Multiplexing TCP services: inetd's tcpmux (Updated)
Have you ever used inetd's built-in tcpmux service? Neither have I,
but after someone asked about it on #NetBSD today, I thought to give
it a try. After not understanding the manpage on first read,
it seems it's a re-invention of rpcbind (or vice versa), where you
can create a TCP connection to the tcpmux port (1/tcp), and ask for
a connection to a service to which you get connected then, by only
knowing a service name, no port number needed.
Here's an example:
(a) --> miyu# egrep '^(tcpmux|ftp)' /etc/inetd.conf
ftp stream tcp nowait root /usr/libexec/ftpd ftpd -ll
tcpmux stream tcp nowait root internal
(b) --> miyu# telnet localhost 1
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
(c) --> ftp
(d) --> -Service not available
Connection closed by foreign host.
(e) --> miyu# egrep ^ftp /etc/inetd.conf | sed 's,^,tcpmux/,' >>/etc/inetd.conf
(f) --> miyu# egrep '^ftp' /etc/inetd.conf
ftp stream tcp nowait root /usr/libexec/ftpd ftpd -ll
tcpmux stream tcp nowait root internal
tcpmux/ftp stream tcp nowait root /usr/libexec/ftpd ftpd -ll
miyu# alias hup
kill -1 `cat /var/run/!*.pid`
(g) --> miyu# hup inetd
(h) --> miyu# telnet localhost 1
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
(i) --> ftp
(j) --> 220-
220 localhost FTP server (NetBSD-ftpd 20050303) ready.
(k) --> ^]
telnet> quit
Connection closed.
miyu#
The example first checks that the FTP and TCPMUX service are enabled
in inetd.conf (a), then connects to the local tcpmux port (b) to ask
for a connection to the "ftp" service (c), which is not available (d).
Now after reading the inetd(8) manpage five times, duplicate the "ftp"
line and prefix it with "tcpmux/" (e), and check that it got added
to inetd.conf (f). Make sure that inetd reads in the changed config
file (g). When connecting now (h) and asking for the "ftp" service
(i), the ftp daemon is started and prints its banner (j).
As I don't really feel like speaking FTP without a client, I disconnect
(k) and exit telnet.
So, the tcpmux service seems quite a nice tool which could be used to
obsolete assigning ports in favour of service names. The problem is
that no TCP service that I've stumbled across ever uses this nice
mechanism. Even more strange, I wonder why things like rpcbind/portmap
were invented in the presence of that serivce (and I guess that tcpmux
does predate rpcbind etc.).
To play a bit more, I built myself a "cookie" service, adding this
to /etc/inetd.conf:
tcpmux/cookie stream tcp nowait root /usr/games/fortune fortune
Now I can run:
miyu# echo cookie | nc localhost 1
'Home, Sweet Home' must surely have been written by a bachelor.
miyu#
Who needs port numbers when you can request a service like this
easily. :-) After reading RFC 1078, I even found that the
protocol could safe one portscanning a machine to find out the
services it runs:
miyu# echo HELP | nc localhost 1
+Available services:
help
cookie
ftp
miyu#
And for the problem of having multiple versions of the same
service running (which rpcbind addresses),
RFC 1078 has a
suggestion, too: ``Multiple versions of a protocol can suffix the service name with a
protocol version number.
''. Easy, huh?
Looking into portmap and friends,
Appendix A of RFC 1047 says that
``The port mapper program maps RPC program and version numbers to
transport-specific port numbers. This program makes dynamic binding
of remote programs possible.''. I.e. reasons for existance of
rpcbind etc. are that that rpcbind does which
are not easily achievable with the inetd/tcpmux combination:
dynamic registration of services, transport over UDP and handling
of broadcast requests.
P.S.: Try running fortune(6) without argv[0] being set. :-)
Update: Aparently the tcpmux service is actualy being used
by Irix 6.5. Martin Neitzel sent me the inetd.conf of such a machine:
% grep tcpmux irix65_inetd.conf
tcpmux stream tcp nowait root internal
tcpmux/sgi_scanner stream tcp nowait root ?/usr/lib/scan/net/scannerd scannerd
tcpmux/sgi_printer stream tcp nowait root ?/usr/lib/print/printerd printerd
tcpmux/sgi_sysadm stream tcp nowait root ?/usr/sysadm/bin/sysadmd sysadmd
tcpmux/sgi_dmusrcmd stream tcp nowait root ?/usr/etc/dmusrcmd /usr/etc/dmusrcmd
[Tags: networking, tcpmux]
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