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Cisco Bug: CSCup77391 - ISDN GW DNS lookup issues when absolute rooted FQDN is not used

Last Modified

May 05, 2015

Products (1)

  • Cisco TelePresence ISDN Gateway

Known Affected Releases


Description (partial)

Symptom: ISDN GW DNS lookup issues when absolute rooted FQDN is not used
- The GW has a domain name (DNS suffix) specified under Network -> DNS
- The GW will always treat an address as an FQDN if the address has a trailing period/dot ('.'). For example: or mcu. (both are treated as FQDN). This is expected behaviour and defined in the RFC. This format is known as an absolute 'rooted' FQDN where the trailing '.' is for DNS root.
- If however, an address doesn't end with a trailing dot, the resolver client will realize that it is not an absolute 'rooted' FQDN. In the case of ISDN GW, this results in its DNS resolver performing the following:
For example, if the user is trying to resolve: and has a DNS suffix configured with,
1) It will first lookup will be:
2) If the first lookup fails, the resolver will then try:

While this is actually correct behaviour, the problem happens if the DNS server returns a successful reply to and it is not what the user is expecting. Some users may expect to be treated as an FQDN, when an absolute FQDN should in fact be: '' . The problem is further exacerbated if the lookup is happening beyond user intervention (e.g. a DNS SRV lookup returns an
FQDN without the trailing '.' due to misconfiguration and the GW then treats this as a non-FQDN address).
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