diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 45199b268..041a4efdb 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -69,7 +69,7 @@ jobs:
 
   package:
     name: package-${{ matrix.compiler }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         include:
@@ -166,7 +166,7 @@ jobs:
 
   clang:
     name: clang-${{ matrix.clang_ver }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         clang_ver: ["6.0", "7", "8", "9"]
@@ -186,7 +186,7 @@ jobs:
 
   clang-m32:
     name: clang-m32-${{ matrix.clang_ver }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         clang_ver: ["6.0", "7", "8", "9"]
@@ -210,7 +210,7 @@ jobs:
           script/check-posix-build
 
   gn:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
@@ -257,7 +257,7 @@ jobs:
         script/check-simulation-build
 
   android:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Build
diff --git a/.github/workflows/docker.yml b/.github/workflows/docker.yml
index c2e540140..d3f369f88 100644
--- a/.github/workflows/docker.yml
+++ b/.github/workflows/docker.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -42,7 +42,7 @@ jobs:
 
   buildx:
     name: buildx-${{ matrix.docker_name }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         include:
diff --git a/.github/workflows/fuzz.yml b/.github/workflows/fuzz.yml
index 818664a84..8f549c505 100644
--- a/.github/workflows/fuzz.yml
+++ b/.github/workflows/fuzz.yml
@@ -30,7 +30,7 @@ name: CIFuzz
 on: [pull_request]
 jobs:
  Fuzzing:
-   runs-on: ubuntu-latest
+   runs-on: ubuntu-20.04
    steps:
    - name: Build Fuzzers
      uses: google/oss-fuzz/infra/cifuzz/actions/build_fuzzers@master
diff --git a/.github/workflows/posix.yml b/.github/workflows/posix.yml
index 8fd18d30d..ea68ed4b8 100644
--- a/.github/workflows/posix.yml
+++ b/.github/workflows/posix.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -42,7 +42,7 @@ jobs:
 
 
   posix-cli:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       PYTHONUNBUFFERED: 1
@@ -94,7 +94,7 @@ jobs:
         path: tmp/coverage.info
 
   posix-ncp:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       NODE_TYPE: ncp-sim
@@ -148,7 +148,7 @@ jobs:
         path: tmp/coverage.info
 
   posix-ncp-rcp-migrate:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
     steps:
@@ -188,7 +188,7 @@ jobs:
         path: tmp/coverage.info
 
   posix-pty:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
     steps:
@@ -196,7 +196,7 @@ jobs:
     - name: Bootstrap
       run: |
         sudo rm /etc/apt/sources.list.d/* && sudo apt-get update
-        sudo apt-get --no-install-recommends install -y socat expect lcov
+        sudo apt-get --no-install-recommends install -y socat expect lcov net-tools
         cd /tmp
         wget https://github.com/obgm/libcoap/archive/bsd-licensed.tar.gz
         tar xvf bsd-licensed.tar.gz
@@ -236,7 +236,7 @@ jobs:
         path: tmp/coverage.info
 
   posix-pty-daemon:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       DAEMON: 1
@@ -245,7 +245,7 @@ jobs:
     - name: Bootstrap
       run: |
         sudo rm /etc/apt/sources.list.d/* && sudo apt-get update
-        sudo apt-get --no-install-recommends install -y socat expect lcov
+        sudo apt-get --no-install-recommends install -y socat expect lcov net-tools
         cd /tmp
         wget https://github.com/obgm/libcoap/archive/bsd-licensed.tar.gz
         tar xvf bsd-licensed.tar.gz
@@ -291,7 +291,7 @@ jobs:
     - posix-ncp-rcp-migrate
     - posix-pty
     - posix-pty-daemon
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
diff --git a/.github/workflows/simulation-1.2.yml b/.github/workflows/simulation-1.2.yml
index b9054e00e..69f71d3da 100644
--- a/.github/workflows/simulation-1.2.yml
+++ b/.github/workflows/simulation-1.2.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -42,7 +42,7 @@ jobs:
 
   thread-1-2:
     name: thread-1-2-${{ matrix.compiler.c }}-${{ matrix.arch }}
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CFLAGS: -${{ matrix.arch }}
       CXXFLAGS: -${{ matrix.arch }}
@@ -112,7 +112,7 @@ jobs:
         path: tmp/coverage.info
 
   packet-verification-low-power:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       REFERENCE_DEVICE: 1
       VIRTUAL_TIME: 1
@@ -170,7 +170,7 @@ jobs:
         path: tmp/coverage.info
 
   packet-verification-1-1-on-1-2:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       REFERENCE_DEVICE: 1
       VIRTUAL_TIME: 1
@@ -211,7 +211,7 @@ jobs:
         path: tmp/coverage.info
 
   expects:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       THREAD_VERSION: 1.2
@@ -299,7 +299,7 @@ jobs:
     - packet-verification-1-1-on-1-2
     - expects
     - thread-1-2-backbone
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
diff --git a/.github/workflows/simulation.yml b/.github/workflows/simulation.yml
index 46d86e2c2..b9727685b 100644
--- a/.github/workflows/simulation.yml
+++ b/.github/workflows/simulation.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -41,7 +41,7 @@ jobs:
       if: "github.ref != 'refs/heads/master'"
 
   distcheck:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CC: clang
       CXX: clang++
@@ -63,7 +63,7 @@ jobs:
         make -f examples/Makefile-simulation distcheck
 
   cli-ftd-otns:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CFLAGS: -m32
       CXXFLAGS: -m32
@@ -117,7 +117,7 @@ jobs:
         path: tmp/coverage.info
 
   packet-verification:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       REFERENCE_DEVICE: 1
       VIRTUAL_TIME: 1
@@ -157,7 +157,7 @@ jobs:
         path: tmp/coverage.info
 
   cli-ftd:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CFLAGS: -m32
       CXXFLAGS: -m32
@@ -193,7 +193,7 @@ jobs:
         path: tmp/coverage.info
 
   cli-mtd:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         message_use_heap: [0, 1]
@@ -234,7 +234,7 @@ jobs:
         path: tmp/coverage.info
 
   cli-time-sync:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CFLAGS: -m32
       CXXFLAGS: -m32
@@ -348,7 +348,7 @@ jobs:
         path: tmp/coverage.info
 
   external-commissioner:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
@@ -385,7 +385,7 @@ jobs:
         path: tmp/coverage.info
 
   multiple-instance:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       MULTIPLE_INSTANCE: 1
@@ -418,7 +418,7 @@ jobs:
         path: tmp/coverage.info
 
   ncp-gcc-m32:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       CFLAGS: -m32
       CXXFLAGS: -m32
@@ -457,7 +457,7 @@ jobs:
         path: tmp/coverage.info
 
   ncp-clang:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     env:
       COVERAGE: 1
       NODE_TYPE: ncp-sim
@@ -506,7 +506,7 @@ jobs:
     - multiple-instance
     - ncp-gcc-m32
     - ncp-clang
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
diff --git a/.github/workflows/size.yml b/.github/workflows/size.yml
index 168923d9b..c498589cc 100644
--- a/.github/workflows/size.yml
+++ b/.github/workflows/size.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -41,7 +41,7 @@ jobs:
       if: "github.ref != 'refs/heads/master'"
 
   size-report:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Bootstrap
diff --git a/.github/workflows/toranj.yml b/.github/workflows/toranj.yml
index 6547ba00a..9db9ab5a0 100644
--- a/.github/workflows/toranj.yml
+++ b/.github/workflows/toranj.yml
@@ -33,7 +33,7 @@ on: [push, pull_request]
 jobs:
 
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
diff --git a/.github/workflows/version.yml b/.github/workflows/version.yml
index 0c71178a4..0dcf05d69 100644
--- a/.github/workflows/version.yml
+++ b/.github/workflows/version.yml
@@ -32,7 +32,7 @@ on: [pull_request]
 
 jobs:
   cancel-previous-runs:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: rokroskar/workflow-run-cleanup-action@master
       env:
@@ -40,7 +40,7 @@ jobs:
       if: "github.ref != 'refs/heads/master'"
 
   api-version:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - name: Check
diff --git a/tests/unit/test_link_quality.cpp b/tests/unit/test_link_quality.cpp
index 025395b66..a8f2290f1 100644
--- a/tests/unit/test_link_quality.cpp
+++ b/tests/unit/test_link_quality.cpp
@@ -333,7 +333,7 @@ void TestRssAveraging(void)
             rss = GetRandomRss();
             IgnoreError(rssAverager.Add(rss));
             sum += rss;
-            mean = static_cast<double>(sum) / j;
+            mean = static_cast<double>(sum) / static_cast<double>(j);
             VerifyOrQuit(ABS(rssAverager.GetAverage() - mean) < 1, "Average does not match the arithmetic mean!");
             VerifyRawRssValue(rssAverager);
             printf("AddRss(%4d) sum=%-5d, mean=%-8.2f RssAverager=", rss, sum, mean);