Networking and SSH

Networking and SSH

XAIOS provides a native IPv4/IPv6 stack, TCP, UDP, DNS, an SSH/SFTP server, and bounded outbound SSH/SCP clients. The server is implemented in XAIOS userspace; it is not a forwarded host sshd.

Connect to QEMU

The default QEMU launcher maps 127.0.0.1:7788 to guest TCP port 22:

ssh -p 7788 admin@127.0.0.1
sftp -P 7788 admin@127.0.0.1

Images do not contain a default password. Provision public keys for routine access. Password authentication requires an explicitly generated PBKDF2 user database and development-image opt-in; release builds reject it.

Supported server behavior

• Ed25519 public-key authentication and optional development password auth.
• Persistent host identity, host-key rotation, revocation, and fail-closed RNG.
• Concurrent SSH connections and multiple channels on one transport.
• PTY shells, one-command execution, terminal resize, rekey, and reconnect.
• Hybrid mlkem768x25519-sha256 key exchange with classical fallback.
• direct-tcpip forwarding for OpenSSH jump-host use and agent forwarding.
• SFTP v3 read, write, positional I/O, stat, list, mkdir, rename, remove, and rmdir with per-process descriptor ownership.
• Stateful per-session cwd, prompt, command status, and terminal applications.
• A 32-transport server ceiling, up to two active channels per transport, and 64 asynchronous child-channel records, with explicit saturation errors and reclamation after disconnect.

Address configuration

Every network device asks the network for an IPv4 address by DHCP before falling back to the compiled-in QEMU address of 10.0.2.15. A guest that assumes that address is simply off-net anywhere else; Virtualization.framework hands out a different subnet entirely.

IPv6 derives a link-local address from the hardware address, then solicits a router and polls briefly for the advertisement, because nothing else reads the interface between bringing it up and starting services. A prefix that is globally routable or unique-local is configured and used as the source address for outbound IPv6. The router that sent the advertisement is kept as the default route for as long as its Router Lifetime says, and traffic to anything outside the advertised /64 is sent to it -- without that a host configures an address and can still only reach its own link, because the neighbour lookup asks about a destination no neighbour will answer for.

Which prefix arrives depends on the network. Virtualization.framework (fd4a:25c::/64) and QEMU's slirp (fec0::/64) both advertise unique-local prefixes, so an address configured on those is unique-local. A bridged Fusion guest on a network with real IPv6 takes a globally routable address, and has been shown end to end on one: ICMPv6, inbound SSH and SFTP, inbound UDP, and outbound SSH and SCP to a host in another country.

Network behavior

The QEMU-tested stack includes IPv4/IPv6 fragment reassembly and source fragmentation, TCP handshake/data retransmission, slow start, congestion avoidance, fast retransmit, out-of-order receive, duplicate-ACK/SACK handling, UDP delivery semantics, asynchronous DNS A/AAAA resolution, bounded TTL caching, DNS-over-TCP fallback, socket ownership, cancellation, and cleanup. DNS requests set EDNS DO and CD; answers are admitted only after XAIOS locally validates the DNSKEY/DS delegation chain from compiled root DS anchors and a matching RRSIG. The upstream resolver's AD bit is not trusted. Unsigned, malformed, expired, unsupported-algorithm, or clock-untrusted replies fail closed and are reported to the caller without waiting for the query timeout. RSA/SHA-256, ECDSA P-256/P-384, and Ed25519 signatures plus SHA-256/SHA-384 DS digests are supported. Signed exact-owner NSEC NODATA proofs are supported; NXDOMAIN, NSEC3, CNAME/DNAME synthesis, wildcard synthesis, and root-anchor rollover policy are deliberately unsupported and fail closed. SNTP applies accepted corrections through a bounded 500-ppm monotonic slew after initial calibration. Runtime-sized CPU/queue metadata avoids a fixed small-core limit.

Boot readiness uses a real IPv4 TCP connection to port 443 before starting sshd; it does not treat a DNS response as proof of Internet reachability. The boot-test image uses the in-guest local-DNSSEC resolver wiring self-test so make qemu-smoke remains deterministic when public DNS is unavailable. The normal nettest application performs an external locally validated lookup.

Outbound clients

From an XAIOS shell:

ssh [-A] [-i KEY] [-p PORT] [-J user@host[:port]] user@host [command]
scp [-r] [-A] [-i KEY] [-P PORT] SOURCE DESTINATION

The dedicated /bin/ssh process supports password, Ed25519 identity-file and forwarded-agent authentication, including passphrase-protected OpenSSH private keys. It verifies Ed25519 host signatures with persistent trust-on-first-use records and connects through IPv4/IPv6 literals or DNS A/AAAA results. Recursive SCP is SFTP-backed. ssh -J user@host[:port] opens a separately authenticated password session to one jump host, requests a bounded direct-tcpip channel, then authenticates the target through that channel. Target password and identity-file authentication are supported; agent authentication with -J, multiple jump hosts, ProxyCommand, and the wider OpenSSH option/algorithm matrix are intentionally out of scope.

Interoperability evidence

Automated suites exercise XAIOS from macOS OpenSSH, Debian 13 OpenSSH, and an official FreeBSD 15.1 VM. They cover valid/invalid authentication, 32 simultaneous sessions under the combined macOS/Debian load gate, reconnects, PTY applications, SFTP lifecycle and isolation, SCP, UDP, IPv6/TCP, malformed traffic, rekey, reboot persistence, and concurrent clients against one guest. The raw Ethernet gates additionally send maximum-size fragmented UDP requests and independently reassemble XAIOS IPv4 and IPv6 replies on AArch64 and x86_64 QEMU.

make qemu-network-adversarial-gate adds sanitizer-backed coverage-guided SSH/SFTP/DNS parser campaigns, packet loss/reordering/corruption cases, connection and channel exhaustion with recovery, concurrent macOS/Debian load, and 20 fresh boots on each of ARM64 and x86_64. This remains emulated correctness evidence rather than physical deployment qualification.

This is protocol correctness evidence under QEMU, not approval for direct Internet exposure. See Security Model, Testing XAIOS, and Current Limitations.