diff --git a/examples/sidecar-nostr-relay/README.md b/examples/sidecar-nostr-relay/README.md
index 22b2dfc..897eb99 100644
--- a/examples/sidecar-nostr-relay/README.md
+++ b/examples/sidecar-nostr-relay/README.md
@@ -187,6 +187,7 @@ docker exec sidecar-nostr-relay-app-1 ping -c1 127.0.0.1
 | `FIPS_TCP_BIND` | `0.0.0.0:8443` | TCP transport bind address |
 | `FIPS_PEER_TRANSPORT` | `udp` | Peer transport type (`udp` or `tcp`) |
 | `FIPS_TUN_MTU` | `1280` | TUN interface MTU |
+| `FIPS_UDP_MTU` | `1472` | UDP transport MTU (default is Docker bridge IPv4 max; set to `1280` for IPv6-min-safe deploys) |
 | `FIPS_NETWORK` | `fips-sidecar-net` | Docker network name (set to join external network) |
 | `FIPS_SUBNET` | `172.20.1.0/24` | Docker network subnet |
 | `FIPS_IPV4` | `172.20.1.20` | Sidecar's IPv4 address on the Docker network |
diff --git a/examples/sidecar-nostr-relay/docker-compose.yml b/examples/sidecar-nostr-relay/docker-compose.yml
index 40afe46..5af4d6f 100644
--- a/examples/sidecar-nostr-relay/docker-compose.yml
+++ b/examples/sidecar-nostr-relay/docker-compose.yml
@@ -31,6 +31,7 @@ services:
       - FIPS_PEER_ALIAS=${FIPS_PEER_ALIAS:-peer}
       - FIPS_UDP_BIND=${FIPS_UDP_BIND:-0.0.0.0:2121}
       - FIPS_TUN_MTU=${FIPS_TUN_MTU:-1280}
+      - FIPS_UDP_MTU=${FIPS_UDP_MTU:-1472}
       - FIPS_PEER_TRANSPORT=${FIPS_PEER_TRANSPORT:-udp}
     volumes:
       - ./resolv.conf:/etc/resolv.conf:ro
diff --git a/examples/sidecar-nostr-relay/entrypoint.sh b/examples/sidecar-nostr-relay/entrypoint.sh
index 06b826d..af40441 100755
--- a/examples/sidecar-nostr-relay/entrypoint.sh
+++ b/examples/sidecar-nostr-relay/entrypoint.sh
@@ -8,6 +8,7 @@ FIPS_NSEC="${FIPS_NSEC:?FIPS_NSEC is required}"
 FIPS_UDP_BIND="${FIPS_UDP_BIND:-0.0.0.0:2121}"
 FIPS_TCP_BIND="${FIPS_TCP_BIND:-0.0.0.0:8443}"
 FIPS_TUN_MTU="${FIPS_TUN_MTU:-1280}"
+FIPS_UDP_MTU="${FIPS_UDP_MTU:-1472}"
 FIPS_PEER_TRANSPORT="${FIPS_PEER_TRANSPORT:-udp}"
 
 mkdir -p /etc/fips
@@ -41,7 +42,9 @@ dns:
 transports:
   udp:
     bind_addr: "${FIPS_UDP_BIND}"
-    mtu: 1472
+    # 1472 = Docker bridge IPv4 max (1500 MTU - 8 UDP - 20 IPv4 header).
+    # Override with FIPS_UDP_MTU=1280 for IPv6-min-safe deploys.
+    mtu: ${FIPS_UDP_MTU}
   tcp:
     bind_addr: "${FIPS_TCP_BIND}"
 
diff --git a/testing/static/configs/node.template.yaml b/testing/static/configs/node.template.yaml
index 73ecd77..ced2104 100644
--- a/testing/static/configs/node.template.yaml
+++ b/testing/static/configs/node.template.yaml
@@ -17,6 +17,8 @@ dns:
 transports:
   udp:
     bind_addr: "0.0.0.0:2121"
+    # Docker bridge supports 1500-MTU IPv4 so 1472 is safe here.
+    # Operator-facing daemon default is 1280 (IPv6 minimum).
     mtu: 1472
     # recv_buf_size: 2097152   # 2 MB (default)
     # send_buf_size: 2097152   # 2 MB (default)