User manual

Etherboot User Manual

techniques involve passing a suitable option on the kernel command line (i.e. "single") or crashing the

ﬁlesystem by power cycling the machine; this in turn will result in fsck being invoked at the next system

start, which will sometimes drop you into single user mode.

Some Linux distributions do not require a password when entering single user mode. While this makes

system administration somewhat easier, it is a considerable security problem. Make sure, that your

system does not suffer from it.

For other security problems with running insecure programs under Linux or using poorly conﬁgured

distributions, refer to the Usenet newsgroups, security mailing lists and choose a distribution whose

manufacturer frequently releases security ﬁxes. It is a fallacy to assume that the unavailability of patches

implies the security and correctness of a software application; as a rule of thumb, a manufacturer who

releases more patches than a different one, probably cares more about the security of your system than

the latter. This also applies to operating systems other than Linux!

C.7. ETHERNET AND ITS PROTOCOLS

The ethernet is extremely vulnerable to attacks from malicious users. Anybody who can gain direct

access to an ethernet segment, can easily monitor all trafﬁc and inject forged data. This is very

dangerous, because many protocols transfer data either un-encoded or in easily decipherable form. Also,

authorization is often based on the assumption that the return address or a session id can be trusted, but

this is no longer true if users gain unlimited access to the ethernet; it does not really matter if this access

is achieved by having physical control over part of the network or by running a compromised or

inherently insecure operating system. There are various attacks from machines that are not directly

connected to your ethernet segment, but the majority of them can be prevented by installing and

maintaining a properly conﬁgured ﬁrewall. For more information, you should regularly monitor security

related newsgroups and mailinglists.

C.8. BOOTP/TFTP

BOOTP and TFTP offer almost no security whatsoever. They basically provide their information to

anybody who asks and solely rely on the assumption that your network is conﬁgured to not make the

server world-accessible. If you install BOOTP gateways, then this assumption is seriously violated. Also,

TFTP server are usually accessible from just about everywhere. You can try to diminish the impact of

this problem by blocking BOOTP and TFTP packets from leaving or entering your network segment, but

this will never be a completely secure solution.

Thus you should always assume that all of the ﬁles that your BOOTP and TFTP server offer are world

readable. They must not contain any sensitive data. Also, the TFTP daemon must be conﬁgured to only

allow access to selected ﬁles. Running it in a chroot’d environment might be a very good idea.

The BOOTP protocol is vulnerable against somebody else impersonating as a BOOTP server. While

security aware operating systems, prevent non-privileged users from starting their own BOOTP servers,

other operating systems do not allow this. This means, if any of your users can launch an arbitrary

program under an insecure operating system on an arbitrary machine connected to your ethernet

segment, then they have full control over the BOOTP boot process.