Port 3495: GnuDIP — Dynamic DNS Finds a Home

What This Port Is

Port 3495 sits in the registered ports range (1024–49151). IANA, the organization responsible for maintaining the official port registry, has not formally assigned this port to any service. On paper, it's empty.

In practice, it isn't.

Port 3495 is the conventional default for GnuDIP, a Dynamic DNS update protocol developed in the early 2000s. GnuDIP solves a specific, common problem: your ISP gives you a different IP address every time you reconnect, but you want a stable hostname — myhome.example.com — that always finds you. GnuDIP lets your router or computer notify a DNS server whenever your IP changes.

How GnuDIP Works

The protocol is straightforward enough to explain in a paragraph. When your client connects to port 3495, the server immediately sends back a random 10-character salt string. Your client takes your password, hashes it with MD5, appends the salt, and hashes it again. That double-hash goes back to the server along with your username and domain. No password ever travels in plaintext.¹

The server replies with a single code: 0 for success, 1 for bad credentials, 2 for successfully marking your hostname offline. That's the whole protocol.

GnuDIP also supports an HTTP-based variant that tunnels the same exchange through a web server — useful when firewalls block direct TCP connections to port 3495.

Current Status

GnuDIP is minimally maintained. The project has been largely superseded by more modern dynamic DNS services and their HTTP/HTTPS APIs, which work through port 443 without needing any special firewall rules. But older routers, particularly some Huawei, LANCOM, and other embedded devices, still ship with GnuDIP client support built in.²

If you see traffic on port 3495, it's almost certainly a GnuDIP update — a device telling a DNS server where to find it.

What's Listening on This Port

To check what process is using port 3495 on your machine:

macOS / Linux:

# Show what's listening on port 3495
sudo lsof -i :3495

# Or with ss (Linux):
ss -tlnp | grep 3495

Windows:

netstat -ano | findstr :3495

If nothing comes back, nothing is using the port. That's the normal state for most systems.

Why Unassigned Ports Exist

The registered range (1024–49151) has 48,128 slots. IANA has formally assigned several thousand of them. The rest are available — services can use them informally, as GnuDIP did with 3495, without any central authority granting permission. This isn't a flaw in the system; it's how the Internet has always worked. Protocols claim a port by convention and usage, not by decree.

The consequence: when you see traffic on an unassigned port, the port number alone won't tell you what it is. You have to look at the process behind it.