Request for Comments: 1537
Common DNS Data File Configuration Errors
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard. Distribution of this memo is unlimited.
This memo describes errors often found in DNS data files. It points out common mistakes system administrators tend to make and why they often go unnoticed for long periods of time.
Due to the lack of extensive documentation and automated tools, DNS zone files have mostly been configured by system administrators, by hand. Some of the rules for writing the data files are rather subtle and a few common mistakes are seen in domains worldwide.
This document is an attempt to list "surprises" that administrators might find hidden in their zone files. It describes the symptoms of the malady and prescribes medicine to cure that. It also gives some general recommendations and advice on specific nameserver and zone file issues and on the (proper) use of the Domain Name System.
1. SOA records
A problem I've found in quite some nameservers is that the various timers have been set (far) too low. Especially for top level domain nameservers this causes unnecessary traffic over international and intercontinental links.
Unfortunately the examples given in the BIND manual, in RFC's and in some expert documents give those very short timer values, and that's most likely what people have modeled their SOA records after.
First of all a short explanation of the timers used in the SOA record:
The SOA record of the primary server is checked every "refresh" time by the secondary servers; if it has changed, a zone transfer is done.
If a secondary server cannot reach the primary server, it tries it again every "retry" time.
If for "expire" time the primary server cannot be reached, all information about the zone is invalidated on the secondary servers (i.e., they are no longer authoritative for that zone).
- Minimum TTL:
The default TTL value for all records in the zone file; a different TTL value may be given explicitly in a record when necessary. (This timer is named "Minimum", and that's what it's function should be according to STD 13, RFC 1035, but most (all?) implementations take it as the default value exported with records without an explicit TTL value).
For top level domain servers I would recommend the following values:
86400 ; Refresh 24 hours 7200 ; Retry 2 hours 2592000 ; Expire 30 days 345600 ; Minimum TTL 4 days
For other servers I would suggest:
28800 ; Refresh 8 hours 7200 ; Retry 2 hours 604800 ; Expire 7 days 86400 ; Minimum TTL 1 day
but here the frequency of changes, the required speed of propagation, the reachability of the primary server etc. play a role in optimizing the timer values.
2. Glue records
Quite often, people put unnecessary glue (A) records in their zone files. Even worse is that I've even seen *wrong* glue records for an external host in a primary zone file! Glue records need only be in a zone file if the server host is within the zone and there is no A record for that host elsewhere in the zone file.
Old BIND versions ("native" 4.8.3 and older versions) showed the problem that wrong glue records could enter secondary servers in a zone transfer.
3. "Secondary server surprise"
I've seen it happen on various occasions that hosts got bombarded by nameserver requests without knowing why. On investigation it turned out then that such a host was supposed to (i.e., the information was in the root servers) run secondary for some domain (or reverse (in- addr.arpa)) domain, without that host's nameserver manager having been asked or even been told so!
Newer BIND versions (4.9 and later) solved this problem. At the same time though the fix has the disadvantage that it's far less easy to spot this problem.
Practice has shown that most domain registrars accept registrations of nameservers without checking if primary (!) and secondary servers have been set up, informed, or even asked. It should also be noted that a combination of long-lasting unreachability of primary nameservers, (therefore) expiration of zone information, plus static IP routing, can lead to massive network traffic that can fill up lines completely.
4. "MX records surprise"
In a sense similar to point 3. Sometimes nameserver managers enter MX records in their zone files that point to external hosts, without first asking or even informing the systems managers of those external hosts. This has to be fought out between the nameserver manager and the systems managers involved. Only as a last resort, if really nothing helps to get the offending records removed, can the systems manager turn to the naming authority of the domain above the offending domain to get the problem sorted out.
5. "Name extension surprise"
Sometimes one encounters weird names, which appear to be an external name extended with a local domain. This is caused by forgetting to terminate a name with a dot: names in zone files that don't end with a dot are always expanded with the name of the current zone (the domain that the zone file stands for or the last $ORIGIN).
zone file for foo.xx:
pqr MX 100 relay.yy. xyz MX 100 relay.yy (no trailing dot!)
When fully written out this stands for:
pqr.foo.xx. MX 100 relay.yy. xyz.foo.xx. MX 100 relay.yy.foo.xx. (name extension!)
6. Missing secondary servers
It is required that there be a least 2 nameservers for a domain. For obvious reasons the nameservers for top level domains need to be very well reachable from all over the Internet. This implies that there must be more than just 2 of them; besides, most of the (secondary) servers should be placed at "strategic" locations, e.g., close to a point where international and/or intercontinental lines come together. To keep things manageable, there shouldn't be too many servers for a domain either.
Important aspects in selecting the location of primary and secondary servers are reliability (network, host) and expedient contacts: in case of problems, changes/fixes must be carried out quickly. It should be considered logical that primary servers for European top level domains should run on a host in Europe, preferably (if possible) in the country itself. For each top level domain there should be 2 secondary servers in Europe and 2 in the USA, but there may of course be more on either side. An excessive number of nameservers is not a good idea though; a recommended maximum is 7 nameservers. In Europe, EUnet has offered to run secondary server for each European top level domain.
7. Wildcard MX records
Wildcard MX records should be avoided where possible. They often cause confusion and errors: especially beginning nameserver managers tend to overlook the fact that a host/domain listed with ANY type of record in a zone file is NOT covered by an overall wildcard MX record in that zone; this goes not only for simple domain/host names, but also for names that cover one or more domains. Take the following example in zone foo.bar:
* MX 100 mailhost pqr MX 100 mailhost abc.def MX 100 mailhost
This makes pqr.foo.bar, def.foo.bar and abd.def.foo.bar valid domains, but the wildcard MX record covers NONE of them, nor anything below them. To cover everything by MX records, the required entries are:
* MX 100 mailhost pqr MX 100 mailhost *.pqr MX 100 mailhost abc.def MX 100 mailhost *.def MX 100 mailhost *.abc.def MX 100 mailhost
An overall wildcard MX record is almost never useful.
In particular the zone file of a top level domain should NEVER contain only an overall wildcard MX record (*.XX). The effect of such a wildcard MX record can be that mail is unnecessarily sent across possibly expensive links, only to fail at the destination or gateway that the record points to. Top level domain zone files should explicitly list at least all the officially registered primary subdomains.
Whereas overall wildcard MX records should be avoided, wildcard MX records are acceptable as an explicit part of subdomain entries, provided they are allowed under a given subdomain (to be determined by the naming authority for that domain).
foo.xx. MX 100 gateway.xx. MX 200 fallback.yy. *.foo.xx. MX 100 gateway.xx. MX 200 fallback.yy. 8. Hostnames
People appear to sometimes look only at STD 11, RFC 822 to determine whether a particular hostname is correct or not. Hostnames should strictly conform to the syntax given in STD 13, RFC 1034 (page 11), with *addresses* in addition conforming to RFC 822. As an example take "c&w.blues" which is perfectly legal according to RFC 822, but which can have quite surprising effects on particular systems, e.g., "telnet c&w.blues" on a Unix system.
9. HINFO records
There appears to be a common misunderstanding that one of the data fields (usually the second field) in HINFO records is optional. A recent scan of all reachable nameservers in only one country revealed some 300 incomplete HINFO records. Specifying two data fields in a HINFO record is mandatory (RFC 1033), but note that this does *not* mean that HINFO records themselves are mandatory.
10. Safety measures and specialties
Nameservers and resolvers aren't flawless. Bogus queries should be kept from being forwarded to the root servers, since they'll only lead to unnecessary intercontinental traffic. Known bogus queries that can easily be dealt with locally are queries for 0 and broadcast addresses. To catch such queries, every nameserver should run primary for the 0.in-addr.arpa and 255.in-addr.arpa zones; the zone files need only contain a SOA and an NS record.
Also each nameserver should run primary for 0.0.127.in-addr.arpa; that zone file should contain a SOA and NS record and an entry:
1 PTR localhost.
There has been extensive discussion about whether or not to append the local domain to it. The conclusion was that "localhost." would be the best solution; reasons given were:
- "localhost" itself is used and expected to work on some systems.
- translating 127.0.0.1 into "localhost.my_domain" can cause some software to connect to itself using the loopback interface when it didn't want to.
Note that all domains that contain hosts should have a "localhost" A record in them.
People maintaining zone files with the Serial number given in dotted decimal notation (e.g., when SCCS is used to maintain the files) should beware of a bug in all BIND versions: if the serial number is in Release.Version (dotted decimal) notation, then it is virtually impossible to change to a higher release: because of the wrong way that notation is turned into an integer, it results in a serial number that is LOWER than that of the former release.
For this reason and because the Serial is an (unsigned) integer according to STD 13, RFC 1035, it is recommended not to use the dotted decimal notation. A recommended notation is to use the date (yyyymmdd), if necessary with an extra digit (yyyymmddn) if there is or can be more than one change per day in a zone file.
Very old versions of DNS resolver code have a bug that causes queries for A records with domain names like "18.104.22.168" to go out. This happens when users type in IP addresses and the resolver code does not catch this case before sending out a DNS query. This problem has been fixed in all resolver implementations known to us but if it still pops up it is very serious because all those queries will go to the root servers looking for top level domains like "3" etc. It is strongly recommended to install the latest (publicly) available BIND version plus all available patches to get rid of these and other problems.
Running secondary nameserver off another secondary nameserver is possible, but not recommended unless really necessary: there are known cases where it has led to problems like bogus TTL values. This can be caused by older or flawed implementations, but secondary nameservers in principle should always transfer their zones from the official primary nameserver.
11. Some general points
The Domain Name System and nameserver are purely technical tools, not meant in any way to exert control or impose politics. The function of a naming authority is that of a clearing house. Anyone registering a subdomain under a particular (top level) domain becomes naming authority and therewith the sole responsible for that subdomain. Requests to enter MX or NS records concerning such a subdomain therefore always MUST be honored by the registrar of the next higher domain.
Examples of practices that are not allowed are:
- imposing specific mail routing (MX records) when registering a subdomain.
- making registration of a subdomain dependent on to the use of certain networks or services.
- using TXT records as a means of (free) commercial advertising.
In the latter case a network service provider could decide to cut off a particular site until the offending TXT records have been removed from the site's zone file.
Of course there are obvious cases where a naming authority can refuse to register a particular subdomain and can require a proposed name to be changed in order to get it registered (think of DEC trying to register a domain IBM.XX).
There are also cases were one has to probe the authority of the person: sending in the application - not every systems manager should be able to register a domain name for a whole university. The naming authority can impose certain extra rules as long as they don't violate or conflict with the rights and interest of the registrars of subdomains; a top level domain registrar may e.g., require that there be primary subdomain "ac" and "co" only and that subdomains be registered under those primary subdomains.
The naming authority can also interfere in exceptional cases like the one mentioned in point 4, e.g., by temporarily removing a domain's entry from the nameserver zone files; this of course should be done only with extreme care and only as a last resort.
When adding NS records for subdomains, top level domain nameserver managers should realize that the people setting up the nameserver for a subdomain often are rather inexperienced and can make mistakes that can easily lead to the subdomain becoming completely unreachable or that can cause unnecessary DNS traffic (see point 1). It is therefore highly recommended that, prior to entering such an NS record, the (top level) nameserver manager does a couple of sanity checks on the new nameserver (SOA record and timers OK?, MX records present where needed? No obvious errors made? Listed secondary servers operational?). Things that cannot be caught though by such checks are:
- resolvers set up to use external hosts as nameservers
- nameservers set up to use external hosts as forwarders without permission from those hosts.
Care should also be taken when registering 2-letter subdomains. Although this is allowed, an implication is that abbreviated addressing (see STD 11, RFC 822, paragraph 6.2.2) is not possible in and under that subdomain. When requested to register such a domain, one should always notify the people of this consequence. As an example take the name "cs", which is commonly used for Computer Science departments: it is also the name of the top level domain for Czecho-Slovakia, so within the domain cs.foo.bar the firstname.lastname@example.org is ambiguous in that in can denote both a user on the host host.cs.foo.bar and a user on the host "host" in Czecho-Slovakia. (This example does not take into account the recent political changes in the mentioned country).
 Mockapetris, P., "Domain Names Concepts and Facilities", STD 13, RFC 1034, USC/Information Sciences Institute, November 1987.  Mockapetris, P., "Domain Names Implementation and Specification", STD 13, RFC 1035, USC/Information Sciences Institute, November 1987.  Partridge, C., "Mail Routing and the Domain System", STD 14, RFC 974, CSNET CIC BBN, January 1986.  Gavron, E., "A Security Problem and Proposed Correction With Widely Deployed DNS Software", RFC 1535, ACES Research Inc., October 1993.  Kumar, A., Postel, J., Neuman, C., Danzig, P., and S. Miller, "Common DNS Implementation Errors and Suggested Fixes", RFC 1536, USC/Information Sciences Institute, USC, October 1993.
Security issues are not discussed in this memo.
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