/// <summary>
/// Reads data from a file that contains database entries that were
/// persisted from a previous session. If the data files cannot
/// be found, the action is aborted. If invalid entries are
/// found in the file, they will be skipped, and the method
/// will continue reading the file after that entry. If
/// any other unforeseen I/O errors occur, the method will
/// simply stop where it is in the file and return.
///
/// It is possible to record any errors reading the file
/// in the system event logs, but this is not done here
/// for simplicity.
/// </summary>
/// <param name="sourcePath">The path to the file that the data should be read from.</param>
public void ReadPersistedData(string sourcePath)
{
try
{
using (StreamReader source = new StreamReader(File.OpenRead(sourcePath)))
{
lock (entryCountLock)
{
while (!source.EndOfStream && this.entryCount < this.maxEntries)
{
MailFilterEntry newEntry = MailFilterEntry.TryParse(source.ReadLine());
if (newEntry != null)
{
this.SaveEntry(newEntry);
}
}
}
}
}
catch (IOException e)
{
Debug.WriteLine(e.ToString());
return;
}
catch (UnauthorizedAccessException e)
{
Debug.WriteLine(e.ToString());
return;
}
}
/// <summary>
/// Records an entry in the database, either by updating an existing
/// entry or by creating a new one.
/// </summary>
/// <param name="entry">The entry to be added to the database.</param>
public void SaveEntry(MailFilterEntry entry)
{
int index = this.CalcIndex(entry.TripletHash);
LinkedList <MailFilterEntry> currentRow = (LinkedList <MailFilterEntry>) this.table[index];
lock (((ICollection)currentRow).SyncRoot)
{
// If the triplet is already there, remove it
// so that you can put it back at the start of the list.
if (currentRow.Contains(entry))
{
currentRow.Remove(entry);
this.DecrementEntryCount();
}
// Insert the triplet at the front of the list.
entry.TimeStamp = DateTime.UtcNow;
currentRow.AddFirst(entry);
this.IncrementEntryCount();
// If the bucket is full, delete the oldest entry in it.
if (currentRow.Count > this.bucketSize)
{
currentRow.RemoveLast();
this.DecrementEntryCount();
}
}
}
/// <summary>
/// An override of the Object class's Equals method.
/// </summary>
/// <param name="obj">The other object to be compared to.</param>
/// <returns>True if the other object is a MailFilterEntry and has the same TripletHash as this.</returns>
public override bool Equals(Object obj)
{
MailFilterEntry entry = obj as MailFilterEntry;
if (entry == null)
{
return(false);
}
return(this.Equals(entry));
}
/// <summary>
/// The core of the MailFilter algorithm. Determines whether
/// a triplet matches a known sender/recipient relationship and should
/// be accepted.
/// </summary>
/// <param name="remoteIP">The remote host's IP address.</param>
/// <param name="sender">The sender's address.</param>
/// <param name="recipient">The recipient's address.</param>
/// <returns>Whether the triplet is verified by the MailFilter.</returns>
private bool VerifyTriplet(IPAddress remoteIP, RoutingAddress sender, RoutingAddress recipient)
{
// Create a MailFilterEntry object for the current session.
// This code uses senderAddress.DomainPart to truncate
// the sender's address to use only the domain. To use the full
// address, use ToString() as with recipient.
UInt64 tripletHash = this.HashTriplet(
remoteIP,
sender.DomainPart,
recipient.ToString());
MailFilterEntry currentEntry = new MailFilterEntry(tripletHash);
// Determine whether a matching entry is in the verified database.
// If it is, save with an updated time stamp.
// This ensures that the most recent entries are
// at the start of the bucket lists in the array tables.
if (this.verifiedDatabase.GetEntry(currentEntry.TripletHash) != null)
{
currentEntry.TimeStamp = DateTime.UtcNow;
this.verifiedDatabase.SaveEntry(currentEntry);
return(true);
}
// If the entry is in the unverified table passed in
// the initial blocking period, remove it.
MailFilterEntry entry = this.unverifiedDatabase.GetEntry(currentEntry.TripletHash);
if (entry != null)
{
if (entry.IsPastPeriod(this.settings.InitialBlockingPeriod))
{
this.verifiedDatabase.SaveEntry(currentEntry);
this.unverifiedDatabase.DeleteEntry(currentEntry);
return(true);
}
else
{
// The entry was in the table of unverified entries,
// but the blocking period has not passed yet.
return(false);
}
}
else
{
// The triplet is not in either database. Send a rejection and
// put it in the unverified database.
this.unverifiedDatabase.SaveEntry(currentEntry);
return(false);
}
}
/// <summary>
/// Removes an entry from the database.
/// </summary>
/// <param name="entry">The entry to be removed.</param>
/// <returns>True if the entry was removed; false if it could not be found.</returns>
public bool DeleteEntry(MailFilterEntry entry)
{
// The return value.
bool retval = false;
// Calculate the hash index.
int index = this.CalcIndex(entry.TripletHash);
LinkedList <MailFilterEntry> currentRow = (LinkedList <MailFilterEntry>) this.table[index];
lock (((ICollection)currentRow).SyncRoot)
{
retval = currentRow.Remove(entry);
}
if (retval)
{
this.DecrementEntryCount();
}
return(retval);
}
/// <summary>
/// Tries to parse the contents of a string into a new entry.
/// </summary>
/// <param name="currentLine">The string to be parsed.</param>
/// <returns>The new entry, or null if unsuccessful.</returns>
public static MailFilterEntry TryParse(String currentLine)
{
// Set up a return value variable.
MailFilterEntry retval = null;
// Split the two values in the line.
string[] splitLine = currentLine.Split(',');
// Verify that you got exactly two values from the line in the file.
if (splitLine.Length == 2)
{
// Try parsing the hash code from the file. If you
// can't parse one, do not return an
// entry object at all.
UInt64 inputHash;
if (!UInt64.TryParse(splitLine[0].Trim(), out inputHash))
{
return(null);
}
long inputTicks;
DateTime inputTime;
if (long.TryParse(splitLine[1].Trim(), out inputTicks))
{
inputTime = new DateTime(inputTicks);
}
else
{
return(null);
}
retval = new MailFilterEntry(inputHash);
retval.timeStamp = inputTime;
}
return(retval);
}
/// <summary>
/// The copy constructor.
/// </summary>
/// <param name="other">The entry object to be copied.</param>
public MailFilterEntry(MailFilterEntry other)
{
this.tripletHash = other.TripletHash;
this.timeStamp = other.TimeStamp;
}
/// <summary>
/// Compares this MailFilterEntry's tripletHash to that of another MailFilterEntry.
/// </summary>
/// <param name="entry">The other MailFilterEntry that this will be compared to.</param>
/// <returns>True if the other entry has the same hash as this entry.</returns>
public bool Equals(MailFilterEntry entry)
{
return(this.tripletHash == entry.tripletHash);
}