QEMU Slirp Network Backend

Slirp APIs in libslirp.h

• slirp_input(): slirp 透過硬體來 send packet

• slirp_output(): 把 packet 送回 guest, 透過 peer 的 receive() function (qemu_send_packet())

struct Slirp

- void* opaque = (SlirpState*) s; // net_slirp_init()
- libslirp.h

  - functions

    - slirp_init()
    - slirp_input()
    - slirp_pollfds_poll(), slirp_pollfds_fill()
    - slirp_add_hostfwd(): socreate() Slirp's socket

  - library user should provide some functions (like callback?)

    - slirp_output(): [net/slirp.c] qemu_send_packet(SlirpState->nc)

SlirpState wraps Slirp API, and provide NetClientInfo interface for QEMU Network architecture.

struct SlirpState

- Slirp* slirp: slirp wrapper in qemu net framework

  - slirp_hostfwd() => Slirp::slirp_add_hostfwd()
  - slirp_guestfwd() => Slirp::slirp_add_guestfwd()
  - slirp_smb() => Slirp::slirp_add_exec()

- NetClientState nc: interface NetClientState

  - nc = qemu_new_net_client(&net_slirp_info, ... )

    - nc->incoming_queue->deliver = qemu_deliver_packet_iov() [1.]
    - nc->info = net_slirp_info; [2.]
    - nc->peer = peer; // external parameter [3.]

  - nc source

    1. qemu_new_net_client() default
    2. struct NetClientInfo net_slirp_info; (slirp provide NetClientInfo interface)

       - .receive = net_slirp_receive; net_slirp_receive() calls Slirp::slirp_input();
       - .cleanup = net_slirp_cleanup; net_slirp_cleanup() calls SlirpState::slirp_smb_cleanup();

    3. from net_slirp_init() function parameters: peer is customizable by function caller.

socket address mapping(NAPT) and host socket API abstraction

struct socket (Slirp socket)

- struct socket *so_next, *so_prev: linked list of sockets
- int s: actual socket fd number
- Slirp* slirp: managing Slirp instance
- struct sbuf so_rcv, so_snd: Recv/Send buffer

- socreate(Slirp* slirp):

  - so->slirp = slirp
  - so->s = socket() syscall

- callers of send() and recv() syscall

  - soread(), sowrite()
  - sosendoob(), sorecvoob()
  - sosendto(), sorecvfrom()
  - slirp_send(struct socket *so, const void *buf, size_t len, int flags): send(so->s, buf, len, flags);

slirp_input()

slirp_input() use system call to send packet to remote OS. for loopback network, slirp_input() may send packet back to peer network.

at least 4 path of slirp_input():

1. __libc_send()
=> slirp_input() => slirp_send() => __libc_send()

2. sosendto(): call sendto() syscall, only udp.c & udp6.c use it.
=> slirp_input() => sosendto() => sendto() # syscall

3. slirp_output()
=> slirp_input() => tcp_input() => tcp_output() => slirp_output()
=> qemu_send_packet() => e1000_receive_iov()

4. slirp_output() by arp?
=> slirp_input() => arp_input() => slirp_output()

• slirp_send() callers

  • sosendoob()

  • sowrite()

  • sbappend()

  • slirp/socket.c and slirp/sbuf.c

Misc

• struct: NICState, NICInfo, NICPeers

• caller of slirp_output()

  • slirp_pollfds_poll() => tcp_output() => slirp_output()

  • ra_timer_handler() => ndp_send_ra() => slirp_output()