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The man page says:

The getaddrinfo(3) function is not limited to creating IPv4 socket address structures; IPv6 socket address structures can be created if IPv6 support is available. These socket address structures can be used directly by bind(2) or connect(2), to prepare a client or a server socket.

What should I do to force getaddrinfo to not create IPv6 socket address structures?

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Is this for a program you're writing? Or are you trying to get an application that calls getaddrinfo with AF_UNSPEC not to use IPv6? – Gilles Aug 16 '12 at 22:58

According to the manpage for getaddrinfo(), you can pass the address family in the hints parameter, so something like

struct addrinfo hints, *result;
int s;

memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET;        /* or AF_INET6 for ipv6 addresses */
s = getaddrinfo(NULL, "ftp", &hints, &result);

I haven't tried this, but the approach seems to be right.

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Interestingly enough, the first question I ever asked on this site turned out to have an answer you might find useful.

To summarize, the file /etc/gai.conf is used by the getaddrinfo() system call to determine how to respond. For your particular case, adding

precedence ::ffff:0:0/96  100

to the end of the config file should be sufficient.

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Thanks for your answer, but in my CentOS 5.5 final, there is no /etc/gai.conf, do I need to create one? – Derui Si Aug 16 '12 at 9:55
Possibly. According to dpkg the file is part of the libc-bin package, which is pretty deep down the essential packages list in Debian... – Shadur Aug 16 '12 at 10:00

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