[Devel] /etc/protocols, getprotobyname, socket(domain, type, protocol)

- Sample content in /etc/protocols:

# # Internet (IP) protocols # ip 0 IP # internet protocol, pseudo protocol number icmp 1 ICMP # internet control message protocol igmp 2 IGMP # internet group multicast protocol tcp 6 TCP # transmission control protocol udp 17 UDP # user datagram protocol raw 255 RAW # RAW IP interface

http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xhtml
"Assigned Internet Protocol Numbers"

In the Internet Protocol version 4 (IPv4) [RFC791] there is a field
called "Protocol" to identify the next level protocol.  This is an 8
bit field.  In Internet Protocol version 6 (IPv6) [RFC2460], this field
is called the "Next Header" field.

http://tools.ietf.org/html/rfc790 ASSIGNED NUMBERS

The networking library needs a way to translate protocol names to protocol numbers understood by the IP layer on other hosts. This is done by looking up the name in the /etc/protocols file.
http://www.tldp.org/LDP/nag2/x-087-2-appl.services.html 

- Check the "Protocol" field in IP packet header: /* use TCP(6) as example */
http://tools.ietf.org/pdf/rfc791.pdf

"include/uapi/linux/ip.h"

struct iphdr {
#if defined(__LITTLE_ENDIAN_BITFIELD)
        __u8    ihl:4,
                version:4;
#elif defined (__BIG_ENDIAN_BITFIELD)
        __u8    version:4,
                ihl:4;
#else
#error  "Please fix <asm/byteorder.h>"
#endif
        __u8    tos;
        __be16  tot_len;
        __be16  id;
        __be16  frag_off;
        __u8    ttl;
        __u8    protocol;
        __sum16 check;
        __be32  saddr;
        __be32  daddr;
        /*The options start here. */
};

- Socket programming:
http://docs.oracle.com/cd/E19620-01/805-4041/6j3r8iu2o/index.html

Selecting Specific Protocols

If the third argument of the socket() call is 0, socket() selects a default protocol to use with the returned socket of the type requested. The default protocol is usually correct, and alternate choices are not usually available. When using "raw" sockets to communicate directly with lower-level protocols or hardware interfaces, it may be important for the protocol argument to set up de-multiplexing. For example, raw sockets in the Internet domain can be used to implement a new protocol on IP, and the socket receives packets only for the protocol specified. To obtain a particular protocol, determine the protocol number as defined in the protocol domain. For the Internet domain, use one of the library routines discussed in "Standard Routines", such as getprotobyname():

#include <sys/types.h> #include <sys/socket.h> #include <netinet/in.h> #include <netdb.h> ... pp = getprotobyname("newtcp"); s = socket(AF_INET, SOCK_STREAM, pp->p_proto);

This results in a socket s using a stream-based connection, but with protocol type of newtcp instead of the default tcp.

- socket creation in kernel:
SYSCALL_DEFINE3(socket, int, family, int, type, int, protocol)
    \retval = sock_create(family, type, protocol, &sock);
        \__sock_create(&init_net, family, type, protocol, res, 1);
            \

- /etc/services:

Define port number for UDP/TCP

Example 12-2. A Sample /etc/services File

# The services file: # # well-known services echo 7/tcp # Echo echo 7/udp # discard 9/tcp sink null # Discard discard 9/udp sink null # daytime 13/tcp # Daytime daytime 13/udp # chargen 19/tcp ttytst source # Character Generator chargen 19/udp ttytst source # ftp-data 20/tcp # File Transfer Protocol (Data) ftp 21/tcp # File Transfer Protocol (Control) telnet 23/tcp # Virtual Terminal Protocol smtp 25/tcp # Simple Mail Transfer Protocol nntp 119/tcp readnews # Network News Transfer Protocol # # UNIX services exec 512/tcp # BSD rexecd biff 512/udp comsat # mail notification login 513/tcp # remote login who 513/udp whod # remote who and uptime shell 514/tcp cmd # remote command, no passwd used syslog 514/udp # remote system logging printer 515/tcp spooler # remote print spooling route 520/udp router routed # routing information protocol

Note that the echo service is offered on port 7 for both TCP and UDP, and that port 512 is used for two different services: remote execution (rexec) using TCP, and the COMSAT daemon, which notifies users of new mail, over UDP (see xbiff(1x) ).