Port 46: MPM-SND — The Message Relay That Time Forgot

Port 46 is assigned to MPM-SND, the "default send" function of the Message Processing Module protocol. It is one of three consecutive ports (44, 45, 46) reserved for MPM, a system designed by Jon Postel in 1979 to route messages between computers across different networks¹.

The protocol never saw widespread adoption. SMTP arrived in 1982 and became the standard for email delivery. But port 46 remains reserved, a permanent entry in the IANA registry under the name of the man who, for most of the Internet's early life, was the registry.

What MPM Was Trying to Do

Before SMTP existed, there was no universal way to send a message from a computer on one network to a computer on another. Different networks had different addressing schemes, different formats, different conventions. Jon Postel's Message Processing Module was an attempt to solve this.

The idea was straightforward: place an MPM process on each host computer. When a message needed to travel between networks, the MPM would make a routing decision¹:

• If the destination was a local user, deliver it directly
• If the destination was on another host in the same network, forward it to that host's MPM
• If the destination was on a different network entirely, forward it to an MPM that was "closer" to the destination

Messages traveled in "bags," each bag addressed to a specific MPM and containing one or more messages for hosts on that MPM's network. Each MPM stamped the message as it passed through, creating a trail of handling marks across the Internet¹.

The three MPM ports divided the work:

Port 46 handled the outbound side: when an MPM needed to push a message toward its destination, it used port 46.

The Man Behind the Port

Jon Postel is listed as the contact for port 46 in the IANA registry. This understates things considerably.

Jonathan Bruce Postel (1943-1998) was one of the people who built the Internet². He edited the RFC document series. He helped create DNS, SMTP, and IP itself. And for decades, he ran IANA, the Internet Assigned Numbers Authority, which is the body that decides which port numbers belong to which protocols.

For much of that time, IANA was Jon Postel. He maintained the registries, responded to requests, and made allocation decisions. When he assigned port 46 to MPM-SND, he was assigning a port number in a system he helped build, for a protocol he designed, in a registry he maintained².

He died on October 16, 1998, at the age of 55. The port assignment outlived the protocol and the man.

Why MPM Lost to SMTP

RFC 753 defined MPM in March 1979¹. RFC 759 refined it in August 1980³. Then in 1982, RFC 821 defined SMTP, and the contest was over.

MPM treated messages as structured objects with type-length-value tuples, requiring a high degree of cooperation between mail systems. SMTP took a simpler approach: plain text commands, plain text responses, messages as streams of bytes. SMTP was easier to implement, easier to debug, and easier to extend¹.

The sendmail documentation notes that MPM "matches sendmail closely in terms of its basic architecture" but "requires a much higher degree of cooperation between mailers than is required by sendmail"⁴. In networking, simpler usually wins.

Security

Port 46 carries no active protocol in modern networks. No legitimate software listens on it. This makes any traffic on port 46 worth investigating.

Some security databases flag port 46 because nearby ports (44 and 48) have been used by trojans like Arctic and DRAT in the past. Port 46 itself has no widely documented malware associations, but any open port with no legitimate purpose is an unnecessary exposure.

If port 46 is open on your system, something is wrong or misconfigured. Close it.

How to Check for Activity on Port 46

Linux:

sudo ss -tlnp | grep :46
sudo netstat -tlnp | grep :46

macOS:

sudo lsof -i :46

Windows:

netstat -an | findstr :46

The Weight of an Empty Port

Port 46 sits in the well-known range (0-1023), the most privileged addresses in the port system. These ports require root or administrator access to bind to on most operating systems. They were meant for the Internet's most important services.

Most of them are. Port 22 carries SSH. Port 53 carries DNS. Port 80 carries HTTP. Port 443 carries HTTPS.

Port 46 carries nothing. It is a reservation that was never cancelled for a protocol that was never widely deployed, held in the name of a man who is no longer alive to release it. The IANA registry is not in the habit of reclaiming port assignments. Once a port is spoken for, it stays spoken for.

There is something fitting about that. Jon Postel spent his career making sure the Internet's naming systems worked, that numbers meant what they were supposed to mean, that assignments were honored. Port 46 honors his assignment. It will keep honoring it, empty and waiting, for as long as the registry exists.