Ubuntu Manpage: getpeername - get information about this or that end of the socket's connection

NAME

       getpeername - get information about this or that end of the socket's connection

       netpipes 4.2

SYNOPSIS

       getpeername [ -verbose ] [ -sock ] [ fd ]

       getsockname [ -verbose ] [ -peer ] [ fd ]

DESCRIPTION

This is not the manual page for the getpeername system call. That manual page is in section 2. You can
access it using a command like "man 2 getpeername" or "man -s 2 getpeername". I apologize for the
confusion.

getpeername performs a getpeername(2) system call on one of its file descriptors specified by fd and
prints out the results. The default fd is 0 (stdin). You may cause getpeername to behave like
getsockname by providing the -sock argument.

getsockname performs a getsockname(2) system call on one of its file descriptors specified by fd and
prints out the results. The default fd is 0 (stdin). You may cause getsockname to behave like
getpeername by providing the -peer argument.

There is a severe limitation of getpeername. If the remote process has closed the connection,
getpeername will fail with a `Socket is not connected' error. This will happen with dismaying frequency
when the remote process is not dependent upon the local process for input and it is only sending small
amounts of output before closing the connection. Hopefully the practical uses of getpeername (if there
are any) will not exercise this problem.

You can use getpeername to find out the address of the opposite end of a socket. You can use getsockname
to find out the address of the local end of a socket. They are in fact the same program with different
names. We will refer to both of them by the name getpeername in the following description.

getpeername knows how to display peer information about UNIX and Internet sockets. If you try to use it
on another type of socket, it will fail with an "unknown address family" error. If you regularly deal
with strange sockets and wish getpeername to work with them, send me email.

If the socket is a UNIX domain socket, then getpeername prints the name of the file (which is the port)
on a single line. If -verbose was specified, getpeername prints a more detailed report consisting of the
word `Unix' on the first line, the word `Port' on the second line, and the name of the file on the third
line.

If the socket is an Internet socket, then getpeername prints the port number on the first line and the
numeric address on the second line. If -verbose was specified, getpeername prints a more detailed report
consisting of the word `Internet' on the first line, the word `Port' on the second line, the port numer
on the third line, the word `Host' on the fourth line. Starting on the fifth line it prints all the
numeric internet addresses returned by the gethostbyaddr(3) library routine. On the rest of the lines it
prints all the host names.

EASTER EGG

       If you specify -verbose twice, the program will print a copyright notice.

EXAMPLES

       I originally designed getpeername so that a faucet-spawned shell script could find out who was talking to
       it  (and  maybe  perform  access  control).   I  added  getsockname for completeness.  Now I realize that
       getsockname is useful for multi-homing services.  However, most software  that  you  want  to  understand
       multi-homing  (httpd,  ftpd)  is  already  capable  of  doing it, and much more efficiently than a script
       wrapper.  Still, it might come in handy some day.

       client$ hose mail.cise.ufl.edu smtp --in ./getpeername
       25
       128.227.205.210

       You connected to mail.cis.ufl.edu on the SMTP port (port 25).  For a verbose report:

       client$ hose mail.cise.ufl.edu smtp --in ./getpeername -v
       Internet
       Port
       25
       Host
       128.227.205.210
       fireant.cise.ufl.edu

       Now let's give an example of a race condition which will cause getpeername to fail:

       client$ hose web.cise.ufl.edu 80 -in ./getpeername
       ./getpeername: getpeername failed on descriptor 0: Socket is not connected

       The HTTP daemon tries to read a request, finds that half of the full duplex  connection  closed  (by  the
       special  behavior of the -in option on hose(1)) and drops the connection before getpeername can query the
       file descriptor.  We can cause the HTTP daemon to wait for us  by  leaving  both  halves  of  the  duplex
       connection open.

       client$ hose web.cise.ufl.edu 80 -fd0 ./getpeername -v
       Internet
       Port
       80
       Host
       128.227.205.206
       flood.cise.ufl.edu

       And, finally, let's extract some useful information from our socket.

       client$ hose web.cise.ufl.edu 80 -fd0 sh -c " ./getpeername -v | \
            tail +5 | egrep -v '^[0-9.]*$' | head -1"
       flood.cise.ufl.edu

ERRORS

       Socket  operation  on  non-socket  The fd you specified does not refer to a socket, or refers to a socket
       that has been closed.  This happens when you run getpeername by itself (it is unlikely that  any  of  the
       file  descriptors  attached  to  an  interactive  shell  are  actually  sockets),  or if you goof up your
       faucet/hose command and forgot to dup(2) one of your descriptors, or if the  remote  machine  manages  to
       close the connection before getpeername could run.

       Bad file number You gave it a bad file number for fd.  If you have enough skill to actually generate this
       error, you probably know what is wrong.

       If you encounter any other errors, clue me in.

BUGS

       These programs are vulnerable to reverse DNS lookup spoofing.  You probably want to add ``nospoof on'' to
       your /etc/host.conf.

NOTES

       Just avoid doing anything funky like passing getpeername strings and it should serve you well.

       DOH!   3.0  didn't  use  the  ntohs  macro  on  the port numbers so the output was bogus on machines with
       non-network-order port numbers (like Linux-i386).  3.1 fixed this.

CREDITS

       "Hi Mom! Hi Dad!"

COPYRIGHT

       Copyright (C) 1995-98 Robert Forsman

       This program is free software; you can redistribute it and/or modify  it  under  the  terms  of  the  GNU
       General  Public License as published by the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.

       This program is distributed in the hope that it will be useful, but WITHOUT ANY  WARRANTY;  without  even
       the  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public
       License for more details.

       You should have received a copy of the GNU General Public License along with this program; if not,  write
       to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

AUTHOR

       Robert Forsman
        thoth@purplefrog.com
        Purple Frog Software
        http://web.purplefrog.com/~thoth/

                                                 March 18, 1998                                   GETPEERNAME(1)