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#!/usr/bin/perl -T
#----------------------------------------------------------------------
# ACME client written with process isolation and minimal privileges in mind
# (webserver component)
# Copyright © 2015-2017 Guilhem Moulin <guilhem@fripost.org>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <https://www.gnu.org/licenses/>.
#----------------------------------------------------------------------
use v5.14.2;
use strict;
use warnings;
# Usage: webserver FD
#
# fdopen(3) the file descriptor FD (corresponding to a listening
# socket), then keep accept(2)'ing connections and consider each
# connection as the verification of an ACME challenge, i.e., serve the
# requested challenge token.
#
# NOTE: one needs to chdir(2) to an appropriate ACME challenge directory
# before running this program, since challenge tokens are (on purpose)
# only looked for in the current directory.
#
# NOTE: one should run this program as an unprivileged user:group such
# as "www-data:www-data"; bind(2)'ing to a privileged port such as 80 is
# not a problem since FD can be bound as root prior to the execve(2).
use Errno qw/EINTR ENOENT/;
use Fcntl qw/O_NOFOLLOW O_RDONLY/;
use Socket qw/AF_UNIX AF_INET AF_INET6/;
# Untaint and fdopen(3) the listening socket
(shift @ARGV // die) =~ /\A(\d+)\z/ or die;
open (my $S, '+<&=', $1+0) or die "fdopen $1: $!";
my $ROOT = '/.well-known/acme-challenge';
close STDIN or die "close: $!";
close STDOUT or die "close: $!";
sub info($$$) {
my ($sockaddr, $msg, $req) = @_;
$req =~ s/\r\n\z// if defined $req;
# get a string representation of the peer's address
my $fam = Socket::sockaddr_family($sockaddr);
my $peer;
if ($fam == AF_UNIX) {
$peer = Socket::unpack_sockaddr_un($sockaddr);
} else {
my (undef, $ip) =
$fam == AF_INET ? Socket::unpack_sockaddr_in($sockaddr) :
$fam == AF_INET6 ? Socket::unpack_sockaddr_in6($sockaddr) :
die;
$peer = Socket::inet_ntop($fam, $ip);
}
$msg .= " from [$peer]" if defined $peer and $peer ne '';
$msg .= ": $req" if defined $req;
print STDERR $msg, "\n";
}
while (1) {
my $sockaddr = accept(my $conn, $S) or do {
next if $! == EINTR; # try again if accept(2) was interrupted by a signal
die "accept: $!";
};
my $req = $conn->getline();
info($sockaddr, "[$$] Incoming connection", $req) if $ENV{DEBUG};
next unless defined $req;
if ($req !~ s/\A(GET|HEAD) (.*) HTTP\/(1\.[01])\r\n\z/$2/) {
info($sockaddr, "Error: Bad request", $req);
next;
}
my ($method, $proto) = ($1, $3);
# Consume (and ignore) the headers
while (defined (my $h = $conn->getline())) { last if $h eq "\r\n" };
my ($status_line, $content_type, $content);
if ($req =~ /\A\Q$ROOT\E\/([A-Za-z0-9_\-]+)\z/) {
# only serve base64-encoded filenames (tokens) in the cwd
# XXX stat(2) followed by open(2) is racy; open(2) would hang if
# an attacker manages to replace a regular file with a FIFO
# between the syscalls, leading to denial of service; however
# there shouldn't be any risk of information disclosure as we're
# not following symlinks (and the cwd is not owned by us)
if (-f $1 and sysopen(my $fh, $1, O_NOFOLLOW|O_RDONLY)) {
($status_line, $content_type) = ('200 OK', 'application/jose+json');
$content = do { local $/ = undef; $fh->getline() };
$fh->close() or die "close: $!";
} elsif ($! == ENOENT) { # errno from either stat(2) or open(2)
$status_line = '404 Not Found';
}
}
$conn->print( "HTTP/$proto ", ($status_line // '403 Forbidden'), "\r\n" );
$conn->print( "Content-Type: $content_type\r\n" ) if defined $content_type;
$conn->print( "Content-Length: ".length($content)."\r\n" ) if defined $content;
$conn->print( "Connection: close\r\n" );
$conn->print( "\r\n" );
$conn->print( $content ) if defined $content and $method eq 'GET';
$conn->close() or die "close: $!";
}
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