Re: Fwd: freeport(1)

[Alex Colomar]

On 10/2/22 00:22, Alejandro Colomar wrote:
[...]

> NAME
>       freeport - get a random unused port number
>
> SYNOPSIS
>       freeport
>
> DESCRIPTION
>       This program prints an available IPv4 port and exits.
>
>       The port is chosen at random from the ephemeral ports range
>       (see ip(7)).
>
> EXIT STATUS
>       0    Success
>       1    Error
>
>       On error, a message is printed on standard error.
>
>
> And the source code is:
>
>
> $ cat freeport.c
> /*
>    * SPDX-License-Identifier:  GPL-2.0-or-later
>    * Copyright 2022  NGINX, Inc.
>    * Copyright 2022  F5, Inc.
>    *
>    * Author:  Alejandro Colomar <alx@nginx.com>
>    */
>
>
> #include <netinet/in.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <strings.h>
> #include <sys/socket.h>
> #include <unistd.h>
>
>
> int32_t get_free_port(void);
>
>
> int
> main(void)
> {
>      int32_t  port;
>
>      port = get_free_port();
>      if (port == -1)
>          exit(EXIT_FAILURE);
>
>      printf("%d\n", port);
>      exit(EXIT_SUCCESS);
> }
>
>
> int32_t
> get_free_port(void)
> {
>      int                 sfd;
>      int32_t             port;
>      socklen_t           len;
>      struct sockaddr_in  addr;
>
>      sfd = socket(PF_INET, SOCK_STREAM, 0);
>      if (sfd == -1) {
>          perror("socket()");
>          return -1;
>      }
>
>      bzero(&addr, sizeof(addr));
>      addr.sin_family = AF_INET;
>      addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
>      addr.sin_port = htons(0);  // random port
>
>      len = sizeof(addr);
>      if (bind(sfd, (struct sockaddr *) &addr, len)) {
>          perror("bind()");
>          goto fail;
>      }
>
>      if (getsockname(sfd, (struct sockaddr *) &addr, &len)) {
>          perror("getsockname()");
>          goto fail;
>      }
>
>      port = ntohs(addr.sin_port);
>
> fail:
>      close(sfd);
>      return port;
> }

And here's a sh(1) implementation:

    $ cat freeport
    #!/bin/sh

    nc -l localhost 0 &

    lsof -i \
    | grep $! \
    | awk '{print $9}' \
    | cut -d':' -f2;

    kill $!;

Cheers,

Alex


--
<http://www.alejandro-colomar.es/>

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