Port 132: Cisco SYSMAINT — The Maintenance Door Nobody Opens

Port 132 is assigned to cisco-sys, Cisco's SYSMAINT (System Maintenance) protocol. It is registered for both TCP and UDP. If you have never heard of it, you are in good company. Even Cisco's own engineers have trouble explaining exactly what it does.

What Cisco SYSMAINT Does

Port 132 is the third in a block of three consecutive ports that Cisco Systems registered with IANA in the early days of TCP/IP networking¹:

• Port 130 — cisco-fna (Cisco FNATIVE)
• Port 131 — cisco-tna (Cisco TNATIVE)
• Port 132 — cisco-sys (Cisco SYSMAINT)

These ports were reserved for Cisco's proprietary internal protocols. FNATIVE and TNATIVE appear to relate to native file transfer and terminal communication between Cisco devices. SYSMAINT, as the name suggests, was intended for system maintenance operations on Cisco networking equipment.

In practice, these ports have been observed in traffic associated with Cisco Catalyst 6000 Network Analysis Modules (NAMs)². But even network administrators running Catalyst 6000 switches without NAMs have reported seeing traffic on these ports, suggesting the protocols may have had broader internal use within Cisco's device ecosystem than the documentation reveals.

The Ghost Protocol

Here is the honest truth about port 132: almost nobody uses it anymore. The protocols that run on ports 130, 131, and 132 are legacy artifacts. Cisco's own documentation (CCO) contains minimal information about them. They remain registered in the IANA database because port assignments, once made, tend to persist indefinitely. They are monuments to a time when networking companies could claim a handful of well-known ports for proprietary use and nobody blinked.

If you see traffic on port 132 in a modern network and you are not running vintage Cisco equipment, you should investigate. Legitimate use is rare enough that unexpected traffic on this port is worth a second look.

The Number 132's Other Life

There is a coincidence worth noting. The number 132 appears in a completely different, far more consequential place in the networking stack: it is the IP protocol number for SCTP (Stream Control Transmission Protocol)³.

IP protocol numbers and port numbers are different things entirely. TCP is protocol number 6. UDP is protocol number 17. And SCTP is protocol number 132. This means that in the protocol field of every IP header carrying SCTP traffic, the value is 132.

SCTP was born from telephone engineers. In the late 1990s, the IETF's SIGTRAN working group needed a way to carry SS7 (Signaling System 7) telephone signaling over IP networks⁴. TCP had problems: head-of-line blocking caused unnecessary delays, its single-homed design couldn't provide the redundancy that telephone signaling demanded, and it was vulnerable to SYN flood attacks. So Randall Stewart, Qiaobing Xie, and a team of engineers from Motorola, Cisco, Siemens, Nortel, Ericsson, and Telcordia designed a new transport protocol⁵.

They originally called it the "Simple Control Transmission Protocol." After two years of design work and twenty revisions, it was renamed the "Stream Control Transmission Protocol" and published as RFC 2960 in October 2000⁶. The name change, as the designers noted, signified an expansion of scope. SCTP was no longer just simple. It offered multi-homing (multiple network paths for resilience), multi-streaming (parallel data channels without head-of-line blocking), and a four-way handshake that resisted SYN flooding.

Today, SCTP carries signaling traffic in mobile networks worldwide, on interfaces like S1-MME and X2³. Every time your phone hands off between cell towers, there is a good chance that SCTP, protocol number 132, is part of what makes it seamless.

Port 132 and protocol 132 have nothing to do with each other technically. But there is something fitting about the parallel: both are about maintenance. One maintains Cisco devices. The other maintains the signaling infrastructure that keeps phone calls connected.

How to Check What Is Listening on Port 132

On Linux:

sudo ss -tlnp | grep :132
sudo netstat -tlnp | grep :132

On macOS:

sudo lsof -i :132

On Windows:

netstat -an | findstr :132

If something is listening on port 132 and you are not running legacy Cisco equipment, treat it as suspicious until proven otherwise.

Security Considerations

Port 132 has been flagged in historical virus and trojan databases⁷. This does not mean the port itself is dangerous, but rather that malware authors have occasionally used unmonitored, low-traffic ports like 132 for command-and-control communication. The logic is straightforward: if nobody expects legitimate traffic on a port, nobody is watching it.

For most networks, blocking port 132 at the firewall is safe and advisable unless you have specific legacy Cisco equipment that requires it.

The Well-Known Range

Port 132 sits in the well-known port range (0 through 1023). These ports are assigned by IANA through IETF Review or IESG Approval¹. The well-known range is reserved for critical, established services: HTTP on 80, HTTPS on 443, SSH on 22, DNS on 53. Getting a port in this range is like getting a street address on Main Street.

Not every address on Main Street has a thriving business behind it. Port 132 is proof that even in the most prestigious range, some doors stay mostly closed. The reservation exists. The protocol behind it has faded. But the number endures, quietly holding its place in the registry while the Internet grows around it.