Advanced UNIX programming

1
Advanced UNIX programming

• Fall 2002
• Instructor Ashok Srinivasan
• Lecture 25

Acknowledgements The syllabus and power point
presentations are modified versions of those by
T. Baker and X. Yuan
2
Announcements

• Reading assignment
• Chapter 9
• Sections 9.1 to 9.9
• Midterm Monday
• Clarifications on project and HW3
• Midterm review

3
Week 9 Topics

• Socket options
• getsockopt, setsockopt
• fcntl
• ioctl
• Introduction to UDP
• TCP vs UDP
• UDP interface

4
Week 9 Topics ... continued

• Name and address conversions
• Domain name system
• gethostbyname
• gethostbyaddr
• uname
• gethostname
• getservbyname
• getservbyport

5
Name and address conversions

• Names are easier to use
• Names add a layer of indirection
• Numeric addresses can be changed without
  modifying the name
• A program can be more portable by using a name
• The mapping between hostnames and IP addresses is
  done by the domain name system (DNS)

6
Domain name system

• Mapping between names and addresses
• xi.cs.fsu.edu lt--gt 128.186.121.41
• References
• See http//www.internic.net/faqs/authoritative-dns
  .html for information on
• DNS organization
• Why they should be the sole authority!

7
Domain name system ... continued

• Hierarchical naming scheme
• Name space partitioned into subdomains
• Top level of domains
• .com, .edu, .gov, .org, .us, .ca, .in, etc
• Each domain can have subdomains
• Distributed delegation of naming authority
• Each domain has the authority to allow names
  within that domain
• Naming follows organization boundary not physical
  network boundary

8
Resource records

• Each server keeps resource records (RRs)
• Information
• IP address, record type, name, etc
• Some relevant RR types
• A Maps a hostname to an IPv4 address
• AAAA Maps a hostname to an IPv6 address
• PTR Pointer records, map IP addresses to
  hostnames
• MX Mail exchanger for a host
• CNAME Canonical name
• Used to assign CNAME records for common services

9
Name servers

• Name server
• When a name server receives a DNS request, if the
  server is the authority for the name, it finds
  the resource record and replies
• Otherwise it recursively asks the next level
• Example gethostbyname(www.cs.yale.edu)
• xi.cs.fsu.edu ---gt cs.fsu.edu --gt
  edu --gt
• yale.edu --gtcs.yale.edu
  --gtyale.edu--gtedu
• --gtcs.fsu.edu--gtxi.cs.fsu.edu
• Optimizations
• Replication and caching

10
gethostbyname

• include ltnetdb.hgt
• struct hostent gethostbyname(const char
  hostname)
• struct hostent
• char h_name
• char h_aliases
• int h_addrtype
• int h_length
• char h_addr_list
• This function queries A or AAAA records

11
gethostbyaddr

• include ltnetdb.hgt
• struct hostent gethostbyaddr(const char addr,
  size_t len, int family)
• addr is a pointer to an in_addr structure
• len 4 for IPv4 and 16 for IPv6
• This function queries PTR records
• See example1.c and example2.c
• These two system calls incur significant
  communication overhead
• You can observe this in example1a.c

12
uname and gethostbyname

• Information on name, os, and hardware
• include ltsys/utsname.hgt
• int uname (struct utsname name)
• struct utsname
• char sysname_UTS_NAMESIZE
• char nodename_UTS_NODESIZE
• char release_UTS_NAMESIZE
• char version_UTS_NAMESIZE
• char machine_UTS_NAMESIZE
• See example2.c
• gethostbyname can be used to get host name

13
getservbyname and getservbyport

• Get service information
• Not querying the DNS, but a system specific file,
  for example /etc/services
• include ltnetdb.hgt
• struct servent getservbyname(const char
  servname, const char protoname)
• struct servent getservbyport(int port, const
  charprotoname)
• struct servent
• char s_name
• char s_aliases
• int s_port
• char s_proto
• See example3.c and example4.c