CSE 7348 - class 10
• Timeouts on sockets.
– Call ALARM which generates the SIGALRM signal when the
specified time has expired..
– Block wait for I/O in select (time limit built in).
– Use the SO_RCVTIMEO or SO_SNDTIMEO socket options.
– All three work with I/O operations (read or write, recvfrom, sendto).
– Need a technique to work with connect.
– Establish the signal handler for SIGALRM.
Sigfunc = Signal (SIGALRM, connect_alarm);
if (alarm(nsec != 0)
connect(…..)
// sets the alarm clock for the process.
Then
CSE 7348 - class 10
• Point of this function is that if the connection hangs the alarm
will fire and the handler will take over.
• Note that the handler restores any other handlers.
• NB: the timeout value can be reduced but it cannot be extended
on a BSD kernel.
• Comments on data queuing (page 365).
• How does a user determine how much data is queued without
reading the data?
• Use the recv function with the MSG_PEEK flag. This is one of
the flags discussed earlier. This return is the number of bytes in
the buffer.
CSE 7348 - class 10
• Be wary that the number of bytes available in the buffer can
change rapidly.
• Some implementations support FIONREAD command of ioctl.
This uses a value-result pointer to return the number of bytes in
sockets receive queue.
• Sockets and standard I/O.
• The reads and writes use descriptors and are effectively system
calls to the Unix kernel.
• It is possible to use the standard I/O library of ANSI C. This is
handy in that buffering of the stream is done automatically.
• The standard I/O library can be used with sockets with some
caveats
CSE 7348 - class 10
• A standard I/O stream can be created from any Descriptor by
calling the fdopen function.
• Or given a standard I/O stream the corresponding descriptor can
be determined by calling fileno. (to use select must convert the
standard I/O stream to a descriptor).
• Standard I/O streams can be made full duplex by opening the
stream with an r+.
– One caveat; an output func cannot be followed by an input func
without a call to fflush, fseek, fsetpos, rewind.
– Neither can an input func be followed by an output func without an
intervening call to fseek, fsetpos, or rewind.
– The problem with these last three is that they all call lseek which
fails on a socket.
CSE 7348 - class 10
• There are three types of buffering performed by the standard I/O
library:
– Fully buffered means that the I/O takes place only when the buffer
is full. (~ 8192 bytes).
– Line buffered (most common) I/O takes place on 0Ah, 0Dh. (LF,
CR).
– Unbuffered where I/O takes place whenever a call is made.
– Most Unix implementations employ the following paradigms.
– Stderr is always unbuffered
– stdin and stdout are always fully buffered (unless a terminal in
which case they are line buffered)
– All other streams are fully buffered (unless a terminal which is line
buffered).
CSE 7348 - class 10
• Unix domain protocols; client server communication on a single
host using the TCP API (socket API).
•
This comes from the need for inter-process communication on a
single host; why not use the TCP API?
• Other schemes are ‘shared memory’, proprietary comm
protocols, polling schemes and a host of other difficult to
understand and pointless to reproduce techniques.
• This leads of course to the basis of distributed computing; the
distribution has to begin with processes on a single platform
able to communicate.
CSE 7348 - class 10
• Two types of sockets in the Unix Domain; stream sockets
(similar to TCP) and datagram sockets (similar to UDP).
– Raw sockets are available but are discouraged
• Unix domain sockets are an alternative to ICP methods (volume
2 starting next week).
• Unix domain sockets are often twice as fast as a TCP socket
(intra host comm). (X windows opens a window it checks the
server’s host name, window and screen; if the same host then it
opens a Unix domain stream to the server).
• There is enhanced security on the newer Unix implementations
to prevent inter-process hacking on a single host.
CSE 7348 - class 10
• Unix domain sockets use a socket address structure.
– The protocol family is AF_LOCAL
– the pathname is stored
struct sockaddr_+un {
uint8_t
sun_len
sa_family_t sun_family;
char
sun_path[104];
// AF_LOCAL
// must be null terminated
The path name is of the form /home / surdenau / fake
The example on page 375 uses the command line argument to get
a pathname to be used in the bind.
CSE 7348 - class 10
• The pathname used must have appropriate file access
permissions set. (turn off world write bit or
system(“o-w”); emply umask per the example.
Ð The default file access should be ugo+rwx then modified by the
umask value.
Ð The pathname must be absolute not relative. No links.
Ð The pathname specified in a call to select must be a pathname
that is currently bound to an open Unix domain socket of the
same type (stream or datagram).
Ð The permission testing is the same as that for a write to a
pathname.
CSE 7348 - class 10
• Unix domain sockets are similar to TCP sockets in that they
provide a stream interface to the process with no record
boundaries.
• If a call to connect for a Unix domain stream socket finds the
listening buffer full, ECONNREFUSED is returned immediately.
• Unix domain sockets are effectively unreliable.
• Sending a datagram on an unbound Unix domain datagram
socket does NOT bind a pathname to the socket.
– This means that the user will be unable to send a reply since he
has no way of knowing from whence the incoming data
CSE 7348 - class 9
• Daemon processes and the inetd superserver. (ch 12).
• A daemon (pronounced demon) is a process that runs in the
background and is independent of control from all terminals.
• Daemons are typically used for various and sundry
administrative tasks in the Unix world.
• Daemon are typically started by the system admin scripts;
(/etc/rc). These daemons have superuser privileges.
– The inetd superserver, a Web server and a mail server will typically
be started by these initialization scripts.
– I.e. daemon is from the Latin daimon which was built on a similar
Greek word. A tutelary deity or spirit.
CSE 7348 - class 9
• Many network servers are started by the inetd superserver.
• Inetd listens for network requests (HTTP, etc) and invokes the
actual server.
• cron is a daemon; programs it invokes do NOT run as daemons
(page 332).
• Daemons can be started from user terminals, but will not display
error messages there or be interrupted by signals from therein.
• Since there must be a way for daemons to output messages the
syslog function is used by the syslogd daemon to intercept
these messages.
CSE 7348 - class 9
• syslogd daemon
• Usually started from the initialization scripts, this daemon reads
from the / etc / syslog.conf file which specifies what to do with
each type of log message to be received.
• In addition a Unix domain socket is created and bound to the
path / var / run / log.
• A UDP socket is created and bound to port 514.
• The path / dev / klog is opened and any error messages from
the kernel appear as input on this device.
• Syslogd runs in an infinite loop using select on the descriptors in
the previous steps to be readable.
CSE 7348 - class 9
• Now for something interesting:
• The individual whom Alan Turing came to visit in 1942, on orders
from the British Foreign Office, where Mr. Turing was employed
as a code breaker, was John VonNuemann.
• Mr. VonNuemann was a legendary gambler, raconteur and world
class mathematician at the Princeton Institute for Advanced
Studies. Mr. VonNuemann’s wit and humor were so special as
to be noted in all remembrances of him.
• Mr. VonNuemann, born in Hungary, is widely accounted as the
inventor of the ‘data flow’ architecture used by most modern
computing machines.
CSE 7348 - class 9
• Exactly what Mr. VonNuemann and Mr. Turing spent their six
months on is not certain.
• It is known that shortly after their visit, Mr. Turing returned to
Bechley Park where he devised systems of computation which
allowed the breaking of the German ‘Enigma’ code.
• After the war, both Mr. Turing and Mr. Von Nuemann designed
computational devices. Both of which were far beyond the
ability of the electronics industry to realize.
• Turing eventually committed suicide, hence the bite (byte) out of
the apple.
CSE 7348 - class 9
• syslog function
• The common technique for logging messages from a terminal is
to call the syslog function
void syslog ( int priority, const char *message…..)
• The priority arg is a combination of a level and a facility.
• The message is similar to a format string for printf with the
addition of the %m specification and the error message
corresponding to the current value of errno.
• Levels vary from 0 to 7 with 0 being the highest priority.
CSE 7348 - class 9
• The facility of log messages is used to identify the type of process
sending the message (LOG_USER is the default).
• An example of a facility is a FTP daemon, or the mail system, or
the cron daemon.
• The purpose of the facility and level is to allow all messages from
a given facility or level to be handled the same.
• As an example a config file will contain the following;
kern.*
local7.debug
/dev/console
/var/log/cisco.log
Which specifies that all kernel messages get logged to the console and
all debug messages from the local7 facility get appended to the file
/var/log/cisco.log.
CSE 7348 - class 9
• When an application calls syslog the first time a Unix domain
datagram socket is created. Then connect is invoked to the well
known pathname of the socket created by the syslogd daemon
(var / run / log).
• This socket remains open until the application terminates.
• As an alternative the openlog and closelog functions can be used.
void openlog (const char *ident, int options, int facility);
void closelog (void);
• ident is a string that is prepended to each log message by syslog.
• options argument is the || of one or more constants (figure 12.3).
• Normally a Unix domain socket is NOT created on a call to
openlog.
CSE 7348 - class 9
• The facility argument of openlog specs a default facility for any
subsequent calls to syslog that do not spec a facility.
• Most daemons call syslog and then only specify a level for any
further logging.
• daemon_init function: an example of how to daemonize a
process.
• Call fork and then parent terminates while child continues. Therefore
child is in background while inheriting parent process group ID (but
maintaining own process ID).
• setsid (posix.1 function) creates a new session; the process now
becomes the session leader and the group leader of a new process
group with no controlling terminal.
• Ignore the SIGHUP signal and fork again. Now the second child is
running. This guarantees that the second child is no longer the
session leader so it cannot acquire a controlling terminal.
CSE 7348 - class 9
• daemon_init (continued)
• set the global daemon, daemon_proc nonzero ( daemon_proc = 1; )
When this value is nonzero it forces a call to syslog instead of stderr.
• Change the working directory to root (generally the working directory
particularly for core files). (if a daemon is in a file system that file
system cannot be unmounted).
• Close all inherited open descriptors. Since there is no limit to the
number of descriptors (or the limit is 32k or 64k) the standard approach
is to close the first 64 descriptors.
• Some daemons open / dev / null for reading and writing and
duplicate the descriptor for stdin, stdout, stderr. Therefore any
library function can read / write without failure.
CSE 7348 - class 9
• daemon_init (continued)
• Call openlog. Specify that the process ID should be appended to
all log messages.
• NB: A daemon should never receive the SIGHUP signal; also the
SIGINT and SIGWINCH signals. Therefore these signals can be
used to notify the daemon that something has changed (such as
the config file).
• Page 338, Steven’s sets up the daytime server as a daemon. An
excellent exercise and a really great test question.
CSE 7348 - class 9
• inetd daemon
• In a typical Unix system the servers are all daemonized and
started at boot.
• All daemons contain nearly the same startup code.
• Each daemon takes a slot in the process paper although it is asleep
most of the time.
• 4.3BSD solved this problem by providing an Internet superserver,
the inetd daemon.
• This daemon can be used by any server that employs TCP or
UDP. It only handles Unix domain sockets.
• Simplifies writing daemon processes - most startup details or
handled by inetd (no longer need to call daemon_init).
• Allows a single process to be waiting for incoming client requests for
multiple services, instead of a process for each service.
CSE 7348 - class 9
• inetd uses the same techniques to establish itself as daemon_init.
• Configuration file is typically in / etc / inetd.conf…..
• The config file specifies the services that the superserver is to handle
and how to handle a service request.
• Sample line; ftp
stream tcp nowait root / usr / bin / ftpd ftpd -1
• Name of the server is always passed as the first arg to a program
when it is exec’ed.
• The names of the fields are service name, socket type, protocol,
wait-flag, login-name, server-program, server-program-arguments.
CSE 7348 - class 9
[/etc]%cat inetd.conf | more
#
#ident "@(#)inetd.conf 1.22 95/07/14 SMI" /* SVr4.0 1.5 */
#
#
# Configuration file for inetd(1M). See inetd.conf(4).
#
# To re-configure the running inetd process, edit this file, then
# send the inetd process a SIGHUP.
#
# Syntax for socket-based Internet services:
# <service_name> <socket_type> <proto> <flags> <user> <server_pathname>
<args>
#
# Syntax for TLI-based Internet services:
#
# <service_name> tli <proto> <flags> <user> <server_pathname> <args>
#
• # Ftp and telnet are standard Internet services.
• #
CSE 7348 - class 9
ftp stream tcp nowait root /usr/sbin/in.ftpd
in.ftpd
telnet stream tcp nowait root /usr/sbin/in.telnetd in.telnetd
#
# Tnamed serves the obsolete IEN-116 name server protocol.
name dgram udp wait root /usr/sbin/in.tnamed in.tnamed
# Shell, login, exec, comsat and talk are BSD protocols.
#
shell stream tcp nowait root /usr/sbin/in.rshd
in.rshd
login stream tcp nowait root /usr/sbin/in.rlogind in.rlogind
exec stream tcp nowait root /usr/sbin/in.rexecd in.rexecd
comsat dgram udp wait root /usr/sbin/in.comsat in.comsat
talk dgram udp wait root /usr/sbin/in.talkd
in.talkd
#
# Must run as root (to read /etc/shadow); "-n" turns off logging in utmp/wtmp.
#
uucp stream tcp nowait root /usr/sbin/in.uucpd
in.uucpd
#
# Tftp service is provided primarily for booting. Most sites run this
# only on machines acting as "boot servers."
CSE 7348 - class 9
• For each service listed in the inetd.conf file, the inetd daemon will
socket, bind, listen (TCP socket).
• The maximum number of servers that inetd can handle depends on
the max number of descriptors that inetd can create. Each new
socket is added to the select descriptor set used by inetd.
• For TCP sockets, listen is called (not done for datagram
sockets).
• After all sockets are created, select is used to wait for a socket to
become readable. inetd spends most of its time blocked on this
call.
• When select returns (socket is readable) accept is called to make
the new connection.
CSE 7348 - class 9
• The inetd daemon forks and the child process handles the service
request. (concurrent server model)
• child closes all descriptors other than the socket descriptor that it is
handling; the new connected socket for TCP.
• The child calls dup three times thereby assigning the socket to
descriptors 0, 1, and 2 (stdin, stdout, stderr).
• The original socket descriptor is then closed; this means that the
child is reading from stdin, writing to stdout or stderr.
• The child calls getpwnam to get the password entry for the loginname specified in the config file.
• If this field is NOT root, then the child becomes the specified user by
executing the setgid and setuid function calls. Inetd is executing
with a user ID of 0 the child process inherits the user ID across the
fork so it is able to become any user it desires.
CSE 7348 - class 9
• The child now does an exec on the appropriate server-program to
handle the request, passing all args delineated in the
configuration file.
• If the socket is a stream socket, the parent must close the
connected socket. The parent loops back to select again waiting
for another socket to become readable.
• The descriptor handling in inetd deserves some attention.
• If inetd directs a connection request to port 21, the child will have a
descriptor for port 21.
• Once the child dups the connected socket to descriptors 0, 1, and 2
the child closes the connected socket.
• This means the client is now connected to fd 0, fd 1, fd 2
CSE 7348 - class 9
• Once the child calls exec all descriptors remain open across the
call so the server uses descriptors 0, 1, and 2 to comm with the
client.
• On page 344 there is a very fine example of the daemon_inetd
function. Review in detail along with Figure 12.12.
• In the Daytime server of 12.12, all of the socket creation code has
been removed; instead we have
daemon_inetd ( argv[0], 0 );
The TCP connection uses descriptor 0.
The infinite for loop is gone (invoked once per client connection).
Call getpeername with descriptor 0 as a way to determine client
protocol address.
Use MAXSOCKADDR since we do not know the size of the socket
address structure.
CSE 7348 - class 9
• What has been done with inetd is actually rather simple; inetd has
become the concurrent server for any and all TCP connections.
• The same concurrent parent/fork child model is still being used
(design pattern??). The model has simply been moved to a
higher level of abstraction and all of the various servers have
been serviced as exec’s within its child processes.
• Most network services are run as daemons (in the background,
independent of terminal control).
• The output from a daemon is usually handled by the syslogd
daemon via the syslog function.
CSE 7348 - class 9
• To daemonize a program;
•
•
•
•
•
•
fork to get in background
call setsid to create a new session and become session leader
fork again to avoid obtaining a new controlling terminal
change the working directory appropriately.
Change the file mode creation mask.
Close all unneeded files.
• Assignment: Problem 12.1. Turn in by next class in hard format.
CSE 7348 - class 9
• Some thoughts on software process.
• In 1908 when Igor Stravinsky debuted ‘le Sacre du Printemps’
rioting broke out at the Paris Opera house.
• Classical music fans are usually not given to fisticuffs.
• The problem is that music and musical beliefs are passionately
held. Stravinsky with his dissonance and revolutionary approach
to formal music upset a lot of people while wildly pleasing others.
He of course changed the course of music in the 20th century.
Stravinsky is why we have Jazz and R&R.
CSE 7348 - class 9
• Similarly with software processes we have zealots and religiously
held beliefs.
• Like all deeply held beliefs they are held on the basis of faith; faith
and faith alone (not unlike anything believed by anybody).
• There is NO demonstrable, repeatable proof or data that any
software process, methodology, language or approach is any
more beneficial than any other (at least in economic terms).
• Which is NOT to say that process, methodology or language are
all interchangeable and without value. Quite to the contrary,
process and methodology will have an economic impact on the
viability of any product; but sales and marketing will have a much
larger impact in many instances.
CSE 7348 - class 9
• So what is the point?
• Temper your beliefs not on the presentations of zealots or other
believers in process X or methodology Y.
• Instead perform as a scientist and engineer and look for the data
which will allow you to accurately quantify the changes wrought
by the adaptation of a method, a process or a language.
• Remember that the software industry is most like the fashion
industry; a large number of customers easily lead by what ever
the couture houses of Paris (or Silicon Valley) are currently
espousing.
• The situation is made frenzied by the billions of dollars at stake.
CSE 7348 - class 9
• And in one final bash of the so-called ‘cleanroom’.
• Even a Level 1 clean room (less than I particle per cubic yard) is
NOT defect free.
• No non-trivial system can EVER be fully or accurately specified.
• Given the provability of the above, and the outlandish vagaries of
the English language, how can a set of requirements, written in
English EVER be the basis for an accurate design?
• The problem is how do we successfully take a set of flawed
requirements, and translate them into a meaningful and accurate
notation in the solution space? This translation must incorporate
an ability to accommodate CHANGE. Which cleanroom really
does not provide.
Class 10
• Frequently, services are performed by so-called
daemons. A daemon is a program that opens a
certain port, and waits for incoming connections. If
one occurs, it creates a child process which accepts
the connection, while the parent continues to listen
for further requests.
• This concept has the drawback that for every service
offered, a daemon has to run that listens on the port
for a connection to occur, which generally means a
waste of system resources like swap space.
• Thus, almost all installations run a super-server
that creates sockets for a number of services, and
listens on all of them simultaneously using the
select(2) system call. When a remote host requests
one of the services, the super-server notices this
and spawns the server specified for this port.
Class 10
• The super-server commonly used is inetd, the
Internet Daemon. It is started at system boot time,
and takes the list of services it is to manage from a
startup file named /etc/inetd.conf. In addition to
those servers invoked, there are a number of trivial
services which are performed by inetd itself called
internal services. They include chargen which simply
generates a string of characters, and daytime which
returns the system's idea of the time of day.
Class 10
• Why should you run the HTTP server standalone as opposed to
running it from inetd? If your server has a lot of CPU cycles to
burn, then there is nothing wrong with running HTTPd from
inetd.
• Most servers don't. If you run the server from inetd, that
means that whenever a request comes in, inetd must first fork
a new process, then load the HTTPd binary. Once the HTTPd
binary is loaded, HTTPd must then load and parse all of its
configuration files (including httpd.conf, access.conf, and
mime.types), which is quite a task to be done for each and
every request that comes in.
• Now, contrast this with running standalone. When HTTPd gets a
request, it makes a copy of itself (which requires no loading of
a binary since shared text pages are used), and the copy
handles the request. The configuration files have already been
loaded on startup, and so we don't reload them every time.
• In fact, with NCSA HTTPd 1.4 and 1.5, it doesn't even have to
make another copy of itself if there is an extra server already
running. The parent will just pass off the request to the free
child.
Class 10
• Common Services
• telnet: allows remote users to connect to a host via telnet.
Since user passwords are transmitted over the wire in plain
text and can therefore be easily sniffed, we recommend
disabling the service and using instead a secure terminal
access mechanism which encrypts the entire session (such as
kerberized telnet or SSH). If telnet access is a must, the
service must be wrapped.
•
• ftp: allows remote users to transfer files to/from a host using
ftp. Since user passwords are transmitted over the wire in plain
text and can therefore be easily sniffed, we recommend
disabling the service and using instead a secure file transfer
mechanism which encrypts the entire session (SSH). Another
solution is to install AFS, thus eliminating the need for an ftp
daemon. If ftpd access is a must, the service must be wrapped.
Class 10
• Common Services (cont)
• shell: allows remote users to run arbitrary commands on a
host via the Berkeley rsh utility (via the trusted hosts
mechanism using the .rhosts file). Highly insecure. Instead,
run the leland kerberalized version of rsh.
• login: allows remote users to use the Berkeley rlogin utility to
log in to a host without supplying a password (via the trusted
hosts mechanism using the .rhosts file). Highly insecure.
Instead, use the leland kerberalized version of rlogin.
• exec: allow remote users to execute commands on a host
without logging in. Exposes remote user passwords on the
network, thus Highly insecure. Instead, use the leland
kerberalized version of rexec.
Class 10
• popper: allows remote users to use POP3 to retrieve their
email from the server. Highly insecure. We recommend that
you use the leland kerberalized version of popper if you have to
run a popper service. Note: If you want to run the kerberalized
popper server, you will need another srvtab specifically for that
service in order for certain mail readers to work. Please contact
srvtab-request@leland for the srvtab.pop srvtab.
• comsat: used for incoming mail notification via biff and is
largely unecessary. We recommend disabling the service.
• finger: allows remote users to use the finger utility to obtain
information about arbitrary users on a host. Highly insecure.
We recommend disabling the service or using a more secure
version such as cfinger.
• talk: allows remote users to use talk to have a real time
conversation with a user on a host. Considered light security
hazard.
Class 10
• tftp: allows remote users to transfer files from a host without
requiring login. Used primarily by X-terminals and routers.
Considered highly insecure. We recommend disabling the
service. If tftp access is needed (for bootp clients), we
recommend that the "-s" option be used and that the service
be wrapped.
• uucp: allows remote users to transfer files to/from using the
UUCP protocol. I can't honestly remember anyone needing this
service who had it enabled. We recommend disabling it and
disabling setuid on any uucp binaries.
• time: Used for clock synchronization. We recommend disabling
the service and using xntp to synchronize your system clock to
stanford.
• echo, daytime, discard, chargen: These services are used
largely for testing and are largely unnecessary. If they are
enabled to use UDP, they can be used for a Denial of Service
attack. We recommend disabling at least the UDP version of
them.
Class 10
• bootp, bootps: used for bootp services. We recommend
disabling it unless you are running a bootp server.
• systat: designed to provide status information about a host.
Considered a potential secucrity hazard. We recommend
disabling the service.
• netstat: designed to provide network status information about
a host to remote hosts. Considered a potential secucrity
hazard. We recommend disabling the service.
• Services based on RPC: RPC based services are used by
protocpls such as NFS and NIS. These protocols are a known
security hazard. our recommendation is against using NIS or
NFS. Kerberos and AFS provide solutions to all the same
problems without the security hazard.
Class 10
• Configuring inetd at home
• Start with an inetd.conf file that is completely commented
out. Then add only the services that you need.
• Services that you probably want are: (taken from a stock
redhat 4.0 machine)
telnet stream tcp nowait root /usr/sbin/tcpd in.telnetd
ftp stream tcp nowait root /usr/sbin/tcpd in.ftpd -l -a
• Things that open your machine up for .rhosts attacks:
shell stream tcp nowait root /usr/sbin/tcpd in.rshd
login stream tcp nowait root /usr/sbin/tcpd in.rlogind
• Things that have no reason running on your machine
gopher stream tcp nowait root /usr/sbin/tcpd gn
Class 10
• First, inetd is dying because it receives a SIGPIPE when it
tries to write to the socket returned by accept since it does
not install a signal handler for it. To fix install a signal
handler for SIGPIPE.
• Now you may be wondering why does a write to the socket
returned by accept() generates a SIGPIPE. This bring us to
the second issue. It seems that at least under Linux 2.0.X
accept() will return a socket in the received queue if it is not
in the SYN_SENT or SYN_RECV state, even when it has
not gone through the ESTABLISHED state. By doing a
stealth scan on the port the socket goes from the
SYN_RECV state to the CLOSED state. When you try to
read from such a socket you get a SIGPIPE. The sematics
of Linux's accept() seems to be non-standard. I wonder what
else breaks by not handling SIGPIPE.
Class 11
– ICMP Protocol Overview
• Internet Control Message Protocol (ICMP), documented in RFC 792, is
a required protocol tightly integrated with IP. ICMP messages,
delivered in IP packets, are used for out-of-band messages related to
network operation or mis-operation. Of course, since ICMP uses IP,
ICMP packet delivery is unreliable, so hosts can't count on receiving
ICMP packets for any network problem. Some of ICMP's functions are
to:

Announce network errors, such as a host or entire portion of the
network being unreachable, due to some type of failure. A TCP or UDP
packet directed at a port number with no receiver attached is also
reported via ICMP.

Announce network congestion. When a router begins buffering too
many packets, due to an inability to transmit them as fast as they are
being received, it will generate ICMP Source Quench messages.
Directed at the sender, these messages should cause the rate of
packet transmission to be slowed. Of course, generating too many
Source Quench messages would cause even more network
congestion, so they are used sparingly.
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
ICMP Overview

Assist Troubleshooting. ICMP supports an Echo function, which just
sends a packet on a round--trip between two hosts. Ping, a common
network management tool, is based on this feature. Ping will transmit
a series of packets, measuring average round--trip times and
computing loss percentages.

Announce Timeouts. If an IP packet's TTL field drops to zero, the
router discarding the packet will often generate an ICMP packet
announcing this fact. TraceRoute is a tool which maps network routes
by sending packets with small TTL values and watching the ICMP
timeout announcements.
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– IGMP Protocol Overview
• Internet Group Management Protocol (IGMP), documented in
Appendix I of RFC 1112, allows Internet hosts to participate in
multicasting. RFC 1112 describes the basic of multicasting IP
traffic, including the format of multicast IP addresses, multicast
Ethernet encapsulation, and the concept of a host group.
• A host group is the set of hosts interested in traffic for a
particular multicast address. Important multicast addresses are
documented in the most recent Assigned Numbers RFC,
currently RFC 1700. IGMP allows a router to determine which
host groups have members on a given network segment.
• The exchange of multicast packets between routers is not
addressed by IGMP.
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Network Working Group
Smoot Carl-Mitchell
Request for Comments: 1027
Texas Internet Consulting John S. Quarterman Texas Internet
Consulting October 1987
Using ARP to Implement Transparent Subnet Gateways
Status of this Memo
• This RFC describes the use of the Ethernet Address Resolution
Protocol (ARP) by subnet gateways to permit hosts on the
connected subnets to communicate without being aware of the
existence of subnets, using the technique of "Proxy ARP" [6]. It
is based on RFC-950 [1], RFC-922 [2], and RFC-826 [3] and is
a restricted subset of the mechanism of RFC-925 [4].
Distribution of this memo is unlimited. Acknowledgment The
work described in this memo was performed while the authors
were employed by the Computer Sciences Department of the
University of Texas at Austin.
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• Introduction The purpose of this memo is to describe in detail
the implementation of transparent subnet ARP gateways using
the technique of Proxy ARP. The intent is to document this
widely used technique.
• 1. Motivation
• The Ethernet at the University of Texas at Austin is a large
installation connecting over ten buildings. It currently has more
than one hundred hosts connected to it [5]. The size of the
Ethernet and the amount of traffic it handles prohibit tying it
together by use of repeaters. The use of subnets provided an
attractive alternative for separating the network into smaller
distinct units. This is exactly the situation for which Internet
subnets as described in RFC-950 are intended. Unfortunately,
many vendors had not yet implemented subnets, and it was
not practical to modify the more than half a dozen different
operating systems running on hosts on the local networks.
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• Therefore a method for hiding the existence of subnets from
hosts was highly desirable. Since all the local area networks
supported ARP, an ARP-based method (commonly known as
"Proxy ARP" or the "ARP hack") was chosen. In this memo,
whenever the term "subnet" occurs the "RFC-950 subnet
method" is assumed.
• 2. Design
• 2.1 Basic method
• On a network that supports ARP, when host A (the source)
broadcasts an ARP request for the network address
corresponding to the IP address of host B (the target), host B
will recognize the IP address as its own and will send a pointto-point ARP reply. Host A keeps the IP-to-network-address
mapping found in the reply in a local cache and uses it for later
communication with host B.
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• If hosts A and B are on different physical networks, host B will
not receive the ARP broadcast request from host A and cannot
respond to it. However, if the physical network of host A is
connected by a gateway to the physical network of host B, the
gateway will see the ARP request from host A.
• Assuming that subnet numbers are made to correspond to
physical networks, the gateway can also tell that the request is
for a host that is on a different physical network from the
requesting host. The gateway can then respond for host B,
saying that the network address for host B is that of the
gateway itself. Host A will see this reply, cache it, and send
future IP packets for host B to the gateway. The gateway will
forward such packets to host B by the usual IP routing
mechanisms. The gateway is acting as an agent for host B,
which is why this technique is called "Proxy ARP"; we will refer
to this as a transparent subnet gateway or ARP subnet
gateway.
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• When host B replies to traffic from host A, the same algorithm
happens in reverse: the gateway connected to the network of
host B answers the request for the network address of host A,
and host B then sends IP packets for host A to gateway. The
physical networks of host A and B need not be connected to
the same gateway. All that is necessary is that the networks be
reachable from the gateway.
• With this approach, all ARP subnet handling is done in the ARP
subnet gateways. No changes to the normal ARP protocol or
routing need to be made to the source and target hosts. From
the host point of view, there are no subnets, and their physical
networks are simply one big IP network. If a host has an
implementation of subnets, its network masks must be set to
cover only the IP network number, excluding the subnet bits,
for the system to work properly.
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• 2.2 Routing As part of the implementation of subnets, it is
expected that the elements of routing tables will include
network numbers including both the IP network number and
the subnet bits, as specified by the subnet mask, where
appropriate.
• When an ARP request is seen, the ARP subnet gateway can
determine whether it knows a route to the target host by
looking in the ordinary routing table.
• If attempts to reach foreign IP networks are eliminated early
(see Sanity Checks below), only a request for an address on
the local IP network will reach this point. We will assume that
the same network mask applies to every subnet of the same IP
network. The network mask of the network interface on which
the ARP request arrived can then be applied to the target IP
address to produce the network part to be looked up in the
routing table.
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• In 4.3BSD (and probably in other operating systems), a default
route is possible. This default route specifies an address to
forward a packet to when no other route is found. The default
route must not be used when checking for a route to the target
host of an ARP request. If the default route were used, the
check would always succeed. But the host specified by the
default route is unlikely to know about subnet routing (since it
is usually an Internet gateway), and thus packets sent to it will
probably be lost. This special case in the routing lookup
method is the only implementation change needed to the
routing mechanism.
• If the network interfaces on which the request was received
and through which the route to the target passes are the same,
the gateway must not reply.
• WHY?????
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• 2.3 Multiple gateways
• The simplest subnet organization to administer is a tree
structure, which cannot have loops. However, it may be
desirable for reliability or traffic accommodation to have more
than one gateway (or path) between two physical networks.
ARP subnet gateways may be used in such a situation: a
requesting host will use the first ARP response it receives, even
if more than one gateway supplies one.
• This may even provide a rudimentary load balancing service,
since if two gateways are otherwise similar, the one most
lightly loaded is the more likely to reply first.
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• 2.4 Sanity checks Care must be taken by the network and
gateway administrators to keep the network masks the same
on all the subnet gateway machines. The most common error
is to set the network mask on a host without a subnet
implementation to include the subnet number. This causes the
host to fail to attempt to send packets to hosts not on its local
subnet. Adjusting its routing tables will not help, since it will
not know how to route to subnets.
• If the IP networks of the source and target hosts of an ARP
request are different, an ARP subnet gateway implementation
should not reply. This is to prevent the ARP subnet gateway
from being used to reach foreign IP networks and thus possibly
bypass security checks provided by IP gateways.