.TH dns_random 3
.SH NAME
dns_random \- return a pseudorandom number
.SH SYNTAX
.B #include <dns.h>

unsigned int \fBdns_random\fP(unsigned int \fIm\fR);

.SH DESCRIPTION
.B dns_random
computes a pseudorandom 32-bit integer and returns that integer modulo
\fIm\fR. It returns 0 if \fIm\fR is 0.

.SH "NOTES ON DNS QUERY SECURITY"

A DNS client will accept any response that shows up at the right time,
is addressed from the IP address of the legitimate server, is addressed
to the UDP port used in the DNS query, repeats the query name and type
used in the DNS query, and repeats the 16-bit ID used in the DNS query.

An active sniffing attacker can easily forge responses by copying
information from queries. Blind attackers need to guess the time, UDP
port, and ID for the targeted query name.

The dns_transmit functions use \fBdns_random\fR to create query IDs and UDP
ports. The \fBdns_random\fR generator is designed to be extremely difficult to
predict for an attacker who cannot guess seed. Note, however, that there
are only about a billion possible ID-port pairs, so a prolonged blind
attack will succeed eventually.

.SH "SEE ALSO"
dns_random_init(3)