utmp/wtmp/lastlog host field too short
In <include/utmp.h> the host field for utmp, wtmp and lastlog is set to 16 characters. This has the effect that both IPv6 addresses and the resolved hostnames sshd puts in lastlog are being truncated. It surfaces as truncated host names in the SSH "Last login: <time> from <host>" message on interactive logins.
IPv6 addresses are max. 39 characters long, 45 if you allow for IPv4 compatible notation: http://stackoverflow.com/questions/166132/maximum-length-of-the-textual-representation-of-an-ipv6-address but since there may be hostnames in there it should probably be MAXHOSTNAME (cf. <sys/param.h>).
Although changing the wtmp and lastlogin file format is probably not easy to achieve safely :(