Re: [PATCH 27/32] tcp: NAT IPv4-mapped IPv6 addresses like IPv4 addresses

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Nov 17, 2022 at 01:15:20AM +0100, Stefano Brivio wrote:
> On Wed, 16 Nov 2022 15:42:07 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > passt usually doesn't NAT, but it does do so for the remapping of the
> > gateway address to refer to the host.  Currently we perform this NAT with
> > slightly different rules on both IPv4 addresses and IPv6 addresses, but not
> > on IPv4-mapped IPv6 addresses.  This means we won't correctly handle the
> > case of an IPv4 connection over an IPv6 socket, which is possible on Linux
> > (and probably other platforms).
> 
> By the way, I really think it's just Linux, I can't think of other
> examples.

Hmm... so descriptions I've seen of the IPv4-mapped IPv6 addresses
seem to imply this is the behaviour in a number of systems. e.g.

https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresses

> > Refactor tcp_conn_from_sock() to perform the NAT after converting either
> > address family into an inany_addr, so IPv4 and and IPv4-mapped addresses
> > have the same representation.
> > 
> > With two new helpers this lets us remove the IPv4 and IPv6 specific paths
> > from tcp_conn_from_sock().
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  inany.h | 30 ++++++++++++++++++++++++++--
> >  tcp.c   | 62 ++++++++++++++++++++++++---------------------------------
> >  2 files changed, 54 insertions(+), 38 deletions(-)
> > 
> > diff --git a/inany.h b/inany.h
> > index 4e53da9..a677aa7 100644
> > --- a/inany.h
> > +++ b/inany.h
> > @@ -30,11 +30,11 @@ union inany_addr {
> >   *
> >   * Return: IPv4 address if @addr is IPv4, NULL otherwise
> >   */
> > -static inline const struct in_addr *inany_v4(const union inany_addr *addr)
> > +static inline struct in_addr *inany_v4(const union inany_addr *addr)
> 
> There must be a reason, but I can't understand why this change is
> needed here.

Because in tcp_snat_inbound() we want to modify, not just examine the
IPv4 address within the IPv6 address.  Ideally the return would be
const if and only if the input is, but C can't express that.  This
appears to be the conventional half-arsed solution (see, e.g. memchr()
or strstr()).

> >  {
> >  	if (!IN6_IS_ADDR_V4MAPPED(&addr->a6))
> >  		return NULL;
> > -	return &addr->_v4mapped.a4;
> > +	return (struct in_addr *)&addr->_v4mapped.a4;
> >  }
> >  
> >  /** inany_equals - Compare two IPv[46] addresses
> > @@ -66,3 +66,29 @@ static inline void inany_from_af(union inany_addr *aa, int af, const void *addr)
> >  		assert(0);
> >  	}
> >  }
> > +
> > +/** inany_from_sockaddr - Extract IPv[46] address and port number from sockaddr
> > + * @a:		Pointer to store IPv[46] address
> 
> This is aa below, I'm not sure why.

Fixed.

> > + * @port:	Pointer to store port number, host order
> > + * @addr:	struct sockaddr_in (IPv4) or struct sockaddr_in6 (IPv6)
> 
> This became sa_ (needless to say, addr would make more sense).

Good call, changed.

> > + */
> > +static inline void inany_from_sockaddr(union inany_addr *aa, in_port_t *port,
> > +				       const void *sa_)
> > +{
> > +	const struct sockaddr *sa = (const struct sockaddr *)sa_;
> > +
> > +	if (sa->sa_family == AF_INET6) {
> > +		struct sockaddr_in6 *sa6 = (struct sockaddr_in6 *)sa;
> > +
> > +		inany_from_af(aa, AF_INET6, &sa6->sin6_addr);
> > +		*port = ntohs(sa6->sin6_port);
> > +	} else if (sa->sa_family == AF_INET) {
> > +		struct sockaddr_in *sa4 = (struct sockaddr_in *)sa;
> > +
> > +		inany_from_af(aa, AF_INET, &sa4->sin_addr);
> > +		*port = ntohs(sa4->sin_port);
> > +	} else {
> > +		/* Not valid to call with other address families */
> > +		assert(0);
> > +	}
> > +}
> > diff --git a/tcp.c b/tcp.c
> > index b05ed6c..fca5df4 100644
> > --- a/tcp.c
> > +++ b/tcp.c
> > @@ -2724,6 +2724,29 @@ static void tcp_connect_finish(struct ctx *c, struct tcp_tap_conn *conn)
> >  	conn_flag(c, conn, ACK_FROM_TAP_DUE);
> >  }
> >  
> > +static void tcp_snat_inbound(const struct ctx *c, union inany_addr *addr)
> 
> What this does is kind of obvious, still a comment would be nice.

Good point, added.  Especially since I'm hoping to share this with UDP
at some later point.

> > +{
> > +	struct in_addr *addr4 = inany_v4(addr);
> > +
> > +	if (addr4) {
> > +		if (IN4_IS_ADDR_LOOPBACK(addr4) ||
> > +		    IN4_IS_ADDR_UNSPECIFIED(addr4) ||
> > +		    IN4_ARE_ADDR_EQUAL(addr4, &c->ip4.addr_seen))
> > +			*addr4 = c->ip4.gw;
> > +	} else {
> > +		struct in6_addr *addr6 = &addr->a6;
> > +
> > +		if (IN6_IS_ADDR_LOOPBACK(addr6) ||
> > +		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr_seen) ||
> > +		    IN6_ARE_ADDR_EQUAL(addr6, &c->ip6.addr)) {
> > +			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
> > +				*addr6 = c->ip6.gw;
> > +			else
> > +				*addr6 = c->ip6.addr_ll;
> > +		}
> > +	}
> > +}
> > +
> >  /**
> >   * tcp_tap_conn_from_sock() - Initialize state for non-spliced connection
> >   * @c:		Execution context
> > @@ -2744,43 +2767,10 @@ static void tcp_tap_conn_from_sock(struct ctx *c, union epoll_ref ref,
> >  	conn->ws_to_tap = conn->ws_from_tap = 0;
> >  	conn_event(c, conn, SOCK_ACCEPTED);
> >  
> > -	if (sa->sa_family == AF_INET6) {
> > -		struct sockaddr_in6 sa6;
> > -
> > -		memcpy(&sa6, sa, sizeof(sa6));
> > -
> > -		if (IN6_IS_ADDR_LOOPBACK(&sa6.sin6_addr) ||
> > -		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr_seen) ||
> > -		    IN6_ARE_ADDR_EQUAL(&sa6.sin6_addr, &c->ip6.addr)) {
> > -			struct in6_addr *src;
> > +	inany_from_sockaddr(&conn->addr, &conn->sock_port, sa);
> > +	conn->tap_port = ref.r.p.tcp.tcp.index;
> >  
> > -			if (IN6_IS_ADDR_LINKLOCAL(&c->ip6.gw))
> > -				src = &c->ip6.gw;
> > -			else
> > -				src = &c->ip6.addr_ll;
> > -
> > -			memcpy(&sa6.sin6_addr, src, sizeof(*src));
> > -		}
> > -
> > -		inany_from_af(&conn->addr, AF_INET6, &sa6.sin6_addr);
> > -
> > -		conn->sock_port = ntohs(sa6.sin6_port);
> > -		conn->tap_port = ref.r.p.tcp.tcp.index;
> > -	} else {
> > -		struct sockaddr_in sa4;
> > -
> > -		memcpy(&sa4, sa, sizeof(sa4));
> > -
> > -		if (IN4_IS_ADDR_LOOPBACK(&sa4.sin_addr) ||
> > -		    IN4_IS_ADDR_UNSPECIFIED(&sa4.sin_addr) ||
> > -		    IN4_ARE_ADDR_EQUAL(&sa4.sin_addr, &c->ip4.addr_seen))
> > -			sa4.sin_addr = c->ip4.gw;
> > -
> > -		inany_from_af(&conn->addr, AF_INET, &sa4.sin_addr);
> > -
> > -		conn->sock_port = ntohs(sa4.sin_port);
> > -		conn->tap_port = ref.r.p.tcp.tcp.index;
> > -	}
> > +	tcp_snat_inbound(c, &conn->addr);
> >  
> >  	tcp_seq_init(c, conn, now);
> >  	tcp_hash_insert(c, conn);
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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