nonamed.8

操作系统设计与实现源码

.\" These numbers should match those in nonamed.c:
.ds TT "1 hour"
.ds ST "two seconds"
.ds MT "four seconds"
.ds LT "five minutes"
.ds NS "five"
.ds NI "256"
.TH NONAMED 8
.SH NAME
nonamed \- not a name daemon, but acts like one
.SH SYNOPSIS
.B nonamed
.RB [ \-d [\fIlevel\fP]]
.RB [ \-p
.IR port ]
.RB [ \-n
.IR address [/\fIport\fP]]
.SH DESCRIPTION
.B Nonamed
is not an Internet name daemon.  When started it immediately tries to find a
real name daemon on the local network.  If it finds one then it will relay
any DNS queries to that name daemon.  If it can't find one then it will do
its best to answer simple queries using the
.B /etc/hosts
table for name to address translation.
.PP
On startup
.B nonamed
broadcasts up to \*(NS simple UDP queries on the local network.  This is a
somewhat nasty thing to do, because it makes all name servers on the net
spring into action to answer the request.  The first name server to answer
is used by
.B nonamed
to answer the queries of its clients.  The broadcast search is repeated
after an hour to give another name server a chance to be abused.  (It is
quite normal for a host to use a remote name server.  The broadcasts are a
bit unfriendly, but they only happen once an hour.)
.PP
If a real name server is found then
.B nonamed
enters "relay mode", passing all queries on.  In relay mode the daemon
cannot be distinguished from a real DNS name daemon. 
.PP
.B Nonamed
accepts both UDP and TCP queries under Minix-vmd.  Under standard Minix
only UDP queries are accepted.  \*(NI relayed UDP queries can be outstanding
before it forgets where the first one came from.
.PP
In answer mode
.B nonamed
can answer simple DNS queries to translate a host name to an IP address, or
an IP address to a host name.  Suppose
.B /etc/hosts
looks like this:
.PP
.RS
.ta +15n
.nf
127.0.0.1	localhost
192.9.200.1	darask.home.cs.vu.nl
192.9.200.2	burask.home.cs.vu.nl
.fi
.RE
.PP
Then queries for the host names listed can be answered with the IP addresses
to the left of them.  If the name can't be found then it is retried with the
domain name stripped of, this is because "localhost" is looked up as
"localhost.home.cs.vu.nl" in the above example.  Reverse lookups are
answered with the first hostname on the line with the IP address.
.PP
UDP queries are immediately answered on startup if the answer can be found
in the hosts file.  Failure answers, and answers to TCP queries are delayed
until it decides that there are no real name servers out there.
.PP
.B Nonamed
employs several timeouts for efficient operation:
.PP
If \*(NS broadcasts, \*(ST apart fail to produce a name server on startup
then it decides that there are no real name servers around.
.PP
After \*(TT it will again look for a name server.  (We don't want to hog the
same one indefinitely.)
.PP
If no UDP reply is seen in \*(MT than a simple probe is sent the name
server.  If that doesn't provoke an answer in \*(ST then it will look for a
new name server.  A failing TCP connection will also invoke a search, the
TCP connection is then made to the new name server.  A client using UDP will
retry eventually, a client using TCP will notice nothing but a short delay.
The daemon drops back into answer mode if the search fails and it has a
hosts file with a "localhost" entry.
.PP
It waits \*(LT for any action on a TCP stream before the connection is
aborted.
.PP
In answer mode it will generate data with a TTL (time to live) of \*(TT.
.SH OPTIONS
The options are only useful when debugging
.BR nonamed ,
although it can be very instructive to watch DNS queries being done.
.TP
.BR \-d [\fIlevel\fP]
Set debugging level to
.I level
(a single digit, by default
.BR 1 .)
Debug mode 1 makes
.B nonamed
decode and display the DNS queries and replies that it receives, sends and
relays.  The decoding only handles the common cases.  In debug mode 2 it
prints tracing information about the internal jobs it executes.  In debug
mode 3 it core dumps when an error causes it to exit.  The debugging level
may also be increased by 1 at runtime by sending signal
.B SIGUSR1
or turned off (set to 0) with
.BR SIGUSR2 .
.TP
.RB [ \-p " \fIport\fP]
Port to use instead of the normal
.B domain
port.
.TP
.RB [ \-n " \fIaddress\fP[/\fIport\fP]]
Use this IP address and port to find a real name server.  It can be used to
make
.B nonamed
relay to another
.BR nonamed .
(They do not listen to broadcasts, of course.)  You can run two daemons on
one machine if you choose a new port number for one of them.  You can force
.B nonamed
out of relay mode by sending it a hangup signal.
.SH FILES
.TP 15n
/etc/hosts
Hosts to address translation table when in answer mode.
.SH "SEE ALSO"
.BR gethostbyname (3),
.BR resolver (3),
.BR hosts (5),
.BR set_net_default (8),
.BR boot (8),
.BR inetd (8),
.BR irdpd (8),
.BR rarpd (8).
.SH NOTES
You can specify a remote name server in
.B /etc/resolv.conf
to circumvent
.BR nonamed .
But then you lose its talent for automatically finding new name servers when
the remote name server becomes unreachable.
.PP
Don't add a "localhost" entry to the hosts file if there are remote name
servers.  It makes
.B nonamed
drop back in answer mode on a flaky network.
.SH BUGS
If you can get a remote
.B nonamed
to listen to your name server then you can make it believe anything you
want to.  You need access to a machine on the same subnet of course.
.SH AUTHOR
Kees J. Bot ([e-mail])