scripts/mosh: Get the server IP from ssh

We get the server IP as seen from the client side using the ssh
ProxyCommand option, and as seen from the server side using the
SSH_CONNECTION environment variable.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

scripts/mosh

@@ -33,7 +33,46 @@ qq{Usage: $0 [options] [user@]host
    --server=PATH        mosh server on remote machine (default: "mosh-server")\n};
 
 GetOptions( 'client=s' => \$client,
-	    'server=s' => \$server ) or die $usage;
+	    'server=s' => \$server,
+	    'fake-proxy!' => \my $fake_proxy ) or die $usage;
+
+if ( defined $fake_proxy ) {
+  use Errno qw(EINTR);
+  use IO::Socket::INET;
+  use threads;
+
+  my ( $host, $port ) = @ARGV;
+
+  # Resolve hostname
+  my $packed_ip = gethostbyname $host;
+  if ( not defined $packed_ip ) {
+    die "$0: Could not resolve hostname $host\n";
+  }
+  my $ip = inet_ntoa $packed_ip;
+
+  print STDERR "MOSH IP $ip\n";
+
+  # Act like netcat
+  my $sock = IO::Socket::INET->new( PeerAddr => $ip,
+				    PeerPort => $port,
+				    Proto => "tcp" )
+    or die "$0: connect to host $ip port $port: $!\n";
+
+  sub cat {
+    my ( $from, $to ) = @_;
+    while ( my $n = $from->sysread( my $buf, 4096 ) ) {
+      next if ( $n == -1 && $! == EINTR );
+      $n >= 0 or die "$0: read: $!\n";
+      $to->write( $buf ) or die "$0: write: $!\n";
+    }
+  }
+
+  my $thr = threads->create(sub { cat $sock, \*STDOUT; $sock->shutdown( 0 ); })
+    or die "$0: create thread: $!\n";
+  cat \*STDIN, $sock; $sock->shutdown( 1 );
+  $thr->join;
+  exit;
+}
 
 if ( scalar @ARGV != 1 ) {
   die $usage;
@@ -41,23 +80,6 @@ if ( scalar @ARGV != 1 ) {
 
 my $userhost = $ARGV[ 0 ];
 
-my ( $user, $host );
-
-# Get username
-if ( $userhost =~ m'(.*?@)(.*)' ) {
-  ( $user, $host ) = ( $1, $2 );
-} else {
-  $user = "";
-  $host = $userhost;
-}
-
-# Resolve hostname
-my $packed_ip = gethostbyname $host;
-if ( not defined $packed_ip ) {
-  die "$0: Could not resolve hostname $host\n";
-}
-my $ip = inet_ntoa $packed_ip;
-
 # Run SSH and read password
 my $pty = new IO::Pty;
 my $pty_slave = $pty->slave;
@@ -72,14 +94,17 @@ if ( $pid == 0 ) { # child
   open STDERR, ">&", $pty_slave->fileno() or die;
   close $pty_slave;
 
-  exec {'ssh'} ('ssh', '-t', $user . $ip, $server, $ip);
+  my $s = q{sh -c 'exec "$@" "`set -- $SSH_CONNECTION; echo $3`"' -- } . $server;
+  exec 'ssh', '-o', "ProxyCommand=$0 --fake-proxy -- %h %p", '-t', $userhost, '--', $s;
   die "Cannot exec ssh: $!\n";
 } else { # server
-  my ( $port, $key );
+  my ( $ip, $port, $key );
   $pty->close_slave();
   LINE: while ( <$pty> ) {
     chomp;
-    if ( m{^MOSH CONNECT} ) {
+    if ( m{^MOSH IP } ) {
+      ( $ip ) = m{^MOSH IP (\S+)\s*$} or die "Bad MOSH IP string: $_\n";
+    } elsif ( m{^MOSH CONNECT } ) {
       if ( ( $port, $key ) = m{^MOSH CONNECT (\d+?) ([A-Za-z0-9/+]{22})\s*$} ) {
 	last LINE;
       } else {
@@ -91,7 +116,7 @@ if ( $pid == 0 ) { # child
   }
   waitpid $pid, 0;
 
-  if ( not defined $port ) {
+  if ( not defined $ip or not defined $port ) {
     die "$0: Did not find mosh server startup message.\n";
   }