Author: Henrik Bengtsson

# port4me: Get the Same, Personal, Free TCP Port over and over

#### Henrik Bengtsson

The port4me tool:

- finds a free TCP port in [1024,65535] that the user can open

- is designed to work in multi-user environments

- gives different users, different ports

- gives the user the same port over time with high probability

- gives different ports for different software tools

- requires no configuration

- can be reproduced perfectly on all operating systems and in all common programming languages

## Introduction

There are many tools to identify a free TCP port, where most of them return a random port. Although it works technically, it might add a fair bit of friction if a new random port number has to be entered by the user each time they need to use a specific tool.

In contrast, port4me attempts, with high probability, to provide the user with the same port each time, even when used on different days. It achieves this by scanning the same deterministic, pseudo-random sequence of ports and return the first free port detected. Each user gets their own random port sequence, lowering the risk for any two users to request the same port. The randomness is initiated with a random seed that is a function of the user’s name (`USER`), and, optionally, the name of the software where we use the port.

The port4me algorithm can be implemented in most known programming languages, producing perfectly reproducible sequencing regardless of implementation language.

## A quick introduction

Assuming we’re logged in as user `alice`, calling `port4me::port4me()` without arguments gives us a free port:

    Sys.info()[["user"]]

    ## [1] "alice"

    port4me::port4me()

    ## [1] 30845

As we will see later, each user on the system is likely to get their own unique port. Because of this, it can be used to specifying a port that some tool should use, e.g.

    shiny::runApp(port = port4me::port4me())

As long as this port is available, `alice` will always get the same port across R sessions and over time. For example, if they return next week and retry, it’s likely they still get:

    port4me::port4me()

    ## [1] 30845

    port4me::port4me()

    ## [1] 30845

However, if port 30845 is already occupied, the next port in the pseudo-random sequence is considered, e.g.

    port4me::port4me()

    ## [1] 19654

To see the first five ports scanned, run:

    port4me::port4me(list = 5)

    ## [1] 30845 19654 32310 63992 15273

## User-specific, deterministic, pseudo-random port sequence

This random sequence is initiated by a random seed that can be set via the hashcode of a seed string. By default, it is based on the name of the current user (e.g. environment variable `$USER`). For example, when user `bob` uses the `port4me` tool, they see another set of ports being scanned:

    Sys.info()[["user"]]

    ## [1] "bob"

    port4me::port4me(list = 5)

    ## [1] 54242  4930 42139 14723 55707

For testing and demonstration purposes, one can emulate another user by specifying argument `user`, e.g.

    Sys.info()[["user"]]

    ## [1] "alice"

    port4me::port4me()

    ## [1] 30845

    port4me::port4me(user = "bob")

    ## [1] 54242

    port4me::port4me(user = "carol")

    ## [1] 34307

## Different ports for different software tools

Sometimes a user would like to use two, or more, ports at the same time, e.g. two ports for two different Shiny apps. In such case, they can specify argument `tool`, which results in a port sequence that is unique to both the user and the tool. For example,

    port4me::port4me()

    ## [1] 30845

    port4me::port4me("myapp")

    ## [1] 55578

    port4me::port4me("demo")

    ## [1] 32273

This allows us to do:

    shiny::runApp(appDir = "myapp", port = port4me::port4me("myapp"))

and

    shiny::runApp(appDir = "demo", port = port4me::port4me("demo"))

## Avoid using ports commonly used elsewhere

Since there is a limited set of ports available (1024-65535), there is always a risk that another process occupies any given port. The more users there are on the same machine, the higher the risk is for this to happen. If a user is unlucky, they might experience this frequently. For example, `alice` might find that the first port (30845) works only one out 10 times, the second port (19654) works 99 out 100 times, and the third one (32310) works rarely. If so, they might choose to exclude the ports that are most likely to be used by specifying them as a comma-separated values via option `--exclude`, e.g.

    port4me::port4me(exclude = c(30845, 32310))

    [1] 19654

An alternative to specify them via a command-line option, is to specify them via environment variable `PORT4ME_EXCLUDE`, e.g.

    {alice}$ PORT4ME_EXCLUDE=30845,32310 R
    ...
    > port4me::port4me()
    [1] 19654

To set this permanently, append:

    ## port4me customization
    ## https://github.com/HenrikBengtsson/port4me
    PORT4ME_EXCLUDE=30845,32310
    export PORT4ME_EXCLUDE

to the shell startup script, e.g. `~/.bashrc`. Alternatively, it can be set specifically for R in `~/.Renviron` as:

    ## port4me customization
    ## https://github.com/HenrikBengtsson/port4me
    PORT4ME_EXCLUDE=30845,32310

This increases the chances for the user to end up with the same port over time, which is convenient, because then they can reuse the same call, which is available in the command-line history, each time without having to change the port parameter.

The environment variable `PORT4ME_EXCLUDE` is intended to be used by the individual user. To specify a set of ports to be excluded regardless of user, set `PORT4ME_EXCLUDE_SITE`. For example, the systems administrator, can choose to exclude an additional set of ports by adding the following to file `/etc/profile.d/port4me.sh`:

    ## port4me: always exclude commonly used ports
    ## https://github.com/HenrikBengtsson/port4me

    PORT4ME_EXCLUDE_SITE=

    ## MySQL
    PORT4ME_EXCLUDE_SITE=$PORT4ME_EXCLUDE_SITE,3306

    ## ZeroMQ
    PORT4ME_EXCLUDE_SITE=$PORT4ME_EXCLUDE_SITE,5670

    ## Redis
    PORT4ME_EXCLUDE_SITE=$PORT4ME_EXCLUDE_SITE,6379

    ## Jupyter
    PORT4ME_EXCLUDE_SITE=$PORT4ME_EXCLUDE_SITE,8888

    export PORT4ME_EXCLUDE_SITE

In addition to ports excluded via above mechanisms, port4me excludes ports that are considered unsafe by the Chrome and Firefox web browsers. This behavior can be controlled by environment variable `PORT4ME_EXCLUDE_UNSAFE`, which defaults to `{chrome},{firefox}`. Token `{chrome}` expands to the value of `PORT4ME_EXCLUDE_UNSAFE_CHROME`, which defaults to the set of ports that Chrome blocks and `{firefox}` expands to to the value of `PORT4ME_EXCLUDE_UNSAFE_FIREFOX`, which defaults to the set of ports that Firefox blocks.

Analogously to excluding a set of ports, one can limit the range of ports to be scanned by specifying command-line argument `include`, e.g.

    port4me::port4me(include = c(2000:2123, 4321, 10000:10999))

    ## [1] 10451

where the default corresponds to `include = 1024:65535`. Analogously to `exclude`, `include` can be specified via environment variables `PORT4ME_INCLUDE` and `PORT4ME_INCLUDE_SITE`.

## Scan a predefined set of ports before pseudo-random ones

In addition to scanning the user-specific, pseudo-random port sequence for a free port, it is possible to also consider a predefined set of ports prior to the random ones by specifying command-line argument `prepend`, e.g.

    port4me::port4me(prepend = c(4321, 11001), list = 5)

    ## [1]  4321 11001 30845 19654 32310

An alternative to specify them via a command-line option, is to specify them via environment variable `PORT4ME_PREPEND`, e.g.

    {alice}$ PORT4ME_PREPEND=4321,11001 R
    ...
    > port4me::port4me(list = 5)
    [1]  4321 11001 30845 19654 32310

The environment variable `PORT4ME_PREPEND` is intended to be used by the individual user. To specify a set of ports to be prepended regardless of user, set `PORT4ME_PREPEND_SITE`.

## Installation

To install the R port4me package, do:

    install.packages("port4me")

To try it out, call:

    port4me::port4me("jupyter-notebook")

    ## [1] 29525

or

    $ Rscript -e port4me::port4me jupyter-notebook
    29525

## The port4me Algorithm

### Requirements

- It should be possible to implement the algorithm using 32-bit unsigned integer arithmetic. One must not assume that the largest represented integer can exceed \(2^{32} - 1\).

- The pseudo-randomized port sequence should sample ports uniformly over \([1024, 65535]\).

- At a minimum, it should be possible to implement the algorithm in vanilla Sh*, Csh, Bash, C, C++, Fortran, Lua, Python, R, and Ruby, with no need for add-on packages beyond what is available from their core distribution. (*) Shells that do not support integer arithmetic may use tools such as `expr`, `dc`, `bc`, and `awk` for these calculations.

- All programming languages should produce the exact same pseudo-random port sequences given the same random seed.

- The implementations should be written such that they work also when sourced, or copy’and’pasted into source code elsewhere, e.g. in R and Python scripts.

- The identified, free port should be outputted to the standard output (stdout) as digits only, without any prefix or suffix symbols.

- The user should be able to exclude a pre-defined set of ports by specifying environment variable `PORT4ME_EXCLUDE`, e.g. `PORT4ME_EXCLUDE=8080,4321`.

- The system administrator should be able to specify a pre-defined set of ports to be excluded by specifying environment variable `PORT4ME_EXCLUDE_SITE`, e.g. `PORT4ME_EXCLUDE_SITE=8080,4321`. This works complementary to `PORT4ME_EXCLUDE`.

- The user should be able to skip a certain number of random ports at their will by specifying environment variable `PORT4ME_SKIP`, e.g. `PORT4ME_SKIP=5`. The default is to not skip, which corresponds to `PORT4ME_SKIP=0`. Skipping should apply after ports are excluding by `PORT4ME_EXCLUDE` and `PORT4ME_EXCLUDE_SITE`.

- New implementations should perfectly reproduce the port sequences produced by already existing implementations.

### Design

- A Linear congruential generator (LCG) will be used to generate the pseudo-random port sequence

  - the next seed, \(s_{n+1}\) is calculated based on the current seed \(s_n\) and parameters \(a, c, m > 1\) as \(s_{n+1} = (a * s_{n} + c) \% m\)

  - the LCG algorithm must not assume that the current LCG seed is within \([0,m-1]\), i.e. it should apply modulo \(m\) on the seed first to avoid integer overflow

  - the LCG algorithm may produce the same output seed as input seed, which may happen when the seed is \(s_n = m - (a - c)\). To avoid this resulting in a constant LCG stream, increment the seed by one and recalculate whenever this happens

  - LCG parameters should be \(m = 2^{16} + 1\), \(a = 75\), and \(c = 74\) (“ZX81”)

    - this requires only 32-bit integer arithmetic, because \(m < 2^{32}\)

    - if the initial seed is \(s_0 = m - (a - c)\), which here is \(m - 1 = 2^{16}\), then the next LCG seed will be the same, which is then handled by the above increment-by-one workaround

- A 32-bit integer string hashcode will be used to generate an integer in \([0,2^{32}-1]\) from an ASCII string with any number of characters. The hashcode algorithm is based on the Java hashcode algorithm, but uses unsigned 32-bit integers in \([0,2^{32}-1]\), instead of signed ones in \([-2^{31},2^{31}-1]\)

- The string hashcode is used as the initial LCG seed:

  - the LCG seed should be in \([0,m-1]\)

  - given hashcode \(h\), we can generate the initial LCG seed as \(h\) modulo \(m\)