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[[http://climate.msrc.sunysb.edu/1984/messages/347.html free sony ringtones]] [[http://brown.cla.sc.edu/projects/pas/bforum/index.php?method=showhtmllist&list=message&rollid=2,26 order levitra]] [[http://mrsrl.stanford.edu/seminar/detailview.php?event_id=313 zanaflex online]] [[http://groups.ku.edu/cgi-bin/cgiwrap/kuksa/forum/ezboard.cgi?db=general&action=read&dbf=44 but phentermine]] [[http://ocw201-1.usc.edu/iPIDD/ju_board/view.php?message_ID=12 celexa online]] Sockets are an API for [[IPC]] or network communication with a [[process]]. For IPC Unix domain sockets are used, for network communication INET sockets are preferred. INET sockets work on [[OSI]] layer 3 and above. Lower layer access is provided by [[bpf]] the Berkeley packet filter. Sockets provide [[descriptors]] to a process with which data or control data can be exchanged with the kernel.

=== Unix domain sockets ===

When a Unix domain socket is set up it is bound to the local systems filesystem. The path it can be bound to is limited to 103 characters (see ''/usr/include/sys/un.h'') instead of the filesystem limit of 1023 characters. This means that a socket should be set up close to the [[root]] perhaps in /tmp (as [[sshd]] does). Unix domain sockets make preferred [[IPC]] in [[OpenBSD]] because of the availability of the [[getpeereid]] syscall which allows a [[daemon]] to check the credentials of who is connecting to the socket. A socket in the [[filesystem]] looks like this:

$ ls -l /tmp/ssh*
total 0
srwxr-xr-x 1 pbug wheel 0 Oct 8 11:27 agent.1327

notice the 's' indicating that this file is a socket.

=== INET sockets ===

In order to communicate with the [[Internet]] a program can communicate with it via the [[Kernel]] which has a built-in [[internet stack]]. Common protocols that one can talk via sockets are [[TCP]] and [[UDP]] as well as [[ICMP]] which is grouped into the raw mode of sockets. When a program is a TCP server the common sequence of [[syscall]]s are socket(2), bind(2), listen(2), and accept(2). When a program is a TCP client the common sequence of [[syscall]]s are socket(2), connect(2). [[TCP]] and [[UDP]] sockets have [[ports]] to identify them. On a system one can use [[netstat]] to see this. In UNIX only [[root]] can bind to ports less than 1024 the rest is available for all users. This is shown here as an example:

$ id
uid=1000(pbug) gid=1000(pbug) groups=1000(pbug), 0(wheel), 5(operator)
$ nc -l 1023
nc: Permission denied
$ nc -l 1024
^C
$

When a server is listening on a certain port it's difficult to regulate who connects to this port. Early implementations messed with [[TCP Wrappers]] which allowed one to set up a simple whitelist or blacklist of who can connect. This didn't cover UDP though and it was still required that the program accepted the connection before closing it. This means that someone could stealth scan a port and know that it was listening. [[Firewall]]s allowed finer control and aren't as revealing over open ports. [[BSD]] has ipfw, ipfw2, ipf and pf as firewalls.

$ fstat | grep traceroute
pbug traceroute 18184 wd /usr 6310091 drwxr-xr-x r 2048
pbug traceroute 18184 0 / 84995 crw--w---- rw ttyp1
pbug traceroute 18184 1 / 84995 crw--w---- rw ttyp1
pbug traceroute 18184 2 / 84995 crw--w---- rw ttyp1
pbug traceroute 18184 3* internet raw icmp 0xfffffe800f85d678
pbug traceroute 18184 4* internet raw reserved 0xfffffe801bc73688

Raw sockets are restricted to the superuser ([[root]]) only. When writing to raw sockets you can manipulate the [[IP]] header which you cannot do with simple [[TCP]] or [[UDP]] sockets ([[setsockopt]] are an exception but require superuser ([[root]]) permissions). The [[traceroute]] program uses raw sockets in a UNIX system. It is possible to read from raw sockets, ie. if you specified a specific protocol you can read any packets that arrive for that protocol. In [[BSD]] and if the protocol is [[ICMP]] the ICMP types echo request (8), timestamp request (13) and address mask request (17) are not passed from the [[kernel]] to the socket. Similarely [[TCP]] and [[UDP]] protocols are not passed to a raw socket, these must use stream or dgram sockets or be read from the [[OSI]] datalink layer (see [[bpf]]).

=== Divert sockets ===

[[FreeBSD]] has a way to read/manipulate/write [[OSI]] layer 3 packets which have been diverted from the [[ipfw]] software. This type of socket is limited to [[FreeBSD]].

=== Packet sockets ===

[[Linux]] has a way to read [[OSI]] layer 2 frames by means of SO_PACKET sockets. This is limited only to Linux.

=== How do I see Internet traffic ===

A packet [[sniffer]] is a tool that allows one to see packets carried through the [[network interface]] of a host computer.

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