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When I tell my Cisco router (IOS) to

(config)# logging syslog.mydomain.com

and then look at the config, the config contains the IP of syslog.mydomain.com, i.e. 1.2.3.4.

Now the whole purpose of DNS is to have a layer of abstraction between service names and actual hosts. In this case, if I move the syslog server to another IP I have to touch the Cisco again. (Looks like that anyway)

Is it normal that it doesn't simply store the syslog HOST name and regularly gets updates for the hostname from DNS?

What do you think?

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1 Answer 1

up vote 3 down vote accepted

It's normal. IOS translates hostnames at the time of configuration and stores the resulting IP address. This is also true for setting ntp servers, aaa, etc.

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Ok it's normal then. But not useful at all ;) –  Marki Jun 22 '12 at 0:16
1  
It depends on your point of view - the router needs its infrastructure services to be stable whether DNS is operational or not. If you want to script things then you can use SNMP or a login script. –  rnxrx Jun 22 '12 at 1:54
    
Sure, either syslogging always works independently of DNS (anyway a sensible DNS client has some kind of cache), or you consider DNS a critical service which gives you the added flexibility to move infrastructure services (like syslog) around transparently without having to touch everything. Can you elaborate on the point about syslog and SNMP/login script, I didn't get that. –  Marki Jun 22 '12 at 9:59
    
There are a number of places in the router configuration where a given service can be specified with either an IP or a hostname. The examples aaa ( TACACS+ / RADIUS), ntp, parts of ACL entries, VPN / crypto definitions and voice peering all behave like the syslog definition. The important point to remember he is that the router can work without DNS, but the inverse is not true. Waiting for queries to time out for the services mentioned so that it falls back to cache means the infrastructure can be completely hanging for some period of time. –  rnxrx Jun 22 '12 at 14:07

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