Network Working Group D. Barr Request for Comments: 1912 The Pennsylvania State University Obsoletes: 1537 February 1996 Category: Informational
Make sure your PTR and A records match. For every IP address, there should be a matching PTR record in the in-addr.arpa domain. If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all. Also, PTR records must point back to a valid A record, not a alias defined by a CNAME. It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data.
DNS domain names consist of "labels" separated by single dots. The DNS is very liberal in its rules for the allowable characters in a domain name. However, if a domain name is used to name a host, it should follow rules restricting host names. Further if a name is used for mail, it must follow the naming rules for names in mail addresses.
Allowable characters in a label for a host name are only ASCII letters, digits, and the `-' character. Labels may not be all numbers, but may have a leading digit (e.g., 3com.com). Labels must end and begin only with a letter or digit. See RFC 1035 and RFC 1123. (Labels were initially restricted in to start with RFC 1035 a letter, and some older hosts still reportedly have problems with the relaxation in RFC 1123. .) Note there are some Internet hostnames which violate this rule (411.org, 1776.com). The presence of underscores in a label is allowed in RFC 1033. , except RFC 1033. is informational only and was not defining a standard. There is at least one popular TCP/IP implementation which currently refuses to talk to hosts named with underscores in them. It must be noted that the language in RFC 1035 is such that these rules are voluntary -- they are there for those who wish to minimize problems. Note that the rules for Internet host names also apply to hosts and addresses used in SMTP (See RFC 821 ).
If a domain name is to be used for mail (not involving SMTP), it must follow the rules for mail in RFC 822 , which is actually more liberal than the above rules. Labels for mail can be any ASCII character except "specials", control characters, and whitespace characters. "Specials" are specific symbols used in the parsing of addresses. They are the characters "()<>@,;:\".[]". (The "!" character wasn't in RFC 822 , however it also shouldn't be used due to the conflict with UUCP mail as defined in RFC 976 ) However, since today almost all names which are used for mail on the Internet are also names used for hostnames, one rarely sees addresses using these relaxed standard, but mail software should be made liberal and robust enough to accept them.
You should also be careful to not have addresses which are valid alternate syntaxes to the inet_ntoa() library call. For example 0xe is a valid name, but if you were to type "telnet 0xe", it would try to connect to IP address 0.0.0.14. It is also rumored that there exists some broken inet_ntoa() routines that treat an address like x400 as an IP address.
Certain operating systems have limitations on the length of their own hostname. While not strictly of issue to the DNS, you should be aware of your operating system's length limits before choosing the name of a host.
Remember that many resource records (abbreviated RR) take on more than one argument. HINFO requires two arguments, as does RP. If you don't supply enough arguments, servers sometime return garbage for the missing fields. If you need to include whitespace within any data, you must put the string in quotes.
Even though some BIND versions allow you to use a decimal in a serial number, don't. A decimal serial number is converted to an unsigned 32-bit integer internally anyway. The formula for a n.m serial number is n*10^(3+int(0.9+log10(m))) + m which translates to something rather unexpected. For example it's routinely possible with a decimal serial number (perhaps automatically generated by SCCS) to be incremented such that it is numerically larger, but after the above conversion yield a serial number which is LOWER than before. Decimal serial numbers have been officially deprecated in recent BIND versions. The recommended syntax is YYYYMMDDnn (YYYY=year, MM=month, DD=day, nn=revision number. This won't overflow until the year 4294.
Choose logical values for the timer values in the SOA record (note values below must be expressed as seconds in the zone data):
As you can see, the typical values above for the timers vary widely. Popular documentation like RFC 1033 recommended a day for the minimum TTL, which is now considered too low except for zones with data that vary regularly. Once a DNS stabilizes, values on the order of 3 or more days are recommended. It is also recommended that you individually override the TTL on certain RRs which are often referenced and don't often change to have very large values (1-2 weeks). Good examples of this are the MX, A, and PTR records of your mail host(s), the NS records of your zone, and the A records of your nameservers.
podunk.xx. in ns ns1.podunk.xx.
in ns ns2.podunk.xx.
ns1.podunk.xx. in a 1.2.3.4
ns2.podunk.xx. in a 1.2.3.5
Here, the A records are referred to as "Glue records".
Glue records are required only in forward zone files for nameservers that are located in the subdomain of the current zone that is being delegated. You shouldn't have any A records in an in-addr.arpa zone file (unless you're using RFC 1101 -style encoding of subnet masks).
If your nameserver is multi-homed (has more than one IP address), you must list all of its addresses in the glue to avoid cache inconsistency due to differing TTL values, causing some lookups to not find all addresses for your nameserver.
Some people get in the bad habit of putting in a glue record whenever they add an NS record "just to make sure". Having duplicate glue records in your zone files just makes it harder when a nameserver moves to a new IP address, or is removed. You'll spend hours trying to figure out why random people still see the old IP address for some host, because someone forgot to change or remove a glue record in some other file. Newer BIND versions will ignore these extra glue records in local zone files.
Older BIND versions (4.8.3 and previous) have a problem where it inserts these extra glue records in the zone transfer data to secondaries. If one of these glues is wrong, the error can be propagated to other nameservers. If two nameservers are secondaries for other zones of each other, it's possible for one to continually pass old glue records back to the other. The only way to get rid of the old data is to kill both of them, remove the saved backup files, and restart them. Combined with that those same versions also tend to become infected more easily with bogus data found in other non- secondary nameservers (like the root zone data).
podunk.xx. IN NS ns1
IN NS ns2
IN CNAME mary
mary IN A 1.2.3.4
This is often attempted by inexperienced administrators as an obvious
way to allow your domain name to also be a host. However, DNS
servers like BIND will see the CNAME and refuse to add any other
resources for that name. Since no other records are allowed to
coexist with a CNAME, the NS entries are ignored. Therefore all the
hosts in the podunk.xx domain are ignored as well!
If you want to have your domain also be a host, do the following:
podunk.xx. IN NS ns1
IN NS ns2
IN A 1.2.3.4
mary IN A 1.2.3.4
Don't go overboard with CNAMEs. Use them when renaming hosts, but
plan to get rid of them (and inform your users). However CNAMEs are
useful (and encouraged) for generalized names for servers -- `ftp'
for your ftp server, `www' for your Web server, `gopher' for your
Gopher server, `news' for your Usenet news server, etc.
Don't forget to delete the CNAMEs associated with a host if you delete the host it is an alias for. Such "stale CNAMEs" are a waste of resources.
Don't use CNAMEs in combination with RRs which point to other names like MX, CNAME, PTR and NS. (PTR is an exception if you want to implement classless in-addr delegation.) For example, this is strongly discouraged:
podunk.xx. IN MX mailhost
mailhost IN CNAME mary
mary IN A 1.2.3.4
RFC 1034 in section 3.6.2 says this should not be done, and RFC 974 explicitly states that MX records shall not point to an alias defined by a CNAME. This results in unnecessary indirection in accessing the data, and DNS resolvers and servers need to work more to get the answer. If you really want to do this, you can accomplish the same thing by using a preprocessor such as m4 on your host files.
Also, having chained records such as CNAMEs pointing to CNAMEs may make administration issues easier, but is known to tickle bugs in some resolvers that fail to check loops correctly. As a result some hosts may not be able to resolve such names.
Having NS records pointing to a CNAME is bad and may conflict badly with current BIND servers. In fact, current BIND implementations will ignore such records, possibly leading to a lame delegation. There is a certain amount of security checking done in BIND to prevent spoofing DNS NS records. Also, older BIND servers reportedly will get caught in an infinite query loop trying to figure out the address for the aliased nameserver, causing a continuous stream of DNS requests to be sent.
Put MX records even on hosts that aren't intended to send or receive e-mail. If there is a security problem involving one of these hosts, some people will mistakenly send mail to postmaster or root at the site without checking first to see if it is a "real" host or just a terminal or personal computer that's not set up to accept e-mail. If you give it an MX record, then the e-mail can be redirected to a real person. Otherwise mail can just sit in a queue for hours or days until the mailer gives up trying to send it.
Don't forget that whenever you add an MX record, you need to inform the target mailer if it is to treat the first host as "local". (The "Cw" flag in sendmail, for example)
If you add an MX record which points to an external host (e.g., for the purposes of backup mail routing) be sure to ask permission from that site first. Otherwise that site could get rather upset and take action (like throw your mail away, or appeal to higher authorities like your parent DNS administrator or network provider.)
HINFO is sometimes abused to provide other information. The record is meant to provide specific information about the machine itself. If you need to express other information about the host in the DNS, use TXT.
podunk.xx. IN NS ns1
IN NS ns2
mary IN A 1.2.3.4
*.podunk.xx. IN MX 5 sue
Mail for mary.podunk.xx will be sent to itself for delivery. Only
mail for jane.podunk.xx or any hosts you don't see above will be sent
to the MX. For most Internet sites, wildcard MX records are not
useful. You need to put explicit MX records on every host.
Wildcard MXs can be bad, because they make some operations succeed when they should fail instead. Consider the case where someone in the domain "widget.com" tries to send mail to "joe@larry". If the host "larry" doesn't actually exist, the mail should in fact bounce immediately. But because of domain searching the address gets resolved to "larry.widget.com", and because of the wildcard MX this is a valid address according to DNS. Or perhaps someone simply made a typo in the hostname portion of the address. The mail message then gets routed to the mail host, which then rejects the mail with strange error messages like "I refuse to talk to myself" or "Local configuration error".
Wildcard MX records are good for when you have a large number of hosts which are not directly Internet-connected (for example, behind a firewall) and for administrative or political reasons it is too difficult to have individual MX records for every host, or to force all e-mail addresses to be "hidden" behind one or more domain names. In that case, you must divide your DNS into two parts, an internal DNS, and an external DNS. The external DNS will have only a few hosts and explicit MX records, and one or more wildcard MXs for each internal domain. Internally the DNS will be complete, with all explicit MX records and no wildcards.
Wildcard As and CNAMEs are possible too, and are really confusing to users, and a potential nightmare if used without thinking first. It could result (due again to domain searching) in any telnet/ftp attempts from within the domain to unknown hosts to be directed to one address. One such wildcard CNAME (in *.edu.com) caused Internet-wide loss of services and potential security nightmares due to unexpected interactions with domain searching. It resulted in swift fixes, and even an RFC ( RFC 1535 ) documenting the problem.
The "classic" lame delegation can be illustrated in this example:
podunk.xx. IN NS ns1.podunk.xx.
IN NS ns0.widget.com.
"podunk.xx" is a new domain which has recently been created, and
"ns1.podunk.xx" has been set up to perform nameservice for the zone.
They haven't quite finished everything yet and haven't made sure that
the hostmaster at "ns0.widget.com" has set up to be a proper
secondary, and thus has no information about the podunk.xx domain,
even though the DNS says it is supposed to. Various things can
happen depending on which nameserver is used. At best, extra DNS
traffic will result from a lame delegation. At worst, you can get
unresolved hosts and bounced e-mail.
Also, sometimes a nameserver is moved to another host or removed from the list of secondaries. Unfortunately due to caching of NS records, many sites will still think that a host is a secondary after that host has stopped providing nameservice. In order to prevent lame delegations while the cache is being aged, continue to provide nameservice on the old nameserver for the length of the maximum of the minimum plus refresh times for the zone and the parent zone. (See section 2.2)
Whenever a primary or secondary is removed or changed, it takes a fair amount of human coordination among the parties involved. (The site itself, it's parent, and the site hosting the secondary) When a primary moves, make sure all secondaries have their named.boot files updated and their servers reloaded. When a secondary moves, make sure the address records at both the primary and parent level are changed.
It's also been reported that some distant sites like to pick popular nameservers like "ns.uu.net" and just add it to their list of NS records in hopes that they will magically perform additional nameservice for them. This is an even worse form of lame delegation, since this adds traffic to an already busy nameserver. Please contact the hostmasters of sites which have lame delegations. Various tools can be used to detect or actively find lame delegations. See the list of contributed software in the BIND distribution.
Make sure your parent domain has the same NS records for your zone as you do. (Don't forget your in-addr.arpa zones too!). Do not list too many (7 is the recommended maximum), as this just makes things harder to manage and is only really necessary for very popular top- level or root zones. You also run the risk of overflowing the 512- byte limit of a UDP packet in the response to an NS query. If this happens, resolvers will "fall back" to using TCP requests, resulting in increased load on your nameserver.
It's important when picking geographic locations for secondary nameservers to minimize latency as well as increase reliability. Keep in mind network topologies. For example if your site is on the other end of a slow local or international link, consider a secondary on the other side of the link to decrease average latency. Contact your Internet service provider or parent domain contact for more information about secondaries which may be available to you.
Don't forget to change the serial number when you change data! If you don't, your secondaries will not transfer the new zone information. Automating the incrementing of the serial number with software is also a good idea.
If you make a mistake and increment the serial number too high, and you want to reset the serial number to a lower value, use the following procedure:
This procedure won't work if one of your secondaries is running an old version of BIND (4.8.3 or earlier). In this case you'll have to contact the hostmaster for that secondary and have them kill the secondary servers, remove the saved backup file, and restart the server. Be careful when editing the serial number -- DNS admins don't like to kill and restart nameservers because you lose all that cached data.
Be consistent with the style of entries in your DNS files. If your $ORIGIN is podunk.xx., try not to write entries like:
mary IN A 1.2.3.1
sue.podunk.xx. IN A 1.2.3.2
or:
bobbi IN A 1.2.3.2
IN MX mary.podunk.xx.
Either use all FQDNs (Fully Qualified Domain Names) everywhere or
used unqualified names everywhere. Or have FQDNs all on the right-
hand side but unqualified names on the left. Above all, be
consistent.
Use tabs between fields, and try to keep columns lined up. It makes it easier to spot missing fields (note some fields such as "IN" are inherited from the previous record and may be left out in certain circumstances.)
Remember you don't need to repeat the name of the host when you are defining multiple records for one host. Be sure also to keep all records associated with a host together in the file. It will make things more straightforward when it comes time to remove or rename a host.
Always remember your $ORIGIN. If you don't put a `.' at the end of an FQDN, it's not recognized as an FQDN. If it is not an FQDN, then the nameserver will append $ORIGIN to the name. Double check, triple check, those trailing dots, especially in in-addr.arpa zone files, where they are needed the most.
Be careful with the syntax of the SOA and WKS records (the records which use parentheses). BIND is not very flexible in how it parses these records. See the documentation for BIND.
It is also highly recommended that you automate this checking, either with software which runs sanity checks on the data files before they are loaded into the nameserver, or with software which checks the data already loaded in the nameserver. Some contributed software to do this is included in the BIND distribution.
primary localhost localhost
primary 0.0.127.in-addr.arpa 127.0
primary 255.in-addr.arpa 255
primary 0.in-addr.arpa 0
These are set up to either provide nameservice for "special"
addresses, or to help eliminate accidental queries for broadcast or
local address to be sent off to the root nameservers. All of these
files will contain NS and SOA records just like the other zone files
you maintain, the exception being that you can probably make the SOA
timers very long, since this data will never change.
The "localhost" address is a "special" address which always refers to the local host. It should contain the following line:
localhost. IN A 127.0.0.1
The "127.0" file should contain the line:
1 PTR localhost.
There has been some extensive discussion about whether or not to
append the local domain to it. The conclusion is that "localhost."
would be the best solution. The reasons given include:
The "255" and "0" files should not contain any additional data beyond the NS and SOA records.
Note that future BIND versions may include all or some of this data automatically without additional configuration.
While running a secondary nameserver off another secondary nameserver is possible, it is not recommended unless necessary due to network topologies. There are known cases where it has led to problems like bogus TTL values. While this may be caused by older or flawed DNS implementations, you should not chain secondaries off of one another since this builds up additional reliability dependencies as well as adds additional delays in updates of new zone data.
Common DNS Operational and Configuration Errors
HTML-Version: Uwe Ohse (14.3.1996)
Changes by Uwe Ohse: