1.\" $OpenBSD: securelevel.7,v 1.12 2001/05/30 03:24:16 millert Exp $ 2.\" 3.\" Copyright (c) 2000 Hugh Graham 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED 15.\" WARRANTIES, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF 16.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 17.\" IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, 18.\" INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES 19.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR 20.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, 22.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 23.\" IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 24.\" POSSIBILITY OF SUCH DAMAGE. 25.\" 26.Dd January 4, 2000 27.Dt SECURELEVEL 7 28.Os 29.Sh NAME 30.Nm securelevel 31.Nd securelevel and its effects 32.Sh SYNOPSIS 33The 34.Ox 35kernel provides four levels of system security: 36.Bl -tag -width flag 37.It \&-1 Em Permanently insecure mode 38.Bl -hyphen -compact 39.It 40.Xr init 8 41will not attempt to raise the securelevel 42.It 43may only be set with 44.Xr sysctl 8 45while the system is insecure 46.It 47otherwise identical to securelevel 0 48.El 49.It \ 0 Em Insecure mode 50.Bl -hyphen -compact 51.It 52used during bootstrapping and while the system is single-user 53.It 54all devices may be read or written subject to their permissions 55.It 56system file flags may be cleared 57.El 58.It \ 1 Em Secure mode 59.Bl -hyphen -compact 60.It 61default mode when system is multi-user 62.It 63securelevel may no longer be lowered except by init 64.It 65.Pa /dev/mem 66and 67.Pa /dev/kmem 68may not be written to 69.It 70raw disk devices of mounted file systems are read-only 71.It 72system immutable and append-only file flags may not be removed 73.It 74kernel modules may not be loaded or unloaded 75.El 76.It \ 2 Em Highly secure mode 77.Bl -hyphen -compact 78.It 79all effects of securelevel 1 80.It 81raw disk devices are always read-only whether mounted or not 82.It 83.Xr settimeofday 2 84may not set the time backwards 85.\"XXX - replace with ipfw when it is in-tree 86.\".It 87.\".Xr ipf 8 88.\"and 89.\".Xr ipnat 8 90.\"rules may not be altered 91.It 92the 93.Va ddb.console 94and 95.Va ddb.panic 96.Xr sysctl 8 97variables may not be raised 98.El 99.El 100.Sh DESCRIPTION 101Securelevel provides convenient means of 102.Dq locking down 103a system to a degree suited to its environment. 104It is normally set at boot via the 105.Xr rc.securelevel 8 106script, or the superuser may raise securelevel at any time by modifying the 107.Va kern.securelevel 108.Xr sysctl 8 109variable. 110However, only 111.Xr init 8 112may lower it once the system has entered secure mode. 113A kernel built with 114.Cm option INSECURE 115in the config file will default to permanently insecure mode. 116.Pp 117Highly secure mode may seem Draconian, but is intended as a last line of 118defence should the superuser account be compromised. 119Its effects preclude 120circumvention of file flags by direct modification of a raw disk device, 121or erasure of a file system by means of 122.Xr newfs 8 . 123Further, it can limit the potential damage of a compromised 124.Dq firewall 125by prohibiting the modification of packet filter rules. 126Preventing 127the system clock from being set backwards aids in post-mortem analysis 128and helps ensure the integrity of logs. 129Precision timekeeping is not 130affected because the clock may still be slowed. 131.Pp 132Because securelevel can be modified with the in-kernel debugger 133.Xr ddb 4 , 134a convenient means of locking it off (if present) is provided 135on highly secure systems. 136This is accomplished by setting 137.Va ddb.console 138and 139.Va ddb.panic 140to 0 with the 141.Xr sysctl 8 142utility. 143.Sh FILES 144.Bl -tag -width /etc/rc.securelevel -compact 145.It Pa /etc/rc.securelevel 146commands that run before the security level changes 147.El 148.Sh SEE ALSO 149.Xr chflags 2 , 150.Xr settimeofday 2 , 151.Xr mem 4 , 152.Xr options 4 , 153.Xr init 8 , 154.Xr rc 8 , 155.Xr sysctl 8 156.Sh HISTORY 157The 158.Nm 159manual page first appeared in 160.Ox 2.6 . 161.Sh BUGS 162The list of securelevel's effects may not be comprehensive. 163