telnet: error: relocation error: symbol krb5int_labeled_fopen, version krb5support_0_MIT not defined in file libkrb5support.so.0 with link time reference (fatal)
Googling hinted at a conflict in the shared library providing kerberos authentication. So I ran telnet under LD_DEBUG=all :
LD_DEBUG=all telnet host port
which, showed me the problem:
29655: symbol=krb5int_labeled_fopen; lookup in file=/usr/local/greenplum-db/lib/libkrb5support.so.0  29655: telnet: error: relocation error: symbol krb5int_labeled_fopen, version krb5support_0_MIT not defined in file libkrb5support.so.0 with link time reference (fatal)
So, an installation of greenplum had inserted its version of the kerberos library ahead of the search path for libraries the linux loader uses. The kerberos version of the library did not export the said function.
This all can be verified quickly :
login@host ~$ nm -D /usr/local/greenplum-db/lib/libkrb5support.so.0 | grep krb5int_labeled_fopen
login@host ~$ nm -D /usr/lib64/libkrb5support.so.0 | grep krb5int_labeled_fopen 00000033aea040b0 T krb5int_labeled_fopen
The greeenplum installation was using the LD_LIBRARY_PATH to allow it preferential status, so inserting the /usr/lib64 before it, was sufficient to help telnet find the right library.
login@host ~$ export LD_LIBRARY_PATH=/usr/lib64:$LD_LIBRARY_PATH
login@host ~$ telnet host port Trying host... Connected to host (ip). Escape character is '^]'.