QA/Networking/Name resolution

Using libc functions

Name resolution features are provided by the GNU C Library (glibc) which is not yet ready for proper IPv6 and dual-stack operation as you can see when performing your tests. The C library comes with its own testing tool getent that has a special database called ahosts that runs getaddrinfo(), the library function that translates names to objects with addressing information. For your testing it is best used together with tools like strace, ltrace or even gdb so that you know exactly what is happening behind the scenes.

As the getent tool is very primitive, we created a tool called getaddrinfo just as the library function that handles a larger subset of the function's API.

Name resolution input

When application requests addressing information for a hostname with an optional service name, the library returns a list of addressing information objects. The order of objects in the list is significant and depends on operating system configuration and connectivity.

From the application

• nodename
• servname
• protocol
• socktype
• flags
  • AI_CANONNAME
  • ...

From local configuration and connectivity checks

• Files in /etc/ including nsswitch, hosts, services and more
• To what extend is IPv4 and IPv6 available

From the outside world

• DNS information
• Multicast DNS information
• LDAP information

Name resolution processing

What is requested

Not all information is requested at all times. Some information like canonical name must be explicitly requested by the application via AI_CANONNAME flag. It may be desirable to suppress other requests by local configuration or connectivity checks, a notable example being suppression of DNS AAAA queries on hosts without global connectivity.

What is passed to the application

Not all information that is learnt via requests is presented to the application. It is typically filtered according to input from the application. It is sometimes also filtered according to connectivity checks but that has caused more problems than improvements.

How it is sorted

There are rules for sorting addressing information returned by getaddrinfo(). One of the basic features is to return global IPv6 destinations before global IPv4 destinations. But when the library detects that IPv6 connectivity is not available, the reverse applies.

Using netresolve

There is an experimental package called netresolve (TODO: not yet in Fedora) that consists of a library somewhat similar to the glibc name resolution API implementation and a set of debugging tools. Any application using libc API and a couple of other APIs can be run using wrapresolve to use the netresolve implementation instead and benefit from some advanced features and more extensive debugging. When using the libc backend, netresolve can be also used to test the glibc implementation. With the nss backend it can be used to test glibc nsswitch backends directly.

$ ./netresolve --node www.nix.cz
response netresolve 0.0.1
name info.nix.cz
ip 2a02:38::1001 any any 0 0 0 21599
ip 195.47.235.3 any any 0 0 0 12589

You can see that netresolve behaves slightly different than getent ahosts and it by default returns one item per IP address. But you can easily tweak it to behave the same way.

 
$ ./netresolve --node www.nix.cz --service ''
response netresolve 0.0.1
name info.nix.cz
ip 2a02:38::1001 stream tcp 0 0 0 21021
ip 2a02:38::1001 dgram udp 0 0 0 21021
ip 2a02:38::1001 raw any 0 0 0 21021
ip 195.47.235.3 stream tcp 0 0 0 12592
ip 195.47.235.3 dgram udp 0 0 0 12592
ip 195.47.235.3 raw any 0 0 0 12592

Note that netresolve is both the name of the command and the name of the library. The above tests are done using the command but the same results would be given to an application using the library. Unless you explicitly request, netresolve uses its internal name resolution modules and not the libc functions.

Comparison of name resolution APIs

Now that there is the good old glibc implementation plus a new name resolution library, I would like to maintain a rather honest comparison table for their features.

For the beginning, I'd like to present API possibilities. Note that glibc is limited at two stages, one is the getaddrinfo API and the other is the nsswitch backend API. Unfortunately, nsswitch backend API has multiple entry points (even for the same task) and none of them provides a superset of the others' functionality.

Feature	getaddrinfo	getaddrinfo_a	asyncns	c-ares	netresolve	notes
POSIX	yes	no	no	no	no
glibc	yes	yes	no	no	no
nonblocking API	no	yes	yes	yes	yes
nonblocking backends	no	no	no	N/A	yes
pollable FD API	no	no	yes	yes	yes
extensible API	no	no	no	N/A	yes
connect/bind wrappers	no	no	no	no	yes
happy eyeballs	no	no	no	no	yes	netresolve optimizes DNS, TCP and (with application support) UDP
SRV records	no	no	DNS API	yes	yes
DNS API	no	no	yes	yes	yes
input: address family	possible	possible	possible	yes	yes	glibc nsswitch broken (gethotbyname2/gethostbyname3 only)
output: ifindex/scope_id	possible	possible	possible	N/A	yes	glibc nsswitch broken (gethostbyname4 only)
TTL	no	no	?	yes	yes	glibc nsswitch broken (gethostbyname3/gethostbyname4 only)
output: validity (e.g. DNSSEC)	no	no	no	yes	planned

And some notable backend implementations

Source	glibc	netresolve	notes
/etc/services	broken	yes	may even return a different address family at times
DNS	yes	yes
custom binary/script	no	yes
TBD...

Test cases

Dual-stack host, destination with global IPv4 and IPv6

On a host with IPv4 and IPv6 connectivity we request addressing information of another host that is announced as dual-stack in DNS.

$ getent ahosts www.nix.cz
2a02:38::1001   STREAM info.nix.cz
2a02:38::1001   DGRAM  
2a02:38::1001   RAW    
195.47.235.3    STREAM 
195.47.235.3    DGRAM  
195.47.235.3    RAW

You can see that getaddrinfo() returned six items for two unique IP addresses with the IPv6 address sorted first. If you only expected only two items, one for each IP address, see upstream bug 14990.

IPv4-only host, destination with global IPv4 and IPv6

We do the same on a host without IPv6 connectivity.

$ getent ahosts www.nix.cz
195.47.235.3    STREAM info.nix.cz
195.47.235.3    DGRAM  
195.47.235.3    RAW    
2a02:38::1001   STREAM 
2a02:38::1001   DGRAM  
2a02:38::1001   RAW    

You can see that the result is the same as before except that IPv4 is sorted first.