| The osf module does passive operating system fingerprinting. This modules |
| compares some data (Window Size, MSS, options and their order, TTL, DF, |
| and others) from packets with the SYN bit set. |
| .TP |
| [\fB!\fP] \fB\-\-genre\fP \fIstring\fP |
| Match an operating system genre by using a passive fingerprinting. |
| .TP |
| \fB\-\-ttl\fP \fIlevel\fP |
| Do additional TTL checks on the packet to determine the operating system. |
| \fIlevel\fP can be one of the following values: |
| .IP \(bu 4 |
| 0 - True IP address and fingerprint TTL comparison. This generally works for |
| LANs. |
| .IP \(bu 4 |
| 1 - Check if the IP header's TTL is less than the fingerprint one. Works for |
| globally-routable addresses. |
| .IP \(bu 4 |
| 2 - Do not compare the TTL at all. |
| .TP |
| \fB\-\-log\fP \fIlevel\fP |
| Log determined genres into dmesg even if they do not match the desired one. |
| \fIlevel\fP can be one of the following values: |
| .IP \(bu 4 |
| 0 - Log all matched or unknown signatures |
| .IP \(bu 4 |
| 1 - Log only the first one |
| .IP \(bu 4 |
| 2 - Log all known matched signatures |
| .PP |
| You may find something like this in syslog: |
| .PP |
| Windows [2000:SP3:Windows XP Pro SP1, 2000 SP3]: 11.22.33.55:4024 -> |
| 11.22.33.44:139 hops=3 Linux [2.5-2.6:] : 1.2.3.4:42624 -> 1.2.3.5:22 hops=4 |
| .PP |
| OS fingerprints are loadable using the \fBnfnl_osf\fP program. To load |
| fingerprints from a file, use: |
| .PP |
| \fBnfnl_osf -f /usr/share/xtables/pf.os\fP |
| .PP |
| To remove them again, |
| .PP |
| \fBnfnl_osf -f /usr/share/xtables/pf.os -d\fP |
| .PP |
| The fingerprint database can be downlaoded from |
| http://www.openbsd.org/cgi-bin/cvsweb/src/etc/pf.os . |
