TCP and UDP Ports
TCP and UDP Ports
1. The TCP part of TCP/IP stands for Transmission Control Protocol,
and it is a reliable transport-oriented way for information to be
communicated.
2. User Datagram Protocol (UDP) is an unreliable transport protocol
that works well with programs that don't rely on the protocol to
make sure their payload gets where it's going.
3. Both TCP and UDP use ports to keep track of communication
sessions.
TCP and UDP Ports
1.
Certain ports are set aside as the particular ones through which to
contact a server running a given service such as HTTP (port 80), FTP (port
21), Telnet (port 23), DNS (port 53), or SMTP (port 25).
2.
The original RFC that documents well-known ports is RFC 1700. However,
for a more up-to-date informative list of all of TCP's and UDP's server-side
ports and the services to which they are assigned,
3.
check
out
this
link
to
the
IANA
website:
http://www.iana.org/assignments/port-numbers. IANA is the Internet
Assigned Numbers Authority the good people who track the number
standards for the Internet as we know it.
TCP and UDP Ports
1. When a client contacts a server, it randomly picks a source port
numbered above 1023 to go out through.
2. Then the client contacts the server on a set port, such as port 23
for Telnet. When the server replies, the information leaves on port
23 and returns to the client on the random greater-than 1023 port
from which it left.
3. This port information is the only way that a packet filter can
determine the service it is filtering.
TCP and UDP Ports
1. For example, you might want to filter out all Telnet traffic; you do
so by blocking all traffic directed at TCP port 23.
2. You might also want to allow all HTTP traffic coming to port 80.
However, if someone is running a Telnet server on port 80
somewhere on your network, and all you have for protection is the
afore mentioned packet filter, the traffic passes.
3. Packet-filtering systems don't have the intelligence to look beyond
the port number to determine what service is running at the
application layer.
TCP and UDP Ports
4. You need to keep this in mind when constructing filtering rules to
block access to a service that you are running on an alternative
port.
5. Sometimes, web servers run on alternative ports, such as 8000,
8080, and the like; if you wanted to allow access to said web
servers, then creating a packet-filtering rule that allows in standard
HTTP traffic on port 80 wouldn't be effective.