docs/network.md

@@ -121,12 +121,12 @@ address arithmetic.
 ### What the Rust side does
 
 `hive-c0re` reads `HIVE_NETWORK_ISOLATION` and passes
-`PRIVATE_NETWORK=1`, `LOCAL_ADDRESS=` (empty), `HOST_ADDRESS=<bridge-ip>`,
+`PRIVATE_NETWORK=1`, `LOCAL_ADDRESS=<deterministic-ip>`,
-and `HOST_BRIDGE=<bridgeName>` via `lifecycle::set_nspawn_flags` when
+`HOST_ADDRESS=<bridge-ip>`, and `HOST_BRIDGE=<bridgeName>` via
-creating or updating containers. `LOCAL_ADDRESS` is left empty so the
+`lifecycle::set_nspawn_flags` when creating or updating containers. Each
-container's dhcpcd acquires an address from the bridge dnsmasq pool
+agent gets a deterministic IP derived from its name so the address is
-(`networking.useDHCP = true` in `harness-base.nix`). This applies uniformly
+reproducible across destroy/recreate. This applies uniformly to all
-to all containers — agents and service containers alike.
+containers — no special case.
 
 `HOST_ADDRESS` is the bridge gateway IP (the address part of
 `HIVE_NETWORK_SUBNET`, via `lifecycle::bridge_gateway_ip` — taken verbatim

hive-c0re/src/lifecycle/host_config.rs

@@ -10,7 +10,7 @@ use hive_sh4re::priv_proto::{BindMount, CredentialMount};
 use crate::coordinator::{AgentPaths, HiveEnv};
 
 use super::{
-    AGENT_PREFIX, CONTAINER_RUNTIME_MOUNT, CONTAINER_SHARED_MOUNT, agent_uid_gid,
+    AGENT_PREFIX, CONTAINER_RUNTIME_MOUNT, CONTAINER_SHARED_MOUNT, agent_network_ip, agent_uid_gid,
     bridge_gateway_ip, container_claude_mount, container_name, validate,
 };
 
@@ -294,6 +294,21 @@ async fn set_nspawn_flags(
         let bridge = std::env::var("HIVE_NETWORK_BRIDGE").unwrap_or_default();
         let subnet = std::env::var("HIVE_NETWORK_SUBNET").unwrap_or_default();
         if isolate && !bridge.is_empty() && !subnet.is_empty() {
+            let Some(agent_ip) = agent_network_ip(agent_name, &subnet) else {
+                tracing::warn!(
+                    %agent_name, %subnet,
+                    "HIVE_NETWORK_SUBNET is set but could not derive a valid IP for agent \
+                     (bad CIDR? prefix too narrow?); skipping PRIVATE_NETWORK write to \
+                     avoid misconfigured isolation"
+                );
+                return crate::priv_client::write_nspawn_flags(
+                    container,
+                    &binds,
+                    None,
+                    &load_creds,
+                )
+                .await;
+            };
             let Some(gateway_ip) = bridge_gateway_ip(&subnet) else {
                 tracing::warn!(
                     %agent_name, %subnet,
@@ -310,10 +325,14 @@ async fn set_nspawn_flags(
                 .await;
             };
             tracing::info!(
-                %agent_name, %gateway_ip, %bridge,
+                %agent_name, %agent_ip, %gateway_ip, %bridge,
-                "network isolation: PRIVATE_NETWORK=1 (DHCP)"
+                "network isolation: PRIVATE_NETWORK=1"
             );
-            Some(hive_sh4re::priv_proto::NetworkIsolation { bridge, gateway_ip })
+            Some(hive_sh4re::priv_proto::NetworkIsolation {
+                agent_ip,
+                bridge,
+                gateway_ip,
+            })
         } else {
             None
         }

hive-c0re/src/lifecycle/mod.rs

@@ -66,7 +66,8 @@ pub const CONTAINER_SHARED_MOUNT: &str = "/shared";
 const WEB_PORT_BASE: u16 = 8100;
 const WEB_PORT_RANGE: u16 = 900;
 
-/// FNV-1a hash of a string — used by `agent_web_port`.
+/// FNV-1a hash of a string — shared by `agent_web_port` and
+/// `agent_network_ip` so the derivation rule is identical.
 fn fnv1a(s: &str) -> u32 {
     let mut hash: u32 = 2_166_136_261;
     for b in s.bytes() {
@@ -89,6 +90,60 @@ pub fn agent_web_port(name: &str) -> u16 {
     WEB_PORT_BASE + u16::try_from(fnv1a(name) % u32::from(WEB_PORT_RANGE)).unwrap_or(0)
 }
 
+/// Deterministic IPv4 address for an agent inside an isolated subnet.
+///
+/// Parses `subnet_cidr` as `<network_ip>/<prefix_len>` (e.g.
+/// `"10.42.0.0/24"`), then computes:
+///
+/// ```text
+/// host_count = 2^(32 - prefix_len)
+/// usable     = host_count - 3  // skip .0 (network), .1 (gateway), .255 (broadcast)
+/// offset     = FNV-1a(name) % usable + 2   // .2 is the first agent slot
+/// agent_ip   = network_base_u32 + offset
+/// ```
+///
+/// Returns `None` when `subnet_cidr` can't be parsed (invalid format,
+/// prefix out of range, etc.) so callers can fall back gracefully.
+/// Collisions are possible (birthday paradox) and the operator resolves
+/// them by renaming an agent, same as for port collisions.
+#[must_use]
+pub fn agent_network_ip(name: &str, subnet_cidr: &str) -> Option<String> {
+    let (ip_str, prefix_str) = subnet_cidr.split_once('/')?;
+    let prefix_len: u32 = prefix_str.parse().ok()?;
+    if prefix_len > 30 {
+        // /31 and /32 have no room for agents; /30 has 1 usable slot.
+        // /0 (the other extreme) is handled further down: host_count
+        // overflows checked_shl(32) → 0 → usable = 0 → None.
+        return None;
+    }
+    // Parse dotted-decimal IPv4.
+    let octets: Vec<u8> = ip_str
+        .split('.')
+        .map(|o| o.parse::<u8>().ok())
+        .collect::<Option<Vec<_>>>()?;
+    if octets.len() != 4 {
+        return None;
+    }
+    let base_u32 = u32::from_be_bytes([octets[0], octets[1], octets[2], octets[3]]);
+    // Mask off host bits to get the true network address.
+    let mask = if prefix_len == 0 {
+        0u32
+    } else {
+        !0u32 << (32 - prefix_len)
+    };
+    let network_base = base_u32 & mask;
+    let host_count: u32 = 1u32.checked_shl(32 - prefix_len).unwrap_or(0);
+    // `.0` = network, `.1` = bridge gateway, last = broadcast → 3 reserved.
+    let usable = host_count.saturating_sub(3);
+    if usable == 0 {
+        return None;
+    }
+    let offset = fnv1a(name) % usable + 2; // +2: skip .0 and .1
+    let ip_u32 = network_base + offset;
+    let [a, b, c, d] = ip_u32.to_be_bytes();
+    Some(format!("{a}.{b}.{c}.{d}"))
+}
+
 /// Extract the bridge gateway IP from `HIVE_NETWORK_SUBNET`.
 ///
 /// `HIVE_NETWORK_SUBNET` carries the host-side bridge address verbatim
@@ -108,8 +163,9 @@ pub fn agent_web_port(name: &str) -> u16 {
 pub fn bridge_gateway_ip(subnet_cidr: &str) -> Option<String> {
     let (ip_str, prefix_str) = subnet_cidr.split_once('/')?;
     // Validate the prefix is a sane IPv4 CIDR length and the address is
-    // dotted-decimal IPv4 so a malformed `HIVE_NETWORK_SUBNET` can't
+    // dotted-decimal IPv4 — same shape `agent_network_ip` accepts — so a
-    // smuggle a bogus HOST_ADDRESS into the nspawn conf.
+    // malformed `HIVE_NETWORK_SUBNET` can't smuggle a bogus HOST_ADDRESS
+    // into the nspawn conf.
     let prefix_len: u32 = prefix_str.parse().ok()?;
     if prefix_len > 32 {
         return None;

hive-c0re/src/lifecycle/tests.rs

@@ -37,6 +37,43 @@ async fn setup_proposed_seeds_flake_nix() {
     assert!(tracked.contains("flake.nix"), "flake.nix not committed");
 }
 
+#[test]
+fn agent_network_ip_is_in_subnet() {
+    // Default subnet 10.42.0.0/24 — agents get .2 to .254.
+    let ip = agent_network_ip("alice", "10.42.0.0/24").expect("should produce an IP");
+    let octets: Vec<u8> = ip.split('.').map(|o| o.parse().unwrap()).collect();
+    assert_eq!(&octets[..3], &[10, 42, 0], "wrong /24 prefix");
+    assert!(
+        octets[3] >= 2 && octets[3] <= 254,
+        "host byte {}",
+        octets[3]
+    );
+}
+
+#[test]
+fn agent_network_ip_stable() {
+    // Same name + subnet must always produce the same IP.
+    let a = agent_network_ip("damocles", "10.42.0.0/24");
+    let b = agent_network_ip("damocles", "10.42.0.0/24");
+    assert_eq!(a, b);
+}
+
+#[test]
+fn agent_network_ip_different_agents() {
+    // Different agent names very likely produce different IPs (not guaranteed,
+    // but for these two names the hashes don't collide).
+    let alice = agent_network_ip("alice", "10.42.0.0/24").unwrap();
+    let bob = agent_network_ip("bob", "10.42.0.0/24").unwrap();
+    assert_ne!(alice, bob, "alice and bob collide — rename one");
+}
+
+#[test]
+fn agent_network_ip_different_subnet() {
+    let ip = agent_network_ip("alice", "192.168.5.0/24").expect("should produce an IP");
+    let octets: Vec<u8> = ip.split('.').map(|o| o.parse().unwrap()).collect();
+    assert_eq!(&octets[..3], &[192, 168, 5]);
+}
+
 #[test]
 fn bridge_gateway_ip_extracts_verbatim_address() {
     // HIVE_NETWORK_SUBNET carries the bridge IP verbatim, not the
@@ -65,6 +102,31 @@ fn bridge_gateway_ip_rejects_bad_input() {
     assert!(bridge_gateway_ip("10.42.0/24").is_none()); // 3 octets
 }
 
+#[test]
+fn agent_network_ip_rejects_bad_input() {
+    assert!(agent_network_ip("alice", "notanip/24").is_none());
+    assert!(agent_network_ip("alice", "10.0.0.0/33").is_none()); // prefix > 32
+    assert!(agent_network_ip("alice", "10.0.0.0/31").is_none()); // too small
+    assert!(agent_network_ip("alice", "10.0.0.0").is_none()); // no prefix
+}
+
+#[test]
+fn agent_network_ip_normalizes_bridge_ip_subnet() {
+    // HIVE_NETWORK_SUBNET carries the bridge IP (10.42.0.1/24), not
+    // canonical network (10.42.0.0/24). Both must produce the same result
+    // after host-bit masking.
+    let from_bridge = agent_network_ip("alice", "10.42.0.1/24");
+    let from_canonical = agent_network_ip("alice", "10.42.0.0/24");
+    assert_eq!(
+        from_bridge, from_canonical,
+        "bridge-IP and canonical-network form should normalize to the same result"
+    );
+    // Result must still be in .2-.254.
+    let ip = from_bridge.unwrap();
+    let last: u8 = ip.rsplit('.').next().unwrap().parse().unwrap();
+    assert!((2..=254).contains(&last), "host byte {last}");
+}
+
 /// `setup_proposed` is idempotent: calling it on an existing repo is a
 /// no-op (the fresh guard skips all writes).
 #[tokio::test]

hive-priv/src/main.rs

@@ -1487,11 +1487,7 @@ fn write_nspawn_flags(
         // setup is skipped, so this only affects the container's route —
         // exactly what we want.
         let _ = writeln!(out, "HOST_ADDRESS={}", iso.gateway_ip);
-        // LOCAL_ADDRESS is intentionally empty: agent containers receive their
+        let _ = writeln!(out, "LOCAL_ADDRESS={}", iso.agent_ip);
-        // IP dynamically via DHCP from the bridge dnsmasq pool.  HOST_ADDRESS
-        // (the gateway IP) is still written so nixos-container's container-side
-        // init installs a default route before the DHCP lease arrives.
-        out.push_str("LOCAL_ADDRESS=\n");
         out.push_str("HOST_ADDRESS6=\n");
         out.push_str("LOCAL_ADDRESS6=\n");
         let _ = writeln!(out, "HOST_BRIDGE={}", iso.bridge);

hive-sh4re/src/priv_proto.rs

@@ -208,21 +208,21 @@ pub struct CredentialMount {
 
 /// Network isolation parameters for `WriteNspawnFlags`. When `Some`,
 /// hive-priv writes `PRIVATE_NETWORK=1` + veth bridge wiring instead
-/// of the default `PRIVATE_NETWORK=0`. Containers receive their IP
+/// of the default `PRIVATE_NETWORK=0`.
-/// dynamically via DHCP from the bridge dnsmasq pool (`networking.useDHCP`
-/// in `harness-base.nix`); no static address is pre-assigned here.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct NetworkIsolation {
+    /// Static IP address to assign to this container on the bridge subnet.
+    pub agent_ip: String,
     /// Host bridge interface name (e.g. `hive0`).
     pub bridge: String,
     /// Bridge gateway IP (the host-side bridge address, e.g. `10.42.0.1`).
     /// Written as `HOST_ADDRESS=` in the nspawn conf so nixos-container's
-    /// container-side setup installs a default route (`default via <gw>`)
+    /// container-side setup installs a default route (`default via <gw>`):
-    /// before DHCP completes: without it the container has no route off
+    /// without it the container comes up with an address but no route off
-    /// the bridge subnet until the DHCP lease arrives. The same IP runs the
+    /// the bridge subnet — no internet, no `api.anthropic.com`. The same IP
-    /// hive dnsmasq resolver, so it's also written into the container's
+    /// runs the hive dnsmasq resolver, so it's also written into the
-    /// `/etc/resolv.conf` (see the isolated-DNS oneshot in `harness-base.nix`,
+    /// container's `/etc/resolv.conf` (see the isolated-DNS oneshot in
-    /// gated on the marker hive-priv drops).
+    /// `harness-base.nix`, gated on the marker hive-priv drops).
     pub gateway_ip: String,
 }
 

nix/modules/hive-ci.nix

@@ -11,6 +11,19 @@ let
   networkCfg = config.services.hyperhive.network;
   tlsCfg = config.services.hyperhive.tls;
 
+  # Static bridge address for the hive-ci container. The hive bridge has
+  # NO DHCP server: agent containers get deterministic static IPs
+  # (`lifecycle::agent_network_ip` hashes each name across .2..broadcast-1)
+  # and dnsmasq on the bridge is DNS-only. hive-ci is the one service
+  # container on the bridge, so it needs a static address too — `useDHCP`
+  # here only hangs the boot waiting for a lease nothing serves. Reserve the
+  # top host address of the (default /24) subnet; a clash with an agent that
+  # happens to hash here is the same rename-to-resolve case as any
+  # agent/agent IP collision. Operators on a non-/24 bridge (or with
+  # `bridgeIp` set to the top address) should pick a free host address.
+  ciBridgeOctets = lib.splitString "." networkCfg.bridgeIp;
+  ciBridgeIp = "${lib.elemAt ciBridgeOctets 0}.${lib.elemAt ciBridgeOctets 1}.${lib.elemAt ciBridgeOctets 2}.254";
+
   # Self-signed TLS is the gateway default (no operator cert / ACME). When
   # active, forgejo's ROOT_URL is `https://forge.<domain>` and the leaf is
   # signed by the host hive CA — so the runner's Node-based actions (e.g.
@@ -409,11 +422,22 @@ in
           # registries, etc.). The bridge→loopback DROP rule does not
           # affect traffic destined for the bridge IP itself.
           networking.nameservers = [ networkCfg.bridgeIp ];
-          # Bridge-attached via privateNetwork=true + hostBridge. The
+          # With privateNetwork=true + hostBridge the container's veth
-          # gateway's dnsmasq serves a DHCP pool covering all usable bridge
+          # (eth0) is bridge-attached. There is no DHCP server on the hive
-          # addresses (see dhcp-range in hive-gateway.nix) — agents and
+          # bridge (dnsmasq is DNS-only; agents use static IPs), so assign a
-          # service containers alike receive IPs dynamically.
+          # static address + default route via the bridge gateway rather than
-          networking.interfaces.eth0.useDHCP = true;
+          # DHCP — `useDHCP` here just hangs boot on a lease that never
+          # arrives. See `ciBridgeIp` above.
+          networking.interfaces.eth0.ipv4.addresses = [
+            {
+              address = ciBridgeIp;
+              prefixLength = networkCfg.bridgePrefixLength;
+            }
+          ];
+          networking.defaultGateway = {
+            address = networkCfg.bridgeIp;
+            interface = "eth0";
+          };
 
           # nspawn containers can't create user-namespaces, so nix
           # sandboxing always fails. Fall back to unsandboxed builds.

nix/modules/hive-gateway.nix

@@ -11,36 +11,6 @@ let
   forgeCfg = config.services.hyperhive.forge;
   networkCfg = config.services.hyperhive.network;
 
-  # DHCP pool covering all usable host addresses on the bridge subnet.
-  # All containers (agents and service containers such as hive-ci) receive
-  # their IPs dynamically; there are no hash-derived static assignments.
-  # Range: .2 to .(hostCount-2) — skipping .0 (network), .1 (gateway/host
-  # bridge), and the broadcast address.
-  #
-  # IPv4 helpers — nix integers are 64-bit so all /0-/32 values are safe.
-  ipToInt =
-    ip:
-    builtins.foldl' (acc: x: acc * 256 + x) 0 (
-      map lib.strings.toIntBase10 (lib.strings.splitString "." ip)
-    );
-  intToIp =
-    n:
-    let
-      a = n / 16777216;
-      b = (n - a * 16777216) / 65536;
-      c = (n - a * 16777216 - b * 65536) / 256;
-      d = n - a * 16777216 - b * 65536 - c * 256;
-    in
-    "${toString a}.${toString b}.${toString c}.${toString d}";
-  # 2^n via recursion (nix has no pow builtin).
-  pow2 = n: if n == 0 then 1 else 2 * (pow2 (n - 1));
-  hostCount = pow2 (32 - networkCfg.bridgePrefixLength);
-  # Mask off host bits to get the network base address.
-  networkBase = builtins.bitAnd (ipToInt networkCfg.bridgeIp) (4294967295 - hostCount + 1);
-  # DHCP range: .2 (first usable after gateway) to .(hostCount-2) (last usable).
-  dhcpStart = intToIp (networkBase + 2); # skip .0 (network) and .1 (gateway)
-  dhcpEnd = intToIp (networkBase + hostCount - 2); # last usable = broadcast - 1
-
   # Dashboard SPA dist, static-served by nginx below. Read in OUTER scope so
   # `config` is the host's (inside the container block it'd be the container's).
   dashboardDist = "${config.services.hyperhive.c0re.servedFrontend}/dashboard";
@@ -1029,13 +999,6 @@ in
               ++ lib.optional (
                 matrixCfg.enable && matrixCfg.gatewayHost != null
               ) "/${matrixCfg.gatewayHost}/${networkCfg.bridgeIp}";
-              # DHCP pool covering all usable host addresses on the bridge subnet.
-              # Range is computed from bridgeIp/bridgePrefixLength at eval time:
-              # .2 (first after gateway) to .(hostCount-2) (last usable before
-              # broadcast). All containers — agents and service containers alike —
-              # receive their IPs dynamically from this pool.
-              dhcp-range = "${dhcpStart},${dhcpEnd},1h";
-              dhcp-leasefile = "/var/lib/dnsmasq/dnsmasq.leases";
             };
           };
         };

nix/templates/harness-base.nix

@@ -1695,10 +1695,6 @@ in
     # disabling dhcpcd itself, so the veth still gets its address); then
     # the hyperhive-isolated-dns oneshot owns resolv.conf. (Same "take
     # resolvconf out of the loop" approach the matrix container uses.)
-    # All agent containers receive their bridge IP via DHCP from the hive
-    # dnsmasq pool (see hive-gateway.nix).  useDHCP runs dhcpcd on every
-    # interface (just eth0 in practice — the nspawn bridge veth).
-    networking.useDHCP = true;
     networking.resolvconf.enable = false;
     networking.dhcpcd.extraConfig = "nohook resolv.conf";