Table of Contents
This is continued from the part 1.
We are adding DHCP support to Pcap4J.
Packet Piece Class
A packet piece class is a Java class which represents a field of a packet. We should create such classes instead of using a primitive types in some cases.
In the case of DHCP, its flags field includes two fields in itself as like below:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|B| MBZ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
B: BROADCAST flag
MBZ: MUST BE ZERO (reserved for future use)Although this flags field is 2 bytes long and can be held by a primitive short variable, it’s better to create a packet piece class to hold it for better usability.
I mean,
boolean broadcast = DhcpV4Packet.getHeader().getFlags().isBroadcast();is better than
boolean broadcast = (0x8000 & DhcpV4Packet.getHeader().getFlags()) != 0;DhcpV4Flags class
Now, let’s write a packet piece class DhcpV4Flags for the flags field. For writing packet piece classes, there is no rule except that they must implement Serializable interface.
package org.pcap4j.packet;
import java.io.Serializable;
public final class DhcpV4Flags implements Serializable {
private static final long serialVersionUID = -7144264525666462708L;
private final short value;
public static DhcpV4Flags newInstance(short value) {
return new DhcpV4Flags(value);
}
private DhcpV4Flags(short value) {
this.value = value;
}
public short value() {
return value;
}
}What we should do to implement Serializable is only adding a field private static final long serialVersionUID in this case because DhcpV4Flags has only one primitive short field.
The short field holds entire value of the flags field.
The constructor receives a short value and simply stores it in the short field.
I made the constructor private and wrote a static factory method newInstance, which is just my taste and not necessary.
Here, let’s remember that the flags field has two fields in itself, which are B (BROADCAST) and MBZ (MUST BE ZERO), and add two methods to get them.
public boolean isBroadcast() { return (value & 0x8000) != 0; }
public short getMbz() { return (short)(value & 0x7FFF); }And, generally speaking, we should always override toString, equals, and hashCode.
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append("[BROADCAST: ")
.append(isBroadcast())
.append("] [value: 0x")
.append(ByteArrays.toHexString(value, ""))
.append("]");
return sb.toString();
}
@Override
public boolean equals(Object obj) {
if (obj == this) { return true; }
if (!this.getClass().isInstance(obj)) { return false; }
DhcpV4Flags other = (DhcpV4Flags)obj;
return this.value == other.value;
}
@Override
public int hashCode() { return value; }The last thing to write is Builder class, which is another way to instantiate DhcpV4Flags objects. The static factory method I wrote above is used when dissecting a real DHCP packet (i.e. a byte array), while the Builder is used when crafting a DHCP packet object.
A Builder class is usually written as an inner class.
public final class DhcpV4Flags implements Serializable {
(snip)
public Builder getBuilder() {
return new Builder(this);
}
public static final class Builder {
private boolean broadcast = false;
private short mbz = 0;
public Builder() {}
private Builder(DhcpV4Flags flags) {
this.broadcast = flags.isBroadcast();
this.mbz = flags.getMbz();
}
public Builder broadcast(boolean broadcast) {
this.broadcast = broadcast;
return this;
}
public Builder mbz(short mbz) {
this.mbz = mbz;
return this;
}
public DhcpV4Flags build() {
return new DhcpV4Flags(this);
}
}
}And, DhcpV4Flags needs a constructor which uses the Builder.
private DhcpV4Flags(Builder builder) {
if (builder == null) {
throw new NullPointerException("builder is null.");
}
if (builder.mbz < 0) {
throw new IllegalArgumentException(
"mbz must be equal or greater than zero but it is: " + builder.mbz
);
}
this.value = builder.broadcast ? (short)(builder.mbz | 0x8000) : builder.mbz;
}That’s all. The entire code is as follows:
package org.pcap4j.packet;
import java.io.Serializable;
import org.pcap4j.util.ByteArrays;
public final class DhcpV4Flags implements Serializable {
private static final long serialVersionUID = -7144264525666462708L;
private final short value;
public static DhcpV4Flags newInstance(short value) {
return new DhcpV4Flags(value);
}
private DhcpV4Flags(short value) {
this.value = value;
}
private DhcpV4Flags(Builder builder) {
if (builder == null) {
throw new NullPointerException("builder is null.");
}
if (builder.mbz < 0) {
throw new IllegalArgumentException(
"mbz must be equal or greater than zero but it is: " + builder.mbz
);
}
this.value = builder.broadcast ? (short)(builder.mbz | 0x8000) : builder.mbz;
}
public short value() {
return value;
}
public boolean isBroadcast() { return (value & 0x8000) != 0; }
public short getMbz() { return (short)(value & 0x7FFF); }
public Builder getBuilder() {
return new Builder(this);
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder();
sb.append("[BROADCAST: ")
.append(isBroadcast())
.append("] [value: 0x")
.append(ByteArrays.toHexString(value, ""))
.append("]");
return sb.toString();
}
@Override
public boolean equals(Object obj) {
if (obj == this) { return true; }
if (!this.getClass().isInstance(obj)) { return false; }
DhcpV4Flags other = (DhcpV4Flags)obj;
return this.value == other.value;
}
@Override
public int hashCode() { return value; }
public static final class Builder {
private boolean broadcast = false;
private short mbz = 0;
public Builder() {}
private Builder(DhcpV4Flags flags) {
this.broadcast = flags.isBroadcast();
this.mbz = flags.getMbz();
}
public Builder broadcast(boolean broadcast) {
this.broadcast = broadcast;
return this;
}
public Builder mbz(short mbz) {
this.mbz = mbz;
return this;
}
public DhcpV4Flags build() {
return new DhcpV4Flags(this);
}
}
}In the next part, we’ll write the DHCP packet class.
