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

[Thomas Huth <thuth@redhat.com>]

On 07.03.2018 04:39, Alexey Kardashevskiy wrote:
> On 08/02/18 15:29, Alexey Kardashevskiy wrote:
>> 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 <aik@ozlabs.ru>
>>> ---
>>>
>>> Does this look any useful?
> 
> Ping, anyone?

Maybe you should make sure to put the SLIRP maintainer on CC: ?

>>
>>
>> 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)
>>>  {
>>>
>>
>>
> 
>