diff --git a/ci/test/00_setup_env.sh b/ci/test/00_setup_env.sh
index 021d5e15976244..94149106ee2089 100755
--- a/ci/test/00_setup_env.sh
+++ b/ci/test/00_setup_env.sh
@@ -64,7 +64,7 @@ export BASE_OUTDIR=${BASE_OUTDIR:-$BASE_SCRATCH_DIR/out}
 # The folder for previous release binaries.
 # This folder exists only on the ci guest, and on the ci host as a volume.
 export PREVIOUS_RELEASES_DIR=${PREVIOUS_RELEASES_DIR:-$BASE_ROOT_DIR/prev_releases}
-export CI_BASE_PACKAGES=${CI_BASE_PACKAGES:-build-essential pkg-config curl ca-certificates ccache python3 rsync git procps bison e2fsprogs cmake}
+export CI_BASE_PACKAGES=${CI_BASE_PACKAGES:-build-essential pkg-config curl ca-certificates ccache python3 rsync git procps bison e2fsprogs cmake net-tools tcpdump}
 export GOAL=${GOAL:-install}
 export DIR_QA_ASSETS=${DIR_QA_ASSETS:-${BASE_SCRATCH_DIR}/qa-assets}
 export CI_RETRY_EXE=${CI_RETRY_EXE:-"retry --"}
diff --git a/ci/test/00_setup_env_i686_centos.sh b/ci/test/00_setup_env_i686_centos.sh
index 22657742d7c0ed..d6bd93187041af 100755
--- a/ci/test/00_setup_env_i686_centos.sh
+++ b/ci/test/00_setup_env_i686_centos.sh
@@ -10,7 +10,7 @@ export HOST=i686-pc-linux-gnu
 export CONTAINER_NAME=ci_i686_centos
 export CI_IMAGE_NAME_TAG="quay.io/centos/amd64:stream9"
 export STREAM_GCC_V="12"
-export CI_BASE_PACKAGES="gcc-toolset-${STREAM_GCC_V}-gcc-c++ glibc-devel.x86_64 gcc-toolset-${STREAM_GCC_V}-libstdc++-devel.x86_64 glibc-devel.i686 gcc-toolset-${STREAM_GCC_V}-libstdc++-devel.i686 ccache make git python3 python3-pip which patch lbzip2 xz procps-ng dash rsync coreutils bison e2fsprogs cmake"
+export CI_BASE_PACKAGES="gcc-toolset-${STREAM_GCC_V}-gcc-c++ glibc-devel.x86_64 gcc-toolset-${STREAM_GCC_V}-libstdc++-devel.x86_64 glibc-devel.i686 gcc-toolset-${STREAM_GCC_V}-libstdc++-devel.i686 ccache make git python3 python3-pip which patch lbzip2 xz procps-ng dash rsync coreutils bison e2fsprogs cmake net-tools tcpdump"
 export PIP_PACKAGES="pyzmq"
 export GOAL="install"
 export BITCOIN_CONFIG="-DWITH_ZMQ=ON -DBUILD_GUI=ON -DREDUCE_EXPORTS=ON"
diff --git a/ci/test/00_setup_env_native_asan.sh b/ci/test/00_setup_env_native_asan.sh
index d7ff7c2c1387fd..8a8b3723a5f0dc 100755
--- a/ci/test/00_setup_env_native_asan.sh
+++ b/ci/test/00_setup_env_native_asan.sh
@@ -33,3 +33,6 @@ export BITCOIN_CONFIG="\
  -DAPPEND_CXXFLAGS='-std=c++23' \
  -DAPPEND_CPPFLAGS='-DARENA_DEBUG -DDEBUG_LOCKORDER' \
 "
+# It is good to have at least one CI task that sets this to 1 so that changes
+# that result in tests connecting to the internet result in a CI failure.
+export CI_FAIL_IF_NO_TCPDUMP_FILE=1
diff --git a/ci/test/02_run_container.sh b/ci/test/02_run_container.sh
index ce01db325cebed..e1539e8bef3a6f 100755
--- a/ci/test/02_run_container.sh
+++ b/ci/test/02_run_container.sh
@@ -91,7 +91,7 @@ if [ -z "$DANGER_RUN_CI_ON_HOST" ]; then
   # Append $USER to /tmp/env to support multi-user systems and $CONTAINER_NAME
   # to allow support starting multiple runs simultaneously by the same user.
   # shellcheck disable=SC2086
-  CI_CONTAINER_ID=$(docker run --cap-add LINUX_IMMUTABLE $CI_CONTAINER_CAP --rm --interactive --detach --tty \
+  CI_CONTAINER_ID=$(docker run --cap-add LINUX_IMMUTABLE --cap-add NET_RAW $CI_CONTAINER_CAP --rm --interactive --detach --tty \
                   --mount "type=bind,src=$BASE_READ_ONLY_DIR,dst=$BASE_READ_ONLY_DIR,readonly" \
                   --mount "${CI_CCACHE_MOUNT}" \
                   --mount "${CI_DEPENDS_MOUNT}" \
diff --git a/ci/test/03_test_script.sh b/ci/test/03_test_script.sh
index 6e77a8927c2220..18f42220cbdd4e 100755
--- a/ci/test/03_test_script.sh
+++ b/ci/test/03_test_script.sh
@@ -146,21 +146,74 @@ if [ "$RUN_CHECK_DEPS" = "true" ]; then
   "${BASE_ROOT_DIR}/contrib/devtools/check-deps.sh" .
 fi
 
+function get_interfaces()
+{
+  set -o pipefail
+  ifconfig | awk -F ':| ' '/^[^[:space:]]/ { if (!match($1, /^lo/)) { print $1 } }'
+  set +o pipefail
+}
+
+function tcpdump_file()
+{
+  echo "/tmp/tcpdump_$1_$2"
+}
+
+function traffic_monitor_begin()
+{
+  test_name="$1"
+  for ifname in $(get_interfaces) ; do
+    tcpdump -nU -i "$ifname" -w "$(tcpdump_file "$test_name" "$ifname")" &
+  done
+}
+
+function traffic_monitor_end()
+{
+  test_name="$1"
+
+  for ifname in $(get_interfaces) ; do
+    f=$(tcpdump_file "$test_name" "$ifname")
+    if [ ! -e "$f" ] && [ "$CI_FAIL_IF_NO_TCPDUMP_FILE" = "1" ] ; then
+      # In some CI environments this script is not running as root and so the
+      # tcpdump errors and does not create $f. Skip silently those, but we
+      # need at least one where tcpdump can run and this is the ASAN one. So
+      # treat the absence of $f as an error only on the ASAN task.
+      continue
+    fi
+    # We are running as root and those files are created with owner:group =
+    # tcpdump:tcpdump and then `tcpdump -r` refuses to read them with an error
+    # "permission denied" if they are not owned by root:root.
+    chown root:root "$f"
+    out="$(tcpdump -n -r "$f" --direction=out tcp or udp)"
+    if [ -n "$out" ] ; then
+      echo "Error: outbound TCP or UDP packets on the non loopback interface generated during $test_name tests:" >&2
+      tcpdump -n -r "$f" tcp or udp
+      exit 1
+    fi
+  done
+}
+
 if [ "$RUN_UNIT_TESTS" = "true" ]; then
+  traffic_monitor_begin "unitparallel"
   DIR_UNIT_TEST_DATA="${DIR_UNIT_TEST_DATA}" LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" CTEST_OUTPUT_ON_FAILURE=ON ctest --stop-on-failure "${MAKEJOBS}" --timeout $(( TEST_RUNNER_TIMEOUT_FACTOR * 60 ))
+  traffic_monitor_end "unitparallel"
 fi
 
 if [ "$RUN_UNIT_TESTS_SEQUENTIAL" = "true" ]; then
+  traffic_monitor_begin "unitsequential"
   DIR_UNIT_TEST_DATA="${DIR_UNIT_TEST_DATA}" LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" "${BASE_OUTDIR}"/bin/test_bitcoin --catch_system_errors=no -l test_suite
+  traffic_monitor_end "unitsequential"
 fi
 
 if [ "$RUN_FUNCTIONAL_TESTS" = "true" ]; then
+  traffic_monitor_begin "functional"
   # parses TEST_RUNNER_EXTRA as an array which allows for multiple arguments such as TEST_RUNNER_EXTRA='--exclude "rpc_bind.py --ipv6"'
   eval "TEST_RUNNER_EXTRA=($TEST_RUNNER_EXTRA)"
   LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" test/functional/test_runner.py --ci "${MAKEJOBS}" --tmpdirprefix "${BASE_SCRATCH_DIR}"/test_runner/ --ansi --combinedlogslen=99999999 --timeout-factor="${TEST_RUNNER_TIMEOUT_FACTOR}" "${TEST_RUNNER_EXTRA[@]}" --quiet --failfast
+  traffic_monitor_end "functional"
 fi
 
 if [ "${RUN_TIDY}" = "true" ]; then
+  traffic_monitor_begin "tidy"
   cmake -B /tidy-build -DLLVM_DIR=/usr/lib/llvm-"${TIDY_LLVM_V}"/cmake -DCMAKE_BUILD_TYPE=Release -S "${BASE_ROOT_DIR}"/contrib/devtools/bitcoin-tidy
   cmake --build /tidy-build "$MAKEJOBS"
   cmake --build /tidy-build --target bitcoin-tidy-tests "$MAKEJOBS"
@@ -185,9 +238,12 @@ if [ "${RUN_TIDY}" = "true" ]; then
   cd "${BASE_ROOT_DIR}/src"
   python3 "/include-what-you-use/fix_includes.py" --nosafe_headers < /tmp/iwyu_ci.out
   git --no-pager diff
+  traffic_monitor_end "tidy"
 fi
 
 if [ "$RUN_FUZZ_TESTS" = "true" ]; then
+  traffic_monitor_begin "fuzz"
   # shellcheck disable=SC2086
   LD_LIBRARY_PATH="${DEPENDS_DIR}/${HOST}/lib" test/fuzz/test_runner.py ${FUZZ_TESTS_CONFIG} "${MAKEJOBS}" -l DEBUG "${DIR_FUZZ_IN}" --empty_min_time=60
+  traffic_monitor_end "fuzz"
 fi