Preprocessor Documentation



Encrypted traffic should be ignored by Snort for both performance reasons and to reduce false positives. The SSL Dynamic Preprocessor (SSLPP) inspects SSL and TLS traffic and optionally determines if and when to stop inspection of it.

Typically, SSL is used over port 443 as HTTPS. By enabling the SSLPP to inspect port 443, only the SSL handshake of each connection will be inspected. Once the traffic is determined to be encrypted, no further inspection of the data on the connection is made.

SSL Detection and Decoding

Each packet containing SSL traffic has an unencrypted portion that provides some information about the traffic itself, and the state of the connection. SSLPP uses this information to determine whether or not a handshake is occurring or if a handshake previously occurred.

By default, SSLPP looks for a handshake followed by encrypted traffic traveling to both sides. If one side responds with an indication that something has failed, such as the handshake, the session is not marked as encrypted. Verifying that faultless encrypted traffic is sent from both endpoints ensures two things: the last client-side handshake packet was not crafted to evade Snort, and that the traffic is legitimately encrypted.

In some cases, especially when packets may be missed, the only observed response from one endpoint will be TCP ACKs. Therefore, if a user knows that server-side encrypted data can be trusted to mark the session as encrypted, the user should use the ‘trustservers’ option, documented below.

Rule Options

SSLPP enables two new rule options: ssl_state and ssl_version.

The ssl_state keyword takes the following identifiers as arguments:


The ssl_version keyword takes the following identifiers as arguments:


More than one identifier can be specified, to either rule keyword, via a comma separated list. Lists of identifiers are OR’ed together, such that if any of them match, the rule option will match.

The option will match if the connection is currently in any one of the OR’ed states. To ensure the connection has reached each of a set of states, multiple rules using the ssl_state rule option should be used.

The rule options support negation. Some examples:

# Not client hello

# server hello OR not client hello

# server hello AND not server key exchange
ssl_state:server_hello; ssl_state:!server_keyx;

# not sslv2

# sslv3 and not sslv2
ssl_version:sslv3; ssl_version:!sslv2;


SSLPP supports the following options:

ports - Space separated list of ports, enclosed in braces

noinspect_encrypted - Disables inspection of encrypted traffic (default off)

trustservers - Disables the requirement that both sides of Application data must be observed (default off) This requires noinspect_encrypted to be useful.

max_heartbeat_length - Maximum length of heartbeat record allowed. This config option is used to detect the heartbleed attacks. The allowed range is 0 to 65535. Setting the value to 0 turns off the heartbeat length checks. For heartbeat requests, if the payload size of the request record is greater than the max_heartbeat_length an alert with sid 3 and gid 137 is generated. For heartbeat responses, if the record size itself is greater than the max_heartbeat_length an alert with sid 4 and gid 137 is generated. Default is off.

For example, to make SSLPP observe ports 443 and 994, and if you trust the servers to not be comprised or potentially malicious, use:

preprocessor ssl: ports { 443 994 }, trustservers, noinspect_encrypted

By default, SSLPP listens on the following ports:

443     HTTPS
465     SMTPS   
563     NNTPS   
636     LDAPS
989     FTPS        
992     TelnetS 
993     IMAPS
994     IRCS        
995     POPS

Important note: Stream4 or Stream5 should be explicitly told to reassemble
                traffic on these ports, or else the SSL preprocessor will
                be trivially evadable.

To add reassembly on port 443 to Stream5, use 'ports both 443' in the 
Stream5 configuration.