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Network Programming

Apr 20, 2025, 5 min read

tags : Networking, Operating Systems, Subnetting, TCP&UDP

Building blocks

File Descriptors

  • See File Descriptors
  • A file descriptor is simply an integer associated with an open file. But (and here’s the catch), that file can be a network connection, a FIFO, a pipe, a terminal, a real on-the-disk file, or just about anything else.

Socket

Means different things

  • Kernel: Endpoint of communication
  • Application
    • socket() return the socket descriptor and we can use send() and recv() socket calls to communicate through it.
    • You can even use read() and write() but send() and recv() gives more control.
    • Client and Server communicate via reading/writing to socket descriptors
  • O and Socket I/O differs in how we open the socket descriptor.

OSI model

  • The actual network hardware and topology is transparent to the socket programmer

Network algorithms

Ports

Meta

  • There is no such thing as a port being open or closed. Ports aren’t real.
  • They are just a two byte label in a network packet that tell both the router and client what to do with it.

Open ports

  • In both TCP and UDP, source and destination ports are of 16bit (2byte)
  • Max number of open ports in an IP is 216 = 65536 => 65535
  • If a computer gets another IP assigned to it, in that case it gets another 65535 ports.
  • So the no. of open ports a computer can have = no. of ip * 65535

Listening ports

  • If you connect to some port which is listening, you don’t consume the remote port. (In case of Web Server there’s additional things that happen)
    • It remains open for others and also for you.
    • Each connection you make, makes a 4-tuple combination and when you make a connection to sever_ip:80
      • connection1: your_local_ip:random_port1 <=> server_ip:80
      • connection2: your_local_ip:random_port2 <=> server_ip:80
      • See this in action w ss -tpn

TCP and UDP

To learn TCP, read the RFCs to understand the design, then dive into the Linux kernel’s TCP code. Build your own TCP stream replay tool (around 1,000 lines of code), and within a year, you’ll be ahead of 99.99% of people in this field.

wangbin579

https://github.com/session-replay-tools/tcpcopy

See TCP&UDP

Different types of sockets

Internet sockets (DARPA sockets, INET Sockets)

Stream Socket (SOCK_STREAM)

  • Reliable two way connected communication
  • Order is maintained
  • Error free
  • Uses TCP and IP
  • send() syscall
  • Examples: telnet, http

Datagram Socket (SOCK_DGRAM)

  • Sometimes called connectionless sockets, but you can use connect() with it.
  • Error free
  • Uses UDP and IP
  • sendto() just encaptulate the data with a method of choice
  • Examples: tftp, dhcpcd, games, streaming audio, video conf etc.

Raw socket

Other sockets

Unix sockets

  • Unix sockets are faster than TCP sockets over loopback
  • path names on a local node (Unix sockets)
  • Unix domain sockets use the file system as their address name space.
  • Processes reference Unix domain sockets as file system inodes
  • In addition to sending data, processes may send file descriptors across a Unix domain socket connection using the sendmsg() and recvmsg() system calls.
  • This allows the sending processes to grant the receiving process access to a file descriptor for which the receiving process otherwise does not have access to.

X.25 sockets

  • Successor to ioctl
  • Netlink is designed and used for transferring miscellaneous networking information between the kernel space and userspace processes.

Network API

Endian ordering

  • When about to transmit data, assume that data is in wrong host byte order; that way each time you transmit put the data in a conversion function that arranges data in big endian order.
  • 32 bit variant examples: htons(), htonl(), ntohs(), ntohl()
  • inet_pton() : converts an IP address in numbers and dots to sin_addr
  • inet_ntop() : converts a sin_addr to printable IP address in numbers and dots
  • Printable to Network (pton)
  • Network to Printable (ntop)
  • getaddrinfo prefered over gethostbyname() ??????
  • getnameinfo prefered over gethostbyaddr() ??????

TODO Data types used by the socket interface

  • a socket descriptor int
  • addrinfo struct : Prepare the socket address structure.
    • getaddrinfo() returns pointer to a LL(Link local??) of this structure
    • ai_addr field is a pointer to struct sockaddr
      • sockaddr : Dealing with sockaddr is done with struct sockaddr_in or sockaddr_in6 for ipv6 -> they can be cast vice versa
        • The connect() system by default takes in sockaddr
        • sockaddr_in is therefore used as an replacement for sockaddr
        • sockaddr_in is padded with 0s in sin_zero[8] field using the memset() function
          • if using sockaddr_in6 there is no sin_zero field
        • sockaddr_in’s sin_port should be in network byte order.
        • sockaddr_in has sin_addr struct which has s_addr field which is a uint32_t

tun/tab/linux

Use-cases

Bidirectional socket

Network Programming for Games

Graph View

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