Chapter 11 Name and Address Conversions Part II

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Chapter 11 Name and Address Conversions(Part
II)

• Presented by
• Erin-Ee-Lin Lau
• - 18th April 2007 -

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Outline

• host_serv Function
• tcp_connect Function
• tcp_listen Function
• udp_client Function
• udp_connect Function
• udp_server Function
• getnameinfo Function
• Re-entrant Function
• gethostbyname_r and gethostbyaddr_r Function

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11.11 host_serv Function

• Two arguments
• Address family and socket type
• host_serv function

Initializes hints structure, calls getaddrinfo
and returns a null pointer if an error occurs
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11.12 tcp_connect Function

• Create a TCP socket and connect to a server

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tcp_connect Function (cont.)
Call getaddrinfo
Try each addrinfo structure until success or end
of list
Returns all the dynamic memory
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Example Daytime client

• Connects to a daytime server
• Retrieves the current date and time

Command line argument - To specify either the
service name or the port number, which allows our
program to connect to other ports
Connect to server
Print servers address
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Example Daytime client (cont)

• tcp_connect does not return the size of socket
  address structure used for connect
• The design goal for tcp_connect is to reduce the
  number of arguments compared to getaddrinfo.
• Able to works with both IPv4 and IPv6
• Host that supports only IPv4
• freebsd daytimetcpcli linux daytime
• connected to 206.168.112.96
• Sun Jul 27 230624 2003
• Host that supports both IPv4 and IPv6
• freebsd daytimetcpcli aix daytime
• connected to 3ffeb801f8d2204acfffe17bf38
• Sun Jul 27 231713 2003
• Force the use of IPv4 address by specifying the
  host-name with -4 suffix
• freebsd daytimetcpcli aix-4 daytime
• connected to 192.168.42.2
• Sun Jul 27 231748 2003

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11.13 tcp_listen Function

• Performs normal TCP server steps
• Create a TCP socket, bind the servers port and
  allow incoming connection requests to be accepted

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11.13 tcp_listen Function
1. Call getaddrinfo
2. Create socket and bind address
3. Check for failure
4. Return size of socket address structure
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Example Daytime Server

• Waits for requests from Client
• Accepts client connections
• Send the current time
• Terminates connection and goes back waiting for
  more connections.

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Example Daytime Server

• Require service name or port no. as command line
  argument

2. Create a listening socket
3. Server loop
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Daytime server with protocol specification

• Previous function exhibits some problem
• Might cause getaddrinfo to return a socket
  address structure with an address family other
  than what is desired
• Clients always specify either an IP address or a
  hostname
• Enter as a command-line argument
• Able to force a given protocol on a server
• If we enter
• server ? it defaults to IPv6 on a dual-stack
  host
• server 0.0.0.0 ? explicitly specifies IPv4
• server 00 ? explicitly specifies IPv6

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Daytime Server (Final Version)

• The only changes is the command-line arguments
• Allows user to specify either a hostname or IP
  
• address for server to bind, in addition to a
• service name or port

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Daytime Server (Final Version)

• Start server with an IPv4 socket and connect to
  server from clients on two other hosts on local
  subnet
• Start server with an IPv6 socket

aix
IPv6
macosx
aix
IPv4
macosx
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Next

• udp_client
• udp_connect
• upd_server

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11.14 udp_client Function

• Creates an unconnected UDP socket
• Returns three items
• Socket descriptor
• saptr address of a pointer to socket address
  structure
• lenp returned the size of socket address
  structure
• Cannot be null pointer
• Length of socket address structure is required in
  any calls to sendto and recvfrom

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11.14 udp_client Unconnected UDP Socket
- Converts hostname and service arguments -
Datagram socket is created - Memory is allocated
for one socket address structure
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Protocol-Independent Daytime Client using
udp_client function
1. Call udp_client function 2. Print IP address
and port of the server
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Protocol-Independent Daytime Client using
udp_client function

• freebsd daytimeudpcli1 aix daytime
• sending to 3ffeb801f8d2204acfffe17bf38
• Sun Jul 27 232112 2003
• ? IPv6 socket
• freebsd daytimeudpclil 192.168.42.2 daytime
• sending to 192.168.42.2
• Sun Jul 27 232140 2003
• ? IPv4 socket

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11.15 udp_connect Function

• Creates a connected UDP socket
• The final 2 arguments required by udp_client are
  no longer needed

• Similar to tcp_connect
• Different
• The call to connect with a UDP socket does not
  send anything to the peer.
• If something is wrong, caller does not discover
  until it sends a datagram to the peer.

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11.16 udp_server Function

• Arguments similar to tcp_listen
• An optional hostname
• A required service (to bound port number)
• An optional pointer to variable in which the size
  of socket address structure is returned

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udp_server function Unconnected socket

• Similar to tcp_listen but without the call to
  listen.
• SO_REUSEADDR socket option are not used for UDP
  socket
• Can allow multiple sockets to bind the same UDP
  port on hosts that support multicasting
• There is no TCPs TIME_WAIT state for UDP socket

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Protocol-independent Daytime Server
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11.17 getnameinfo Function

• Complement of getaddrinfo
• Takes a socket address and returns character
  strings describing host and service
• Translate a socket address to a node name and
  service location
• Protocol-independent fashion
• sockaddr points to socket address structure
  containing the protocol address to be converted
  into a human-readable string
• addrlen length of structure (normally returned
  by accept, recvfrom, getsockname, or getpeername)

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11.17 getnameinfo Function

• Caller allocates space for two human-readable
  strings
• Host string host and hostlen
• Service string serv and servlen
• 0 if caller does not want the host string or
  information on service to be returned.
• sock_ntop vs. getnameinfo
• sock_ntop does not involve DNS and just returns
  a printable version of IP address and port number
• getnameinfo tries to obtain a name for both host
  and service

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11.17 getnameinfo Function

• Six flags arguments to change the default
  operation of getnameinfo
• NI_DGRAM
• Specified when caller knows it is dealing with a
  datagram socket (SOCK_DGRAM)
• Given only IP address and port number in socket
  address structure, getnameinfo cannot determine
  the protocol (TCP or UDP)
• Eg Port 514 rsh service for TCP syslog
  service for UDP
• NI_NAMEREQD
• Causes an error to be returned if the hostname
  cannot be located using DNS
• NI_NOFQDN
• Causes returned hostname to truncated at the
  first period
• Eg192.168.42.2
• gethostbyaddr return a name of aix.unpbook.com
• If this flag is specified to getnameinfo, return
  hostname ? aix

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11.17 getnameinfo Function

• NI_NUMERICHOST
• Numeric representation of IP address shall be
  returned instead of its name
• NI_NUMERICSERV
• Specifies the numeric form of the service address
  shall be returned (for example, its port number)
  instead of its name
• NI_NUMERICSCOPE
• Specifies numeric form of scope identifier to be
  returned instead of its name
• NI_NUMERICSERV
• Specify by server as client port numbers
  typically have no associated service name they
  are ephemeral port

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11.18 Re-entrant Functions

• A function that can be safely called while it's
  already executing is said to be re-entrant

Re-entrancy problem!
Called and main flow control is temporarily
stopped
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11.19 gethostbyname_r and gethostbyaddr_r

• To make a nonre-entrant function becomes
  re-entrant
• Four additional arguments
• Result
• Buffer
• buflen buffer size
• h_errnop to return error code when error occurs

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- Thank You -