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You can change the type of information you receive from a query. The
default query type is A. Table 5-14 lists the different types of
query information.
5.8.5.1 A Query Type
This is the default NSLOOKUP query type. It returns the name and IP address of a host. The following NSLOOKUP session shows a query for the host apple . The query to the server condor.lgk.dec.com is successful, and the server returns the IP address 16.99.208.10.
$ NSLOOKUP Default Server: condor.lgk.dec.com Address: 16.99.208.53 > apple Server: condor.lgk.dec.com Address: 16.99.208.53 Name: apple.lgk.dec.com Address: 16.99.208.10 > |
If you enter a domain name without a trailing period, NSLOOKUP appends the default domain to the name. You can change the default domain with the set domain or set srchlist commands.
To look up a host not in the current domain, append a period to the name, as shown in the following example:
$ NSLOOKUP apple.koz.dec.com. |
To obtain the host name for an IP address, change the query type to PTR and enter the IP address, as shown in the following example:
> set type=ptr > 16.99.208.189 Server: condor.lgk.dec.com Address: 16.99.208.53 Name: dove.lgk.dec.com Address: 16.99.208.189 |
You can also use the PTR query type to obtain more information about a domain, as as shown in the following example:
> lgk.dec.com
Server: condor.lgk.dec.com
Address: 16.99.208.53
lgk.dec.com
origin = condor.lgk.dec.com
mail addr = postmaster.lgk.dec.com
serial = 1998101948
refresh = 3600 (1H)
retry = 300 (5M)
expire = 604800 (1W)
minimum ttl = 43200 (12H)
>
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To obtain information about mail exchange records, set the query type to MX and enter a domain. The output tells you which hosts handle mail for the specified domain, as shown in the following example.
> set type=mx > lgk.dec.com Server: condor.lgk.dec.com Address: 16.99.208.53 lgk.sea.com preference = 200, mail exchanger = crl.sea.com lgk.sea.com preference = 50, mail exchanger = collie.lgk.sea.com lgk.sea.com preference = 100, mail exchanger = mail13.digital.com lgk.sea.com preference = 100, mail exchanger = mail11.digital.com lgk.sea.com preference = 200, mail exchanger = mail2.digital.com lgk.sea.com nameserver = collie.lgk.sea.com lgk.sea.com nameserver = condor.lgk.sea.com lgk.sea.com nameserver = hageln.lgk.sea.com crl.sea.com internet address = 192.58.206.2 collie.lgk.sea.com internet address = 16.99.208.100 mail13.digital.com internet address = 192.208.46.30 mail2.digital.com internet address = 204.123.2.56 condor.lgk.sea.com internet address = 16.99.208.53 hageln.lgk.sea.com internet address = 16.99.208.10 |
This query type returns the domain's start-of-authority information.
> set type=soa
> microsoft.com
Server: condor.lgk.sea.com
Address: 16.99.208.53
microsoft.com
origin = dns1.microsoft.com
mail addr = msnhst.microsoft.com
serial = 1998101204
refresh = 7200 (2H)
retry = 1800 (30M)
expire = 2592000 (4w2d)
minimum ttl = 86400 (1D)
microsoft.com nameserver = dns3.nwnet.net
microsoft.com nameserver = dns4.nwnet.net
microsoft.com nameserver = dns1.microsoft.com
microsoft.com nameserver = dns2.microsoft.com
microsoft.com nameserver = dns1.moswest.msn.net
microsoft.com nameserver = dns2.moswest.msn.net
dns3.nwnet.net internet address = 192.220.250.7
dns4.nwnet.net internet address = 192.220.251.7
dns1.microsoft.com internet address = 131.107.1.7
dns2.microsoft.com internet address = 131.107.1.240
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To obtain information about the name servers for a particular zone, set the query type to NS and then enter the zone you want. The following example shows the name servers for the microsoft.com zone.
> set type=ns > microsoft.com Server: condor.lgk.sea.com Address: 16.99.208.53 Non-authoritative answer: microsoft.com nameserver = dns2.microsoft.com microsoft.com nameserver = dns1.moswest.msn.net microsoft.com nameserver = dns2.moswest.msn.net microsoft.com nameserver = dns3.nwnet.net microsoft.com nameserver = dns4.nwnet.net microsoft.com nameserver = dns1.microsoft.com Authoritative answers can be found from: dns2.microsoft.com internet address = 131.107.1.240 dns3.nwnet.net internet address = 192.220.250.7 dns4.nwnet.net internet address = 192.220.251.7 dns1.microsoft.com internet address = 131.107.1.7 > |
If you want to use another name server as your default server, use the server command.
$ NSLOOKUP Default Server: condor.klg.sea.com Address: 16.99.208.53 > server ns01.koz.sea.com Default Server: ns01.koz.sea.com Address: 16.99.9.20 |
If for some reason the default server is not responding, you can always use the lserver command to change the default server. The lserver command uses the initial default name server to look up the IP address of the new server.
> lserver collie.klg.sea.com Default Server: collie.klg.sea.com Address: 16.99.208.10 |
Or, if you already know the IP address of the new server, you can use the server command to reset the default server.
>server 16.99.208.10 > server 16.99.99.226 Default Server: beagel.zok.sea.com Address: 16.99.99.226 |
The ls command lists information about a domain. This command is useful for:
Table 5-15 describes options to the ls command.
| Option | Function |
|---|---|
| -a | Lists aliases of hosts in the domain (CNAME entries). |
| -d | Lists all the entries in the domain. |
| -h | Lists CPU and operating system information for the domain (HINFO entries). |
| -m | Lists mail exchange (MX) entries in the domain. |
| -s | Lists well-known services (WKS) in the domain. |
| -t | Lists a specified entry type. |
The following example shows the use of the ls command to obtain address records for all hosts within a zone.
> ls -t a lgk.sea.com [condor.lgk.sea.com] @ 12H IN A 16.99.208.208 dhcp-253 12H IN A 16.99.208.253 ucxv4a 12H IN A 16.99.208.129 beavis 12H IN A 16.99.208.90 boxmor 12H IN A 16.99.208.30 kempo 12H IN A 16.99.208.47 pacnet 12H IN A 16.99.208.84 kwai 12H IN A 16.99.208.63 alxica 12H IN A 16.99.9.37 ppponvms 12H IN A 16.99.208.104 a71kt 12H IN A 16.99.208.142 peteathome 12H IN A 16.99.208.101 larisa 12H IN A 16.99.208.49 pigdog 12H IN A 16.99.208.140 ntruder 12H IN A 16.99.208.110 |
In the following example, the ls command displays alias records for hosts within the lgk.sea.com domain.
> ls -a lgk.sea.com [condor.lgk.sea.com] $ORIGIN LGK.SEA.COM. celics 12H IN CNAME celtics news 12H IN CNAME nntpd.KLG.SEA.COM. tiger 12H IN CNAME ntruder console 12H IN CNAME bblts.KLG.SEA.COM. deebug 12H IN CNAME dot ayla 12H IN CNAME ayla.KLG.SEA.COM. cscibm 12H IN CNAME cscibm.KLG.SEA.COM. > |
Using the -m option obtains the MX records for hosts within the lgk.sea.com domain, as shown in the following example.
> ls -m lgk.sea.com
brigit 12H IN MX 10 brigit
12H IN MX 100 mail1.digital.com.
12H IN MX 100 mail2.digital.com.
12H IN MX 200 crl.SEA.com.
piglet 12H IN MX 10 piglet
12H IN MX 100 mail1.digital.com.
12H IN MX 100 mail2.digital.com.
12H IN MX 200 crl.SEA.com.
tieta 12H IN MX 10 tieta
12H IN MX 100 mail1.digital.com.
12H IN MX 100 mail2.digital.com.
12H IN MX 200 crl.SEA.com.
sherry 12H IN MX 10 sherry
12H IN MX 100 mail1.digital.com.
12H IN MX 100 mail2.digital.com.
12H IN MX 200 crl.SEA.com.
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In the following example, using the -s option displays the well-known services for a domain.
> ls -s lgk.sea.com [condor.lgk.sea.com] WKStesthave 12H IN WKS 16.99.208.255 21 ( ) WKStesthavenot 12H IN WKS 16.99.208.255 255 ( ) WKStestnumbers 12H IN WKS 16.99.208.255 255 ( 21 23 ) |
You can redirect the output from this command to a file. This method is helpful when the domain consists of a large number of hosts. Once the file is created, you can look at its contents with the DCL command TYPE. For example:
> ls -t a klg.sea.com > systems.txt [condor.klg.sea.com] ############# Received 932 answers (0 records). $ TYPE SYSTEMS.TXT > ls -t a klg.sea.com [condor.klg.sea.com] $ORIGIN KLG.SEA.COM. @ 12H IN A 16.99.208.208 dhcp-253 12H IN A 16.99.208.253 ucxv4a 12H IN A 16.99.208.129 beavis 12H IN A 16.99.208.90 boxmor 12H IN A 16.99.208.30 kempo 12H IN A 16.99.208.47 pacnet 12H IN A 16.99.208.84 kwai 12H IN A 16.99.208.63 alxica 12H IN A 16.99.9.37 ppponvms 12H IN A 16.99.208.104 |
To solve BIND server problems, refer to the following sections:
A missing client name in the BIND server's database files results in lack of service to that client. If records that point to the name servers (NS records) in a domain are missing from your server's database files, you might see the following messages:
%TCPIP-W-BIND_NOSERVNAM, Server with address 199.85.8.8 is not responding %TCPIP-E-BIND_NOSERVERS, Default servers are not available %TCPIP-W-NORECORD, Information not found -TCPIP-E-BIND_NOSERVERS, Default servers are not available |
When the CONVERT/ULTRIX BIND /DOMAIN command creates the .DB files from the hosts database, it cannot detect the existence of or the names of name servers in a domain. Therefore, it does not add NS records for the name servers to the .DB files.
To solve the problem, follow these steps:
The serial number mismatch log=xxx, zone=xxx message indicates that the BIND server was unsuccessful in an attempt to load a particular dynamic update from the update log file. This can occur because the update is old and no longer valid. The serial number for the zone has since been incremented and is no longer in synchronization with the serial number attached to the logged update.
This chapter describes how to use DNS to balance the network traffic on a multihomed host or on network servers when you have multiple systems providing the same network service.
TCP/IP Services provides two methods for balancing work load using DNS:
This chapter discusses how to use DNS to balance server work load and includes the following topics:
TCP/IP Services defines the term DNS cluster to refer to
several A resource records for a single host name. This could be the A
resource records for a multihomed host or the A resource records for
one or more servers which are to share a work load.
6.2 Round-Robin Scheduling
Round-robin scheduling is the default load-sharing method used by a DNS server. If multiple resource records satisfy a query, the BIND server returns them each time in a round-robin order. The round-robin scheme is a simple rotation where client requests are passed from one cluster member to the next. The round-robin scheme is also useful for MX records to share mail loads among multiple equivalent gateways of the same MX preference.
Unlike the traditional load-balancing method, round-robin does not take into account the current work load on the DNS cluster members and does not know whether these hosts are up or down.
The following example demonstrates how round-robin load sharing works.
In the example, the DNS cluster alias is defined as robin . When the DNS server receives queries for robin , it shuffles the A resource records in a round-robin manner.
;
; TCP/IP DNS cluster load sharing - round robin method
; DNS cluster alias: "robin"
robin IN A 9.20.208.47
IN A 9.20.208.30
IN A 9.20.208.72
;
birdy IN A 9.20.208.47
seagull IN A 9.20.208.30
owl IN A 9.20.208.72
;
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A user enters the TELNET command, specifying the DNS cluster alias ROBIN. The first query to the DNS name server results in the following TELNET session:
$ TELNET ROBIN %TELNET-I-TRYING, Trying ... 9.20.208.47 %TELNET-I-SESSION, Session 01, host birdy, port 23 -TELNET-I-ESCAPE, Escape character is ^] |
The TELNET client connects to host birdy at IP address 9.20.208.47, the first resource record in the list.
The second query to the name server results in the following TELNET session:
$ TELNET ROBIN %TELNET-I-TRYING, Trying ... 9.20.208.30 %TELNET-I-SESSION, Session 01, host seagull, port 23 -TELNET-I-ESCAPE, Escape character is ^] |
The TELNET client connects to host seagull at IP address 9.20.208.30, the next resource record in the list.
The third query to the name server results in the following TELNET session:
$ TELNET ROBIN %TELNET-I-TRYING, Trying ... 9.20.208.72 %TELNET-I-SESSION, Session 01, host owl, port 23 -TELNET-I-ESCAPE, Escape character is ^] |
TELNET connects to host owl at IP address 9.20.208.72, the next resource record in the list.
The fourth query to the name server results in the following TELNET session:
$ TELNET ROBIN %TELNET-I-TRYING, Trying ... 9.20.208.47 %TELNET-I-SESSION, Session 01, host birdy, port 23 -TELNET-I-ESCAPE, Escape character is ^] |
TELNET again connects to host birdy at IP address 9.20.208.47. This is the start of the cycle repeating. The cycle repeats for the subsequent queries.
The SHOW HOST display for this DNS name server shows the shuffling effect in greater detail. Notice that the output displays the cluster alias name for each host involved in round-robin scheduling.
TCPIP> SHOW HOST ROBIN
BIND database
Server: 9.20.208.72 owl.ucx.ern.sea.com
Host address Host name
9.20.208.47 robin.ucx.ern.sea.com
9.20.208.30 robin.ucx.ern.sea.com
9.20.208.72 robin.ucx.ern.sea.com
TCPIP> SHOW HOST ROBIN
BIND database
Server: 9.20.208.72 owl.ucx.ern.sea.com
Host address Host name
9.20.208.30 robin.ucx.ern.sea.com
9.20.208.72 robin.ucx.ern.sea.com
9.20.208.47 robin.ucx.ern.sea.com
TCPIP> SHOW HOST ROBIN
BIND database
Server: 9.20.208.72 owl.ucx.ern.sea.com
Host address Host name
9.20.208.72 robin.ucx.ern.sea.com
9.20.208.47 robin.ucx.ern.sea.com
9.20.208.30 robin.ucx.ern.sea.com
TCPIP> SHOW HOST ROBIN
BIND database
Server: 9.20.208.72 owl.ucx.ern.sea.com
Host address Host name
9.20.208.47 robin.ucx.ern.sea.com
9.20.208.30 robin.ucx.ern.sea.com
9.20.208.72 robin.ucx.ern.sea.com
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If you want to disable round-robin scheduling, use the DCL command DEFINE to define the following logical:
$ DEFINE/SYSTEM TCPIP$BIND_ROUND_ROBIN_OFF |
TCP/IP Services provides a configurable, calculated, load-balancing mechanism called the load broker for distributing the load across systems in a DNS cluster.
Unlike round-robin scheduling (the default method used by most DNS name
servers), the load broker takes into account the load on all DNS
cluster participants. The load broker polls DNS cluster members and
updates the DNS namespace accordingly.
6.3.1 How the Load Broker Works
When the load broker starts, it reads its configuration file and starts polling DNS cluster members. The load broker exchanges messages with DNS cluster members that run the metric server. The metric server ( Section 6.3.2) calculates the current rating and reports it when polled by the load broker. Periodically, the load broker sorts the list of addresses based on metric rating reports, drops the systems that are not responding after being polled three times, and takes a subset of the list and compares it to the name server information. To do the comparison, the load broker sends a host lookup request to the specified name server. If the lists are the same, the load broker does not make any change. If the lists are different, the load broker updates the name server data by sending a dynamic update request to the specified name server.
The name server uses round-robin scheduling to further balance the load across the members of a DNS cluster. So every consecutive request for translating the DNS cluster name results in a list being returned, rotated by one.
The dns-ttl value stored in the load broker configuration file governs how long the record is to be cached by other name servers. If some intermediate name server caches the "A" resource records for a given DNS cluster name, it caches it for the period of time defined by the dns-ttl value. The default dns-ttl value is 45 seconds. If less time is required, you can set dns-ttl to a smaller value. To suppress any caching, you can set the dns-ttl to 0.
The dns-refresh time specifies how often the DNS information for a given DNS cluster is refreshed. The default is 30 seconds. If you want to quickly pick up changes in the system load (reported by metric servers), set dns-refresh to a smaller number.
If the load broker has not received a response from a metric server after three polling intervals, the load broker marks the address for removal from the DNS alias. This removal will occur at the next dns-refresh interval.
For earliest possible detection of this failure, the polling-interval should be set to a value that is less than one-third of the value specified as the dns-refresh period.
polling-interval < (dns-refresh) / 3
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The masters list specifies the name server to use for dynamic updates. This must point to the master name server. The name server must be set up to allow dynamic updates from the system that runs the load broker. For how to configure dynamic updates, see Section 5.3.6.
TCP/IP Services supports dynamic updating of only one master server in a DNS cluster environment.
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