webserver: allow listening to multiple addresses.

(Useful when dual-stack IPv4/IPv6 is not supported.)  Also, change the
default to listen to a UNIX-domain socket </var/run/lacme.socket>.

Moreover temporary iptables rules are no longer installed.  Hosts
without a public HTTP daemon listening on port 80 need to set the
'listen' option to [::] and/or 0.0.0.0, and possibly set the 'iptables'
option to Yes.

5 files changed, 212 insertions, 118 deletions

diff --git a/Changelog b/Changelog
index b23191f..fdb0775 100644
--- a/Changelog
+++ b/Changelog
@@ -3,6 +3,13 @@ lacme (0.3) upstream;
   + When parsing config-cert files and directories (default "lacme-certs.conf
     lacme-certs.conf.d"), import the default section of files read earlier.
   + new-cert: create certificate files atomically.
+  + webserver: allow listening to multiple addresses (useful when
+    dual-stack IPv4/IPv6 is not supported).  Listen to a UNIX-domain
+    socket by default </var/run/lacme.socket>.
+  + webserver: don't install temporary iptables by default.  Hosts
+    without a public HTTP daemon listening on port 80 need to set the
+    'listen' option to [::] and/or 0.0.0.0, and possibly set the
+    'iptables' option to Yes.
   - Ensure lacme's config file descriptor is not passed to the accountd
     or webserver components.
   - new-cert: sort section names if not passed explicitely.
diff --git a/config/lacme.conf b/config/lacme.conf
index c5efb03..874bb1f 100644
--- a/config/lacme.conf
+++ b/config/lacme.conf
@@ -16,18 +16,16 @@
 # since the two processes communicate through a socket pair.  See the
 # "accountd" section below for details.
 #
-#socket = /run/user/1000/S.lacme
+#socket =
 
 # username to drop privileges to (setting both effective and real uid).
 # Preserve root privileges if the value is empty (not recommended).
-# Default: "nobody".
 #
-#user = lacme
+#user = nobody
 
 # groupname to drop privileges to (setting both effective and real gid,
 # and also setting the list of supplementary gids to that single group).
 # Preserve root privileges if the value is empty (not recommended).
-# Default: "nogroup".
 #
 #group = nogroup
 
@@ -35,11 +33,11 @@
 #
 #command = /usr/lib/lacme/client
 
-# Root URI of the ACME server.  NOTE: Use the staging server for testing
-# as it has relaxed rate-limiting.
+# Root URI of the ACME server.  NOTE: Use the staging server
+# <https://acme-staging.api.letsencrypt.org/> for testing as it has
+# relaxed rate-limiting.
 #
 #server = https://acme-v01.api.letsencrypt.org/
-#server = https://acme-staging.api.letsencrypt.org/
 
 # Timeout in seconds after which the client stops polling the ACME
 # server and considers the request failed.
@@ -61,17 +59,17 @@
 
 [webserver]
 
-# Specify the local address to listen on, in the form ADDRESS[:PORT].
+# Comma- or space-separated list of addresses to listen on, for instance
+# "0.0.0.0:80 [::]:80".
 #
-#listen = 0.0.0.0:80
-#listen = [::]:80
+#listen = /var/run/lacme.socket
 
-# If a webserver is already running, specify a non-existent directory
-# under which the webserver is configured to serve GET requests for
-# challenge files under "/.well-known/acme-challenge/" (for each virtual
-# hosts requiring authorization) as static files.
+# Non-existent directory under which an external HTTP daemon is
+# configured to serve GET requests for challenge files under
+# "/.well-known/acme-challenge/" (for each virtual host requiring
+# authorization) as static files.
 #
-#challenge-directory = /var/www/acme-challenge
+#challenge-directory =
 
 # username to drop privileges to (setting both effective and real uid).
 # Preserve root privileges if the value is empty (not recommended).
@@ -92,7 +90,7 @@
 # ADDRESS[:PORT] specified with listen.  Theses rules are automatically
 # removed once lacme(1) exits.
 #
-#iptables = Yes
+#iptables = No
 
 
 [accountd]
@@ -103,13 +101,13 @@
 # username to drop privileges to (setting both effective and real uid).
 # Preserve root privileges if the value is empty.
 #
-#user = root
+#user =
 
 # groupname to drop privileges to (setting both effective and real gid,
 # and also setting the list of supplementary gids to that single group).
 # Preserve root privileges if the value is empty.
 #
-#group = root
+#group =
 
 # Path to the lacme-accountd(1) executable.
 #
diff --git a/lacme b/lacme
index a8c25fe..b4d09e8 100755
--- a/lacme
+++ b/lacme
@@ -30,7 +30,8 @@ use File::Temp ();
 use Getopt::Long qw/:config posix_default no_ignore_case gnu_getopt auto_version/;
 use List::Util 'first';
 use POSIX ();
-use Socket qw/AF_UNIX PF_INET PF_INET6 PF_UNIX PF_UNSPEC INADDR_ANY IN6ADDR_ANY
+use Socket qw/AF_UNIX AF_INET AF_INET6 PF_UNIX PF_INET PF_INET6 PF_UNSPEC
+              INADDR_ANY IN6ADDR_ANY IPPROTO_IPV6
               SOCK_STREAM SOL_SOCKET SO_REUSEADDR SHUT_RDWR/;
 
 use Config::Tiny ();
@@ -95,12 +96,12 @@ do {
             map {$_ => undef} qw/server timeout SSL_verify SSL_version SSL_cipher_list/
         },
         webserver => {
-            listen                => ':80',
-            'challenge-directory' => '/var/www/acme-challenge',
+            listen                => '/var/run/lacme.socket',
+            'challenge-directory' => undef,
             user                  => 'www-data',
             group                 => 'www-data',
             command               => '/usr/lib/lacme/webserver',
-            iptables              => 'Yes'
+            iptables              => 'No'
 
         },
         accountd => {
@@ -270,12 +271,9 @@ sub set_FD_CLOEXEC($$) {
 
 
 #############################################################################
-# Try to spawn a webserver to serve ACME challenges, and return the
-# temporary challenge directory.
-# If a webserver is already listening, symlink the 'challenge-directory'
-# configuration option to the temporary challenge directory.
-# Otherwise, bind(2) a socket, pass its fileno to the webserver
-# component, and optionally install iptables rules.
+# If 'listen' is not empty, bind socket(s) to the given addresse(s) and
+# spawn webserver(s) to serve ACME challenge reponses.
+# The temporary challenge directory is returned.
 #
 sub spawn_webserver() {
     # create a temporary directory; give write access to the ACME client
@@ -283,73 +281,151 @@ sub spawn_webserver() {
     my $tmpdir = File::Temp::->newdir(CLEANUP => 1, TMPDIR => 1) // die;
     chmod 0755, $tmpdir or die "Can't chmod: $!";
     if ((my $username = $CONFIG->{client}->{user}) ne '') {
-        my $uid = getpwnam($username) // die "Can't getgrnam($username): $!";
+        my $uid = getpwnam($username) // die "Can't getpwnam($username): $!";
         chown($uid, -1, $tmpdir) or die "Can't chown: $!";
     }
 
     my $conf = $CONFIG->{webserver};
-    my ($fam, $addr, $port) = (PF_INET, $conf->{listen}, 80);
-    $port = $1 if $addr =~ s/:(\d+)$//;
-    $addr = Socket::inet_ntop(PF_INET, INADDR_ANY) if $addr eq '';
-    $fam = PF_INET6 if $addr =~ s/^\[(.+)\]$/$1/;
-
-    my $proto = getprotobyname("tcp") // die;
-    socket(my $srv, $fam, SOCK_STREAM, $proto) or die "socket: $!";
-    setsockopt($srv, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) or die "setsockopt: $!";
-    $addr = Socket::inet_pton($fam, $addr) // die "Invalid address $conf->{listen}\n";
-    my $sockaddr = $fam == PF_INET  ? Socket::pack_sockaddr_in($port,  $addr)
-                 : $fam == PF_INET6 ? Socket::pack_sockaddr_in6($port, $addr)
-                 : die;
-
-    # try to bind aginst the specified address:port
-    bind($srv, $sockaddr) or do {
-        die "Can't bind to $conf->{listen}: $!" if $! != EADDRINUSE;
-        print STDERR "[$$] Using existing webserver on $conf->{listen}\n" if $OPTS{debug};
-        my $dir = $conf->{'challenge-directory'};
+
+    # parse and pack addresses to listen to
+    my @sockaddr;
+    foreach my $a (split /[[:blank:],]\s*/, $conf->{listen}) {
+        my $sockaddr;
+        if ($a =~ /\A\//) { # absolute path to a unix domain socket
+            $sockaddr = Socket::pack_sockaddr_un($a);
+        } elsif ($a =~ /\A(\d+(?:\.\d+){3})(?::(\d+))?\z/) {
+            my $n = Socket::inet_pton(AF_INET, $1);
+            $sockaddr = Socket::pack_sockaddr_in($2 // 80, $n) if defined $n;
+        } elsif ($a =~ /\A\[([[:xdigit:]:.]{2,39})\](?::(\d+))?\z/) {
+            my $n = Socket::inet_pton(AF_INET6, $1);
+            $sockaddr = Socket::pack_sockaddr_in6($2 // 80, $n) if defined $n;
+        }
+        die "Invalid address: $a\n" unless defined $sockaddr;
+        push @sockaddr, $sockaddr;
+    }
+
+    # symlink the 'challenge-directory' configuration option to the
+    # temporary challenge directory (so an existing httpd can directly
+    # serve ACME challenge reponses).
+    if (defined (my $dir = $conf->{'challenge-directory'})) {
+        print STDERR "[$$] Using existing webserver on $dir\n" if $OPTS{debug};
         symlink $tmpdir, $dir or die "Can't symlink $dir -> $tmpdir: $!";
         push @CLEANUP, sub() {
             print STDERR "Unlinking $dir\n" if $OPTS{debug};
             unlink $dir or warn "Warning: Can't unlink $dir: $!";
+        }
+    }
+    elsif (!@sockaddr) {
+        die "'challenge-directory' option of section [webserver] is required when 'listen' is empty\n";
+    }
+
+    # create socket(s) and spawn webserver(s)
+    my @sockaddr4;
+    foreach my $sockaddr (@sockaddr) {
+        my $domain = Socket::sockaddr_family($sockaddr) // die;
+        socket(my $sock, $domain, SOCK_STREAM, 0) or die "socket: $!";
+        setsockopt($sock, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))
+            if $domain == AF_INET or $domain == AF_INET6;
+
+        my $p; # pretty-print the address/port
+        if ($domain == AF_UNIX) {
+            $p = Socket::unpack_sockaddr_un($sockaddr);
+        } elsif ($domain == AF_INET) {
+            my ($port, $addr) = Socket::unpack_sockaddr_in($sockaddr);
+            $p = Socket::inet_ntop($domain, $addr).":$port";
+        } elsif ($domain == AF_INET6) {
+            my ($port, $addr) = Socket::unpack_sockaddr_in6($sockaddr);
+            $p = "[".Socket::inet_ntop($domain, $addr)."]:$port";
+        }
+
+        if ($domain == AF_UNIX) {
+            # bind(2) with a loose umask(2) to allow anyone to connect
+            my $umask = umask(0111) // die "umask: $!";
+            my $path = Socket::unpack_sockaddr_un($sockaddr);
+            bind($sock, $sockaddr) or die "Couldn't bind to $p: $!";
+            push @CLEANUP, sub() {
+                print STDERR "Unlinking $path\n" if $OPTS{debug};
+                unlink $path or warn "Warning: Can't unlink $path: $!";
+            };
+            umask($umask) // die "umask: $!";
+        }
+        else {
+            bind($sock, $sockaddr) or die "Couldn't bind to $p: $!";
+        }
+
+        listen($sock, 5) or die "listen: $!";
+
+        # spawn a webserver component bound to the given socket
+        my $pid = fork() // "fork: $!";
+        unless ($pid) {
+            drop_privileges($conf->{user}, $conf->{group}, $tmpdir);
+            set_FD_CLOEXEC($sock, 0);
+            $ENV{DEBUG} = $OPTS{debug};
+            # use execve(2) rather than a Perl pseudo-process to ensure that
+            # the child doesn't have access to the parent's memory
+            exec $conf->{command}, fileno($sock) or die;
+        }
+
+        print STDERR "[$$] Forking ACME webserver bound to $p, child PID $pid\n" if $OPTS{debug};
+        set_FD_CLOEXEC($sock, 1);
+        push @CLEANUP, sub() {
+            print STDERR "[$$] Shutting down ACME webserver bound to $p\n" if $OPTS{debug};
+            shutdown($sock, SHUT_RDWR) or warn "shutdown: $!";
+            kill 15 => $pid;
+            waitpid $pid => 0;
         };
-        return $tmpdir;
-    };
-    listen($srv, 5) or die "listen: $!";
 
-    # spawn the webserver component
-    my $pid = fork() // "fork: $!";
-    unless ($pid) {
-        drop_privileges($conf->{user}, $conf->{group}, $tmpdir);
-        set_FD_CLOEXEC($srv, 0);
-        $ENV{DEBUG} = $OPTS{debug};
-        # use execve(2) rather than a Perl pseudo-process to ensure that
-        # the child doesn't have access to the parent's memory
-        exec $conf->{command}, fileno($srv) or die;
+        # on dual-stack ipv4/ipv6, we'll need to open the port for the
+        # v4-mapped address as well
+        if ($domain == AF_INET6) {
+            my $v6only = getsockopt($sock, Socket::IPPROTO_IPV6, Socket::IPV6_V6ONLY)
+                // die "getsockopt(IPV6_V6ONLY): $!";
+            my ($port, $addr) = Socket::unpack_sockaddr_in6($sockaddr);
+            my $mask   = "\xFF" x 12              . "\x00" x 4;
+            my $prefix = "\x00" x 10 . "\xFF" x 2 . "\x00" x 4;
+
+            if (unpack('i', $v6only) == 0) {
+                if ($addr eq IN6ADDR_ANY) {
+                    push @sockaddr4, Socket::pack_sockaddr_in($port, INADDR_ANY);
+                } elsif (($addr & $mask) eq $prefix) {
+                    my $v4 = substr($addr, 12);
+                    push @sockaddr4, Socket::pack_sockaddr_in($port, $v4);
+                }
+            }
+        }
     }
 
-    print STDERR "[$$] Forking ACME webserver, child PID $pid\n" if $OPTS{debug};
-    set_FD_CLOEXEC($srv, 1);
-    push @CLEANUP, sub() {
-        print STDERR "[$$] Shutting down ACME webserver\n" if $OPTS{debug};
-        shutdown($srv, SHUT_RDWR) or warn "shutdown: $!";
-        kill 15 => $pid;
-        waitpid $pid => 0;
-    };
+    # allow incoming traffic on the given addresses
+    if (lc ($conf->{iptables} // 'No') eq 'yes') {
+        iptables_save(AF_INET,  @sockaddr, @sockaddr4);
+        iptables_save(AF_INET6, @sockaddr);
+    }
+
+    return $tmpdir;
+}
+
 
-    return $tmpdir if lc ($conf->{iptables} // 'Yes') eq 'no';
+#############################################################################
+# Save current iptables/ipv6tables to a temporary file and install
+# temporary rules to open the given addresses/ports.
+sub iptables_save($@) {
+    my $domain = shift;
+    my @sockaddr = grep { Socket::sockaddr_family($_) == $domain } @_;
+    return unless @sockaddr; # no address in that domain
 
     # install iptables
-    my $iptables_bin = $fam == PF_INET ? 'iptables' : $fam == PF_INET6 ? 'ip6tables' : die;
+    my $iptables_bin = $domain == AF_INET ? 'iptables' : $domain == AF_INET6 ? 'ip6tables' : die;
     my $iptables_tmp = File::Temp::->new(TMPDIR => 1) // die;
     set_FD_CLOEXEC($iptables_tmp, 1);
 
-    my $pid2 = fork() // die "fork: $!";
-    unless ($pid2) {
+    my $pid = fork() // die "fork: $!";
+    unless ($pid) {
         open STDIN,  '<',  '/dev/null'   or die "Can't open /dev/null: $!";
         open STDOUT, '>&', $iptables_tmp or die "Can't dup: $!";
         $| = 1; # turn off buffering for STDOUT
         exec "/sbin/$iptables_bin-save", "-c" or die;
     }
-    waitpid $pid2 => 0;
+    waitpid $pid => 0;
     die "Error: /sbin/$iptables_bin-save exited with value ".($? >> 8) if $? > 0;
 
     # seek back to the begining, as we'll restore directly from the
@@ -359,32 +435,39 @@ sub spawn_webserver() {
     seek($iptables_tmp, SEEK_SET, 0) or die "Can't seek: $!";
 
     push @CLEANUP, sub() {
-        print STDERR "[$$] Restoring iptables\n" if $OPTS{debug};
-        my $pid2 = fork() // die "fork: $!";
-        unless ($pid2) {
+        print STDERR "[$$] Restoring $iptables_bin\n" if $OPTS{debug};
+        my $pid = fork() // die "fork: $!";
+        unless ($pid) {
             open STDIN, '<&', $iptables_tmp or die "Can't dup: $!";
             open STDOUT, '>', '/dev/null'   or die "Can't open /dev/null: $!";
             exec "/sbin/$iptables_bin-restore", "-c" or die;
         }
-        waitpid $pid2 => 0;
+        waitpid $pid => 0;
         warn "Warning: /sbin/$iptables_bin-restore exited with value ".($? >> 8) if $? > 0;
     };
 
-    # it's safe to install the new iptables to open $port now that the
-    # restore hook is in place
-    my $mask = $fam == PF_INET  ? ($addr eq INADDR_ANY  ? '0' : '32')
-             : $fam == PF_INET6 ? ($addr eq IN6ADDR_ANY ? '0' : '128')
-             : die;
-    my $dest = Socket::inet_ntop($fam, $addr) .'/'. $mask;
 
-    system ("/sbin/$iptables_bin", qw/-I INPUT  -p tcp -m tcp -m state/,
-            '-d', $dest, '--dport', $port,
-            '--state', 'NEW,ESTABLISHED', '-j', 'ACCEPT') == 0 or die;
-    system ("/sbin/$iptables_bin", qw/-I OUTPUT -p tcp -m tcp -m state/,
-            '-s', $dest, '--sport', $port,
-            '--state',     'ESTABLISHED', '-j', 'ACCEPT') == 0 or die;
+    # it's safe to install the new iptables to open $addr:$port now that
+    # the restore hook is in place
 
-    return $tmpdir;
+    foreach my $sockaddr (@sockaddr) {
+        my ($port, $addr, $mask);
+        if ($domain == AF_INET) {
+            ($port, $addr) = Socket::unpack_sockaddr_in($sockaddr);
+            $mask = $addr eq INADDR_ANY ? '0' : '32';
+        } elsif ($domain == AF_INET6) {
+            ($port, $addr) = Socket::unpack_sockaddr_in6($sockaddr);
+            $mask = $addr eq IN6ADDR_ANY ? '0' : '128';
+        }
+
+        my $dest = Socket::inet_ntop($domain, $addr) .'/'. $mask;
+        system ("/sbin/$iptables_bin", qw/-I INPUT  -p tcp -m tcp -m state/,
+                '-d', $dest, '--dport', $port,
+                '--state', 'NEW,ESTABLISHED', '-j', 'ACCEPT') == 0 or die;
+        system ("/sbin/$iptables_bin", qw/-I OUTPUT -p tcp -m tcp -m state/,
+                '-s', $dest, '--sport', $port,
+                '--state',     'ESTABLISHED', '-j', 'ACCEPT') == 0 or die;
+    }
 }
 
 
diff --git a/lacme.md b/lacme.md
index aec2cd1..94b40cb 100644
--- a/lacme.md
+++ b/lacme.md
@@ -51,13 +51,12 @@ with its own executable:
  4. For certificate issuances (`new-cert` command), an optional
     webserver (specified with the *command* option of the [`[webserver]`
     section](#webserver-section) of the configuration file), which is
-    spawned by the “master” `lacme` process when no service is listening
-    on the HTTP port.  (The only challenge type currently supported by
-    `lacme` is `http-01`, which requires a webserver to answer
-    challenges.) That webserver only processes `GET` and `HEAD` requests
-    under the `/.well-known/acme-challenge/` URI.
-    By default some [`iptables`(8)] rules are automatically installed to
-    open the HTTP port, and removed afterwards.
+    spawned by the “master” `lacme`.  (The only challenge type currently
+    supported by `lacme` is `http-01`, which requires a webserver to
+    answer challenges.)  That webserver only processes `GET` and `HEAD`
+    requests under the `/.well-known/acme-challenge/` URI.
+    Moreover temporary [`iptables`(8)] rules can be automatically
+    installed to open the HTTP port.
 
 Commands
 ========
@@ -228,18 +227,19 @@ This section is used for configuring the [ACME] webserver.
 
 *listen*
 
-:   Specify the local address to listen on, in the form
-    `ADDRESS[:PORT]`.  If `ADDRESS` is enclosed with brackets ‘[’/‘]’
-    then it denotes an IPv6; an empty `ADDRESS` means `0.0.0.0`.
-    Default: `:80`.
+:   Comma- or space-separated list of addresses to listen on.  Valid
+    addresses are of the form `IPV4:PORT`, `[IPV6]:PORT` (where the
+    `:PORT` suffix is optional and defaults to the HTTP port 80), or an
+    absolute path of a UNIX-domain socket (created with mode `0666`).
+    Default: `/var/run/lacme.socket`.
 
 *challenge-directory*
 
-:   If a webserver is already running, specify a non-existent directory
-    under which the webserver is configured to serve `GET` requests for
-    challenge files under `/.well-known/acme-challenge/` (for each
-    virtual hosts requiring authorization) as static files.
-    Default: `/var/www/acme-challenge`.
+:   Specify a non-existent directory under which an external HTTP daemon
+    is configured to serve `GET` requests for challenge files under
+    `/.well-known/acme-challenge/` (for each virtual host requiring
+    authorization) as static files.
+    This option is required when *listen* is empty.
 
 *user*
 
@@ -263,10 +263,10 @@ This section is used for configuring the [ACME] webserver.
 
 *iptables*
 
-:   Whether to automatically install [`iptables`(8)] rules to open the
-    `ADDRESS[:PORT]` specified with *listen*.  Theses rules are
+:   Whether to automatically install temporary [`iptables`(8)] rules to
+    open the `ADDRESS[:PORT]` specified with *listen*.  The rules are
     automatically removed once `lacme` exits.
-    Default: `Yes`.
+    Default: `No`.
 
 `[accountd]` section
 ---------------------
diff --git a/webserver b/webserver
index e97fe00..7914762 100755
--- a/webserver
+++ b/webserver
@@ -38,12 +38,9 @@ use warnings;
 # not a problem since FD can be bound as root prior to the execve(2).
 
 use Errno 'EINTR';
-use Socket qw/AF_INET AF_INET6/;
+use Socket qw/AF_UNIX AF_INET AF_INET6/;
 
 # Untaint and fdopen(3) the listening socket
-# TODO: we could even take multiple file descriptors and select(2)
-# between them; this could be useful to listen on two sockets, one for
-# INET and one for INET6
 (shift @ARGV // die) =~ /\A(\d+)\z/ or die;
 open my $S, '+<&=', $1 or die "fdopen $1: $!";
 my $ROOT = '/.well-known/acme-challenge';
@@ -57,13 +54,22 @@ sub info($$$) {
 
     # get a string representation of the peer's address
     my $fam = Socket::sockaddr_family($sockaddr);
-    my (undef, $ip) =
-        $fam == AF_INET  ? Socket::unpack_sockaddr_in($sockaddr)  :
-        $fam == AF_INET6 ? Socket::unpack_sockaddr_in6($sockaddr) :
-        die;
-    my $addr = Socket::inet_ntop($fam, $ip);
+    my $peer;
 
-    print STDERR $msg." from [$addr]".(defined $req ? ": $req" : "")."\n";
+    if ($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) {