Does your program use gethostbyname() ?

There has been some discussion on the Debian IPv6 list about how the function gethostbyname() has changed.  The manual page says this function is obsolete and with good reason.

A new feature of the function is that if a hostname has both IPv4 and IPv6 entries then the order of the return values is not predefined.  In fact it used to be you’d get the IPv4 addresses first, then the IPv6.  That has now changed and with the more recent glibc you will get an IPv6 address first.  Quite often old code doesn’t even check the address family or the size of address structure but just copies the relevant 4 bytes.  You then get the first part of an 16-byte IPv6 address wedged into a 4-byte IPv4 address structure which basically means you get a mess.

The fix is pretty simple by changing the system call from gethostbyname() and using getaddrinfo() instead.  If you only want the IPv4 addresses, perhaps because other parts of the code have not been checked or changed, then set the hints variable to return only IPv4 addresses.

If one of your packages is starting to play up and unable to connect to certain remote places and is instead trying to get to some “random” address, have a quick check for gethostbyname(). A quick grep across the source code may save a lot of debugging time.