Document jail networking on a single public IP

The --ip same-subnet rule from earlier is necessary but not sufficient
once a host only has one public address: jails shouldn't get public IPs
at all (only nginx needs one), but a private range aliased onto the same
interface needs its own outbound NAT rule for jail builds to reach the
internet - nothing upstream translates it the way a NAT'd virtual network
(like the dev VM) does for free. Skipping it fails the same silent way as
the out-of-subnet case: packets leave the jail, nothing answers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

1 files changed, 39 insertions, 3 deletions

diff --git a/USER_GUIDE.md b/USER_GUIDE.md
index 587a625..8cc62c3 100644
--- a/USER_GUIDE.md
+++ b/USER_GUIDE.md
@@ -368,7 +368,43 @@ bypassing `/etc/jail.conf` entirely. Only the boot-time, no-name form of
 stays invisible until the host reboots and no app jail comes back up on
 its own - see the [Troubleshooting](#troubleshooting) entry below.
 
-#### 4. Nginx
+#### 4. Jail networking on a single public IP
+
+Classic (non-VNET) jails share the host's network stack - `--ip` just
+adds an address to the host's existing interface (see [Provisioning a new
+app](#provisioning-a-new-app)) - so on a host with only one public
+address, jails shouldn't get public IPs at all. Only nginx needs one,
+listening on 80/443; every app's vhost `proxy_pass`es to its jail's own
+private address.
+
+Alias a private range onto the same interface as the public IP, once:
+
+```
+# ifconfig vtnet0 inet 10.0.0.1/24 alias
+```
+
+and hand out jail `--ip`s from within it (`10.0.0.2`, `10.0.0.3`, ...).
+Inbound just works from there - nginx reaching a jail is on-link delivery
+on the same NIC, no gateway involved, the same same-subnet rule covered
+above.
+
+Outbound doesn't, without one more step. A jail's build step needs real
+internet access (module/package downloads - see
+[Requirements](#requirements)), and a private range has nothing upstream
+translating it to the public internet the way, say, a NAT'd virtual
+network would - the host itself has to do that translation for its own
+jails. Add a NAT rule to `/etc/pf.conf`:
+
+```
+nat on vtnet0 from 10.0.0.0/24 to any -> (vtnet0)
+```
+
+Skip this and jail builds fail exactly like the out-of-subnet case above
+- packets leave the jail fine, `tcpdump` shows them, nothing ever answers
+- just for a different reason (no return NAT, instead of no route at
+all).
+
+#### 5. Nginx
 
 Add once to `/usr/local/etc/nginx/nginx.conf`, inside the `http` block:
 
@@ -385,7 +421,7 @@ include /usr/local/etc/nginx/skiff.d/*.conf;
 Skiff only ever writes files under `skiff.d/` and reloads - it never
 touches `nginx.conf` itself.
 
-#### 5. TLS (Let's Encrypt)
+#### 6. TLS (Let's Encrypt)
 
 Skiff issues certificates itself via `certbot certonly` (webroot
 validation, never certbot's own `--nginx` plugin - Skiff owns
@@ -416,7 +452,7 @@ every script under this directory after any successful renewal:
 # chmod +x /usr/local/etc/letsencrypt/renewal-hooks/deploy/reload-nginx
 ```
 
-#### 6. Install skiff
+#### 7. Install skiff
 
 Cross-compile from any machine with Go, then copy the binary over: