When I dig the IP addresses for the different OS types, I get a different set than for nat.travisci.net:
diff -u <(dig +short nat.travisci.net | sort | uniq) <(dig +short nat.gce-us-central1.travisci.net nat.macstadium-us-se-1.travisci.net nat.gce-us-central1.travisci.net | sort | uniq)
--- /dev/fd/63 2019-06-27 10:09:13.628876175 +0100
+++ /dev/fd/62 2019-06-27 10:09:13.629876180 +0100
@@ -1,12 +1,13 @@
104.154.113.151
104.154.120.187
+104.198.131.58
207.254.16.35
207.254.16.36
207.254.16.37
207.254.16.38
207.254.16.39
-34.233.56.198
-34.234.4.53
+34.66.178.120
+34.68.144.114
35.184.226.236
35.184.96.71
35.188.1.99
@@ -14,12 +15,7 @@
35.192.136.167
35.192.187.174
35.192.85.2
+35.193.14.140
35.193.7.13
35.202.145.110
35.224.112.202
-52.3.55.28
-52.45.185.117
-52.45.220.64
-52.54.31.11
-52.54.40.118
-54.208.31.17
So perhaps the correct set of NAT IP addresses is this:
dig +short nat.gce-us-central1.travisci.net nat.macstadium-us-se-1.travisci.net nat.gce-us-central1.travisci.net | sort | uniq
104.154.113.151
104.154.120.187
104.198.131.58
207.254.16.35
207.254.16.36
207.254.16.37
207.254.16.38
207.254.16.39
34.66.178.120
34.68.144.114
35.184.226.236
35.184.96.71
35.188.1.99
35.188.73.34
35.192.136.167
35.192.187.174
35.192.85.2
35.193.14.140
35.193.7.13
35.202.145.110
35.224.112.202