Table of Contents
Short term rate and resources limiting
In November 1975, Jon Postel issued the historic RFC 706 On the Junk Mail Problem. His goal was clearly to warn people about the possibility of Denial of Service generated by Junk Mail. Many people refers to this RFC as being one of the first publications about spam. But this RFC presents two ideas to solve the problem : a distributed database of misbehaving hosts (Hmmmm…. looks like a realtime blacklist…), and the evaluation unwanted message rates !
- 706 - On the Junk Mail Problem
A Host might make use of such a facility by measuring, per source, the number of undesired messages per unit time, if this measure exceeds a threshold then the Host could issue the "refuse messages from Host X" message to the IMP.
Jon Postel not only devised the problem, but also proposed solutions, which seemed to be completely ignored.
As long as the author knows, ze-filter was the first filter to implement (June 2002) the idea of measuring rates by mail filters, and limiting resources granted to SMTP clients, based on their consumption and behaviour. This idea was implemented, almost in the same time, by Claus Assmann on sendmail X (which was renamed later MeTA1). Connection rate and simultaneous open connections limiting per IP address by sendmail 8 is a contribution done by the author.
ze-filter and sendmail X/MeTA1, please let me know, so I can mention it here.
ze-filter evaluate some “resource” rates on a 10 minutes sliding time window - this is the “Short Term Rate Control” - the filter administrator can set limits based on some parameters :
- Connection Rate
- Message Rate
- Recipient Rate
Other limits can be sets to some simultaneous parameters :
- Simultaneous open connections
- Messages in the same connection
- Recipients in the same message
- These limits are set to ALL sources : well and bad behaving SMTP clients.
- The filter administrator can assign different limits to different sources or different network classes.
ConnRate:10.3.3.3 0 ConnOpen:10.3.3.3 0 RcptRate:10.3.3.3 0 MaxRcpt:10.3.3.3 0 MsgRate:10.3.3.3 0 MaxMsgs:10.3.3.3 0
Connection Rate
CHECK_CONN_RATE YES MAX_CONN_RATE 15
- database default values
ConnRate:DEFAULT 15 ConnRate:127.0.0.1 1000 ConnRate:LOCAL 300 ConnRate:DOMAIN 100 ConnRate:FRIEND 30 ConnRate:dnsbl 3
Simultaneous Open Connections
CHECK_OPEN_CONNECTIONS YES MAX_CONN_OPEN 10
- database default values
ConnOpen:DEFAULT 10 ConnOpen:127.0.0.1 30 ConnOpen:LOCAL 15 ConnOpen:DOMAIN 15 ConnOpen:FRIEND 10 ConnOpen:dnsbl 2
Message Rate
CHECK_MSG_RATE YES MAX_MSG_RATE 100
- database default values
MsgRate:DEFAULT 15 MsgRate:127.0.0.1 1000 MsgRate:LOCAL 300 MsgRate:DOMAIN 100 MsgRate:FRIEND 30 MsgRate:dnsbl 3
Recipient Rate
CHECK_RCPT_RATE NO MAX_RCPT_RATE 100
- database default values
RcptRate:DEFAULT 300 RcptRate:127.0.0.1 1000 RcptRate:LOCAL 1000 RcptRate:DOMAIN 1000 RcptRate:FRIEND 300
Recipient count per message
CHECK_NB_RCPT YES MAX_RCPT 200
- database default values
MaxRcpt:DEFAULT 100 MaxRcpt:127.0.0.1 300 MaxRcpt:LOCAL 300 MaxRcpt:DOMAIN 250 MaxRcpt:FRIEND 250 MaxRcpt:dnsbl 10
Messages per connection
CHECK_NB_MSGS YES MAX_MSGS 100
- database - default values
MaxMsgs:DEFAULT 10 MaxMsgs:127.0.0.1 50 MaxMsgs:LOCAL 50 MaxMsgs:DOMAIN 30 MaxMsgs:FRIEND 20 MaxMsgs:dnsbl 1
