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|
#! /usr/local/bin/perl -w
# The Emergency Daemon - wait for simple emergency commands and execute them
# David A. Madore <URL: http://www.madore.org/~david/ > - Public Domain
# *** The emergency daemon protocol ***
#
# Commands are sent through UDP datagrams, normally addressed to port
# 911. Commands consist of three parts separated or terminated by '|'
# or \n (but \r\n is also accepted). First part is the command
# proper. Second is a UTC timestamp in yyyy-mm-ddThh:mm:ssZ format
# (here 'T' and 'Z' are literal 'T' and 'Z'). Third is the
# HMAC-SHA256, in hexadecimal, of command|timestamp (separated by '|',
# even if they were separated differently in the input, and not
# terminated by anything), with a HMAC key shared by client and
# server.
#
# The server response consists of one or more lines terminated by \n.
# If the command is PING, the server responds PONG. If the command is
# DATE, the server responds DATE followed by the current time and the
# timestamp of the last authenticated command. Apart from PING and
# DATE, all other commands are authenticated: if the HMAC does not
# match what it should be, the server responds !MAC. If the timestamp
# does not match the current date +/- 30 seconds, the server responds
# !DAT; it also does so if the timestamp is not (strictly) greater
# than the timestamp of the last request.
#
# The NOOP command does nothing (but must still be authenticated): the
# server responds NOOP. The DPID command returns the daemon's PID:
# the server responds DPID and then the PID itself (on a separate
# line). The DIE! command causes the daemon to respond BYE! and then
# quit. The RKEY command causes the daemon to reread its key file:
# the server responds either DONE or !ERR in case of error, in which
# case the next line in the response is a human-readable error line.
# The SYRQ command is followed by whitespace and then by data to be
# written to /proc/sysrq-trigger: the server responds either DONE or
# !ERR in case of error, in which case the next line in the response
# is a human-readable error line.
# *** The emergency daemon itself ***
#
# The daemon understands three options:
#
# -p <number> indicates which port it should bind to. The daemon
# binds to the IPv6 unspecified address with the IPV6_V6ONLY option
# set to 0, thus listening on both IPv6 and IPv4 families.
#
# -k <filename> specifies the keyfile to use. This file contains one
# or more keys, one per line, which will all be equally valid when
# computing the MAC.
#
# -f requests that the daemon ignore HUP and INT signals, and fork
# once it has successfully set up its listening socket: the father
# then prints its child's PID and exits successfully.
use strict;
use warnings;
use Digest::SHA qw(hmac_sha256_hex);
use Socket;
use Socket6;
use POSIX ();
use Getopt::Std;
use constant {
DEFAULT_PORT => 911
};
my %opts;
getopts("k:p:f", \%opts);
my @authorized_keys;
my $keyfilename = $opts{k};
die "No key file specified (use -k option)" unless defined($keyfilename);
sub readkeys {
open my $keyfile, "<", $keyfilename
or die "Cannot open key file $opts{k}: $!";
@authorized_keys = ();
while (<$keyfile>) {
chomp;
push @authorized_keys, $_;
}
close $keyfile;
}
readkeys;
my $proto = getprotobyname("udp") or die "Can't resolve udp protocol: $!";
my $port;
if ( defined($opts{p}) ) {
$port = $opts{p};
$port =~ /^\d+$/ or die "Invalid port number (-p option) $port";
} else {
$port = DEFAULT_PORT;
}
socket my $socket, PF_INET6, SOCK_DGRAM, $proto or die "Can't create socket: $!";
if ( defined(IPV6_V6ONLY) ) {
setsockopt $socket, IPPROTO_IPV6, IPV6_V6ONLY, 0 or die "Can't set IPV6_V6ONLY option to 0: $!";
}
bind $socket, sockaddr_in6($port, in6addr_any) or die "Can't bind socket: $!";
if ( $opts{f} ) {
$SIG{HUP} = "IGNORE";
$SIG{INT} = "IGNORE";
my $childpid = fork;
die "Can't fork: $!" unless defined($childpid);
if ( $childpid ) {
print "$childpid\n";
exit 0;
}
}
sub curtime {
my $fiddle = shift // 0;
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = gmtime(time+$fiddle);
return sprintf("%04d-%02d-%02dT%02d:%02d:%02dZ",$year+1900,$mon+1,$mday,$hour,$min,$sec);
}
my $mintime = "0";
PACKET:
while (1) {
my $buf;
my $sender = recv($socket, $buf, 16384, 0);
my @lines = split /\015*\012|\|/s, $buf;
my $command = $lines[0] // "";
my $timestamp = $lines[1] // "";
my $maccheck = $lines[2] // "";
next PACKET if $command eq "";
if ( $command eq "PING" ) {
send $socket, "PONG\n", 0, $sender;
} elsif ( $command eq "DATE" ) {
send $socket, ("DATE\n".curtime."\n".$mintime."\n"), 0, $sender;
} else {
my $validate = "$command|$timestamp";
my $macchecked = 0;
foreach my $key ( @authorized_keys ) {
if ( $maccheck eq hmac_sha256_hex($validate, $key) ) {
$macchecked = 1;
}
}
unless ( $macchecked ) {
send $socket, "!MAC\n", 0, $sender;
next PACKET;
}
my $datechecked = ($timestamp ge curtime(-30))
&& ($timestamp le curtime(30))
&& ($timestamp gt $mintime);
unless ( $datechecked ) {
send $socket, "!DAT\n", 0, $sender;
next PACKET;
}
$mintime = $timestamp;
if ( $command eq "NOOP" ) {
send $socket, "NOOP\n", 0, $sender;
} elsif ( $command eq "DPID" ) {
send $socket, "DPID\n$$\n", 0, $sender;
} elsif ( $command eq "DIE!" ) {
send $socket, "BYE!\n", 0, $sender;
exit 0;
} elsif ( $command eq "RKEY" ) {
my $resp = "DONE\n";
eval { readkeys };
if ( $@ ) {
$resp = "!ERR\n$@";
}
send $socket, $resp, 0, $sender;
} elsif ( $command =~ /^SYRQ\s+(.*)$/ ) {
my $s = $1;
my $resp = "DONE\n";
eval {
open my $sysrq_trigger, ">", "/proc/sysrq-trigger" or die "Couldn't open /proc/sysrq-trigger for writing: $!";
print $sysrq_trigger $s or die "Couldn't write to /proc/sysrq-trigger: $!";
close $sysrq_trigger;
};
if ( $@ ) {
$resp = "!ERR\n$@";
}
send $socket, $resp, 0, $sender;
} else {
send $socket, "!UNK\n", 0, $sender;
}
}
}
|
