getsockname [ -verbose ] [ -peer ] [ fd ]
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. On the fifth and following lines it prints all of the numeric internet addresses followed by all the host names returned by the gethostbyaddr(3) library routine.
If you specify -verbose twice, the program will print a copyright notice.
I have a feeling any practical uses of the getpeername program are fairly complicated. If you actually do use it in a non-trivial way, drop me a line.
client$ hose mail.cis.ufl.edu smtp -in ./getpeername
25
128.227.224.13
You connected to mail.cis.ufl.edu on the SMTP port (port 25). For a verbose report:
aviator:80 $ ./hose mail.cis.ufl.edu smtp -in ./getpeername -v
Internet
Port
25
Host
128.227.224.13
128.227.100.196
inlet.cis.ufl.edu
Now let's give an example of a race condition which will cause getpeername to fail:
client$ hose www.cis.ufl.edu 80 -in ./getpeername
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 www.cis.ufl.edu 80 -fd0 ./getpeername -v
Internet
Port
25
Host
128.227.224.13
128.227.100.196
inlet.cis.ufl.edu
And, finally, let's extract some useful information from our socket.
client$ hose www.cis.ufl.edu 80 -fd0 sh -c " ./getpeername -v | \
tail +5 | egrep -v '^[0-9.]*$' | head -1"
sand.cis.ufl.edu
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.
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!"
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