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Re: [Qemu-devel] [RFC PATCH qemu] slirp: Update forwarding IP address if


From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [RFC PATCH qemu] slirp: Update forwarding IP address if guest receiver non-default IP
Date: Thu, 8 Feb 2018 15:29:02 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 01/02/18 20:36, Alexey Kardashevskiy wrote:
> If we run QEMU with -netdev user,id=USER0,hostfwd=tcp::2222-:22, it starts
> a DHCP server and starts allocating client IPs from 10.0.2.15 so
> this is what the guest normally receives. Since QEMU automatically adds
> the DHCP starting address into the forwarding table, everything works.
> This is the table before guest started:
> 
> (qemu) info usernet
> VLAN -1 (USER0):
>   Protocol[State]    FD  Source Address  Port   Dest. Address  Port RecvQ 
> SendQ
>   TCP[HOST_FORWARD]  11               *  2222       10.0.2.15    22     0     > 0
> 
> However if the guest happens to have DHCP lease (for example, 10.0.2.16),
> the forwarding stops working. The guest can still reach the outer world
> (which is expected).
> 
> This updates the forwarding table when QEMU confirms the requested IP
> to the guest.
> 
> Signed-off-by: Alexey Kardashevskiy <address@hidden>
> ---
> 
> Does this look any useful?


It does not seem like it does very much but .... :)


> 
> Sure I can remove /var/lib/dhcp/dhclient.enp0s1.leases in the guest or
> start QEMU with the DHCP start address equal to what the guest wants to
> reserve but it is quite confusing why such a simple config just does not
> work.
> 
> Found this with the brand new Ubuntu 17.10 which runs dhcp and something
> called "netplan" and the guest ends up with 2 IPs from 10.0.2.x network.
> After disabling netplan, the lease remains and it is not 10.0.2.15 but
> rather .16 or .17.
> 
> Comments? Thanks.
> 
> ---
>  slirp/libslirp.h |  2 ++
>  slirp/bootp.c    |  2 ++
>  slirp/slirp.c    | 27 +++++++++++++++++++++++++++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/slirp/libslirp.h b/slirp/libslirp.h
> index 540b3e5..6779081 100644
> --- a/slirp/libslirp.h
> +++ b/slirp/libslirp.h
> @@ -33,6 +33,8 @@ int slirp_add_hostfwd(Slirp *slirp, int is_udp,
>                        struct in_addr guest_addr, int guest_port);
>  int slirp_remove_hostfwd(Slirp *slirp, int is_udp,
>                           struct in_addr host_addr, int host_port);
> +void slirp_update_hostfwd(Slirp *slirp, struct in_addr old_guest_addr,
> +                          struct in_addr new_guest_addr);
>  int slirp_add_exec(Slirp *slirp, int do_pty, const void *args,
>                     struct in_addr *guest_addr, int guest_port);
>  
> diff --git a/slirp/bootp.c b/slirp/bootp.c
> index 5dd1a41..5876004 100644
> --- a/slirp/bootp.c
> +++ b/slirp/bootp.c
> @@ -225,6 +225,8 @@ static void bootp_reply(Slirp *slirp, const struct 
> bootp_t *bp)
>      /* Update ARP table for this IP address */
>      arp_table_add(slirp, daddr.sin_addr.s_addr, client_ethaddr);
>  
> +    slirp_update_hostfwd(slirp, slirp->vdhcp_startaddr, daddr.sin_addr);
> +
>      saddr.sin_addr = slirp->vhost_addr;
>      saddr.sin_port = htons(BOOTP_SERVER);
>  
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 1cb6b07..a9d8a16 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -1061,6 +1061,33 @@ int slirp_add_hostfwd(Slirp *slirp, int is_udp, struct 
> in_addr host_addr,
>      return 0;
>  }
>  
> +static void slirp_do_update_hostfwd(Slirp *slirp, struct socket *head,
> +                                    struct in_addr old_guest_addr,
> +                                    struct in_addr new_guest_addr)
> +{
> +    struct socket *so;
> +    char oldaddr[17], newaddr[17];
> +
> +    for (so = head->so_next; so != head; so = so->so_next) {
> +        if ((so->so_state & SS_HOSTFWD) &&
> +            so->lhost.sin.sin_addr.s_addr == old_guest_addr.s_addr) {
> +            strncpy(oldaddr, inet_ntoa(old_guest_addr), sizeof(oldaddr) - 1);
> +            strncpy(newaddr, inet_ntoa(new_guest_addr), sizeof(newaddr) - 1);
> +            DEBUG_ARGS((dfd, "Updating forwarding from %s:%d to %s:%d\n",
> +                       oldaddr, ntohs(so->lhost.sin.sin_port),
> +                       newaddr, ntohs(so->lhost.sin.sin_port)));
> +            so->lhost.sin.sin_addr = new_guest_addr;
> +        }
> +    }
> +}
> +
> +void slirp_update_hostfwd(Slirp *slirp, struct in_addr old_guest_addr,
> +                          struct in_addr new_guest_addr)
> +{
> +    slirp_do_update_hostfwd(slirp, &slirp->udb, old_guest_addr, 
> new_guest_addr);
> +    slirp_do_update_hostfwd(slirp, &slirp->tcb, old_guest_addr, 
> new_guest_addr);
> +}
> +
>  int slirp_add_exec(Slirp *slirp, int do_pty, const void *args,
>                     struct in_addr *guest_addr, int guest_port)
>  {
> 


-- 
Alexey



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