Archivi tag: tcp

Enterprisely sending your hostname to DHCP

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DHCP protocol supports sending the client host name to the server via the DHCP HOST NAME option.
See RFC2132 appendix and the original
DHCP Vendor Extension at RFC1497

Further info on the option structure is in the bootp documentation:

You can configure it

On debian:
uncomment the ‘send host-name’ directive in /etc/dhcp3/dhclient.conf

In RHEL:
add the following line to the interface with the ip you want to associate

vim /etc/sysconfig/network-scripts/ifcfg-eth0
DHCP_HOSTNAME=your-nice-hostname

TCP to UDP: a top-down focus on the differences – I

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While writing a simple UDP server, I had the idea to focus on the differences with a TCP one.

The standard TCP Server flow is the following:

  1. create TCP socket()
  2. bind() it to an ip:port address
  3. listen() for incoming connection
  4. accept() them

The 1. is done via a socket(.., STREAM, TCP) system call. Replacing STREAM, TCP with DATAGRAM, UDP, and going thru point 2. to 4. we’ll get the following errors:

  • listen(): Operation not supported
  • accept(): Operation not supported

What happened? Yep, easy: as UDP is a connection-less protocol, we cannot “listen for” or “accept” connection.

While TCP establishes a fixed connection between client and server, an UDP server just gets every single packet which arrives from a client.

You can check this behavior running netcat in udp mode:

#nc -u localhost 12345

This command is successful even if there’s nothing listening on that port: it’s like a cannon ready to fire everything against a castle’s port.

Each time you send something, it fires. And you need to switch it off to “close” it!