CISC 4700 L01 Network & Client-Server Programming Spring Harold, Chapter 4: Internet Addresses

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1 CISC 4700 L01 Network & Client-Server Programming Spring 2016 Harold, Chapter 4: Internet Addresses

2 IPv4 address: four bytes ( ) IPv6 address: sixteen bytes (2001:0250:02FF:0210:0250:8BFF:FEDE:67C8) DNS: mechanism for translating back and forth between IP addresses in dotted quad form and hostnames erdos.dsm.fordham.edu 2400:cb00:2048:1:0:0:6ca2:c Previous mechanism: centrally located HOSTS.TXT file Some hosts have multiple names (e.g., the "www" and "ftp" conventions). Some have multiple IP addresses (e.g., on two networks) erdos.dsm.fordham.edu Std DNS software package: BIND, which includes named and the like. Configuring DNS: a major pain. Rather than each host running BIND software, it has access to one or more nameservers (specified in the /etc/resolv.conf file in the Unix world).

3 The InetAddress Class (in the java.net package) Encapsulates Internet address.... extends java.lang.object... final class (can't be subclassed) Used by other networking classes (Socket, ServerSocket, URL,... ). Internally represents Internet address as String hostname and int address. These fields have package visibility; they may be accessed via getters and setters. Creating New InetAddress Objects No public ctors!! Assuming import java.net.*; use factory methods InetAddress address = InetAddress.getByName(" or InetAddress address = InetAddress.getByAddress(" ");

4 The tostring() method produces a representation hostname/dotted quad, thus: String dottedquadaddress = " "; try { InetAddress address = InetAddress.getByName(dottedQuadAddress); System.out.println(address); catch (UnknownHostException e) { System.out.println("Could not find " + dottedquadaddress); would print sobolev.dsm.fordham.edu/ when run. Note: getbyname doesn't do DNS lookup. This only happens when DNS lookup is actually required (implicit or explicit). Hence can create InetAddress objects for non-existent hosts.

5 getallbyname: used to get *all* Internet addresses. String hostname=" try { InetAddress[] addresses = InetAddress.getAllByName(hostName); for (int i = 0; i < addresses.length; i++) { System.out.println(addresses[i]); catch (UnknownHostException e) { System.out.println("Could not find " + hostname); getlocalhost: find address of local machine try { InetAddress address = InetAddress.getLocalHost(); System.out.println(address); catch (UnknownHostException e) { System.out.println("Could not find computer's address.");

6 Caching Purpose: save time (IP address shouldn t change while app is running.) Java only caches negative results for 10 seconds (reconfigurable). Java can t control non-local caching (e.g., DNS server). Security Issues Main idea: an untrusted applet can't perform arbitrary DNS lookups for third-party hosts, since Evil Ones can use this to set up a covert channel allowing applet to talk to a third party. Example: Applet downloaded from wants to send msg " is vulnerable" to evilgenius.org. Applet only needs to create DNS request " This fails, but the msg appears in the evilgenius.org DNS error log. See pp for further discussion.

7 Getter Methods gethostname(), gethostaddress(): return hostname or dotted quad address of IP address represented by an InetAddress object. String dottedquadaddress = " "; try { InetAddress ia = InetAddress.getByName(dottedQuadAddress); System.out.println(ia.getHostName()); catch (Exception e) { System.err.println(e); prints sobolev.dsm.fordham.edu when run try { InetAddress me = InetAddress.getLocalHost(); String dottedquad = me.gethostaddress(); System.out.println("My address is " + dottedquad); catch (UnknownHostException e) { System.out.println("I don't know my own address."); prints

8 getaddress(): returns IP address as 4-byte (IPv4) or 16-byte (IPv6) array of byte. One might think that this is OK: try { InetAddress me = InetAddress.getLocalHost(); byte address[] = me.getaddress(); System.out.print("My IP address: "); for (int i = 0; i < 4; i++) { System.out.print((int)address[i]); if (i < address.length - 1) System.out.print("."); System.out.println(); catch (UnknownHostException ex) { System.err.println(ex); It implicitly assumes IPv4. What about IPv6? Also, when run, this produces My IP address:

9 Let s write the following useful helper functions: public class AddressTests { public static int getversion(inetaddress ia) { byte[] address == ia.getaddress(); if (address.length == 4) return 4; if (address.length == 1) return 6; return -1; public static char getseparator(inetaddress ia){ int version = getversion(ia); if (version == 4) return : ; if (version == 6) return : ; return? ;

10 Problem? bytes in the IP address are unsigned, in the range Unlike C, Java doesn't have an unsigned byte type. So bytes in the range are treated as negative numbers; you need to add 256 to them to get the proper values: InetAddress me = InetAddress.getLocalHost(); byte address[] = me.getaddress(); char separator = AddressTests.getSeparator(me); System.out.print("My IP address: "); for (int i = 0; i < address.length; i++) { int thebyte = (int)address[i]; if (thebyte < 0) thebyte += 256; System.out.print(theByte); if (i < address.length - 1) System.out.print(separator); System.out.println(); When run, this produces My IP address: as expected.

11 More useful: determining the class (A, B, C, D, E) of an IPv4 address. Here, class A addresses begin with 0 network: bits 1-7 of byte 0; host: bytes 1-3 class B addresses begin with 10 network: bits 2-7 of byte 0, with byte 1; host: bytes 2-3 class C addresses begin with 110 network: bits 3-7 of byte 0, with bytes 1 and 2; host: byte 3 class D addresses begin with 1110 multicast group: bits 4-7 of byte 0, with bytes 1-3 class E addresses begin with bits 5-7 of byte 0, with bytes 1-3, are reserved for future use public static char getclass(inetaddress ia) { byte[] address = ia.getaddress(); int firstbyte = address[0]; if ((firstbyte & 0x80) == 0) return 'A'; else if ((firstbyte & 0xC0) == 0x80) return 'B'; else if ((firstbyte & 0xE0) == 0xC0) return 'C'; else if ((firstbyte & 0xF0) == 0xE0) return 'D'; else if ((firstbyte & 0xF8) == 0xF0) return 'E'; else return 'F';

12 Address Types In Java public boolean isanylocaladdress() public boolean isloopbackaddress() public boolean islinklocaladdress() public boolean issitelocaladdress() public boolean ismulticastaddress() public boolean ismcglobal() public boolean ismcorglocal() public boolean ismcsitelocal() public boolean ismclinklocal() public boolean ismcnodeocal() Sample program: test characteristics of an IP address import java.net.*; public class IPCharacteristics { public static void main(string[] args) { try { InetAddress address = InetAddress.getByName(args[0]);

13 if (address.isanylocaladdress()) { System.out.println(address + " is a wildcard address."); if (address.isloopbackaddress()) { System.out.println(address + " is loopback address."); if (address.islinklocaladdress()) { System.out.println(address + " is a link-local address."); else if (address.issitelocaladdress()) { System.out.println(address + " is a site-local address."); else { System.out.println(address + " is a global address.");

14 if (address.ismulticastaddress()) { if (address.ismcglobal()) { System.out.println(address + " is a global multicast address."); else if (address.ismcorglocal()) { System.out.println(address + " is an organization wide multicast address."); else if (address.ismcsitelocal()) { System.out.println(address + " is a site wide multicast address."); else if (address.ismclinklocal()) { System.out.println(address + " is a subnet wide multicast address."); else if (address.ismcnodelocal()) { System.out.println(address + " is an interface-local multicast address."); else { System.out.println(address + " is an unknown multicast address type.");

15 else { System.out.println(address + " is a unicast address."); catch (UnknownHostException ex) { System.err.println("Could not resolve " + args[0]);

16 Testing Reachability Check to see whether a host is reachable: public boolean isreachable(int timeout) throws IOException public boolean isreachable(networkinterface interface, int ttl, int timeout) throws IOException Attempts to connect to remote host's echo port. Object Methods InetAddress inherits from java.lang.object. It overrides three methods from Object: equals(): an object o equals an InetAddress object ia iff o is an InetAddress object o and ia have same IP address hashcode(): returns four bytes of the address, converted to an int tostring(): returns host name/dotted quad address if no name, then dotted quad/dotted quad

17 Example: Test to see whether or not two hostnames are the same import java.net.*; public class AreAliases { public static void main (String args[]) { if (args.length!= 2) { System.err.println("Usage: AreAliases address1 address2"); System.exit(1); try { InetAddress address0 = InetAddress.getByName(args[0]); InetAddress address1 = InetAddress.getByName(args[1]); System.out.print(args[0] + " is "); if (!address0.equals(address1)) System.out.print("not "); System.out.println("the same as " + args[1]); catch (UnknownHostException e) { System.out.println("Host lookup failed.");

18 Inet4Address and Inet6Address Subclasses of InetAddress. Generally, app program shouldn't care whether an IP address is IPv4 or IPv6. Inet4Address overrides several methods in InetAddress, but doesn t change their behavior in any significant way. Inet6Address has one new method not found in InetAddress: public boolean isipv4address() If true, address has form ::xxxx, can create new InetAddress object from last four bytes of array returned by getbytes().

19 NetworkInterface Class Represents local IP address (physical or virtual). Factory methods public static NetworkInterface getbyname(string name) throws SocketException Returns NetworkInterface object representing network interface with this name (or null, if no such) Typical names: Unix: "eth0", "em1", "enp5s0f1" ; "lo" Windows: "CE31", "ELX100" try { NetworkInterface ni = new NetworkInterface.getByName("eth0"); if (ni == null) System.err.println("No such interface: eth0"); catch (SocketException ex) { System.err.println("Could not list sockets");

20 public static NetworkInterface getbyaddress (InetAddress address)throws SocketException Returns NetworkInterface object representing network interface bound to given IP address (or null, if no such) Example: Find network interface for local loopback address try { InetAddress local = InetAddress.getByName(" "); NetworkInterface ni = NetworkInterface.getByInetAddress(local); if (ni == null) { System.err.println( "That's weird. No local loopback address."); catch (SocketException ex) { System.err.println("Could not list sockets."); catch (UnknownHostException ex) { System.err.println( "That's weird. No local loopback address.");

21 public static Enumeration getnetworkinterfaces() throws SocketException Returns java.util.enumeration listing all network interfaces on local host. Example: List all network interfaces on local host import java.net.*; import java.util.*; public class InterfaceLister { public static void main(string[] args) throws Exception { Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces(); while (interfaces.hasmoreelements()) { NetworkInterface ni = interfaces.nextelement(); System.out.println(ni);

22 When run on sobolev.dsm.fordham.edu: $ java InterfaceLister name:em1 (eth0) name:lo (lo) When run on erdos.dsm.fordham.edu: $ java InterfaceLister name:enp5s0f0(enp5s0f0) name:enp7s0 (enps70) name:lo (lo)

23 Getter Methods public Enumeration getinetaddresses() returns Enumeration of all IP addresses associated with the interface (usually only one, sometimes more than one) Example: NetworkInterface em1 = NetworkInterface.getByName("em1"); Enumeration addresses = em1.getinetaddresses(); while (addresses.hasmoreelements()) System.out.println(addresses.nextElement()); Output when run on sobolev.dsm.fordham.edu: /fe80:0:0:0:ea39:35ff:fe49:cde2%2 / public String getname() returns name of a NetworkInterface object (e.g., "em1") public String getdisplayname() supposedly returns more human-friendly name for NetworkInterface

24 Some Useful Programs SpamCheck Real-time blackhole lists (RTBLs): monitor spammers For example: sbl.spamhaus.org To identify whether a given IP address is a spammer: reverse the bytes in IP address prepend this to sbl.spamhaus.org do a DNS lookup this succeeds iff the IP address is listed as a spammer So to check , do a DNS lookup on sbl.spamhaus.org sobolev@dsm:src$ nslookup sbl.spamhaus.org Server: Address: #53 ** server can't find sbl.spamhaus.org: NXDOMAIN Look at

25 HostLookup nslookup is a Unix utility that does hostanme <-> IP address conversion: $ nslookup sobolev.dsm.fordham.edu Server: Address: #3 Name: sobolev.dsm.fordham.edu Address: $ nslookup > sobolev.dsm.fordham.edu Server: Address: #3 Name: sobolev.dsm.fordham.edu Address: > (typed control-d here) See

26 Processing Web Server Log Files A web server log file contains lines such as [15/Feb/2001:09:40: ] "GET / HTTP 1.0" where the fields are remotehost: usually in dotted quad form rfc931 specification of user on remote system (usually a dash) authuser: authenticated remote username (usually a dash) [date] "HTTP request" status bytes sent It would be nicer if the remotehost were a hostname. That's what the Weblog application does. It takes one command line parameter (the name of the web server log file) and prints each line in the logfile, with each IP address replaced by a hostname.

27 Pseudocode: bin = BufferedReader built from args[0]; entry = null string; // needed to get the loop started for (String entry = bin.readline(); entry!= null; entry = bin.readline()) { break entry into ip (IP address) and therest (everything else); find the hostname; print hostname and therest; See

28 Author suggests threading to improve performance. But first, a detour... ThreadPools (from Chapter 3 of Harold): Adding additional threads to a program can improve performance. But threads have their own overhead: starting thread cleanup when thread dies cost of switching between (CPU-bound) threads Java can let you use a pool of reusable worker threads. Executors class (in java.util.concurrent): makes it easy to set up a thread pool: Submit each task as a Runnable or Callable object to the pool. A Callable can have a return value, returned by its call() method. Get back a Future object, which you can use to check progress of task.

29 Example: Find the largest element in an array of integers, by splitting the work into two threads (may be faster on multiple-core chip). import java.util.concurrent.callable; class FindMaxTask implements Callable<Integer> { private int[] data; private int start; private int end; FindMaxTask(int[] data, int start, int end) { this.data = data; this.start = start; this.end = end; public Integer call() { int max = Integer.MIN_VALUE; for (int i = start; i < end; i++) { if (data[i] > max) max = data[i]; 2 9

30 import java.util.concurrent.*; public class MultithreadedMaxFinder { public static int max(int[] data) throws InterruptedException, ExecutionException { if (data.length == 1) { return data[0]; else if (data.length == 0) { throw new IllegalArgumentException(); // split the job into 2 pieces FindMaxTask task1 = new FindMaxTask(data, 0, data.length/2); FindMaxTask task2 = new FindMaxTask(data, data.length/2, data.length); // spawn 2 threads ExecutorService service = Executors.newFixedThreadPool(2); Future<Integer> future1 = service.submit(task1); Future<Integer> future2 = service.submit(task2); return Math.max(future1.get(), future2.get()); 3 0

31 Code at My own experiment (logfile with around 350,000 entries): time java Weblog ~/tmp/ssl_access_log > /dev/null real 0m59.291s user 0m1.662s sys 0m0.213s time java PooledWeblog ~/tmp/ssl_access_log > /dev/null real 0m49.902s user 0m3.857s sys 0m0.355s 3 1

Ch.4 Internet Addresses

CSB541 Network Programming 網路程式設計 Ch.4 Internet Addresses 吳俊興國立高雄大學資訊工程學系 Outline 4.1 The InetAddress Class 4.2 Inet4Address and Inet6Address 4.3 The NetworkInterface Class 4.4 Some Useful Programs 2 Internet