The lesson: solving the problem isn't enough. Timing matters. Deployment friction matters. Existing alternatives matter. And sometimes, the world solves the problem differently than you expected.
Cloud sync won. Not because it's more elegant than ACAP—it often isn't. But because it's invisible. Users don't configure servers or install clients. They sign in, and it works.
ACAP asked users to set up infrastructure. The cloud asked users to create an account. That difference was everything.
Related Ports
- Port 143 — IMAP, the email protocol ACAP was designed to complement
- Port 389 — LDAP, the directory protocol that competed with ACAP for configuration storage
- Port 993 — IMAPS (IMAP over TLS), secure email access
Frequently Asked Questions About Port 674
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