/etc/services Is Made of Ports (and People!)

Hospital switchboard, 1970s
I’ve been using a switchboard/phone extension metaphor to explain ports to my non-engineer friends who have been asking about my talk progress. Feels fitting to start the blog version of it with this switchboard professional.

This is the expanded blog counterpart of a talk I gave at !!con West in February 2019. I’ll add a video link here once it’s available.

I learned about /etc/services in my first year of engineering. I was working with another engineer to figure out why a Jenkins master wasn’t connecting to a worker. Peering through the logs and netstat output, my coworker spied that a service was already listening on port 8080. “That’s Jenkins,” he said.

“But how did you know that?”

“Oh, /etc/services,” he replied. “It has all the service-port pairings for stuff like this.”

Jenkins is not, in fact, in /etc/services, but http-alt is listed at port 8080. The more immediately relevant answer was probably “through experience, because I’ve seen this before, o junior engineer,” but his broader answer got me curious. I spent some time that afternoon scrolling through the 13,000-plus-line file, interested in the ports but especially curious about the signatures attached to so many of them: commented lines with names, email addresses, and sometimes dates, attribution for a need as yet unknown to me.

I got on with the business of learning my job, but /etc/services stayed in my head as a mystery of one of my favorite kinds: everyone was using it, but many well-qualified engineers of my acquaintance had only partial information about it. They knew what used it, or they at least knew that it existed, but not where it came from. The names in particular seemed to surprise folks, when I asked colleagues for their knowledge about this as I was doing this research.

This post, a longer counterpart to my !!con West talk on the same subject, digs into a process and a file that was once commonplace knowledge for a certain kind of back-end and network engineer and has fallen out of more regular use and interaction.  I’ll take you through some familiar services, faces old and new, correspondence with contributors, and how you – yes, you – can make your mark in the /etc/services file.

What is it, where does it live

In *nix systems, including Mac OS, /etc/services lives exactly where you think it does. Windows also has a version of this file, which lives at C:\Windows\System32\drivers\etc\services. Even if you’ve never opened it, it’s been there, providing port name and number information as you go about your life.

The file is set up like this: name, port/protocol, aliases, and then usually a separate line for any comments, which is where you’ll often find names, email addresses, and sometimes dates. Like so:

ssh      22/udp  # SSH Remote Login Protocol

ssh      22/tcp  # SSH Remote Login Protocol

#                   Tatu Ylonen <ylo@cs.hut.fi>

The most common protocols are UDP and TCP, as those were the only ones you could reserve until a few years ago. However, as of an August 2011 update to RFC 6335 (more on that later), you can now snag a port to use with SCTP and/or DCCP as well. This RFC update added more protocols, and it also initiated a change from the old practice of assigning a port for both UDP and TCP for a service to only allocating the port for the protocol requested, and just reserving it for the others, though they’ll only be used if other port availability dwindles significantly.

Incidentally, the presence of a service in /etc/services does not mean the service is running on your computer. The file is a list of possibilities, not attendance on your machine (which is why your computer is probably not currently on fire).

Going through the first 500-odd lines of the file will show you some familiar friends. ssh is assigned port 22. However, ssh also has an author: Tatu Ylonen. His bio includes a lot of pretty typical information for someone who’s listed this far up in this file: he designed the protocol, but he has also authored several RFCs, plus the IETF standards on ssh.

Jon Postel is another common author here, with 23 entries. His representation in this file just hints at the depth of his contributions – he was the editor of the Request for Comment document series, he created SMTP (Simple Mail Transfer Protocol), and he ran IANA until he died in 1998.  A high /etc/services count is more a side effect of the enormity of his work rather than an accomplishment unto itself.

It’s cool to see this grand, ongoing repository of common service information, with bonus attribution. However, that first time I scrolled (and scrolled, and scrolled) through the entirety of /etc/services, what stayed with me were how many services and names I wasn’t familiar with – all this other work, separate of my path in tech thus far, with contact information and a little indicator of what that person was up to in, say, August 2006.

For instance: what’s chipper on port 17219? (It’s a research rabbit hole that took me about 25 minutes and across Google translate, the Wayback Machine, LinkedIn, Wikipedia, a 2004 paper from The European Money and Finance Forum, AMONG OTHER RESOURCES. Cough.) chipper, by Ronald Jimmink, is one of two competing e-purse schemes that once existed in the Netherlands; the longer-lasting competitor, Chipknip, concluded business in 2015. The allure of these cards, over a more traditional debit card, was that the value was stored in the chip, so merchants could conduct transactions without connectivity for their card readers. This was a common race across Europe, in the time before the standardization of the euro and banking protocols, and chipper is an artifact of the Netherlands’s own efforts to find an easier way to pay for things in a time before wifi was largely assumed.

Then there’s octopus on port 10008 (a port which apparently also earned some notoriety for being used for a worm once upon a time). Octopus is a a professional Multi-Program Transport Stream (MPTS) software multiplexer, and you can learn more about it, including diagrams, here.

There are, of course more than 49,000 others; if you have some time to kill, I recommend scrolling through and researching one whose service name, author, or clever port number sparks your imagination. Better still, run some of the service names by the longer-tenured engineers in your life for a time capsule opening they won’t expect.

Port numbers and types

Ports are divided into three ranges, splitting up the full range of 0-65535 (the range created by 16-bit numbers).

  • 0-1023 are system ports (also called well-known ports or privileged ports)
  • 1024-49151 are user ports (or registered ports)
  • And 49152-65535 are private ports (or dynamic ports)

Any services run on the system ports must be run by the root user, not a less-privileged user. The idea behind this (per W3) is that you’re less likely to get a spoofed server process on a typically trusted port with this restriction in place.

Ok, but what actually uses this file? Why is it still there?

Most commonly, five C library routines use this file. They are, per (YES) the services man page:

  • getservent(), which reads the next entry from the services database (see services(5)) and returns a servent structure containing the broken-out fields from the entry.
  • getservbyname(), which returns a servent structure for the entry from the database that matches the service name using protocol proto
  • getservbyport(), which returns a servent structure for the entry from the database that matches the port port (given in network byte order) using protocol proto
  • setservent(), which opens a connection to the database, and sets the next entry to the first entry
  • endservent()

The overlapping use of these routines makes service name available by port number and vice versa. Thus, these two commands are equivalent:

  • telnet localhost 25
  • telnet localhost smtp

And it’s because of information pulled from /etc/services.

The use  you’ve most likely encountered is netstat, if you give it flags to show service names. The names it shows are taken directly from /etc/services (meaning that you can futz with netstat’s output, if you have a little time.)

In short: /etc/services used to match service to port to give some order and identity to things, and it’s used to tell developers when a certain port is off limits so that confusion isn’t introduced. Human readable, machine usable.

Enough about ports; let’s talk about the people

First, let’s talk about the method. Shortly after getting the acceptance for my !!con talk, I went through the entire /etc/services file, looking for people to write to. I scanned for a few things:

  • Email addresses with domains that looked personal and thus might still exist
  • Interesting service names
  • Email addresses from employers whose employees tended to have long tenures
  • Anything that sparked my interest

I have a lot of interest, and so I ended up with a list of 288 people to try to contact. The latest date in my local copy of /etc/services is in 2006, so I figured I’d be lucky to get responses from three people. And while more than half of the emails certainly bounced (and the varieties of bounce messages and protocols had enough detail to support their own fairly involved blog post), I got a number of replies to my questions about how their work came to involve requesting a port assignment, how it was that they knew what to do, and how the port assignment experience went for them.

I will say that the process revealed an interesting difference between how I’d write to folks as a writer and researcher (my old career) vs. how one writes questions as an engineer. As a writer working on a story that would eventually involve talking to people about a subject, I would research independently and only later approach the people involved; my questions would start a few steps back from what I already knew to be true from my research. This allows room for people to feel like experts, to provide color and detail, and to offer nuance that doesn’t come out if the person asking questions charges in declaring what they know and asking smaller, more closed questions.

This is… not the case in computer science, when questions are typically prefaced with a roundup of all 47 things you’ve googled, attempted, and wondered about, in the interest of expediency. This meant that my very second-person questions, in the vein of “how did you do this” and “what was the nature of your process,” sometimes were taken as some email rando not being able to, how you say, navigate the internet in a most basic way.

The more you know.

Happily, I got more than three responses, and people were incredibly generous in sharing their experiences, details of their work, and sometimes relevant messages from their astonishingly deep email archives.

bb, port 1984: Sean MacGuire

The first response I got, delightfully, was for the service named after my initials: bb, on port 1984. More delightfully, this turned out to be the port reserved for software called Big Brother, “the first Web-based Systems and Network Monitor.” Its author, Sean MacGuire, figured out the process for reserving a port after researching IANA’s role in it. At the time (January 1999), it was approved in 12 days. He described it as “totally painless.” Fun fact: Sean also registered the 65th registered domain in Canada, which required faxing proof of the company and making a phone call to the registrar.

The thing I started to learn with Sean’s response was how this was, at one point, pretty ordinary. Most web-based services restrict themselves to ports 80 and 443 now, in large part because a lot of enterprise security products clamp down on access by closing less-commonly used ports, so reserving a port for your new service isn’t always a necessary step now.

In which I am written to by one of the chief architects of the internet as we know it

The next response I got was from a little further back in computer science history. For context, I’ll tell you how I went about contacting people: back in December, after my talk was accepted for !!con West, I went through the /etc/services file on my computer and selected people to contact. I picked people whose email address domains looked like they might still be around, who worked for companies where people tend to have long tenures, or who were contacts tied to interesting-sounding services.

I did this across about ten days, which meant that, by the time I got to the end, I couldn’t have recounted to you the first folks I selected, particularly as I’d chosen 288 people to try to reach in all. Incidentally, about half of those bounced – not nearly as many as I expected.

This is all to say that I was a little startled to read this response:

> How did you come to be the person in charge of reserving the port

I designed the HTTP protocol

Which reminded me that I had indeed selected this entry as one worth diving into, when I was first getting a handle on this research:

http          80/udp www www-http # World Wide Web HTTP

http          80/tcp www www-http # World Wide Web HTTP

#                       Tim Berners-Lee <timbl@W3.org>

He was, I am pleased to say, impeccably polite in his brief response, and he recommended his book, Weaving the Web, which is such a wonderful look at finding compromise between competing standards and design decisions across dozens of institutions, countries, and strong-willed people. As he said, more information on his place within this work and that file can be found there, and if you’re at all curious, I so recommend it.

More contributors

I also liked that some people had fun with this or considered it, as Christian Treczoks  of Digivote, port 3223, put it, a “real “YEAH!” moment.” Barney Wolff, of LUPA, worked a few layers of meaning into his assignment of port 1212: “I picked 1212 because the telephone info number was <area>-555-1212. And LUPA (an acronym for Look Up Phone Access) was a pun on my last name. I don’t know if my bosses at ATT or anyone at IANA ever noticed that.”

Christian Catchpole claimed port 1185, appropriately named catchpole. He requested a low port number in the interest of claiming something memorable. He explained: “The original project back in 2002 involved a compact wire protocol for streaming objects, data structures and commands etc. While the original software written is no longer in the picture, the current purpose of the port number involves the same object streaming principal.  I am currently using the port for async communications for my autonomous marine robotics project.” The original uses of many ports have shifted into computer science history, but Christian’s projects live on.

Alan Clifford (mice, port 5022) claimed his space for a personal project; approval took 21 days. (He, like several people I contacted, keeps a deep and immaculate email archive.) Mark Valence (keysrvr at 19283 and keyshadow at 19315) recounted his involvement thusly: “I was writing the code, and part of that process is choosing a port to use.” He ended up in /etc/services around 1990 or 1991, when his team was adding TCP/IP as an option for their network service a year or so prior, enabling Macs, PCs, and various Unix systems to communicate with each other.

Ulrich Kortenkamp (port 3770, cindycollab) was one of two developers of Cinderella, and he claimed a port in /etc/services to codify their use of a private port for data exchange. He added: “And I am still proud to be in that file :)”

Greg Hudson’s contributions date to his time as a staff engineer at MIT, when he became a contributor to and then a maintainer of the school’s Zephyr IM protocol (zephyr-hm in the file) and then similarly with Subversion, the open-source version control system now distributed by Apache. His name is connected to ports 2102-2104 for Zephyr and port 3690 for Subversion.

Jeroen Massar has his name connected to four ports:

He noted that AURORA also has an SCTP allocation too, which is still fairly rare, despite that protocol being available since 2011. He remarked, “[This] is actually likely the ‘cool’ thing about having ‘your own port number’: there is only 65536 of them, and my name is on 4 of them ;)”

I asked people how they knew what to do; some were basically like :shrug: “The RFC?” But others explained their context at the time. Mostly, folks seemed to have general industry awareness of this process and file just because of the work they did. (“I was the ‘Network Guy’ in the company,” said Christian Treczoks.)  Some knew the RFC or knew to look for it; others had been involved with the IETF and were around for the formation of these standards. My anecdotal impression was that it was, at that point, just part of the work. If you were on a project that was likely to need a port, it was known how you’d go about getting it.

Who controls this file? Where does the official version come from?

Like so many things in the big world of the internet, /etc/services and its contents are controlled by IANA. The official version varies; what’s on IANA’s official and most up-to-date version deviates some from what you might find locally. The version of /etc/services on my Mac, as I’ve mentioned, is about 13 years out of date. However, people are still claiming ports, and you can see the most current port assignments at IANA’s online version.

On most Unixes, the version of /etc/services you see is a snapshot of the file taken from IANA’s official version at the time that version of the OS was released. When installing new services, often you’ll want to tweak your local copy of /etc/services to reflect the new service, if it’s not already there, even if only as a reminder.

However, updates vary between OSes; the version included with the Mac OS is not the most current, and how updates are added and communicated can vary widely. Debian, for instance, furnishes /etc/services as part of the netbase package, which includes “the necessary infrastructure for basic TCP/IP based networking.” If the included version of /etc/services got out of date, one could file a bug to get it updated.

To learn how /etc/services managed and how to contribute, the golden standard is:

RFC 6335

No, seriously. The Procedures for the Management of the Service Name and Transport Protocol Port Number Registry has most of what you would need to figure out how to request your own port. While the process is less common now, it’s still regimented and robustly maintained. There’s a whole page just about statistics for port request and assignment operations. While this isn’t as commonly used as it once was, it’s still carefully governed.

RFC 7605, Recommendations on Using Assigned Transport Port Numbers, includes guidance on when to request a port). 7605 and 6335 are concatenated together as BCP 165, though 6335 is still referred to frequently and is the most commonly sought and cited resource.

How can you get your piece of /etc/services glory?

There’s real estate left to claim; as of this writing, more than 400 ports are still available. Others have notes that the service claims are due to be removed, with timestamps from a few years ago, and just haven’t yet.

If you have a service in need of a port, I am pleased to tell you that there is a handy form to submit your port assignment request. You’ll need a service name, transport protocol, and your contact information, as you might guess. You can also provide comments and some other information to bolster your claim. The folks managing this are pretty efficient, so if your request is valid, you could have your own port in a couple of weeks, and then you could be hiding inside of your computer many years from now, waiting to startle nosy nerds like me.

Despite the shift to leaning more heavily on ports 80 and 443, people are absolutely still claiming ports for their services. While the last date in my computer’s /etc/services file is from about a decade ago, the master IANA list already has a number of dates from this year.

So: make your service, complete the form, and wait approximately two weeks. Then cat /etc/services | grep $YourName, and your immortality is assured (until IANA does an audit, anyway).  

And if you do, please let me know. Enabling people to do great, weird, and interesting things is one of my favorite uses of time, and it’d make my day. Because there are no computers without people (or not yet, anyway), and a piece of that 16-bit range is waiting to give your work one more piece of legitimacy and identity.

Thanks to everyone who wrote back to me for being so generous with their experiences, to Christine Spang for the details about Debian /etc/services updates, to the patient overseers of RFC 6335, and the authors of all the RFCs and other standards that have managed to keep this weird world running.