To build clientside on Debian sid, try redirect to newest Ubuntu release.

clientside/event/linktest/iperf/GNUmakefile.in

 #
-# Copyright (c) 2000-2011 University of Utah and the Flux Group.
+# Copyright (c) 2000-2018 University of Utah and the Flux Group.
 # 
 # {{{EMULAB-LICENSE
 # 
@@ -29,7 +29,7 @@ SYSTEM         := $(shell $(SRCDIR)/../../../tmcc/osstuff.sh -o)
 IPERFVERSION    = 2.0.2
 ifeq ($(SYSTEM),Linux)
 OSTAG=$(shell $(SRCDIR)/../../../tmcc/osstuff.sh -t)
-ifneq (,$(filter $(OSTAG),alpine debian9))
+ifneq (,$(filter $(OSTAG),alpine debian9 debianS))
 IPERFVERSION    = 2.0.10
 # If 2.0.10, we have to disable 64-bit seq numbers; they mess up the
 # client_hdr size in a backwards-incompat way.

clientside/tmcc/GNUmakefile.in

@@ -112,6 +112,11 @@ endif
 ifeq ($(MDSUBDIR),debian9)
 MDSUBDIR  = ubuntu16
 endif
+# Just point debian sid to the latest Ubuntu supported and hope for
+# the best.
+ifeq ($(MDSUBDIR),debianS)
+MDSUBDIR  = ubuntu16
+endif
 # We don't want to have a dir named "arch"; that would be confusing.
 # Hence archlinux.
 ifeq ($(MDSUBDIR),arch)

clientside/tmcc/osstuff.sh

 #!/bin/sh
 #
-# Copyright (c) 2007-2016 University of Utah and the Flux Group.
+# Copyright (c) 2007-2018 University of Utah and the Flux Group.
 # 
 # {{{EMULAB-LICENSE
 # 
@@ -100,6 +100,12 @@ Linux)
 	    if [ -n "$trel" ]; then
 		dist="Debian"
 		rel=$trel
+	    else
+		grep PRETTY_NAME /etc/os-release | grep -q sid
+		if [ $? -eq 0 ]; then
+		    rel=S
+		    tag=debianS
+		fi
 	    fi
 	fi
     fi