Which hostname will a web server determine for a client via reverse DNS?

Question

Excerpt from the book I am reading:

Most web servers can be configured to convert client IP addresses into client hostnames, using “reverse DNS.” Web servers can use the client hostname for detailed access control and logging.

I am a little confused about client hostname identification. I thought the clients don't even have hostnames, just IP addresses. I understand that servers are identified by hostnames a.k.a. domain names, but I can't make the connection between identifying browser by hostname. What exactly do you identify when identifying client hostname? Just the IP address?

Answer 1

DNS servers can be configured to provide not only hostname -> address mappings but also address -> hostname mappings.

Typically, the reverse mappings (address -> name) are the same as the name -> address mappings. So if a normal DNS lookup for www.example.com returns 192.0.2.1, a reverse DNS lookup for 192.0.2.1 returns www.example.com.

So yes, if the DNS server that is authoritative for the reverse mapping records for your block of IPv4 addresses is configured well, then when another host asks the DNS system what hostname is associated with your IP address, it will get back an answer of a full DNS-style hostname (that is, a "fully qualified domain name" or FQDN, like "somehost5.example.com"), and a lookup of that FQDN will return your IP address as well.

Answer 2

• "hostname aka domain name": Read more about these terms. There's a huge difference between a hostname and a domain name.
• "browser by hostname": The browser has absolutely nothing to do with this, although in principle, it might reveal the local hostname in the header of its request. But that's not happening too often these days and is completely unrelated to reverse DNS.
• The server will only see the public IP address of the NATting gateway which comes last on the route from client to server; read about gateways and NAT. Public gateways usually have names. E.g. if a home is connected via DSL, the home router will be assigned a public IP address with a name, such as "dsl-pool-1234.kingfisher-networking.dunno". Comapnies also use gateways. Only in rare cases, there'll be no name available.