Diagnostic aids for IPv6 support

For information about configuring an IPv6 Daemon, see z/OS Communications Server: IP Configuration Guide.

In addition, to handle network variation, the following are useful.

Failed to open socket.
When invoking sendmail, if it fails to open a socket, the following log message is displayed:
```
opendaemonsocket: daemon < MTA_name>: cannot create server SMTP socket"
opendaemonsocket: daemon <MTA_name>: problem creating SMTP socket"
```
Consider the following to solve this problem:
- Is the TCP/IP stack enabled for IPv4 or IPv6?
- Is the DaemonPortOption in sendmail configuration file (sendmail.cf) properly set? (Remember that an IPv6 daemon option cannot run on a IPv4-only stack.)
DNS support.
When sendmail runs as a IPv6-enable daemon, it needs to do two things:
- Receive mails with long-type address
- Make AAAA type queries with DNS
In some database files (for example, aliases, relay-domains, or access), if mail which is targeted to a legal IPv6 site always fails to be sent, check whether name server supports IPv6 (AAAA type queries).
If DNS queries are failing, see RESOLVER trace (SYSTCPRE) for information about how to run a resolver trace.
To determine whether the name server is IPv6-capable, issue the following:
```
dig @<address_of_name_server> <host_name_of_target> aaaa
```
If this does not return an IPv6 address, either the name server is not IPv6-capable or the name server is not configured properly.
To determine whether the name server is IPv6-capable and the name server is a bind-based name server (not Microsoft), issue the following:
```
dig @<address_of_name_server> version.bind chaos any
```
If the version of bind returned is 9.0 or greater, the name server is IPv6-capable, so it is likely not configured properly for IPv6. DNS administrators can restrict the name server from giving out its bind version, but if any type of an answer is received other than a failed query response, the name server is IPv6-capable. If the query fails, the name server cannot support IPv6.