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

netpipes 4.2


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

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


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.


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


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 smtp --in ./getpeername

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

client$ hose smtp --in ./getpeername -v

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

client$ hose 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 80 -fd0 ./getpeername -v

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

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


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.


netpipes (1), faucet (1), hose (1), sockdown (1), socket (2), shutdown (2),


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


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.


"Hi Mom! Hi Dad!"


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.


Robert Forsman
Purple Frog Software