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.
SEE ALSO
netpipes (1), faucet (1), hose (1), sockdown (1), socket (2), shutdown
(2),
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)