Updates for peering page. (#1316)

* Updates for peering page.

Signed-off-by: Matt Nelson <85905982+non-fungible-nelson@users.noreply.github.com>

* wording tweak

Signed-off-by: Matt Nelson <85905982+non-fungible-nelson@users.noreply.github.com>

* lint

Signed-off-by: Matt Nelson <85905982+non-fungible-nelson@users.noreply.github.com>

* minor edits

Signed-off-by: Alexandra Tran <alexandra.tran@consensys.net>

---------

Signed-off-by: Matt Nelson <85905982+non-fungible-nelson@users.noreply.github.com>
Signed-off-by: Alexandra Tran <alexandra.tran@consensys.net>
Co-authored-by: Alexandra Tran <alexandra.tran@consensys.net>

docs/public-networks/how-to/troubleshoot/peering.md

@@ -18,6 +18,9 @@ If you have peering issues, start by [configuring your ports](../connect/configu
     - Your ports aren't open in your firewall and/or router.
     - Your node is sending large numbers of DNS requests.
       See [issue #4375](https://github.com/hyperledger/besu/issues/4375).
+    - You're using [checkpoint sync](../../get-started/connect/sync-node.md#checkpoint-synchronization),
+      which doesn't download all historical block data, so your peers may disconnect you when
+      fetching those blocks.
     - Your node is experiencing the normal behavior of peers connecting and disconnecting.
       This is especially normal soon after you start your node.
 
@@ -27,7 +30,7 @@ If you have peering issues, start by [configuring your ports](../connect/configu
       inbound connections.
     - Restart Besu.
       This can take a while to build up again.
-    - Set `-Xdns-enabled` to `true`.
+    - Set `-Xdns-enabled` to `true` (only for private networks).
     - Set `-Xp2p-peer-lower-bound` to a minimum number of peers.
     - Delete the node key (which is autogenerated in your data directory).
       There are two reasons that this might help find more peers:
@@ -40,6 +43,18 @@ If you have peering issues, start by [configuring your ports](../connect/configu
     [Prysm EL and CL peering documentation](https://docs.prylabs.network/docs/prysm-usage/p2p-host-ip)
     for more information.
 
+??? question "What network or router/modem settings should I check?"
+    Check the following settings:
+
+    - Your machine and router's specified DNS should support TCP.
+      You can check your DNS online for TCP support.
+      Google and Cloudflare, 8.8.8.8 and 1.1.1.1, support TCP over port 853.
+      Other DNS might as well.
+    - The appropriate ports should be open on your router, or your router should have UPNP enabled.
+      See the next FAQ for more information on router settings.
+    - If you use [Docker](https://docs.docker.com/network/network-tutorial-host/) or virtualization,
+      the container should be able to create outbound connections on the host machine.
+
 ??? question "How do I open/forward my ports?"
     If you’re behind NAT, you probably need to set up port forwarding in your router.
     You might also need to configure your firewall.