nsupdate only takes one optional argument and has four command-line options. The optional argument is the name of a file containing commands to be sent to the DNS server. If called without an argument, nsupdate accepts commands on standard input. Commands are batched and sent after supdate reads a blank line. This allows you to send multiple groups of commands in one file or session and make changes dependent on certain prerequisites being present.

nsupdate accepts four options - d, y, k, v. -d makes nsupdate run in debug mode. -y allows you to specify a key name and the key itself on the command line. The format is -y keyname:secret. Using the -y option is discouraged because the secret key would then be available via ps or your history file. The preferred way to use a secret key is the -k option. With the -k option, nsupdate reads the key from the .private file that was created when you generated your key. According to the man page, the .key file must also be present in order to use the -k option. Finally, -v makes nsupdate use TCP to send the update. This may be necessary if you are sending a large batch of commands that are too big to fit in a single UDP packet.

nsupdate accepts quite a few commands but you will most likely only need to use the update command. update can be used to add or delete resource records in a zone.

When deleting a record, update's syntax is update delete domain-name [class] [type [data ...]]. class is only required if you are making updates to a zone that is not in the internet class. type specifies the type of resource record you want to delete (i.e., A, MX, CNAME). If data is present the delete will only take place if their is a resource record of type whose RDATA matches data.

To add a record, update's syntax is update add domain-name ttl [class] type data. ttl is the Time-to-Live for the record (just like you would specify in the zone file). When adding a record, type and data are required as they specify the resource record to be added.

In order to add an entry in DNS for your broadband-connected machine you'll want to add an A record specifying that your hostname has the IP address you received via dhcp. You probably also should delete any A record that currently exist for your hostname (perhaps from a previous update). An nsupdate session to do this would look like the following:

# nsupdate -k Khome-dns.shakabuku.org.+157+48378.private
> update delete home.d.shakabuku.org A
> update add home.d.shakabuku.org 86400 A 192.168.1.10
>
> ^C

Automating nsupdate

Listing 5. /etc/dhcpc/dhcpcd.exe

01 #!/bin/bash
02 
03 TTL=86400
04 KEYFILE=/var/named/Khome.d.shakabuku.org.+157+14660.key
05 NSUPDATE=/usr/bin/nsupdate
06
07 exec >> /var/log/dhcpcd.log 2>&1
08 
09 if [ $# -lt 2 ]; then
10   echo "$(date +\%b\ \%e\ \%H:\%M:\%S) dhcpcd.exe: wrong usage"
11   exit 1
12 fi
13
14 hostinfo=$1
15 state=$2
16
17 # Reading HostInfo file for configuration parameters
18 . $hostinfo
19
20 case $state in
21   up) echo "$(date +\%b\ \%e\ \%H:\%M:\%S) dhcpcd.exe: interface $INTERFACE has been configured with old IP=$IPADDR"
22       HOSTNAME=$(hostname)
23       delete="update delete $HOSTNAME A\n"
24       update="update add $HOSTNAME $TTL A $IPADDR\n\n"
25       update=${delete}${update}
26       ;;
27
28   new) echo "$(date +\%b\ \%e\ \%H:\%M:\%S) dhcpcd.exe: interface $INTERFACE has been configured with new IP=$IPADDR"
29        HOSTNAME=$(hostname)
30        delete="update delete $HOSTNAME A\n"
31        update="update add $HOSTNAME $TTL A $IPADDR\n\n"
32        update=${delete}${update}
33         ;;
34   down) echo "$(date +\%b\ \%e\ \%H:\%M:\%S) dhcpcd.exe: interface $INTERFACE has been brought down"
35         update="update delete $HOSTNAME A\n\n"
36         ;;
37 esac
38
39 echo -e $update | $NSUPDATE -k $KEYFILE
40
41 exit 0

Lines 9-12 check to make sure the script was passed enough parameters. Lines 14 and 15 set some variables based on the parameters the script was passed. Line 18 sources the file containing information for the interface that has changed. That file contains the IP address, subnet mask, etc.

Lines 20-37 are the meat of the script. They act on the state change of the interface, building a command string that will update the dynamic DNS record. Line 39 wraps things up by calling nsupdate with the TSIG key file and piping the commands to nsupdate.

Summary

©2001, 2002 Jason Bodnar