diff --git a/tests/letstest/scripts/boulder_install.sh b/tests/letstest/scripts/boulder_install.sh
index 7e298783f..f997268bd 100755
--- a/tests/letstest/scripts/boulder_install.sh
+++ b/tests/letstest/scripts/boulder_install.sh
@@ -5,5 +5,6 @@
 # Check out special branch until latest docker changes land in Boulder master.
 git clone -b docker-integration https://github.com/letsencrypt/boulder $BOULDERPATH
 cd $BOULDERPATH
-sed -i 's/FAKE_DNS: .*/FAKE_DNS: 172.17.42.1/' docker-compose.yml
+FAKE_DNS=$(ifconfig docker0 | grep "inet addr:" | cut -d: -f2 | awk '{ print $1}')
+sed -i "s/FAKE_DNS: .*/FAKE_DNS: $FAKE_DNS/" docker-compose.yml
 docker-compose up -d