Chapter 15. Programming with the Resolver and Name Server Library Routines

15.1.1. A Typical Problem

Before you write a program, you need a problem to solve. Let's suppose you want your network management system to watch over your primary master and slave name servers. You want it to notify you of several problems: a name server that isn't running (it might have died), a name server that is not authoritative for a zone it is supposed to be authoritative for (the config file or zone data file might have been messed up), or a name server that has fallen behind in updating its zone data (the primary master's serial number might have been decreased accidentally).

Each of these problems is easily detectable. If a name server is not running on a host, the host sends back an ICMP port unreachable message. You can find this out with either a query tool or the resolver routines. Checking whether a name server is authoritative for a zone is easy: ask it for the zone's SOA record. If the answer is nonauthoritative or the name server does not have the SOA record, there's a problem. You'll have to ask for the SOA record in a nonrecursive query so that the name server doesn't go off and look up the SOA record from another server. Once you have the SOA record, you can extract the serial number.

15.1.2. Solving This Problem with a Script

This problem requires a program that takes the domain name of a zone as an argument, looks up the name servers for that zone, and then queries each of those name servers for the SOA record for the zone. The response will show whether the name server is authoritative, and it will show the zone's serial number. If there is no response, the program needs to determine if there's even a name server running on the host. Once you write this program, you should run it on each zone you want to watch over. Since this program looks up the name servers (by looking up the NS records for the zone), we assume that you have listed all your name servers in NS records in your zone data. If that's not the case, you will have to change this program to read a list of name servers from the command line.

Let's write the basic program as a shell script that uses nslookup. First, we figure out what the output of nslookup looks like so that we can parse it with Unix tools. We'll look up NS records to find out which name servers are supposed to be authoritative for the zone, both when the server is authoritative for the zone that contains the NS records and when it isn't:

% nslookup
Default Server:  relay.hp.com
Address:  15.255.152.2

> set type=ns

Find out what the response looks like when the name server is not authoritative for the NS records:

> mit.edu.
Server:  relay.hp.com
Address:  15.255.152.2

Non-authoritative answer:
mit.edu nameserver = STRAWB.MIT.EDU
mit.edu nameserver = W20NS.MIT.EDU
mit.edu nameserver = BITSY.MIT.EDU

Authoritative answers can be found from:
MIT.EDU nameserver = STRAWB.MIT.EDU
MIT.EDU nameserver = W20NS.MIT.EDU
MIT.EDU nameserver = BITSY.MIT.EDU
STRAWB.MIT.EDU  internet address = 18.71.0.151
W20NS.MIT.EDU   internet address = 18.70.0.160
BITSY.MIT.EDU   internet address = 18.72.0.3

Then find out what the response looks like when the name server is authoritative for the NS records:

> server strawb.mit.edu.
Default Server:  strawb.mit.edu
Address:  18.71.0.151

> mit.edu.
Server:  strawb.mit.edu
Address:  18.71.0.151

mit.edu nameserver = BITSY.MIT.EDU
mit.edu nameserver = STRAWB.MIT.EDU
mit.edu nameserver = W20NS.MIT.EDU
BITSY.MIT.EDU   internet address = 18.72.0.3
STRAWB.MIT.EDU  internet address = 18.71.0.151
W20NS.MIT.EDU   internet address = 18.70.0.160

You can see from this output that we can grab the domain names of the name servers by looking for the lines that contain nameserver and saving the last field. When the name server wasn't authoritative for the NS records, it printed them twice, so we'll have to weed out duplicates.

Next, we look up the SOA record for the zone, both when the server is authoritative for the zone that contains the SOA record and when it isn't. We turn off recurse so the name server doesn't go off and query an authoritative name server for the SOA:

% nslookup
Default Server:  relay.hp.com
Address:  15.255.152.2

> set type=soa
> set norecurse

Find out what the response looks like when the name server is not authoritative and does not have the SOA record:

> mit.edu.
Server:  relay.hp.com
Address:  15.255.152.2

Authoritative answers can be found from:
MIT.EDU nameserver = STRAWB.MIT.EDU
MIT.EDU nameserver = W20NS.MIT.EDU
MIT.EDU nameserver = BITSY.MIT.EDU
STRAWB.MIT.EDU  internet address = 18.71.0.151
W20NS.MIT.EDU   internet address = 18.70.0.160
BITSY.MIT.EDU   internet address = 18.72.0.3

Then find out what the response looks like when the name server is authoritative for the zone:

> server strawb.mit.edu.
Default Server:  strawb.mit.edu
Address:  18.71.0.151

> mit.edu.
Server:  strawb.mit.edu
Address:  18.71.0.151

mit.edu
        origin = BITSY.MIT.EDU
        mail addr = NETWORK-REQUEST.BITSY.MIT.EDU
        serial = 1995
        refresh = 3600 (1H)
        retry   = 900 (15M)
        expire  = 3600000 (5w6d16h)
        minimum ttl = 21600 (6H)

When the name server was not authoritative for the zone, it returned references to other name servers. If the name server had previously looked up the SOA record and cached it, the name server would have returned the SOA record and said that it was nonauthoritative. We need to check for both cases. When the name server returns the SOA record and it is authoritative, we can grab the serial number from the line that contains serial.

Now we need to see what nslookup returns when no name server is running on a host. We'll change servers to a host that does not normally run a name server and look up an SOA record:

% nslookup
Default Server:  relay.hp.com
Address:  15.255.152.2

> server galt.cs.purdue.edu.
Default Server:  galt.cs.purdue.edu
Address:  128.10.2.39

> set type=soa
> mit.edu.
Server:  galt.cs.purdue.edu
Address:  128.10.2.39

*** galt.cs.purdue.edu can't find mit.edu.: No response from server

Last, we need to see what nslookup returns if a host is not responding. We can test this by switching name servers to an unused IP address on our LAN:

% nslookup
Default Server:  relay.hp.com
Address:  15.255.152.2

> server 15.255.152.100
Default Server:  [15.255.152.100]
Address:  15.255.152.100

> set type=soa
> mit.edu.
Server:  [15.255.152.100]
Address:  15.255.152.100

*** Request to [15.255.152.100] timed-out

In the last two cases, the error message was written to stderr.[104] We can make use of that fact when writing our shell script. Now we are ready to compose the shell script. We'll call it check_soa:

[104]Not all versions of nslookup print the last error message for a timeout. Be sure to check what yours prints.

#!/bin/sh
if test "$1" = ""
then
    echo usage: $0 zone
    exit 1
fi
ZONE=$1
#
# Use nslookup to discover the name servers for this zone ($1).
# Use awk to grab the name server's domain names from the nameserver lines.
# (The names are always in the last field.)  Use sort -u to weed out
# duplicates; we don't actually care about collation.
#
SERVERS=`nslookup -type=ns $ZONE |\
                 awk '/nameserver/ {print $NF}' | sort -u`
if test "$SERVERS" = ""
then
    #
    # Didn't find any servers.  Just quit silently; nslookup will
    # have detected this error and printed a message.  That will
    # suffice.
    #
    exit 1
fi
#
# Check each server's SOA serial number.  The output from
# nslookup is saved in two temp files: nso.$$ (standard output)
# and nse.$$ (standard error).  These files are rewritten on
# every iteration.  Turn off defname and search since we
# should be dealing with fully qualified domain names.
#
# NOTE: this loop is rather long; don't be fooled.
#
for i in $SERVERS
do
  nslookup >/tmp/nso.$$ 2>/tmp/nse.$$ <<-EOF
    server $i
    set nosearch
    set nodefname
    set norecurse
    set q=soa
    $ZONE
EOF
  #
  # Does this response indicate that the current server ($i) is
  # authoritative?  The server is NOT authoritative if (a) the
  # response says so, or (b) the response tells you to find
  # authoritative info elsewhere.
  #
  if egrep "Non-authoritative|Authoritative answers can be" \
                                          /tmp/nso.$$ >/dev/null
  then
    echo $i is not authoritative for $ZONE
    continue
  fi
  #
  # We know the server is authoritative; extract the serial number.
  #
  SERIAL=`cat /tmp/nso.$$ | grep serial | sed -e "s/.*= //"`
  if test "$SERIAL" = ""
  then
    #
    # We get here if SERIAL is null.  In this case, there should
    # be an error message from nslookup; so cat the "standard
    # error" file.
    #
    cat /tmp/nse.$$
  else
    #
    # Report the server's domain name and its serial number.
    #
    echo $i has serial number $SERIAL
  fi
done  # end of the "for" loop
#
# Delete the temporary files.
#
rm -f /tmp/nso.$$ /tmp/nse.$$

Here is what the output looks like:

% check_soa mit.edu
BITSY.MIT.EDU has serial number 1995
STRAWB.MIT.EDU has serial number 1995
W20NS.MIT.EDU has serial number 1995

If you are pressed for time, this short tool will solve your problem, and you can go on to other work. If you find that you are checking lots of zones and that this tool is too slow, you'll want to convert it to a C program. Also, if you want more control over the error messages -- rather than relying on nslookup for error messages -- then you'll have to write a C program. We'll do just that later in this chapter.