Why can't Emacs talk to certain hosts (or certain hostnames)?

The problem may be that Emacs is linked with a wimpier version of gethostbyname than the rest of the programs on the machine. This is often manifested as a message on startup of `X server not responding. Check your DISPLAY environment variable.' or a message of `Unknown host' from open-network-stream.

On a Sun, this may be because Emacs had to be linked with the static C library. The version of gethostbyname in the static C library may only look in /etc/hosts and the NIS (YP) maps, while the version in the dynamic C library may be smart enough to check DNS in addition to or instead of NIS. On a Motorola Delta running System V R3.6, the version of gethosbyname in the standard library works, but the one that works with NIS doesn't (the one you get with -linet). Other operating systems have similar problems.

Try these options:

• Explicitly add the host you want to communicate with to /etc/hosts.
• Relink Emacs with this line in src/config.h:

    #define LIBS_SYSTEM -lresolv
  

• Replace gethostbyname and friends in libc.a with more useful versions such as the ones in libresolv.a. Then relink Emacs.
• If you are actually running NIS, make sure that `ypbind' is properly told to do DNS lookups with the correct command line switch.
• Use tcp.el and tcp.c from GNUS.This has the additional advantage that you can use numeric IP addresses instead of names. open-network-stream currently can't handle numeric addresses. Brian Thomson <thomson@hub.toronto.edu> has a enhancement to open-network-stream to allow it to handle numeric addresses.