My first approach to this was to condition the use of SO_PEEK_OFF with 
tcpv4, e.g., basically
a test like if (v4 && peek_offset_cap) {...} everywhere, but then I made 
an interesting discovery.

It turns out that, unless the ´-4' option is explicitly given on the 
command line, all sockets are
v6, even those that are later used as v4 sockets. So, the set_peek_off() 
call failed even
for supposedly v4 sockets.

I checked this by adding a printout to the tcp_listen_handler(), and 
noticed that all returns from
the accept4() call goes into the AF_INET6 branch, even when the client 
(iperf3) call is using an IPv4 address.
During traffic, the very same socket is marked as v4 in the tcp_tap_conn 
structure, and this seems to
have worked just fine until I added the set_peek_offset call().

I believe this is an issue that has been introduced during the last 
months, since I didn't start
using the ´-4' option consistently until some months ago, and then it 
worked.

Happy summer
///jon


On 2024-07-20 09:54, Jon Maloy wrote:
...
  The recently added socket option SO_PEEK_OFF is not
supported for
 TCP/IPv6 sockets. Until we get that support into the kernel we need to
 test for support in both protocols to set the global 'peek_offset_cap´
 to true.

 Signed-off-by: Jon Maloy <jmaloy(a)redhat.com>
 ---
   tcp.c | 36 +++++++++++++++++++++++++-----------
   1 file changed, 25 insertions(+), 11 deletions(-)

 diff --git a/tcp.c b/tcp.c
 index c5431f1..32026ca 100644
 --- a/tcp.c
 +++ b/tcp.c
 @@ -2717,6 +2717,28 @@ static void tcp_sock_refill_init(const struct ctx *c)
   	}
   }
   
 +/**
 + * tcp_probe_peek_offset_cap() - Check if SO_PEEK_OFF is supported by kernel
 + * @af:		Address family, IPv4 or IPv6
 + *
 + * Return: true if supported, false otherwise
 + */
 +bool tcp_probe_peek_offset_cap(int af)
 +{
 +	bool ret = false;
 +	int s, optv = 0;
 +
 +	s = socket(af, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
 +	if (s < 0) {
 +		warn_perror("Temporary TCP socket creation failed");
 +	} else {
 +		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
 +			ret = true;
 +		close(s);
 +	}
 +	return ret;
 +}
 +
   /**
    * tcp_init() - Get initial sequence, hash secret, initialise per-socket data
    * @c:		Execution context
 @@ -2725,8 +2747,7 @@ static void tcp_sock_refill_init(const struct ctx *c)
    */
   int tcp_init(struct ctx *c)
   {
 -	unsigned int b, optv = 0;
 -	int s;
 +	unsigned int b;
   
   	ASSERT(!c->no_tcp);
   
 @@ -2752,15 +2773,8 @@ int tcp_init(struct ctx *c)
   		NS_CALL(tcp_ns_socks_init, c);
   	}
   
 -	/* Probe for SO_PEEK_OFF support */
 -	s = socket(AF_INET, SOCK_STREAM | SOCK_CLOEXEC, IPPROTO_TCP);
 -	if (s < 0) {
 -		warn_perror("Temporary TCP socket creation failed");
 -	} else {
 -		if (!setsockopt(s, SOL_SOCKET, SO_PEEK_OFF, &optv, sizeof(int)))
 -			peek_offset_cap = true;
 -		close(s);
 -	}
 +	peek_offset_cap = tcp_probe_peek_offset_cap(AF_INET) &&
 +		tcp_probe_peek_offset_cap(AF_INET6);
   	info("SO_PEEK_OFF%ssupported", peek_offset_cap ? " " : " not
");
   
   	return 0;